A study of the effects of temperature and photoperiod on vegetative growth and seed production of leaf lettuce (Lactuca sativa L ) : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Seed Technology

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(2) A. STUDY OF THE EFFECTS OF TEMPERATURE AND. PHOTOPERIOD ON VEGETATIVE GROWTH AND SEED PRODUCTION OF LEAF LETTUCE (LACTUCA SATIV A L.). A thesis presented in partial fulfilment of the requirement for the Degree of Doctor of Philosophy in Seed Technology at Massey University. SUTEVEE SUKPRAKARN. 1985.

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(4) iii. ABSTRACT. Thr e e Gr and. leaf. l e t t u c e ( Lac t u c a s a t iva L . ). c u l t ivar s ,. Ra p i d s and S l obo l t were gr own in a c on tr o l l e d. Th a i ,. env i ro nme n t. a t e i ther 2 5 / l 5 ° C or 3 0 / 2 0 ° C d a y / n igh t t emper a t ur e s a n d a t 1 0 , 1 2 or 14 hour s d a y l en g t h . and. lea f. L e a f fr e sh we i gh t , dr y we igh t , l e a f ar e a. number a l l inc r e a s e d w i th inc r e a s ing. p h o t oper i od .. S l obo l t. p roduced a h i gher vege t a t ive. Th a i and Gr and Rap i d s . s t em. e l ong a t ion .. t emper a t u r e. and. yield. t han. Long day s and h i gh t emper a tur e s ha s t ened. F l owe r ing in Tha i and Gra nd Ra p i d s was m a i n l y. inf luenc e d b y h i gh t emper a ture wh er e a s S l ob o l t r eq u i r e d b o t h h i gh t emper a t ur e s and l on g d ay s . de l ayed. in. S l ob o l t howeve r ,. wa s. con s i d e r ab l y. bo l t ing a n d f l owe r ing compar e d wi th the. o t her. two. c u l t ivar s. Le t tuce p lan t s showed two d e f in i t e f lower ing peaks ov e r a 5 0 S lobo l t pr oduced o n l y one peak a t 2 5 / l 5 ° C. - day f l ower ing p e r i o d .. due t o i t s s l ow b o l t ing charac t e r . S e ed in. bo t h. hour s. deve l o pment s tud i e s were c a rr i ed out. g r own. the 2 5 / l 5 ° C and 3 0 / 2 0 ° C t empe r a tu re reg ime s and. day l ength .. ma tur i ty ,. on p l an t s. The. germ ina t i on. ph y si o l og i c a l. at. h i gher t emper a t u re r e g ime h as tened and. s ha t t er ing .. Le t t u ce. s e ed. 12 seed. r e ached. m atu r i t y 1 1 days af t e r an t h e s i s at 3 0 / 20·c and. 13. day s a ft er an the s i s a t 2 5 / l 5 ° C , when the s e ed head h ad j u s t b e gun to. t u rn. brown - gr e en .. ma x imum a t t h i s s t age.. Germinat ion. c a p a c i ty a l s o. S h at t er ing however ,. reached. its. occurr e d about. two. days a f t er s e ed m a t u r i t y at 2 0% mo i s t ur e c on t en t at 3 0 / 2 0 ° C and 4 -. 5 days. l a t er a t 2 5 / l 5 ° C;. The thr e e l e t t u c e cu l t ivar s u s e d in t h i s s t udy prod u c e d s e e d s u c c e s s fu l ly f o und. to. sl igh t l y. in. a l l tr e a tmen t s .. occur. wh en. br own. in. Opt imum t ime o f. the maj o r i t y of c o l our. and. wa s. seed a lso. heads r e duced. harve s t. was. had. tur ned. by. h i gher.

(5) iv. t emp e r a t ur e s. l on g e r day l e n g t h s due to e a r l i e r. Highe s t s e e d y i e l d. f l owe r ing . hours ) .. and. b o l t ing. and. was o b t a ined unde r l ong day s ( 14. Grand Rap i d s produced h i gh e r s e e d yi e l d s th an Tha i. and. S l ob o l t . H i gh. s e e d y i e l d was. re l a t e d to inc r e a s e d b r anch and f l owe r. numbe r s , p e r c en t age o f s e e d s e t o r s e e d numb e r s p er head and t ime of. harve s t .. Good. seed. s e t wa s. obt a ined. on l y. d a y l en g t h s. a t h i gh t empe r a t u r e wh i l e a t 2 5 / l 5 °C,. re�ative ly. un impo r t ant.. by. t empe r a t u re. or. und e r. d a y l ength. F i na l s e e d germina t i on wa s. d ay l ength .. Prac t i c a l. l onger wa s. una f f e c t ed. app l i c a t ion. of. the. r e su l t s wh ich are r e l evant t o Tha i land cond i t ions are d i s c us s e d ..

(6) V. ACKNOWLEDGEMENTS. I. w i s h to expr e s s my s inc e r e t hanks t o Dr M J H i l l f o r. s u pe rv i s i on ,. his. und e r s t and ing and k ind encouragement t h r oughout. s tu d y and e s pe c i a l ly f o r h i s cons t r u c t ive c r i t ic i s m , i n t e r p r e t ing. my. p a t i ence i n W i thout. the r e s u l t s a n d c o r r e c t ion o f m y Eng l i s h .. his h e l p , this wo rk wou l d n o t have been po s s i b l e. S in c e re thanks are a l s o due t o : Profe s s or c o n s t ruc t ive. B. R. Wa tk in,. for. his. va luab l e. s ugg e s t ions ,. c r i t i c i s m , h e l p in d i s c u s s ing the r e s e ar c h p r ob l ems. and a l s o r e ad ing the manu s c r i p t . Mr I J War r ing t on , c o n s tant. P l ant Phy s i o l ogy D i v i s ion , DS I R f o r h i s. h e l p and in provid ing in forma t i on and. wi l l in g l y. go ing. t hrough the d r a f t of the t he s i s . Pro f e s s o r T W T i bb i t t s ,. Ho r t i cu l tu re D epar tmen t , Un iv e r s i ty. o f W i s cons in , for h i s in t e r e s t and advice a t the b e g inn ing o f t h e s tudy. Mr s. D. E. th e i r f r i end ly. M Me e c h and s ta f f o f S e ed Techno l o gy. Cent re. for. encour a g ement a n d a l l the t e chn ic a l h e l p . I owe a. g r e a t d e b t t o them . Dr D C Mo rgan and s t a f f o f th e Cl ima t e Labora t o ry , t he i r. h e l p and ma intenance o f th e cont ro l le d. DSIR for. env i r onmen t. r ooms. u s e d in t h i s s t udy . P r o f e s s o r RJ Town s l e y and Dr I L Gordon for th e i r a d v i c e in s t a t i s t i ca l prob l ems. Mr. R. C S e ddon f o r h i s h e l p in c ompu t ing th e d a t a and. a lso. r e a d ing t h e manu s c r i p t . Mr L Ma iden and s t a f f o f C e n t r a l Pho t o gr aph i c Un i t f o r t h e i r ph o t ogr aph i c wo rk . Mrs. A M Dav i e s and Mr s. t y p ing t h i s the s i s.. Mr G. F. S W i c he r t s f o r the i r. pa t i en ce. Ha l l igan for th e i l l u s t r a t ions .. in.

(7) vi. The. New. Zea l and Gove rnment for prov i d ing. the. S ch o l a r s h i p. wh i c h enab l e d m e t o und e r t ake t h i s s tudy . I e s pec i a l ly wan t t o t hank my hus band and ch i l dren for t h e i r und e r s t anding .. Mr s K . M c dona l d f o r her k indne s s i n p rov i d ing me .. wi th a rea l h ome dur ing my s t ay in New. Zea l and .. fr i end for h e r mo r a l support and encouragement .. Sunan t a ,. my.

(8) vii. CONTENTS. Page Ab s t r a c t. iii. Ac know le dgeme n t s. V. L i s t o f Tab l e s. X. L i s t o f F i gu r e s. xi. L i s t of P l at e s. xii. L i s t o f Append i c e s. x iv. I NTRODUCTI ON. 1. CHAPTER 1. 4. REV I EW OF L I TERATURE. 1.. Gen e r a l descr i p t ion o f l e t t u ce . . . . . . . . . . . . . . . . . . .. 2.. Tempe ra ture and p h o t oper iod e f fec t s on growth and deve l opmen t . . . . . . . . . . . . . . . . . . .. ,. ..... .. . . . . . . . .. . .. 3.. Tempera ture and pho t oper i od e f fec t s on f l ower ing .. 4.. Temperature and pho t oper i od i n t e rac t ions w i th verna l i z a t ion on f l owe r ing. .. .. .. 5.. S e e d qua l i ty and g e rminat i on .. 6.. S e e d pr oduct ion pract ice s .. 7.. S e e d deve l opmen t .. CHAPTER 2. .. .. .. .. .. PRE L IMINARY STUDY. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .... . . .. .. .. ... .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. . ·. .. .. .. . . . . .. .. .. .. .. .. •. .. ... ... .. .. ;. 5 7 8 11 12 16 22 26. MATERIALS AND METHODS. 28. RE S ULTS AND D I S CU S S I ON. 30.

(9) vi i i. CHAPTER 3. VEGETATIVE GROWTH. 40. MATERIALS AND METHODS. 41. RESULTS AND DI SCUSS ION. 45. 1.. P l ant f r e s h we i gh t . . . . . . . . . . . . . . . . . . . . . . . . . .. 45. 2.. P l ant dry we i gh t . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 50. 3.. L e a f area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 53. 4.. L e a f numbe r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 56. 5.. S t em l en g t h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 59. 6.. Numbe r o f s hoo t s . . . . . . . . . .. 64. CHAPTER 4. .. ... · .. .. .. .. ... .. . . . . .. REPRODUCT IVE GROWTH , FLORAL I N I T IATION , FLOWERING PATTERN AND SEED PRODUCTION. 66. MATERIALS AND METHODS. 67. RESULTS AND D I SCUSS ION. 75. 1.. F l ower ing pat t e rn . . . . . . . . . . . . . . . . . . . . . . . . . . .. 75. 2.. S e e d product ion . . ; . . . . . . . . . . . . . . . . . . . . . . . . . .. 83. CHAPTER 5. SEED MATURAT I ON AND V IAB I LITY. 88. MATERIALS AND METHODS. 89. RESULTS AND D I SCUSS ION. 91. 1.. Change s in s e e d we i gh t . . . . . . . . . . . . . . . . . . . . . . .. 91. 2.. Mo i s ture. content s . . . . . . . . .. . . . . . . . . . . . . . . . . . . . .. 92. 3.. S e e d s ha t te r ing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 95. 4.. S e e d co l our chang es . . . . . . . . . . . . . . . . . . . . . . . . . .. 102. 5.. Germina t ion and v i ab i l i t y . . . . . . . . . . . . . . . . . . . .. 103.

(10) iX. CHAPTER 6. SEED YIELD AND QUAL ITY. 109 l lO. MATER IALS AND METHODS. . l l2. RE SULTS AND D I SCUS S ION 1.. He i g h t. 2.. B r anch numbe r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . 1 14. 3.. Seed yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 6. 4.. G e rmina t i on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i 2 1. 5.. S e e d we i gh t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2 3. CHAPTER 7. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. GENERAL D I SCUSS ION AND C ONCLU S I ON. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. 112. 126. B I BLI OGRAPHY. 140. APPENDICES. 156.

