Root dry we i ght Leaf dry weight

S t ern dry we i ght

Nod u l e dry we i ght

Total p l ant. N i trogen

c 2H 2 r educt i on a ct i v i t y

P o d dry we i ght

After

function

0

function

1

0 . 7 4 4

0 . 7 2 9

0 . 6 2 0

0 . 6 1 7

0 . 4 9 7

0 . 4 2 7

0 . 2 9 0

0 . 1 1 3

Chi-square

5 3 . 2 9 8

df

8

Significance

* * * * N 0

1 9 76 ) . Although , there was greater compet i t ion between vegetat ive and reproduct ive growth in the indeterminat e cult ivar than in t he

determinate cultivar , Egli and Leggett ( 1 9 7 3 ) found no d i f ference between soybean cultivars in the total amount of dry-ma t t e r produced .

I n the present experiment however , vegetative dry-ma t t e r

component s of t h e indeterminat e cult ivar viz . lea f , stem a n d r o o t were signif icant ly higher than tho se o f the determinat e cultiv a r ( Table 4 . 1 ) . This could have part ly been due to the indeterminat e cultivar reaching the f lowering stage one week later than the determinat e cultivar . However , c a lculations t o assess the impact o f this extra week showed that the indetermina t e cult ivar had produced only about 4 . 4 g dry-matter in tha t t ime whereas , by the end of the sampling period the indeterminat e cult ivar accumulated about 20 g dry-ma t t e r more than that o f the determinat e cultivar . Henc e , the l a t e r

f lowering c a n not fully account f o r the d i f ferenc e . In a growing plant , where photo synthetic t issues make the maj or contribut ion t o dry-mat ter production , the absolute growth r a t e at any t ime is t he produc t o f the rate of increase in weight per unit of l e a f ( net assimilation rate , NAR) and the amount of leaf present ( J ackson , 1 9 63 ) . The differenc e in dry-mat ter yield in this pre sent experiment therefore , was largely the re sult of the indet erminate c u l t ivar having more leaf area per plant than the determinat e cult ivar at every growth stage from first f lowering onwards ( Table 4 . 1 ) . Furthermo r e , the indet erminate cultivar also had a higher NAR at the grain f i l ling stage ( Figure 4 . 4b ) . The NAR values in this experiment were also very similar to the value s reported by Sivakumar and Shaw ( 1 9 78 ) using the same calculation technique .

Root growth was also different between the two c u l t ivars . Root dry weight o f the det erminat e cultivar began to plateau soon after f lowering , whereas for the indeterminate cultivar i t c ontinued to increase for a further two weeks after initial f lowering before leve lling off by the f inal sampling date ( Figure

4 . 1 ) .

This higher root growth in the indeterminat e cultivar was a l s o reflect e d in t he lower shoot : root ratio at the two growth stages c ompared a ft er f lowering ( Figure

4 . 5 ) .

The shoot : root ratio values from the indeterminat e cu1i t

�

ar were quit e similar t o the values from f i e ld grown soybean cultivar ' Wayne ' ( Sivakumar

e t a Z . , 1 97 7 )

whi c h had valu e s ranging f rom

6 . 8

to

9 . 5 .

When the ratio of root dry-ma t t e r t o total dry-matter was compared , i t indicated that more dry-ma t t e r was part i tioned t o t he root component o f the indeterminat e cul t ivar t han that of the determinat e cultivar ( Figure

4 . 6 ) .

This would sugges t that t he indeterminat e cultivar had more pho t o synthate ava ilable to promot e more root growth over a longer period o f t ime than t he determinate cult ivar . Thi s could have impli c a t ions for nitrogen fixa t ion , a s wil l be discussed be low

( 4 . 4 . 3 ) .

There

is

evidenc e that the root sys t em may be a yield-limiting f a c t or in soybean . For example , Sanders and Brown

( 1 9 76 )

found in their graf ting experiment that increasing the root : shoot ratio resulted in increased growt h , nut rient uptake and yield of Lee

68 '

soybeans ; increasing t he number of sho o t s per root ( increasing the shoot : root ratio ) has much sma ller effec t s . Silberbush and Barber

( 1 984 )

a lso reported that soybean cultivars with more extensive root systems were found t o absorb more P and K and these cultivars were those with highe s t yield s .