(11) X. TABLES. Ta b l e 2. 1. Varie t a l charac t e r i s t i c s in a range of d i f f e r e n t l e a f l e t tu c e c u l t iva r s. 29. 3.1. E f fe c t o f t empe r a t u r e , pho t o pe r iod and cu l t ivar on fre s h we ight at each harve s t. 46. 3.2. E f fe c t s o f t empe r a t u r e , pho t o p e r i o d and c u l t ivar on f r e s h we i gh t a t each h a rve s t. 51. 3.3. E f fe c t s o f t empe r a t u r e , pho t o pe r i od and c u l t ivar on l e a f area a t each harve s t. 54. 3.4. E f fec t s o f t empera ture , pho t op e r i o d and c u l t ivar orr l e a f numb e r at ea ch harve s t. 57. 3.5. E f fec t s o f t emper a t u r e , pho t op e r i od and cu l t ivar on s tem l ength at e ach h a rve s t. 60. 3.6. Numb e r o f shoo t s 4 0 days a f t e r s ow ing. 65. 4. 1. E f fe c t s o f tempe ra ture , pho t o p e r i o d and c u l t ivar on l e t tuce f l ower i ng. 76. 4. 2. E f fec t s of tempera ture , day l ength and c u l t ivar on f lo r e t numbers , s e e d numb e r s per head a n d p e r cen t age o f s e e d s e t a t e a c h p e ak f l ower in g. 84. 6.1. E f fe c t s o f t empe rature , daylength and cu 1 t ivar on p l ant h e ight at each peak f l owe r ing. 1 13. 6.2. E f fe c t s of t empe rature , day l eng t h , cu l t ivar and ha rve s t t ime on b ranch numb e r s. 6.3. E f fec t s o f tempe rature , dayleng t h , cu l t ivar and harve s t t ime on s e e d y i e l d. 6.4. E f f e c t s o f t empe rature , day leng t h , cu l t ivar and ha rve s t t ime on g ermina t ion pe rcen tage. 122. 6.5. E f f e c t s of t empe rature , day leng t h , cu l t ivar and harve s t t ime on s e e d we ight. 124. 115. 117.

(12) xi. F IGURES. Figure 3. 1. Number of. green lettuce leaves for each. cultivar at different. tern perature. 58. and. daylength 3. 2. Stem length. of three lettuce cultivars. each tern perature 4. 1. Diagrammatic and. 4. 2 4. 3. branching. of the innorescences. anthesis and peak. Percentage and. 5. 4. 6. 1. 93. physiological components. dtn'ing. 94. at. 30/20 C •. of germination. viability at at each. (normal se edling). 104. 30/2o·c. Percentage of germination. Seed yield. 81. 25/15•C. seed viability at. and seed. 80. during. seed development 5. 3. 77. 30/20 •C. Changes in physioloigcal corn ponents. Changes in. 74. 25/15•C. Flowering pattern of three leaf lettuce cultivars. seed development at 5. 2. nowering. pattern of three leaf lettuce cultivars. under different daylengths at 5. 1. 61. of leaf lettuce. under different daylength at 4. 4. at. daylength. representation. Days to bolting, Flowering. and. (normal seedling). 105. 25/15 C •. daylength,. temperature. and time of harvest in three lettuce cultivars. 118.

(13) xii. PLATES. P la t e 2.1. B l ack s e e de d l e t t u c e ( Th a i cv . ) a ) in the f ie l d b ) as p r e s e n t e d f o r the mark e t. 2.2. Typ i c a l Tha i d i she s p r e pared f o r the t ab l e. 2.3. A p l ant o f c u l t ivar a ) Tha i b ) Grand Rap i d s a t 6 0 d a f t e r s owing. 32. 2.4. A p l an t of c u l t ivar a ) S l ob o l t b ) Wa l dmanmn ' s G reen a t 6 0 d a f t e r s owing. 34. 2.5. A p l an t cu l t ivar a ) Deep Red b ) P r i z e head at 6 0 d a f t e r s owing. 35. 2.6. A p l ant cu l t ivar a) Br a z i l 48 b ) Br a z i l 2 2 1 a t 6 0 d a f t e r s owing. 36. 2.7. A p l an t o f c u l t ivar a ) Summe r Queen b ) Tan i a at 60 d a f t e r s owing. 37. 2.8. A p l an t of c v . S a l a d Bow l at 60 d a f t e r s owing. 39. 3.1. V i s ua l appearance of l e t tuce p l an t s a t 4 0 d a f t e r s owing. 49. 3.2. S t em and s h oo t d eve l o pmen t charac t ers in thre e l e a f l e t t u c e c u l t iv a rs und er 10 and 14 h day l en g th s at 2 5 / l 5 ° C and 3 0 / 2 0 ° C. 62. 4.l. F l ora l in i t i a t ion. 68. 4.2. F l ower deve l opme n t. 69. 4.3. Le t t u c e f lower h e ad. 70. 4.4. Le t t u c e s e e d. 71. 4.5. An ind iv i du a l l e t tu c e p l ant inf l o r e s cence w i t h a c lu s t e r of s eed head s in v a r ious s t ag e s o f deve l o pmen t. 73. 4.6. Ind i v i dua l s eed heads. 73. 27. 31.

(14) xiii. P late 5.1. Tagged f l ower o f l e a f l e t t uce cv . Grand Ra p i d s a few hours a f t e r anthe s i s. 5.2. Chang e s in s e ed head and s ee d co lour at 25 / l 5 ° C in a ) Tha i b ) Grand Rap i d s c ) S lobo l t. 96 97 98. Change s i n s eed head and s e e d c o l o u r at 30/20°C in a ) Tha i b ) Grand Rap ids c ) S lo b o l t. 99 100 101. 5.3. 5 .4. Examp l e s o f the t ype s o f abnorma l s e e d l ings f o und in s e eds removed from l e t t u c e p l an t s dur ing the s t age of seed deve l o pmen t. 90. 108.

(15) xiv. APPENDICES. App end ix l.. C l ima t i c d a t a at Pakchong Hor t i c u l tu r a l. Re s e arch S t at ion. 2.. C l ima t i c S t at ion. 3.. The l eve l s of pho t o s yn the t i c rad i a t i on of t rea tment in con t r o l l ed env i ronment ro oms d i f ferent t empe r a t u re and day length t re a tmen t s. 4.. C l ima t e nu t r i e n t. 5.. Re l a t ive g r owth l e a f area ( RLGR ). 6.. Tempe r a t u re , daylength we ight ( g / p l ant ). and cu l t ivar e f f e c t s. on f r e s h. 7.. Temp e r a ture , day length we ight ( g / p l ant ). and c u l t ivar e f f e c t s. on dry. 8.. Tempe r a t u r e , daylength area ( cm 2 / p l an t ). and c u l t ivar e f fec t s. on l e a f. 9.. Tempe r a t u r e , f e r t i l i ty .. and cu l t ivar e f f e c t s. on s e e d. data. at. Lab. Roya l Angkhang. - mod i f i ed. H i gh l and. ha l f - s t r ength. f r e s h we i gh t (mg ) a t d i f ferent. each unde r. Ho a g l and ' s. r a t e s o f p l an t dry we igh t. d a y l ength. Re s e arch. ( RG R ). and. 10 .. 1 00 s e e d anthe s i s. 11 .. 100 s e e d anthe s i s. 12.. Mo i s t u r e content ( % ) a t d i f f e r e n t. 13 .. Germin a t ion percentage ( norma l s e e d l ing s ) a t d i f fe re n t d a y s a f t e r anthe s i s. 14 .. Germin a t ion percentage ( abno rma l s e edl ings ) a t d i f fe r e n t d a y s a f t e r an t he s i s. 15 .. S e e d v i ab i l i ty a t d i f fe rent days a f ter anthe s i s. 16.. Tempe r a t u r e , day l ength y i e l d ( g / p l ant ). 1 7.. Tempe r a t u r e , day leng th germina t i on percentage. and. c u l t ivar. e f fec t s. on. 18 .. Tempe r a tu r e , day length and 1000 s e e d dry we ight (mg ). cu l t ivar. e f fe c t s. on. dry. we ight (mg ) a t d i f ferent days. day s. a f ter. days. a f te r. a f t e r anthe s i s. and cu l t ivar e f fec t s on. s eed.

(16) 1. I N TRODUCT I ON. Lea f l e t tu c e i s the mo s t popu lar s a lad c r op in Tha i l and . is. gr own. both. comme r c ia l ly throughou t the year and i s. the loca l marke t and for home u s e. It. produced. for. in a reas th roughout. the. I t s produc t ion and consump t ion have r i s en marke d ly over. coun t ry .. recent years in l ine with popu l a t ion increa s e s . Le a f. let tuce. d eve lo p s. best. mo d e r a t e . s e a s on". is. dur ing. early the. ma t u r ing and c o o l month s. The c o o l mon t h s are a lway s in. Tha i l and ,. mak ing. them. easy. when. to. the. g row .. It. r a in f a l l. is. a s s o c iated w i t h also. s u i t a b le. the "dry for. seed. produ c t i on .. Howeve r , pr o f i t is a maj o r c oncern t o Tha i fa rmer s ,. who. to. pre fer. consump t ion t han. impor ted ,. on. growing. l e t tuce. w i t h i t s as s o c i a t e d qu i ck f inan c i a l s ee d .. for. c oncen t r a t e. for. ma rke t. re turn ,. Mo s t o f the l e t tu c e s ee d s own in. rather. Tha i l and. is. a l though s ome sma l l quan t i t i e s o f seed are produced in. t h e coun t ry . S ince l e a f l e t t u c e i s n o t a da p ted t o l ong d i s t ance o r t r ans por t , po s s ib l y to. g rea t d i s t an c e from the marke t a r e. b e t t e r s u i t ed t o s eed produc t ion r a ther t han a t t emp t in g. su p p l y. pr oduce s ee d. t h o s e areas a t. s h i pmen t. the f r e s h marke t .. Some s e e d g r owe rs have. tried. l e t tuce s ee d b u t have genera l ly b e e n d i s couraged b y. y i e l d s and qua l i t y .. I t i s l i ke l y t h a t t h i s. l ow. d i s appoint ing. re t urn from s e e d produc t i on in l e a f l e t t u c e ha s occurred l ack. to. through. o f informa t i on about the d e s i r ab l e a g ronomic cond i t ions and. r e q u ir emen t s. for maximi s ing s e e d y i e ld .. ove r c ome th e s e d i f f icu l t ie s ,. I f s eed growe r s. c ou l d. l e t t uce s e e d produc t i on in Tha i l and. m i ght be more s o c i a l ly a c c e p t ab l e and f inanc ia l ly succe s s fu l . A from. numbe r o f inve s t iga t o r s have s tud i e d the. fl owe r ing. v i ew. point. of enhanc ing. seed. l e t tuce s t alk. produc t i on. forma t ion. and. or d e v i s ing me thods of t re a t ing l e t tuce p l an t s to make.