Nodule dry weight chang e s in thi s experiment ( Figure

4 . 7 )

indica t ed that the indetermina t e cult ivar still had high nodul e g rowth after the pod development stage , while it began to decline at t h i s time in the determina t e cultivar . I n legumes , either a reduced photo synthate supply or an increa sed level of combined nit r o gen can cause the senescenc e o f previously active nodules ( Sutton ,

1 983 ) .

I n the present experiment , the early nodule senescence of the d e t e rmina t e cultivar was probably due to the reduct i on o f photo synthat e supp l y . Thi s could in part be supported by the f act that there was

signif icant ly less dry-matter part i tioned to the roo t s of t he determinate cultivar a s compared with that of the indeterminate cultivar ( Figure

4 . 6 ) .

4 . 4 . 2

YIELD AND YIELD C OMPONENTS : I t has been sugges t ed t ha t competition between vegetative and reproductive growth during

f lowering and pod development may reduce the number of pods set ( Greer and Anderson ,

1 96 5 ) .

Thi s led t o the suggestion that cult ivars wit h a determinate growth type may exhibit less competit ion during thi s

period and thus produc e higher yields ( Shibles

et a l . , 1 9 7 5 ) .

However , there are disagreement s over these points in the l i t e r ature . For example , Egli and Leggett

( 1 9 7 3 )

and Beaver

et a l . ( 1 9 8 5 )

recorded simi lar values of grain yie ld between the two growth typ e s in soybean , whereas Hicks

et a l . ( 1 9 6 9 )

report ed higher values in the d e t ermina t e

type o f soybean and Chaturvedi

et a l . ( 1 980 )

found a higher value in an indet erminat e type of cowpea . Furthermore , Beaver and J ohns o n

( 1 98 1 )

found tha t the det ermina t e cultivars passed l e s s predic t i b l e performance s than t h e indet ermina t e cultivars when they c ompared yie ld

stability o f two growth types adapted to the northern Uni t e d Stat e s . Thus the yield differenc e between the two growth types may not only be affected by gene tic factors but also by o ther factors such as

environment a l condit ions , cultural prac t i c e s and their interactions as well . In t he present experiment , the indeterminate cultivar produced twice a s much seed yield as the det erminat e cultivar ( Table

4 . 2 ) .

A c ompari son of the yield c omponent s indicated that t he main yielding advantage o f the indetermina t e cult ivar over the d e terminat e cultivar came f rom the number of pods s e t . Pod number has previou sly been reported a s the most important c ompo nent in determining grai n yield in soybean ( e . g . Greer and Anderson ,

1 9 6 5 ;

Shibles

e t a l . , 1 9 7 5 ;

Laohasiriwong ,

1 982 ) .

When the yie ld and yield component s f rom the main s t em and the branches were c ompared seperately between t he growth type s , it was c lear that the dif f erence was mainly due to the

difference in branching ( Table

4 . 3

) . Beaver

et a l. ( 1 985 )

a l s o f ound that t he determinate cultivars set as many pods on the mai n s t em a s the indeterminat e cultivar . In c ontrast t o the result of this pre sent experiment , they found that the determina t e cult ivars

C

E l f ' and

I _'

C lark ) produced a greater number of pods and more seed we ight from branches than the indeterminat e cultivar�Wil liams ' . However , the i r experiment was carried o u t i n the field where they noted t hat the cond i tions were 'les s than produc t ive ' . The morphologi c a l behavior of the two growth types in their experiment may not be direc t ly comparable t o those of the present experiment which was conducted under contro lled environment condi tions .