(17) 2. Al thou gh. f lower .. t hem. pho t op e r i o di c T incker ,. e f fec t. 1933 ) ,. pho t oper iod. in. it. early. l e t tuce. has. o b s ervat ions ( Ga rner. been. and. shown. r e s pons e i s cond i t i oned by t empe rature. S t ruckmeye r ,. 1 9 38 ) . wh i l e. f l owe r ing. 1 9 20 ;. A l l ard ,. s u b s e quen t ly. a. sugge s t e d that. t he. ( Ro b e r t. and. Hi gher t emp e r a t u r e s acce l e r a t e bo l t ing and long. days. promo t e. and. sho r t. days. d e l ay. re produ c t ive deve l o pment ( Thomps on and Kno t t , 1 9 3 3 ; Kr i ck l , 1 9 5 4 ; Rappapo r t and W i t twe r , 1 9 5 6 , a , b ; Ra l e i gh , 1 9 5 9 ; I t o e t a l . , 1 9 6 3 ; C y t ov i c , 19 6 7 a ; H i rao ka , 1 9 6 7 , 1 9 6 9 ) . Jones. ( 1927 ). and S o f fer and Smi t h. l e t tuce. showed. e i ther. t emp e r a t u r e. f l owe r ing. de f in i t e peak f l owe r ing or. pa t t e rn .. t empe ratures und er. a. ( 1 9 74a ). l i ght. Be s t. and. in t ens i t y. repor t e d. that. sugge s te d. that. might. i n f l uence. s e ed s e t h a s b een obs erved. at. b e twe en 1 7 - 2 3 ° C unde r s hor t days a f t e r. long. da ys. ( Ko l le r ,. h a s t ened by h i gh t empe r a t u r e s. 1962 ) ,. a l thou gh. ( Jone s ,. s eed. 1927 ;. the n i gh t. indu c t ion. ma t u r i t y. G l ob e r s on ,. is. 1981 ) .. D i f f i cu l t i e s. in t h e germina t ion o f l e t t u c e seeds have a l s o. r e por t e d. by. B o r t hwick and Robb ins ( 1 9 2 8 ) and by Ha rring ton. Thomp s on. ( 1 9 5 2 ) b ut can b e overcome b y u s ing only. fu l ly. b een and. ma t u r e. s e e d ( Thomp s on , 1 9 3 6 ) , s ince maximum s e e d g ermina t ion capac i ty i s no rma l ly. reached. one. or. two. days. b e fore. s eed. ma t u r a t ion. ( G l ob e r s on , 1 9 8 1 ) . t he s e. Al though. t ime. fac t o r s undoub t e d l y a f f e c t s eed of. harve s t can a l s o. be. the. pr odu ced. fr om. p r oduced. later. d e l ay ing. harve s t much o f the seed y i e ld can be l o s t. sha t t e r ing eme rge. f lowe r s. a re. in the s e a s on ( So f fer. ( Hawt horn. and Po l lard ,. and. 1 9 54 ) .. heav ier Smi th , La rger. and Seeds. impor ta n t .. qua l i ty ,. e a r ly- formed. yield. thos e. than 1 9 74 a ) .. because s eeds. By of also. fas t e r and produce more v i g o r o u s s eed l ings ( Sm i t h e t a l . ,. 1 9 7 3b ) . Under comme r c i a l harv e s t ing cond i t ions , hand l e d. at. popu l a t ion. the. one t ime .. As a r e s u l t ,. the en t i re crop plants. have. a. is seed. t h a t h a s been e xposed t o d i f ferent we ath er cond i t ions.

(18) 3. for. vary ing. d i f feren t. p e r i od s o f t ime and a l s o b e a r s eeds s tages. of. seed. ma tur i ty .. wh ich. are. at. Th i s. can. resu l t. in. on. t he. e f fec t. of. s i gn i f icant d i f f e r ence s in s eed qua l i t y . Little. work. environment l e t t uce .. on. has. b e en. c ar r i ed. out. l e t tuce s eed produc t ion , par t i cu l a r l y w i t h. leaf. However , the env i ronment inc lud e s s o many i n t e rr e l a t e d. f a c tors t h a t i t i s o f ten d i f f i c u l t t o d e t e rmine t h e ma j o r. factor. or fac t o r s a f fe c t in g p l an t growth and d ev e l opment .. Th i s sugg e s t s. that. env i ronmen t a l. in. a t t emp t ing t o s e p a r a t e and ana l y s e the s e. f a c tors e f fec t ive l y , i t i s o f ten nece s s ary t o condu c t exper imen t s under cont r o l l e d environment cond i t ions . The ob j e c t i v e s o f t h i s s t udy we re t o d e t e rmine how var i a t i on in. t emp e r a t ur e. and. pho t ope r i od. a ffected. ve ge t a t ive. g r owt h ,. f l owe r ing and f l owe r ing pa t t e rn , s e e d deve l opmen t , s e e d y i e l d and seed. qua l i ty. in. leaf. l e t tuce. c u l t ivar s .. The. a l t e rna t ing. t emperatures and r anges o f day l eng t h u s e d were chosen to s imu l a t e a s c l o s e l y a s p o s s i b l e the type s o f env i ronmen t a l range l ik e l y t o be expe r ienced i n Tha i l and . the. o p t imum. qua l i t y . becau s e Tha i land .. t ime The. of. of. At t emp t s were a l s o made t o d e t e rmine. harve s t to ensure. c u l t iv a r s. h i gh. s eed. u s e d i n t h i s e xper iment. yie ld were. and. chosen. t h e i r s imi l a r i t y t o l o c a l l e t tuce cu l t i va r s u s e d. in. Thr e e c u l t ivar s ( Th a i , Grand Rap ids and S l ob o l t ) we r e. cho s en for fur t h e r s tudy f o l l owing in i t i a l s creen ing t r i a l s o n number o f d i f f e r e n t l e a f l e t t u c e cu l t ivar s .. a.

(19) 4. CHAPTER. REV I EW. The qui t e. amount la rge .. c oncen t ra t e d. OF. 1. L I TERATURE. o f work on l e t tuce ci t ed in the. li t e ra ture. The. however ,. ma j ori ty. of. t hi s. work ,. on t h e r e a c ti on o f l e t tu c e p l an t s t o. f a c to r s - p a r ti cu la r ly t o day l ength and t o t hi s. wo rk. t he s e. has. envi r onmen t a l. t empe r a t u r e .. h a s c ri ti c a l l y exami ned the r e a c ti on. is. of. Much o f. l e t tu c e. to. vari a b l e s during v ege t a tive growth and thei r e f f e c t o n t h e. d e t e rmining the ons e t o f f l o r a l ini ti a ti on . Compar a ti v e l y c ri ti c a l ly. li t t le. eva l uat e d. the. work. h a s b een. i n f luence. of. pub li shed. whi ch. has. envir onmen t. on. seed. d eve l o pment o r e f fe c t s o f envi r onmen t a l condi ti ons o n s ee d yi e l d , yi e l d compone n t s and s ee d qua li t y . A and. know l edge o f the c ondi ti ons whi ch pred e t e rmine. s eed. l e t tuce. f o rmati on s e ed .. c ommon. i s u s e fu l in t h e s u c c e s s fu l. B o l ting. o r pr ema ture s e e di ng i n. f l oweri ng. p rodu c ti on l e t tu c e. of. is. a. sou r c e o f lo s s si nce i t d e s t ro y s market a c c e p t abi li t y and. there fore the va lue of the vege t a tive p l ant . charac teri s ti c. is. a. na tur a l. one whi ch i s. Neve r the l e s s e s s en ti a l. fo r. t hi s the. produ c tion o f s e ed . Le t t u c e g r owing d emands c lo s e a t t en ti on to de t ai l a s r e g a r d s cul tural. me t hod s ,. impo r t ant. part .. invo lve. the. whi l e. c limati c c ondi ti ons a l s o p l ay. a. very. The c u l ture o f l e t t u c e for marke t u s e d o e s n o t. p r odu c ti on o f the. i n f l o r e s cence. and. s eed .. Seed. gr owe r s , by c o n t ra s t , have t o c a r r y thei r crops through a f u r t h e r s t age. o f growth in o r d e r t o o b t ain a s a leab le produ c t. and P o l lard , 1 9 54 ) .. ( Hawthorn. .I.

(20) 5. 1 . Gene r a l De s c r ip t ion of Le t tuce ( Le t tu c e Its. has. or i g in. is. been in cu l t iva t i on f o r a t l e a s t b e l i eved t o be in inner. Turk i s tan ( Du r s t , b e longs. L. ). Lactuca sativa. 1 9 15 ;. He d r i c k ,. As i a. 1919 ) .. 2 5 00. Min or ,. years .. I r an. and. I t is an annu a l wh i c h. t o t he s un f l owe r fam i ly o r C ompo s i tae fami l y. ( Mc C o l l um ,. 1 9 80 ) and i s one o f about 3 0 0 s p ec i e s ( Ko s t e r , 1 9 7 6 ) in the genus Lac tuca ( m i l k - forming ) . Th e r e. are s ix morpho l o g i c a l type s. of. l e t t uc e ;. c r i s phe a d ,. but t e rhead , c o s , le a f , s tem and l a t in ( Thomp s on , 1 944 ; Rodenbu r g , 1 9 60 ) .. The. c r i s pa ). are g r own pr inc i pa l ly in the t r o p i c s .. heads. leaf. or. bunch ing. cu l t ivars ( Lactuca s a t iva v a r . They do n o t f o rm. but d eve l o p a more open type o f g rowth .. produce rs ,. and. are. easy. to. grow. under. They ad ve r s e. e s pec i a l l y in s ummer ( He rk l o t s , 1 9 7 2 ; Ryd er , 1 9 7 9 ) . cu l t ivars Rap i d s ,. in. this. Pr i ze. group inc lude B l a ck. Head ,. Domineer ,. a re. Seeded. Ruby , Sa lad. early. c ond i t i ons Be t te r known. S impson ,. Bowl. and. Grand S l obo l t. ( S hoemake r , 1 9 5 3 ; Herklo t s , 1 9 7 2 ; McC o l l um , 1 9 80 ) . Le t tu c e at. r a inf a l l max imum. day. requires ha rve s t. l ong wa rm g r owing. a. t empe r a t u r e s o f about 2 2 ° C ,. gr ows. but. coo l. at. l ow mean. n i gh t s. a re. howeve r ,. The l e a f l e t t u ce ,. 1974 ) .. a. and. Mo s t cu l t ivars grow b e s t. t ime .. e s s en t i a l ( Wh i t aker et a l . ,. s e a s on. mo r e s u c c e s s fu l l y a l l year round and i s not heat s ens i t ive. ( He rkl ot s , 1 9 7 2 ; Ryd e r , 1 9 7 9 ). Cu l t iva t e d long - day. p l an t. tempe r a t u r e s. l e t tuce at. ( Lactuca s a t iva L . ). ( V in c e - Prue , p r ima r i ly. pho tope r i od. is. less. by. S e ed - s ta lk. 1975 ) . h igh. e f fec t ive. ( Thomp s on and Knot t , 1 9 3 3 ) . wh i le. a quan t i t a t ive. h i gh tempe r a t u r e and i s day neu t ra l. i n f l uenced. deve lopment. is. t emperature in. a f fe c t ing. at. deve l o pment ( 21 - 26°. C). l ow is and. s ee d. fo rma t ion. Long days ( 1 6 h ) promo t e. s ee d- s ta lk. s h o r t days ( 9 h ) d e l ay i t in Gr e a t Lake s. l e t tuce ( Rappaport and W i t twer , 1 9 5 6 a) .. head.

(21) 6. The head s , wh ich. i n f l o r e sc ence i s a pan i c l e cons i s t ing of a e a c h c on t a in ing f r om 1 5 to 2 5 or mo re f l ore t s .. t e rmina t e s the i n f l o r e s cence is t h e o l d e s t .. l ateral and. c lu s t e r. head s a r i s e l a t e r in the ax i l s of the. Pol lard ,. 1954 ) .. The. of. The head. S e c on da ry or. leav e s. ( Hawthorn. l a t e r a l branches a l s o have. t e rmina l. f l owe rs wh ich a r e the f i r s t to open on t h e i r re spec t ive b r anche s . P o s i t ion. on. the. in f lo r e s cence. is. genera l ly. not. re l a t e d. to. f l owe r ing t ime ( So f f e r and Smi t h , 1 9 7 4a ) . The f l owe r s are per fec t and the y e l l ow cor o l l a i s symp e t a l o u s and f ive l ob e d .. The r e are f ive s t amens , each a t tached s ep a ra t e ly The anthers are un i t e d t o fo rm. a t the ba s e o f the coro l la tube .. The ovary i s one - c e l l e d ,. a she at h . c a rpe l s .. The. s ty l e. is h i- l obed .. i n fe r i or ,. and con t a in s two. The fru i t is. a chene w i t h a t u f t o f h a i r s a t t ached ( Jone s ,. a. 1927 ) .. one - s ee d e d The s e e d i s. a s p ind le form w i th a numb er o f long i tu d ina l r i bs o n the. s u r f ac e. ( I sely , 1947 ) . Cu l t iva t e d 1 - 6. is. a lmo s t en t ir e l y se l f - po l l in a te d b u t. percent c r o s s po l l ina t i on b y 1 9 34 ) .. ( Thomp s on ,. i n s ec t s. h as. b e en. reported. The f l ower s a re genera l ly open for on l y 1 - 2 h. Howeve r , on b r ight days they may only b e open fo r h a l f. each day . an. let tuce. for. hour whe r e a s on coo l c loudy morning they may r emain open. s evera l hours ( Jone s , 1 9 2 7 ) . F l owe r ing period. and. cont inu e s. on a s in g l e p l an t over a. s hows d e f i n i t e peaks .. fa i r l y. Over 9 0% of the. s ee d. l ong y ie l d. c ome s f r om f l owers wh i c h open dur ing t h e f i r s t 3 5 day s ( d ) S o f fer and. Smi th ,. anthe s i s ( J one s , at. 1 9 7 4a ) .. S e e d s a re r i pe w i t h in 1 2 - 1 3 and 1 7 d a f t e r. f r om p l a n t s h arve s t ed a t 2 0 ° C 1 9 2 7 ; G l ob e r s on , 1 9 8 1 ) .. and. 25°C ,. r e s pe c t ive l y. Be s t s e ed s e t h a s been r e p o r t e d. nigh t t empe r a t u r e s be tween 1 7 a n d 2 3 ° C under s h o r t d a y ( 8. cond i t i ons. ( Ko l le r ,. 1962 ) .. The. numb er o f norma l ly. h). d ev e l op ed. achene s av e r ag e d 1 6 . 2 s e e d s /head ( Jone s , 1 9 2 7 ) . In genera l , t h e l e a f type s o f l e t tuce produce mu ch mo r e s ee d than d o t h e h e ad ing t ype s .. Re s p e c t iv e ave rage s ee d y i e l d s o f 5 0 0.

(22) 7. kg/ha. and. 1 0 0 kg/ha have been repo r t ed (Hawt horn. and. Po l l a rd ,. 19 5 4 ) . Germina ti on temperature s . t emperature. of. l e t tu c e. The of. o p timum. about. seed for. is. inhi bi t e d. g erminati on. 24 ° C .. Above. 26°C. is. by a. hi gh. c on s t a n t. g ermi n a ti on. is. increa si n g l y inhi bi t ed ( Bor thwi c k and Robbins , 1 9 2 8 ) .. 2.. and. Tempe r a t u r e. Pho t operi od. E f f ec t s. on. and. Growth. Deve l opment Si nce impor tance c arri e d. the c u l ti vati on of l e t t u c e unde r g l a s s is of d u ri ng t h e win t e r peri od ,. many experimen t s have b een. out wi t h the purpo s e o f o b t ai ni ng optima l. vari a b l e. c ondi ti ons. i n t ens i ty .. M a j or. of. great. t empe r a t u r e ,. growth. pho t operi od. under. and. li ght. emphasi s in thi s r e s earch has been p l a c e d. on. b u t t e rhead l e t tuce . Brouwe r. and Huy s k e s ( 1 9 6 8 ) s tu di e d the b ehavi our o f. p l an t s. o f the c u l ti v a r Rapi d e and the hybri d Rapi d e x Hamadan , g rown in contro l l e d c limate rooms , d ay l engths l e ve l s , B o th. a t a t empe r a ture o f 2 0 ° C and und e r two. ( 8 or 1 6 h ) each combi ne d wi th three li gh t. and. under. c u l ti v a r s. a 1 2 h day l e n g t h wi th one. r e s ponde d. t o i n c r e a sing. li gh t. li gh t. i n t en si ty i n t en si ty .. i n t ensi t y. more. e f f e c ti ve l y when grown und e r long day condi ti ons t han s h o r t days . Growth cases .. r a t e s incre a s ed wi t h i n c r e a si n g li gh t int ensi ti e s in Be n si nk. t emper a t u r e , growth rooms .. ( 1958 ,. 196 1 ,. L e a f produc ti on wa s approxima t e ly cons t ant , i ncreased. wi t h. t emperature. c ons t an t. li ght i n t en si ty .. posi t ive l y. of. dayleng t h , li ght i n t en si ty and other fac t o r s o n the. c ons tan t. l e ngth. 1 9 7 1 ) exp lored the i n f l uence. of but t erhead cu l tivars Mei koningen and Rapi d e in. produc tion. all. and. i n c r e a si ng wi t h. li gh t. i n c reasing. growth. the r a t e o f. i n t ensi t y. t empe r a t u r e. at at. a a. L e a f wi d th and leng t h b o t h r e s ponde d. t o inc r e a sing daylength and li gh t intensi t y b u t. w a s more r e s p onsi ve t han l e a f wi d th t o change s und e r. li ght leve l s or und e r shor t days .. S ub s e quent deve l o pmen t. leaf l ow und er.

(23) 8. l ight i n t en s i t y o r und er long day c ond i t ions ,. h igh the. le ave s b e c oming s u c ce s s ive l y b road e r .. and. sho r t. r e s u l t ed. Low l i gh t. in t ens i t y. d a y s tended t o ma in t a in the produ c t i on o f. long narrow l e a ve s .. r e l at iv e l y. The e f fe c t o f t empe rature was a l s o d e p endent. u pon l i gh t i n t en s i ty . leaf. in. At h i gh l igh t l e ve l s , there was a pos i t ive. e f fe c t. on. wid th with inc e a s ing t empe ra ture. e f fect. a t l ow l igh t i n t ens i ty .. ( 1973). demons t r a t e d. L ikew i s e ,. and. ne g a t ive. B ierhu i zen. that a rapid l inear r i s e. in. et. fresh. al . we i gh t. o c curred w i th a n inc r e a s e in rad i a t ion and h igh tempe r a t u r e . T ime to. fu l l. a l so. p l ant deve l o pment wh ich make s i t s u i tab l e. depended o n s ub s e quent t o t a l rad i a t ion .. numbe rs. and. d i f ferent. l e a f a r e a o f head. l e t tu c e. cv .. for. harve s t leaf. In cont r a s t B l ond ie. we re. b e tween tha t at 2 5 / l 7 ° C and 2 1 / l 3 ° C i f rad i an t. not. energy. was ma inta ined at a low l e ve l of 1 2 . 8 W/m2 ( Verkerk and S p i t t e r s , 1973) . S c a i fe Cobham. ( 1 9 7 3 ) a l s o s howed that the r e l a t ive growth r a t e. Green wa s not a f f e c t ed s i gn i f i c an t l y by t emperat u r e. the range 10 t o l 4 ° C . S o f f e et a l . and · l e a f a r e a o f l e t tuce cv . the. day l e n g t h. f r om. 12. h. of over. (1 9 7 7 ) found that dry w e i gh t. K l oek wa s increas ed by. ex t en d ing. ( 1 1 5 W/m2 ) to 16 h w i th. 4. h. of l ow. intens i ty ( 3 W/m2 ) when grown a t l 5 ° C . Cracker and S e i be r t ( 1 9 8 3 ) r e p o r t e d that the vege t a t ive we igh t. and. inf luenced. leaf by. numb e r o f Grand Rapids l e t tu c e. the. length. of. pho toper iod ,. o c cu rr ing unde r long photope r iods . gr own. at. the. h ighe s t. ir rad iance. w i th. prima r i ly. mo s t. g rowth. By 59 d a f t e r s owing , p l an t s leve l. �u...�W-M-'i-Ct� -h�. hou r s pho toperiodA. wa s. dry. (113. �. W/m2 ). w i th. 24. d, -....y ��. The h i ghe s t t o t a l l e a f area wa s I o rmed unde r. the l owe s t irrad iance l eve l ( 14 . 1 W/m2 ) . 3,. T emp e rature and Pho t op e r iod E f f e c t s on F l ower ing F l owe r ing. in. many. p l an t s i s. not. d e t e rmined. solely. by. gene t i c cons t i tu t i on ,. but i s cont r o l l e d by env i ronmen t a l fac t o r s. wh ich. gene t ic con s t i t ution i n a. interact. ( Z eevaar t ,. w i th. 1963 ) .. s pec i f i c. The two ma in c l ima t i c fac t o rs wh ich. manner cont r o l.

(24) 9. f l owe r ing. in l e t t uce a r e t empe r a tu re and ph o t operiod ( Garner and. A l l a rd, 1 9 2 0 ; Ro bert and S t ruckmey er , 1 9 3 8 ) . Day length de ve l opment. is. of. of. many. pr ima ry. impo r t ance. p l an t s ,. in. part icularly. the with. growth. and. respect. to. f l owe r ing ( Ga rner and A l l a rd , 1 9 20 ) . Tincker l e t tuce r ap i d in. ( 1 9 3 3 ) o b s e rved t h a t E a r l y Par i s and S t an s t e ad Park. p l an t s. r e s ponded to inc r e a s e d pho t o p e r i o d s. deve l o pment of f l ower s t a lk s ,. daylength by inc r e a s ed vege t a t ive. by. deve l opment .. Rud o r f. not. r e s pond ,. and. expo s e d. to d i f fe r e n t p e r i o d s of day leng t h s , 1 6 . 5 , 12 and 8 h . d id. more. and r e s ponded t o a d e c r e a s e. S t e l zn e r ( 1 9 34 ) grew s ix va r i e t i e s o f l e t t u ce , and c u l t ivars. a. them. The s umme r. b u t t h e other c u l t iva r s. s h owed. a. d im in i shed tendency t o shoot t o s ee d w i th short day l e ng t h . and Kn o t t. Thomp s on dur ing. early. the. of. pr oduc t i on. o f growth. s t age s. s e ed - s t a lk s (21-26°C). t empe r a t u r e. ( 1 9 3 3 ) r e po r t ed t h a t later .. wa s. an. was They. impo r t an t. h igh. t e mp e r a t u r e. re spons i b l e a l so. fo r. su g ge s t e d. p l an t s in. in. In this. went t o s ee d und e r h igh t emperature even und e r. re l at ive l y s h o r t day s ( 1 0 to 1 2 h ) . rapid l y. that. i nv o lved. fac t o r. premature s ee d ing in head l e t t u c e cu l t ivar Wh i t e Bo s t on, c u l t ivar ,. the. S e e d - s t a lk s e l onga t e d. p l an t s that had been g rown cont inuou s l y. at. more. 2 1 - 26°C. t emper a t u r e and long days ( 1 5 h ) .. L ikewi s e , long. requ i r e d s e e d - s t a lk n i gh t s. Rappapor t. and W i t twer ( 1 9 5 6 b ) s howed that. pho t ope r i od s. deve l o pmen t .. ( 1 6 h ) and warm n i gh t s. G r and Ra p id s f lowered on. or in l ong pho t oper i o d s ,. B i bb ,. ( 2 1 ° C) e i t h er. wh i l e the Tend e r green. for warm. c u l t ivar. f l owered pr ima r i l y in long pho t o pe r iod s . Ra l e i gh. ( 1 9 5 9 ) a l s o found t h a t s e e d - s t a lk. enhanc e d by h i ghe r temp e r a t u r e s in head l e t tuce .. e l on ga t ion. wa s. The s e ed - s t a lk. was s ign i f i c an t l y l onger at 2 6 / 2 1 oC t han at 2 6 / 1 5oC; at 2 1 /2 1 oC than 2 6 / 1 5°C and 2 1 / 1 5°C than at 2 6 / 1 0°C,.