Severa l authors have conc luded that the dif ferenc e in the o�eY

duration which seed accumulates dry mat t e r account s for more yield

variation among cult ivars than do differenc e s in the rate o f seed dry mat ter accumulation ( e . g . Hanway and Weber , 1 9 7 1 a; Egli a nd Legget t ,

1 97 3 ) . In t he present experiment both cultivars had a similar

duration o f seed dry-ma t ter accumulation

(4

weeks from grai n f i l l i ng stage ) . Henc e the yield advantage of the indeterminat e c u lt ivar was mainly due t o more pods developing from a larger source c apacity

( leave s ) . As Tanaka ( 1 980 ) exp l ained , in grain legumes during the period from f lowering t o the early stage of pod deve lopment , the

source capac i t y i s the key factor of pod setting . This i s because the veget a t ive , reproduct ive and ripening pha s e s overlap each o t her a fter the commenc ement o f f lowering and pods start t o grow whil e both new leaves and f l ower primodia continue to differentiate on t he growing stems for some t ime . The sink ( young pods ) o f this growth s t age , and the potential s ink ( f lowers ) and source ( le ave s ) of the next growth ( ripening ) pha s e grow s imultaneously , and the s e organs may c ompet e with each other when the sour c e c apacity i s insuf ficient t o meet a l l the demands of t he se sinks . The sink capacity during the ripening phase ( pod number ) is decided by the condition of this comp e t i t i o n during a shor t period a t or a f ter f lowering .

The proportion of the dry-matter o f a grain crop that i s harvested ( se ed ) relat ive to t h e total above ground bioma s s ,

i . e . ,

harvest index , i s an important parameter describing the e f f iciency o f grain yield ( Gi f f ord and Evans , 1 98 1 ) . I t has been sugges t ed that future improvement s in yield may come primarily from improvement s in the partitioning o f a s s imilate into the harvested fraction ( Loomi s

et

a l . ,

1 9 79 ) . The resul t s in this experiment indicated that the

in the determinat e cultivar than in the indeterminate cultivar ( Figure

4 . 6 ) .

The f inal harvest indices of the two cultivars were also

signi ficant ly different ( Table

4 . 2

) . However , the difference in the f inal harvest indices was relative ly sma ller than the proport ion o f dry-mat ter which was part i t ioned to the pods a t the grain f i lling stage . This could be due t o d i f f erent s t rategies of reproduc t ive growth betwe en the two growth type s ,

i . e . ,

the determinat e soybeans produce most of their vegeta t ive growth first , and then their

reproduc tive growt h while the indetermina t e soybeans overlap their vegetative and reproduct ive periods ( f lowering continues until the plant stops produc ing new nodes and by then seed development has already begun on the lower node s ) . Even though pods at the basal node s begin developing earlier than pods at the distal nodes a ll the pods mature at about the same t ime due to a faster growth rate in t he upper pods ( Johnson and Dunphy ,

1 9 83 ) .

4 . 4 . 3

N ITROGEN FIXATION : There are relat ively f ew published studies on the e f fect of growth typ e s on nitrogen f ixation . Bello

et a l .

( 1 980 )

found that indeterminat e cultivar 'Williams ' f ixed more

nitrogen a f t er pod f i lling than other cultivars with semi-determi na t e or determinate growth types . However , they did not discuss the cha nge of ace tylene activity in relation to the other growth stages of the different growth types . I n general , the values o f acetylene

reduct ion activity in t he present experiment were higher than tho s e reported by Skrdleta

et a l . ( 1 9 7 8 )

when compared at similar growth stages with their glas shouse-grown soybean .

The patt erns of acetylene reductio n activity of the two growth types in thi s experiment were markedly d i f ferent . The sharp decr e a s e i n activity of the indeterminate cult ivar a f t e r f lowering probably came from the s trong c ompet ition for pho t osynthate between vegeta t ive and reproduct ive organs at this t ime ( Figure

4 . 8

) . In c ontrast t he

acetylene

reduction activity of the determinat e cultivar increased markedly unti l t he plant reached the pod development stage a f t e r which the rate dec lined , subsequent ly recovered and then dec l ined

l��

again at grain f i lling stage . However , it is not c lear why these A

changes occured a s will be shown i n t h e next Chapter tha t in t he determinat e cultivar when the a c t ivity dec lined , it did not go up again .