(25) lQ. Sub s e quen t l y t empe r a t ur e s 3 0 ° C,. et. I to. ( 1963). al .. s ugg es ted. that. h igh. above 2 0 ° C are e f fec t ive for f l ower induc t i on . p l an t s. let tuce. need fewe r days o f. exposure. for. At. f l owe r. ind u c t ion than a t 2 3 ° C .. At l 7 ° C b o t h s t ern e l onga t i on and f l ower. induc t i on. L ong days ( 1 6 h ) w i th h igh t empe r a t u r e s. led. a r e de l ayed .. t o f l owe r indu c t ion b u t pho t ope r i o d s l e s s than 8 h. w i th. h igh. indu c t ion . supply. t empe r a tur e s Fu l l. has ten. induc t i on .. result. s un l i gh t ,. the. long days ,. p l ant growth , Shad ing ,. in. fai lure. and. t o gether. of. abundan t. f l owe r n i t rogen. b e i ng fo l l owed by ea r l i er. short. days ,. poor. n i t rogen. f l ower. supply. and. d e fo l i a t i on a l l r e t a rd both p l ant g r owth and f l owe r indu c t ion . Cytovic 10,. 12. or. ( 1 9 6 7 a ) s tud i ed f ive l e t tu c e c u l t iva r s g rown a t 24 h. day length .. The. l e a fy. cu l t ivars. 8,. Mos kov s k i j ,. Parnikovyi and Boul and the head l e t t u ce B ibb reac t ed s t rong l y t o reduced. day l ength .. reduced. the. H i r aoka. ( 1 9 6 7a ,. long. days. whe re a s. In pa r t i cu l a r ,. numb e r. 8 t o 1 2 h d ay l en g th. o f p l a n t s a f f e c ted b y. bac t e r i a l. also. d i s ea s e s .. 1 9 6 7 b ) showed tha t h i gh t empera ture ( 2 5 ° C ). ( 1 6 h ) a c c e l e ra t e d. b o l t ing ,. budd ing. and. and. f l owe r in g. s h o r t days ( 8 h ) at lower t emperature encouraged h e ad ing. and de layed b o l t ing .. The Prem i e r G r e a t Lake s and G rea t Lake s 3 6 6. cu l t ivars we r e apparen t l y l e s s s en s i t ive t o pho t o p e r i o d than. wa s. the Edogowa s t ra i n o f the cv . Wayah ea d but the numbe r o f b r anche s wa s. h igher a t h igh temperatur e s and an 8 h. day l eng t h .. H i r aoka. ( 1 9 6 9 ) a l s o c on f i rmed tha t the h i gher the t emper a t u r e , the l onger wa s the s e e d - s t a lk and the e a r l i e r was f l ower bud d i f f eren t i a t ion .. S e ve r a l be. s tud i e s have ind i c a t e d tha t c u l t iv a t ed l e t t u c e. induced t o b o l t by t reatment w i th c e r t a in. ( Frank l in , 1957 ;. 1948 ;. Thoma s ,. G i bbere l l i c. C l ark and. 1968 ;. gr owth. c an. r e gu l a t o r s. Wi t twe r , 1 9 4 9 ; Wi t twer and B ukova c ,. Nor thrn ann ,. 1 9 7 3 ; Gu t terrnan e t a l . , 1 9 7 5 ) .. a c i d ( GAJ ) i s mo s t e f fe c t ive i n promo t in g. f l ower ing. and s e e d y i e l d ( Ha r r in g t on ,. 1 9 6 0 ; H i l lrn an , 1 9 6 2 ; H i raGka , 1 9 6 7 a ;. C y t ov i c ,. and. W i t twer. 1967b ;. G lob e r s on. Ventur a ,. 197 3 ) .. Bukovac. and. ( 1 9 5 8 ) f ound that GA3 p romo t ed f l owe r ing in G r e a t Lake s ,. head l e t tuce and t he e f f e c t s were a d d i t ive t o tho s e o f long. days.

(26) 1 1. (18. h),. h igh. G ibberel l in produced flower. (19-2l. C ). temperature treated. e l ongated. p lant s s tems. pr imordia .. formed. and no. prior to the. Furthermore,. s eed. verna l izat ion.. vege tat ive appearance. heads of. and. v i s ible. Cytovic (1967b ) reported. that. double s t a lk format ion was found in 60% of the GA3 t reated p lant s of. Premier. Great Lake s and that,. following. treatment,. immed iate ly bolt ed, produc ing flower s and seeds . the. rate. s tage s Lakes. p lant s. GA3 s prayed at. of 3 to 10 �1/1 on le t tuce plan t s a t the 4 and. of. growth s ignifican t ly increased s eed. head. l e t tuce .. yield. 8 leaf. in. Great. Harr ing ton (1960 ) a l s o reported tha t. the. s eed crop ma tured about two weeks earl ier, with ext reme ly uniform maturity among treated plants. (1973 ). S imi lar ly, G loberson and Ventura. applied 5 �1/1 of GA3 at the 2,. 4 and 6 leaf. s t ag e s. to. bu t t erhead No .. 615 and c r i s phead No. 923 (bo th bo l t ing r e s i s tant. c u l t ivars ). a. flowered yield. in. c ommerc ial f ield .. The. GA3. t reated. p l an t s Seed. e arl ier and were more unifo rm than the contro l s .. wa s als o highe r in treated plo t s,. with maximum y i e l d s. of. 5 6 4 kg/ha in No . 615 and 260 kg/ha i n No . 923 . 4. Temperature and Pho toperiod Interact ions with Verna l izat ion on Flowe r ing M i l t horpe. and Horowitz (1943 ) inve s t igated the effe c t of. a. low temperature treatment a t 5 ° C for 10 t o 20 d dur ing the period of germin a t ion and early growth. s t imulated. They found that such c onditions. s eed-s talk development in le t tuce provided tha t. they. were fol lowed by high temperature s and a long phot operiod .. Thi s. effec t. head. has. been s imilarly demons trated i n. Wh i t e. Bos t on,. l e t tuce (Kno t t et� . , 1937) and in Imperial (Gray, 1942 ) . Andrew (195 3 ). also. and. d evelopment p lant s. repor ted. were. a. cons i s tent. promo t ion. of. f lowe ring in le t tuce 456 and Gre a t. s ee d-s t a lk Lakes. grown under long day and at a temperature. of. when 15 ° C. fol lowing seed vernal iza t ion. With found. Great. Lake s cu l t ivar,. c ons iderab le. photoper iod. Rappaport and W i t twer. interac t ion be t ween. verna l izat ion. and temperature dur ing subs equent growth.. (196Sa ) and In 16. the h.

(27) 12. days ,. and. at. a. mi nimum. glas shouse. t empe rature. of. verna li z a tion. c a u s e d a r educ ti on o f 28 d i n the time from s owing. t o anthe si s .. In 1 6 h days and a t l 6 ° C ,. and. in. 9. h days a t 2 1 ° C ,. ve rna li z a ti o n anthe si s . its. c au s e d. no. the reduc ti on wa s 2 2 In 9 h days. i t wa s 7 d . si gni fi c an t. re duc tion. at. in. d. l6° C ,. time. to. C le a r ly , f l owering wa s has t ened b y verna li z a ti on , b u t. expr e s si on cou l d b e subs equen t ly reduced b y s h o r t. days. and. preve n t e d by low t empe r a t ure . P rince ( 1 9 8 0 ) c onc luded t h a t ve rna li z a ti on of s e e d s i n L a c tuca. s a tiva. produ c ti on , and. mo s t. L . cu l tiva r s p rovi d ed a means o f ha s t ening. a s l o n g a s o ther c ondi ti ons ,. t empe r a ture ,. we r e. sui t a b l e for. s eed. particu l a r l y d ay length. f l owe ri ng .. The. great e s t. re s p ons e wa s found among the c ri s phead le t tuce cu l tivars b u t. the. vari e t i e s which s howed li t t l e re s pons e to verna li z a tion t en d e d to have. an. e x t r eme r e qui r emen t fo r l ong days in order. V ern a li z a ti on. di d. no t. to. ha s t en b o l ting i n S l obo l t and. f lowe r . Cos. 3288. c u l tiva r s ( Thomps o n and Ko s a r , 1 9 48 ) . 5.. S e e d Q u a li ty and Germina ti on G e rminati on ,. of Seed. puri ty. and h e a l t h a r e the three mai n c ri t e ri a. s e e d qua li ty whi ch. are we l l e s t ab li sh ed b y the T e s ti ng As s o ci a ti on ( I S TA . ,. a f fe c t e d by a numb e r o f di s tinc t , inc luding. 1976 ) .. I n t e rn a ti on a l. Seed qua li t y. can. be. but o f t en inter a c ting fac t o r s. gen e ti c fac t o r s ( s pe ci e s and vari e t a l ) ,. phy si o lo gi c a l. fac t o r s ( p rema t u r e harv e s t o f t h e s e e d s ) and patho l o gi c a l fac t o r s ( s e e d - borne di s ea s e s and mechani c a l damage ). ( Heydecke r , 1 9 69 ) .. L e t tuce s e e d quali t y wa s ex p lo red by Scai fe and Jone s ( 1 9 7 0 ) who s howed t h a t under uni form condi ti on s , i n t e r p l an t Borough. c ompe ti ti on ,. the. and in the a b s ence. f r e s h weight o f. l e t tuce. Wonde r at harve s t wa s di rec t ly and li ne a r l y. of. c u l tivar. r e l a t ed. to. the wei gh t of the s e e d s . Smi t h � a l . ( 1 9 7 3 a ) dev e loped a s l ant t e s t for s ee d vi gour , in. whi ch. s e e d s we re germina t ed unti l maximum radi c l e. e x tension.

(28) D. wa s. r e ached .. s l ante d. The s ee ds o f Ca lmar c u l t ivar we re. b l o t t e r s immersed in wa t e r i n the dark ,. g ermina t e d. on. e l imina t in g the. e f fec t s of nu t r i t ion supp l y out s id e t he s e ed i t s e l f .. With. this. me thod s e e d we igh t wa s found t o b e more imp o r t an t than s e e d w i d t h or. An a i r co lumn s ep a r a t o r wa s. t h i c kne s s in p r e d i c t ing v igour .. shown. to. be. we i gh t .. an e f fe c t ive way o f s eparat ing. Smith e t a l .. s e eds. vary ing. in. ( 1 9 7 3b ) s howed in f i e l d t r i a l s that l ight. ( l ow v i gour ) seeds emerged s lowe r , w i th l e s s t o t a l eme rg en c e , and produ c e d. sma l l e r s e ed l ings at th inn ing t ime,. v i gour). s e e ds .. Head. s i ze. a t harve s t and. than the. he avy. ( h i gh. propo r t ion. of. marke t a b l e heads i n the c rop we re g r eat e r from h i gh v i gour s ee d s . S o f fer. and. Smith ( 1 9 7 4a ) s tu d ied the f l owe r in g pa t t e rn. l e t tuce and i t s r e l a t ion t o s e e d y i e l d and qua l i t y .. Ove r 9 0% o f. the s e e d y i e l d wa s from f l owe r s o p ening dur ing the f i r s t 3 5 d a 7 0 d f l owering per iod . f l owe r ing. peaks. were heav i e r than l a t e r produced. seeds .. Seed. Seed yield. and. qua l i t y we r e una f fe c t e d by ea r ly h a rve s t o r by wi thho l d ing nu t r i e n t s. F l ower ing. dur ing. rate ,. of. S e e d s p r oduced dur ing the f i r s t o f two. s i z e was unre l a t ed t o numb er o f s eeds p er head . or. in. t h e l a t t e r ha l f o f t h e. f l ower ing. wat e r p e r i od .. s e e d y i e l d and q ua l i t y we r e not r e l a t e d t o. air. tempe r a tu r e i n t h e r ange 20 t o 3 5 ° C . S o f fe r. and. Smi t h. ( 1 9 74b ). showed tha t. the. magn i tude. of. c o r r e l a t i o n be tween s e e d we i gh t and s ee d l ing vigour ranged wide l y Corr e l a t ions. among s e e d s f rom d i f ferent mo t h er p lant s . indiv idua l s ee d l ing. s eed vigour. b e tween. me asuremen t s and embryo phy s i c a l p a rame t e r s we r e. s ign i f ica n t but s ee d. s ize. and. to. s e ed l ing. v i gour we re not a s s o c i a t e d w i th e i ther head we ight o r uni formi t y . Sof fer. and. Smi t h. ( 1 9 7 4c ). also. s tu d i ed. nut r i t i ona l. e f fe c t s;. i n c r e a s ed nu t r i en t s supp l i e d t o p l an t s grown in s o i l r e s u l t ed i n c r e a s e d s e e d y i e l d b u t no s ee d vigou r . grown v i gour .. s e ed. wa s. heavier than s o i l gr own. in. Howeve r , hydropon i c a l ly s eed ,. but. it. l acked. Neve r the l e s s there wa s a l inear incre a s e in se e d y ie l d ,. we i gh t / s eed. and s e e d l ing v i gour w i t h increase in n i t r ogen .. The. r e s ul t s showed that the r e l a t i o n s h i p b e tween s e e d s i z e and v igour wa s d e pendab l e only w i t h in s e e d lo t s ..

(29) 14. Renard s eeds. ( 1978). r e p o r t e d tha t the mid d l e grades. ( fr om s i x grade s ) gave the be s t germina t ion. the s hor te s t germinat ion t ime . d amaged ,. and. o f ten. pr oduced. l a rger p l an t s .. of. l e t tu c e. capac i ty. and. The l a r ge s t s ee d s we re s ome t ime s. showed reduced g ermina t i on. prope r t i e s. but. The sma l l e s t s e e d s we re the wor s t f r om. a l l poin t s o f v i ew . Many. fac tors. germinat i on. have. b een shown to. i n f luence. inc luding a r e a o f produc t i on ,. s e ed. l e t tuce age ,. etc .. l i ght ,. t empe r a tu re , ca rbon d i oxide and oxygen. ( Bo r thwick and Robb ins ,. 1 9 36 ,. 1938 ;. Gu t t e rman , wh ich. Ha r r ington. 1973) .. h ave. L i ght. been. c u l t ivar ,. nu t r i t i on o f. ph y s i o l o g i c a l age o f p l an t at t ime o f s e ed harve s t , the parent p l an t ,. s ee d. 1 9 28 ;. and. Thorn ton ,. Thomp s on ,. 1936 ;. 195 2 ;. Ko l l e r�. and t empe ra tu re are the. s tud i ed. mo s t. e x t en s ive ly. Thomps on ,. two. 1962 ; a s pe c t s. ( To o l e ,. 1973 ;. Vanderwoude and To o l e , 1 980 and Vande rwoude , 1 9 82 ) . F l in t ( 1 934 ) and F l in t and McA l i s t e r gre a t e s t. promot ive r e s ponse t o l e t tu c e. ( 1 9 3 5 ) showe d t h a t the s e ed germinat i on f r om. irrad iance o c curred in the r e d r e g ion ( 6 0 0 - 7 00 nm) o f. g iven. Maximum. s pe c t rum . f ar- red pr omo t i on. the. inh i b i t i on o c cu r r e d from i rra d i an c e s in. range ( 7 2 0 - 7 6 0 nm).. the. The ac t ion s pe c t rum curve for. and inh i b i t ion of germina t ion of Grand Rap i d s. a. the. l e t tuce. ha s b e en d e f ined by Bor thw i c k e t a l. ( l 9 5 2 , 1 9 5 4 ) . In. t e rms. s a t i s fac t ory inhi b i ted ,. of. t empe ra t u r e ,. at 15 t o 20° C . de pend ing. Bor thwick and Rob b in s , is. l e t tuce. s eed. g ermina t ion. is. A t 2 6 ° C o r above ,. g erminat ion. is. upon cu l t ivar ( Thompson 1 9 28 ) .. and. Horn ,. 1 9 44 ;. Th i s s ens i t iv i t y to t empe rature. mo s t pronounced w i th freshly harve s t ed s e ed. ( Ha r r i n g t on. and. Thomp s on , 1 9 5 2 ) . Thomp s on. ( 1936 ). f ound. that fu l ly ma tured s e e d s. Ra p i d s ge rmina t e d in d i f fu s ed l igh t at 15 t o 2 0 ° C and in. of. Grand. da rkne s s. a t 1 0 t o l 5 ° C but that g e rmina t i on capac i ty dropped s i gn i f i can t l y when t h e tempe rature in d arkne s s wa s incre a s ed t o Z 5 ° C .. Imma t u r e.

(30) 15. s e e d s fa i le d t o g e rminat e in the abs ence o f l ight a t e i t h e r 1 0 t o 1 5 ° C o r 25 ° C .. The Hi l d e cu l t ivar a l s o g e rmina t e d s a t i s f a c t o r i ly. a t 1 5- 2 0 ° C in d arkne s s ( Kre s t chme r , 1 9 7 8 ) . Gray King. ( 1 9 7 7 ) in wo rk w i t h s e e d s o f c u l t ivars H i l d e ,. F e l t ham. and Avon c r i s p s howe d tha t the f ir s t 4 h o f imb i b i t ion. and. the pha s e b e tween the on s e t o f mi t o s i s and r a d i c le eme rgence were not. s ens i t ive t o h igh t empe r a tu re .. The s e f ind ings. agree. with. s imi lar work b y I kuma and Th imann ( 1 9 6 4 ) w i t h Grand Rap id s . Hadnagy. ( 1979 ). r e c ommende d. s ee d s o f c u l t ivars At t r a c t i on , 2. d. the need t o. prech i l l. l e t tu ce. Nansen and S o r o k s a r i a t 4 - 6 ° C f o r. fo l l owed b y g e rmina t i on a t 2 0 ° C with l igh t to. o b t a in. fu l l. g e rmina t i on capac i t y . t he. In. a c ce l e r a t e d e i ther bu t. c u l t ivar by. a l one. wa s. is. germin a t ion. apparen t ly. expo s u r e o f s e e d t o f luor e s cent l ight for or f o l l owed by 24 h da rk / l ight p e r i o d s a t. d e layed 197 7 ). Pu j a r i ,. S l obo l t ,. when s eed was germina t ed a t. 30°C. 24. h. 20- 25°C. ( Ve rma. c omp a r e d w i t h a s imi l a r e f fe c t in G r e a t Lake s. and at. 3 2 ° C ( Dunlap and Mo rgan , 1 9 7 7 ) . L i g h t e f fec t s can b e mod i f ied by the add i t ion o f GA3. ove r c ome s the e f f e c t o f f a r - red l i gh t. However ,. Burde t t. e thy l ene. and. chemica l s .. ( Kahn e t a l . ,. and V idaver ( 1 9 7 1 ) h ave shown the. g ib b e r e l l in t o ge the r in promo t ing. far- r e d l i ght over that of g i bb er e l l in a l one .. 1957 ) .. e f fe c t s. of. g e rmina t ion. in. The s e e f fec t s are. even g r e a t e r in r e d l i ght . t empe r a t u r e d o rmancy has b e en shown t o be ove r c ome. H i gh. the add i t ion o f th iourea ( Th omp s on and and. by. Horn , 1 9 44 ) and by e th re l 1 9 7 3 and H a rs h. et. ad d i t i ona l env i r onmen t a l e f fe c t on t h e germinab i l i ty. of. k i net in ( Sm i th e t a l . ,. 1968 ;. Sharp l e s ,. �·' 1 9 7 3 ) . An l e t tuce found. seeds that. h a s b e en d e s c r ibed b y G i rad and Monin ( 1 9 7 7 ). the l e t tuce c u l t ivar Merve i l le. des. Gua t re. who. S a i s ons.

(31) 16. r a i s ed. in. s ho r t. days p r oduced s eed wh ich wa s mor e. d o rmant. C onve r s e ly ,. harve s t than s e e d s f rom p l an t s r a i s e d i n l ong days . a. at. change in day leng th dur ing t he l a s t 1 2 d of s e ed. r i pening. is. appar en t ly s u f f i c ient t o a f fe c t the g e rminab i l i ty o f Grand Rap i d s seed .. S imi l a r l y. s eeds. deve l oped and r i pened under. short. day. ( 8 h ) germ in a t e d fa s t er than s ee d s from long days ( 20. cond i t i ons. 1973 ) ,. h ) ( Gu t t e rman ,. wh i l e the ma tura t i on o f Grand Rap i d s s ee d. und e r h i gh t empe r a t ur e s o r in cont inuous l i gh t ( 24 h ) c a n b e u s e d to. incr e a s e. the. h igh. t empe ra tu re t o l erance. of. s e ed. dur ing. g erminat ion both in cont inuou s dar k and a f ter a sho r t l i gh t - b reak (Ko l l er , 1 9 6 2 ) .. 6.. Seed Produ c t ion Prac t i c e s Produ c t ion. favour a b l e e n s ure. of. h igh. qua l i t y. s e e d i s po s s i b l e. env i ronme n t w i t h good growing cond i t ions. that. uni fo rmi t y. harve s t ing .. The r e. of. t h e c rop. can. be. only. in. wh i c h. a ch ieved. wi l l b e fore. appear s t o be amp l e l i t e ra tu re ava i l ab l e. l e t tuce p l ant produc t ion b u t ,. a. on. u n t i l r e cent l y , i n fo rma t ion on the. pr oduc t ion o f l e t t u c e s e ed c r o p s has b e en c ompar a t iv e l y l im i t e d . 6.1. C l ima t i c and So i l Re qui reme n t s The r e i s l i t t le d i f ference in. or for marke t .. g rowing l e t t u c e f o r s e ed. Up to the s t age when the marke t cr o p is r e ady. harve s t, t h e s ame gene r a l rec omme nd a t ions can be mad e .. to. Fr om then. on, the s e ed gr ower is c oncerned w i t h the growing and h and l in g o f the. cr o p. un t i l. r e qu ir e s fr om. the crop i s ma tur e .. a long ,. L e t t uc e. warm growing s e a s on wh ich is. r a in a t harve s t ing t ime ( Hawt horn and. coo l p e r iod a t the t ime o f s owing ,. s ee d. produ c t ion. r e l a t iv e l y. Pol l ard ,. 1 9 54 ) .. free A. f o l l owed by a s t eady inc r e a s e. in t empe r a t ur e s o t h a t f l owe r in g o c cur s unde r warm cond i t ions , i s b e s t f o r s e e d ing o f l e t tuce ( M i l t ho rpe and Hor ow i t z , 1 94 3 ) . Soils. s elected. for. growing. l e t tuce f o r. f er t i l e and have a h i gh o r gan i c ma t ter conten t . let tuce. s ee d. s h ou l d. Becau s e o f. be the. p l ant ' s d emand for a p l en t i fu l sup p l y of mo i s tu re dur ing. the g rowing s e a s on ,. h e avy s i l t and c l ay loam s o i l s ar e pre ferr e d.

(32) 17. ( Thomps on ,. 195 1 ) .. The. s o i l r e a c t ion shou l d be near l y. neu t r a l. be tween p h 6 and ph 7 ( Wh i taker e t a l . , 1 9 7 4 ) . 6.2. Cu l t u r a l Procedures L e t tuce. p l an t s. a re genera l l y produced in t he. bed p r i o r to t rans p l an t ing in t o t he f i e ld . e s pe c i a l ly The. t he. seed. l e a f type s ,. However , s ome type s ,. may be s own d ir e c t ly in the. s h ould be p l anted t h in ly a t the r a t e o f later ,. t he. nu r s ery field .. 1-1 . 5. s e e d l ings are t h inned. S evera l. we eks. S pac ing. is. he ading. type s and 1 5 - 20 cm apa r t in the l e a f t ypes d epend ing. us ua l ly about 25- 3 0 cm in the row. the var i e t y ( Hawt horn and Po l l a rd ,. 1 9 5 4 and. in. kg/ha .. wi t h. the t he. Duncan ,. row . l arge. 196 5 ) .. on In. Grand Rap i d s , Pokanan ( 1 9 8 1 ) o b t a ined the h i ghe s t s e e d y i e l d ( 3 18 kg/ha ) from a 30 x 7 5 cm p l an t s pac ing .. Maximum s ee d y i e l d s p er. p l ant ( 1 2 . 5 g ) we r e ob t a ined a t a 5 0 x 75 cm p l ant s pac ing . D i f f icu l t ie s. are. e l ong a t ion in l ea f ,. not n o rma l ly encoun t e r ed. with. s e e d s t a lk. c o s or mo s t but t e rh ea d varie t ie s .. With the. c r i s phead t ype howeve r , nece s s ary. pa r t ia l head remov a l or t r e a tment ma y b e. to a l l ow s e ed- s ta lk ' e s cape ' a n d d eve l o pment .. i s n o t d one ,. I f this. the s e e d - s t a lk tends t o cur l ins ide t h e head. or be. de l ayed. i n i t s matur i t y a s we l l a s re s u l t ing i n a. reduc t i on. in. yield .. I f h e a ds a r e s p l i t t o o e a r l y the p l ant wi l l c on t inue. to. h e a d and i f d e l ay e d t o o l ong ,. s e ed s t a lk inj ury. comm on. method. l eave s. on each p l ant by hand s o a s t o expo s e the. but. wi l l r e s u l t .. wh i c h ove r c ome s t h i s prob l em i s t o pe e l back. t h i s i s l abour i o� s a n d cos t ly ( Ha r r ing ton ,. growing 1960 ;. A t he. point. Hawthorn. and Po l l ard , 1 9 5 4 ) . 6.3. Harve s t in g. Le t tuce. s eeds. r i pen uneven ly .. d e ve l o p to f u l l ma tur i t y , There. I f s e e d i s a l l owed t o. l o s s e s f rom s h a t t e r i n g may b e. are d e f in i t e flowe r ing peaks f o r ind ividua l l e t t u ce. s ev e r e . p lan t s. and approxima t e ly 1 2 d i s nec e s sa ry f rom anthe s i s t o the matu r i t y of when. an. ind iv i dua l s e e d .. I t i s apparen t l y adv i s ab l e t o. the p l an t s are s howing about 5 0% " f eathe r ". pappu s ). ha rve s t. ( s howing. even th ough i t h a s been o b s e rv e d tha t b e t t e r y i e l d s. wh i t e are.