There are numerous studies indicating that the nitrogen fixation potent ial of the plant is limited by i t s ability t o provide photosynthat e to the nodules ( se e Bergersen ,

1 9 7 5 ;

Edwards ,

1 9 75 ) .

The result s from the present experiment indicated that the t o t a l nitrogen f ixed by the indetermina t e cultivar was greater than that o f the determina t e cultivar ( e . g . higher 'integra t ed acetylene reduct ion act ivity ' Figure

4 . 9 ) .

Since the acetylene reduc tion ac tivity per unit nodule dry weight of the determinat e cultivar was genera lly

higher than those of the indeterminat e cultivar ( Figure

4 . 8 ) ,

i t c ould be conc luded that the superiority of the acetylene reduc t ion activity in the indet erminat e cultivar came largely from a rapid increase in the nodule mass , which c ertainly c ame from the indeterminat e

cultivar ' s ability to supply more pho t o syntha t e to the nodules . The result s of

1 4c

movement , which showed that a higher percentage of

1 4c

that of the determinat e cultivar suppor t s that conclusion indirec t ly ( Table 4 . 5 ) .

The distribution of assimil a t e s in soybean is predominant ly localized about the source leaf ( S tephenson and Wilson , 1 9 7 7a ) . The maj or portion of the a s s imilate accumulates in the axillary pods o f the source leaf , with leaves lower in the canopy contribu t i ng very little e lsewhere ( Stephenson and Wilson , 1 9 7 7 a , 1 9 7 7b ) . The resul t s from this experiment supported their finding , with about 3 0 % o f 1 4c found in the axil lary pods l e s s than 1% e lsewhere ( Ta b l e 4 . 5 ) .

The significantly higher amount of 1 4c f ound in t he internode below the labelled leaf in the indeterminat e cult ivar ( Table 4 . 5 ) may be an indication that t here is a greater downward movement o f

photosynthate i n this than i n the det ermina te cultivar . Similar trends had been reported in other species . For example , Baker

e t a Z ·

( 1 983 ) found a 4 . 6% distribut ion o f 1 4c in the indetermina t e f i e ld bean

( Vicia faba )

compared wi th only 2 . 5% in the determinat e c u l t ivar

t��

when the leaf at first f lowering node was labelled .

A

Ac etylene reduct ion ac t ivity i s an instantaneous e s t imat e o f nitrogen fixat ion One way of e s t imating the total activity i s by

summing the area under the values for instantaneous ac etylene

reduct ion activity determined at each succes sive harves t ( Pa t te r son and LaRue , 1 9 83 ) ; thi s is the 'int egrat ed acetylene reduct io n

activity ' . For plant s that depended solely on symbiotic n i t ro g e n , the relationship of this 'integrated acetylene reduct ion act iv i t y ' and total plant nitrogen should be linear . A linear relat ionship was found between the two parame t e r s for each cultivar in this experiment ( Figure 4 . 9b ) . Pat terson and LaRue ( 1 983 ) also report e d signif i c ant

correlations between total plant nitrogen , 'integrated ace tylene reduc tion act ivity ' , isotope dilu t ion and N-differenc e when they

studied the seasonal nitrogen fixation of 2 1 soybean cult ivars . Their 'int egra t ed acetylene reduction a c t ivity ' value s , however , were less

than the values reported in this experiment ( 5-2 0 mmol plant - 1

compared with 2 0-50 mmol plant - 1 ) . Thi s may be due t o the difference in environmental c onditions ( e . g . t emperature ) , since the ir experiment was conducted in the field whereas thi s experiment was conduc ted in

controlled environmental condit ions .

4 . 4 . 4 NITROGEN DI STRIBUTION : The soybean seed has a high nitrogen

In document A study of nitrogen fixation, nitrogen distribution and seed yield of selected legumes with two different growth types : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Palmerston N (Page 143-154)