(33) 18. o b t a ined. when. the. f e a t he r". s t age .. crop i s l e f t unt i l i t. re ache s. the. ''fu l l. The l onger the s e e d s are l e f t on t h e p l an t the. g r e a t e r the chance of s ha t t e r ing b y wind o r rain ( J one s , 1 9 2 7 ) . For cornrn e r i c a l s e e d produ c t ion ( Thomp s on , 1 9 3 8 ) a dv i s ed t h a t t wo harve s t s o f seed shou l d b e made a l t hough i t wa s b e l ieved t h a t t h e s e c ond harve s t s e ed re covered w a s i n f e r ior t o t h a t f i r s t.. harve s t ed. So f f e r and Smi t h ( 1 9 7 4a ) s howed that in le t tu ce ,. 9 0% o f. t he. seed y i e l d comes from f lowe r s wh i ch open dur ing the f i r s t 3 5. d.. H i ghe s t s eed y i e l d s a re o b t a ined by hand harve s t ing. whe r e. f l owe r head wer e shaken i n t o a bag .. me thod s. Th i s p r o c e s s can. repea te d t w o o r t h r e e t ime s dur ing a s e a s on ( Duncan , 1 9 6 5 ) . ha rve s t ed. be Seed. i n t h i s mann e r i s much e a s ier t o c l ean s i nce t h e r e. is. l i t t l e cha f f mixed w i t h i t ( Hawthorn and Po l lard , 1 9 5 4 ) . Doug l a s s. ( 1 9 3 9 ) and Shoemak e r ( 1 9 5 3 ) s ugge s t e d t h a t l e t tu ce. p l an t s s h ou ld b e pu l l e d o u t o f the ground when the h e a d s b e g i n t o swe l l The At. and. t urn y e l l ow o r wh i t e and s t acked o n a. canv a s. sha t t e r ing o f the e a r l y s ee d w i l l then fa l l on. canva s .. the s e e d heads a re s t i l l ab l e t o d r aw on. the s ame t ime ,. mo i s ture. the. shee t .. a n d nut r i e n t s ava i l ab l e in the cut p l a n t .. the. Th i s a l lows. t hem to c on t inue d eve l o pment un t i l the seed reache s ma t ur i ty . Duncan ( 1 9 6 5 ) r e commended , f o r l arge l e t tuce s e e d produc t i on areas , t h a t p l an t s shou l d are. ye l low i s h i n co lour .. b e c u t when the ma j o r i ty o f the s e e d s T h e s ee d s c a n be t h r e shed a s s oon. he ads and out e r - mo s t b r anches have d r i ed s u f f i c i en t ly . wi thou t. s e ed i n j ury is b e s t a c c omp l i shed by p u t t ing the. as. Thr e s h ing p l ant s. through a comb ine harve s t e r wh i c h h a s s ome t e e t h removed from t he pe g dr um . I n genera l ,. t he y i e ld o f l e t tuce obviou s l y d e pends on. fac t or s ,. inc lud ing. Howeve r ,. ma j or. yie ld. d i f f e r e n t c u l t ivar s . has. l o c a l i ty ,. s pac ing ,. d i f ferences. have. The New York No .. b e en shown t o produce 2 0 0 kg / ha. soil b een. many. f er t i l i ty. etc .. recorded. with. 1 2 c u l t ivar f o r examp le ,. ( C laypool�. 1933 ). compa red.

(34) 19. with. on l y. 1960) .. In. I s r ae l ,. ave r a g e s e e d y i e l d s a r e 2 5 0 k g / ha in but terhead No .. 615. and. kg / h a. 70. 1973) .. lOO. kg / h a in G r e a t in. c r i s phead N o .. Lake s 923. ( Harr ing ton , ( G lob e r s on. and. Ventu r a ,. In Th a i l and , Grand Rap i d s h a s b e en s hown to be capab l e o f. produc ing about 300 kg/ha o f s e ed ( P okanan , 1 9 8 1 ) . 6.4. D i s e a s e and P e s t s Even. t r oub led are. though. the p r oduc t i on o f l e t t uce s e e d. by a large numb e r of d i s ea s e s and ins e c t s ,. impor tan t. may. b e c ome so s e r i ou s tha t they. is. not. t h o s e wh ich. may. l im i t. the. produ c t ion of s e e d . Downy and. m i ldew. ( Bremia la ct u ca e R ) cau s e s the. p l an t s t o s l igh t ly s tun t ( D ixon ,. disease. is. 1960 ). Ro d e r ,. 1938) .. d amage. to. c ond i t ions. l eave s. this. Rap i d s. l e t tu c e and. It. can. and keep ing s o i l P ig l ionica ,. p l ant may b e a f fe c t e d. bu t te rh e a d. by. be. mo i s ture. 1 9 64 ) .. Und er. or f o l l ow ing ove r - irr i ga t ion o f. ( S c l e r o t i n i a s c l e r o t i orum Li b ) . c r i s phead. G r and. par t ic u l a r l y in head l e t tuce .. un i form t o prevent t ipburn ( E l i a and the. of. S o f t r o t ( Bo t ryt i s s pp . ) r e s u l t s i n con s i derab le. b y e a r l y r emova l o f h e a d s. mo i s t. par t s. o r Me ikon ingen but terhead l e t t u c e ( S chu l t z and. p l an t s ,. con t ro l l e d. soils ,. Cont r o l. by u s ing r e s i s t an t cu l t iva r s such as. ( Verhoe f f ,. co o l ,. 1981 ) .. f l owe r. h e avy. s c lerot inia. rot. Hawthorne ( 1 9 7 4 ) a l s o f ound t h a t. l e t tuce p l an t s wh ich h ave. t he. l ower. l y ing f l at on the ground are mo re l iab le to i n f e c t ion. sc l e ro t i n i a. than. cos. l e t tuce wh i ch. have. l e ave s .. If. may. a f f e c t e d b y powde ry m i l d ew ( Erys iphe. be. mor e. upr i gh t. by. lower. the we a t h e r is r e l a t ive ly warm and h umi d , t he p l an t s c i chorace arum. DC ). e i ther in the f i e l d or i n the g re enhou s e ( S chnathor s t , 1 9 � 0 ) . S evera l. s pe c i e s o f aph i d s s pread l e t tu ce mo s a i c v i r u s ,. u l t ima t e e f fec t on growth and y i e l d be ing dependent upon t ime infec t i on ; infec t e d 1 961 ) . areas. the of. l a t e r i n fe c t ion c au s e s l e s s p l an t d amage wh i le p l an t s early. I n fec t e d a l ong. the. b e c ome. ye l low and s tunted. ( Z ink. a snd. p l an t s that b e c ome reprodu c t ive s how s e e d s t a lk and in t h e s e e d. hea d s .. Kimb l e , ne c r o t i c. Ryd e r. and.

(35) 20. Du f fu s ( 1 9 6 6 ) showed t h a t l e t tuce mo s a i c v i ru s d e layed f lowe r in g , r e du c ed. s e e d s t a l k he i g h t and r e du c e d t o t a l s ee d y i e l d. bu t. d id. n o t a f f e c t s eed ge rm ina t i on . As t e r. ye l l ows ,. impor t ant bu t. s po t t e d. wi l t. and. ' b ig. t r ansmi t t ed. are. d i f ficult. to. are. t he. other. They are no t s e ed. v i r a l d i s e a s e s o f l e t tuce .. s ome t imes. ve in '. con t r o l .. As t e r. borne ,. ye l l ows. b y l e a f h o ppe r s and caus e s t h e f l ower p a r t s t o. is form. no s e ed ( Jagger and Chand l e r , 1 9 34 ; Ryder , 1 9 7 9 ) . Aph i d s There. are. y i e ld . The. can. increase. on. l e t tu ce .. o f ten t ime s when they may cau s e a reduc t ion. The. use. be a mo s t t r oub l e s ome ins e c t pe s t. of. l e t tu c e s ee d s t em a ph i d i s. p ar t icu l a r l y. insec t i c i de such a s Parath ion. has. in. s ee d. d ama g ing .. b een. s hown. to. s eed y i e l d s compared t o un t r e a t e d aphid - in fe c t e d p lan t s. ( Hawthorn and P o l l a r d , 1 9 5 4 ) . 6 . 5 Ti pbu rn Le t tu c e. c o n t a i n s a s ub s t ance c a l l e d l a c tuc in wh ich i s a. s l e e p - induc ing drug ( Ry de r ,. 1979 ) .. Th i s l a t ex - t yp e o f ma t e r i a l. occurs in a s y s t em o f d u c t s c a l l e d l a t i c i fe r s .. T i pburn , a non. pathogenic d i s ord e r o f l e t tuce , i s man i fe s t e d by nec ro s i s o f l e a f ma rgins on r e l a t iv e l y imma ture l e ave s o f ma turing l e t tuce p l an t s . Ru ptu r e. o f the l a t ic i f e r s near the l e a f margins ,. o cc lus i on o f xy l em e l emen t s , and. caus ing c o l l a p s e o f parenchyma , mo s t. s i gn i f ican t l y ,. latici fer marg i n ,. s y s t em. c oagu l a t ion. be tween. r e s u l t ing. r e l ea s e s l a t e x. of. latex. the p o i n t o f. w i t h in. rupture. the. and. in dark b rown s po t s ( T ibb i t t s e t. entire. the. leaf. al . ,. 1965 ;. O l s en e t � . , 1 9 6 7 ) . In may. both g l a s shouse and growth chamb er. t i pburn. deve lop on young p lan t s when e n l a r g ing l eave s begin t o_. inwards , po in t . near. s i tua t ions ,. bend. par t i a l ly e nc l o s ing the young l e av e s around the growing Unde r. p l an t. ove rma tu r i t y. f ie ld c ond i t ions t ipburn usua l ly deve l o p s ma tur i ty ( Te rmoh len. and and. is. mor e. Van. de. l ike l y Hoeven ,. to. appear. 1966 ) .. at. or with. T i p burn.

(36) 21. deve lopment that. has. b e en a s s o c i a t e d w i t h. env i ronment a l. encourage r a p i d dry ma t t e r ac cumu l a t ion and a. for. c a l c ium. quickly. in. in. e xpand ing lea f t i s s u e .. y oung p l an t s when. intens i t ie s ,. ext ended. The and. h i gh. d e mand. s ymptoms. they a re expo s ed. pho tope r i o d s. cond i t ions. to. h igh. deve lo p. h i gh. l i gh t. t empe r a tures. ( T ibb i t t s and Roa , 1 9 6 9 . , Tibb i t t s and B o t t enberg , 1 9 7 6 ; T i b b i t t s and Re ad , 1 9 7 6 ; C a l c ium. Cox e t a l . , 1 9 7 6 .. nu t r i t ion. i s in s ome manner Howeve r ,. even t s l ead ing to t i pburn . in. a s s o c ia t e d. with. the. much confu s ion i s a p parent. e xp l an a t ions o f the role of env i ronment a l i n f luenc e .. Krug er. ( 1 9 6 6 ) f ir s t preven t e d t ipburn b y a p p lying c a l c ium s a l t s d i rec t ly to the s en s i t ive t i s sue . genera l ly. accepted. r e l a t e d d i s order . con t r o l growth. theory. Mino t t i ( 1 9 6 9 ) l e d. tha t l e t tuce t i pburn. is. a. the. c a l c i um. L ikewi s e , Ashkar and R i e s ( 1 9 7 1 ) we r e a b l e t o. t i pburn in head l e t tuce g rowing i n nu t r ie n t s o l u t ion chamb e r s by add ing a t l ea s t. also. The y. Th ibodeau and. S mM. c a l c ium t o t h e. f ound h i gher t i pburn inc idence. with. in. s o lu t ion .. h ighe r. nitrate. add i t ions . Crisp et wh ich. al.. ( 1 9 7 6 ) indu c e d t i pburn b y wi t hho l d ing. i n turn incre a s ed the end o genous aux in. fac t or. in. i n c idence. near l y of. all. r e l a t i onsh i p s. be tween. leve l . growth. t ipburn i s tha t t h i s cond i t ion inc r e a s e s. boron. A. c ommon and. with. the an. inc r e a s e in gr owth rate ( Co x � a l . , 1 9 7 6 ) . The. mo s t. e f fe c t ive proce dure for t i pburn cont r o l has. b e en. the u s e o f c u l t iva r s t o l erant t o the i n j ury ( Co x and McKee , 1 9 7 6 ) or. by. deve l o p ing. e i ther chemic a l. or. phys i o lo g i c a l. s creen ing. procedures for use on ind iv idua l s e e d l ings ( Co l l ie r et a l . , 1 9 7 7 ) At. p r e s en t ,. there a r e no r e c ommended procedu r e s the grower. can. use tha t w i l l guaran t e e t i pburn prevent ion in f i e l d - g rown l e t tuce ( Co l l ier and Tibb i t t s , 1 9 8 2 ) ..

(37) 22. 7.. S e e d Dev e l opme n t Y i e ld. is. me a s ur in g. an extreme ly impor t ant and c ommon. parame t e r. To ob t a in d i f fer ence s in. gene t i c d i f ference s .. among varie t i e s they are u s ua l ly grown toge ther under cond i t i ons ( Kr ame r , a s s oc i a t ed produc e d. 1978 ) .. y i e ld. cont r o l l e d. Howev er , d i f fe renc e s i n y i e l d may b e. w i t h the l o c a t ion in wh ich the s e e d s we re and. for. o r i g ina l l y. the var i a t ion in s ee d qua l i t y wh ich can. oc cur. at. d ev e lo pment. in. any t ime during s eed deve lo pmen t . A l t hough. t he. amount o f l i t e ra tu re on s e e d. g r a s s e s and l e g umes. i s vo l uminou s , r e l a t ive ly l i t t le informa t i on. ap pea r s on l e t tuce s e e d deve l opmen t . l i ke carrot ( Bo r t hwi ck ,. 1931 ) ,. beet ( Ar t s chwe ager and S t a re t t , s e em. to. indicate. mo rpho log i c a l. S tud i e s on o t h e r vege tab l e s. cabbage ( Thompson ,. 1 9 3 3 ) , sugar. 1 9 3 3 ) and pepper ( Cockran , 1 9 3 7 ). t h a t t h e i r pu r p o s e wa s. to. ob t a in. a. bet ter. unde r s t and ing o f f l owe r and s e e d deve lopmen t p r i or. t o the und e r t ak ing o f a b r e e d ing or crop improvemen t programme . S e e d deve l opmen t i s c oncerned w i t h t he various proce s s e s and s t ages. o c curr ing dur ing t he p e r i od from fe r t i l i z a t ion unt i l. the It. s e ed is fu l ly formed and r eady to harve s t ( Cope l and ,. 1976 ) .. is. nu t r i t iona l. charac t e r i s e d. by. i n t e r r e l a t ionships Central. to. a c omp lex o f. b e tween. the. s t ruc tu ra l. emb ryo and. and. end o s p e rm. t i s su e s .. the s e i n t e r r e l a t ionships i s t he deve lopment. of. endos perm and i t s s ub s e quent d i g e s t ion by the deve lo p ing Endo s pe rm. t he. embryo .. d eve l opment i s ob l iga t ory t o s eed deve lopmen t ( Po l lo ck. and Roo s , 1 9 7 2 ) . Dur ing ana t om i c a l produce d. mat ura t i on ,. s ee d s. pass. through. seve r a l. and phy s i o l o g i c a l s t ages un t i l t h e ma tu re d ry s ee d i s or. ( Ha r r i ng t on , produ c e. s e ed unt i l 1972) .. seeds. phy s i o l o g i c a l ly. the. s ee d reaches. its. max imum. dry. we i gh t. Any a t t empt t o a c c e l e r a t e t he s e chan g e s may. wh i c h ,. when. dry ,. appear. norma l. but. impe r fec t and incapab le o f in i t ia t ing rapid. c oord ina ted growt h .. are and.

(38) 23. Ko l l e r. ( 1962). maturat ion. of. Harr ing t on. s howed. Grand. tha t warm. Rap i d s. seed. t empe ra tures reduced. dur i ng. s eed. the. d orman c y .. ( 1 9 6 0 ) a l s o found t h a t d e f ic ienc i e s o f po t a s s ium. and. c a l c ium in l e t tuce d e c r e a s e d germina t i on and r�duced s e e d s t o ra g e l i fe .. Such. d e f ic i enc i e s a l s o produce s evere p lan t s ymp t oms but. do not pr event seed produc t i on . Seed. ma tur i t y i s u s ua l ly re f le c t e d in mean s e ed d r y. or s i z e ( Aus t in , 1 9 7 2 ) .. we i g h t. The s i z e o f l e t tuce s e e d s ha s b een found 1 9 7 0 ; Renar d ,. to a f fe c t b o t h grow t h and y i e l d ( S c a i fe and Jone s , 1 9 7 8 ; Sm i t h � a l . , 1 9 7 3 b ; S o f fe r and Sm i t h 1 9 7 4a ) . Seed 1981 ) . from. deve lopmen t. o c cur s i n two d i s t in c t. Dur i ng t he f i r s t pha s e , the. parent. pha s e s. food ma t e r i a l s are. p l an t and a c c umu l a t e d in the. ( Mu l l e t t , t ran s po r t e d. see d .. A. ra p i d. inc r e a s e in the d r y we i ght o f t h e s e ed o c curs at t h i s s t age . second pha s e , t rans p o r t sugar s. known as the ma t u ra t i on pha s e ,. from. and. the p a rent p l an t ha s. o c c u r s a f t er. c e a s ed .. The. o t h e r s imple c ompoun d s a r e conve r t e d i n t o. for the deve l opme n t of the embryo .. cons i de rab le seed. loss. s t arche s ,. seeds. wh i l e the dry we ight i s. wh i ch d o n o t h ave end o s perm ,. the. unchanged .. In. the embryo deve l o p s a t. The. s tores. any s u r p lu s to i t s i �e d i a t e energy r e qu i r emen t or. u t i l i s e s s ome t ran s po r t ed food. seed. In. l e ave s .. seeds. with. a. of. t ime .. c o t y ledons. s e ed. prov ide. Dur ing t h i s t ime. o f wa t e r o ccu rs and t he fres h we i gh t. de c l ines s ha r p l y ,. food. t rans por t e d. f a t s and s t orage pr o t e ins t o b e c ome the endos perm and t o pro t e in. The. this. mat e r i a l s in. min ima l. and the. emb ryo. deve lopment , the s t orage ma t e r i a l be c omes a s e p a r a t e non - c e l lu l ar end o s pe rm ,. wh ich. i s t h e food supply for fur ther deve l opment. of. the embryo dur ing g e rmina t i on . Seed overa l l. deve l o pment loss. of. and. mo i s tu r e .. h i ghe s t mo i s ture c on t en t . et. of. al .. ma tur a t i on are. as sociated. U s ua l ly very young. seed. with has. an the. Le i n inger and U r i e ( 1 9 6 4 ) and Ke r s t ing. ( 1 9 6 1 ) found a s t e ady dec l ine in the p e r c e n t age. r i pen ing s e e d s f r om an in i t ia l 8 0 - 9 0% t o 1 0 - 20% .. mo i s t u r e Au s t in. et.

(39) 24. ( 1 9 69 ). al. wh ich. found. the. further. w i t h c a r r o t s e e d s that a f t e r a pe r i od. mo i s ture. growth. c ontent d e c l ined to. in embryo s i z e .. 5 0 - 6 0%. dur ing. there. De lou che ( l 9 8 0 ). wa s. reported. no that. seeds a t t a in phy s i o l o g i c a l ma tur i t y a t mo i s t u r e conten t s rang ing f r om 3 2 - 3 5% ( e . g .. c orn , s o rphum , r i c e ) t o 5 0 - 5 5% ( e . g . s oybean s ,. peanu t s , beans ) . Jones h i s t o ry. ( 1927 ). of. s tudy on t h e I c eberg .. po l l ina t i on. He found. that. and the. l i fe brac t s. the f l owe r head s t ar t t o open a t the s ummi t b eau s e o f. deve l opment. e l onga t e. a. l e t t u c e cv .. s u b t en d ing the. made. of. the i nd iv idua l. f lower. bud s .. The s e. bud s. The. s tyle. r a p i d ly dur ing the 24 h p r i o r t o anthe s i s .. and s t i gma o f t h e p i s t i l a r e covered w i th sma l l brush h a i r s .. The. anthe r s. the. dehi s c. pistils .. to. the ins i d e pr io r to the. As t h e p i s t i l s e l onga t e ,. e l onga t ion. of. the ha i r s on t he s id e o f t h e. p i s t i l b r us h the po l len up out o f the p o l l e n s ac s o f the deh i s ce d an t he r s . fa l l. The l obe s o f t h e s t i gma s e para t e ,. a l lowing p o l len. to. on the s t i gma t ic p ap i l l ae loca t ed on the ins ide su r face. of. A s s o on a s t h e lobes have e xp anded ,. the s t i gma . cu r l. they b e g in t o. P o l l in a t i on probab ly t a k e s p l ace when t h i s. backward .. has. oc curre d . The. l e t t uc e. p l ant. as. a. who le. f l owe r ing peak s over a 7 0 d a y per i o d . a. u s ua l l y. shows. de finite. Th is i s o f ten f o l lowed by. c e s s a t ion of f l owe r ing a l t hough in s ome p l an t s there may b e. cont inua t i on. of. f l owe r ing. for s ever a l days. and. t hen. ano t h er. later .. The. g en e r a l. f l owe r ing. peak. f l owe r ing. cu rve may c o n t a in minor i r r e g u l ar i t i e s wh ich. ma inl y. to. o f l e s s magn i t ude a mon t h. a. f l u c tuat i on s in t empe r a t u r e ( Jone s ,. are. 192 7 ). or. two. only ,. due l ight. inten s i t y ( So f f e r and Sm i t h , 1 9 7 4a ) . The. f l owe r s. us ua l l y. open for an hour. or. t he. l i gu l ate c o ro l l a then f o l d s t igh t ly t o ge ther and d o e s not r eopen . W i thin two or three days the c o ro l l a , syt les t i ght ly. deh i s ce d s t amen s , w i t h e r e d. a n d s t igma a r e s h e d in a c lu s t e r . about. t h e deve lop ing achene s .. The b r a c t s t h e n c l o s e The beak o f. the. y oung.

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