EXPERIMENT

CONTROL LOW P LOW P LOW P ANTHESI S A + 20

t

t

c:

TIME AFTER SOWING (d a y s a t 18/13°C)

100 F i g u r e 6 . 1 D e s i g n s o f e x p e r i m e n t s 1 a n d 2. B r o a d l i n e s = d a i l y a p p l i c a t i o n s o f p h o s p h o r u s v i a t h e r o o t s . T h i n l i n e s = n o p h o s p h o r u s in n u t r i e n t s o l u t i o n . A r r o w s i n d i c a t e f o l i a r a p p l i c a t i o n s o f p h o s p h o r u s . A + 2 0 , A + 2 2 a n d A + 3 0 i n d i c a t e h a r v e s t t i m e s ( d a y s a f t e r a n t h e s i s ) a n d M = m a t u r i t y .

86.

-1 -1

d a y . In all f o l i a r t re a tm e n ts h e re a to ta l o f 80 pg P leaf was a p p lie d in 3 a p p lic a tio n s o v e r 6 d a y s .

T h e p h o s p h o r u s was a p p lie d as a 0.08M ammonium d i h y d r o g e n o r th o p h o s p h a t e s o lu tio n c o n ta in in g 0.1% Tw een 20, and a d ju s te d to pH 6 .5 w i th ammonium h y d r o x i d e . T h i s was b r u s h e d o n to th e p h o t o s y n t h e t i c a l l y a c tiv e p o r t io n o f th e a d a xia l s u r fa c e o f th e fla g leaf o r th e glum es o f th e c e n tr a l f e r t i l e s p i k e le ts . A p r e l i m i n a r y c h e c k c o n firm e d t h a t t h i s p h o s p h o r u s s o lu tio n d id n o t damage th e w h e a t leaf (see also B a re l and B la c k , 1 97 9b). T h e p h o s p h o r u s a p p lie d to th e

32

ear was tr a c e d u s in g P and f o u n d to remain p r e d o m i n a n t ly w i th in th e e a r.

6 . 2 . 4 A sse ssm en ts

P la n t senescence was fo llo w e d b y m e a s u rin g th e n e t c a rb o n e x c h a n g e ( N . C . E . ) o f th e fla g leaf ( C h a p t e r 2 . 5 . 5 ) , o r b y v i s u a l l y e s tim a tin g th e p r o p o r t io n o f th e leaf s t i l l g re e n and c a lc u la tin g N . C . E . u s in g r e la t io n s h ip s e s ta b lis h e d in t h i s and an e a r li e r s t u d y ( F ig u r e 6 . 2 ) .

G ra in d r y w e ig h t ch an ges w e re fo llo w e d b y t a k i n g g r a in s fro m t h e A and B f l o r e t s o f c e n tr a l s p ik e le ts b e fo re and 10 d a y s a f t e r th e i n it ia l a p p lic a tio n o f p h o s p h o r u s ( E x p e r im e n t 1 ) , and a t 22 d a y s a f t e r a n th e s is and a t m a t u r i t y in E x p e rim e n t 2. T h e g r a in s ta k e n 22 d ays a f t e r a n th e s is w e re a n a ly s e d f o r to ta l p h o s p h o r u s and n itr o g e n ( C h a p t e r 2 . 7 . 1 ) and th e n u m b e r of e n d o s p e rm cells was d e te rm in e d b y th e m ethod o f R ijv e n and W ardlaw (1 9 6 6 ).

25

F ig u r e 6 .2 R e la tio n s h ip betw een a v is u a l s co re o f senescence and p h o t o s y n t h e s is o f th e fla g leaf o f low P p la n ts in e x p e r im e n t 1 o f t h i s c h a p t e r OB) and e x p e r im e n t 2 o f C h a p t e r 3 ( # ).

Zero N . C . E . o c c u rs when a b o u t 25% o f th e leaf is s t i l l g re e n due to t h e t e c h n i q u e used ( F i g u r e 4 . 1 ) . . ? r o

N . C . E . = -2 .5 4 5 + 0.115X -6 .0 8 x 1 0 X + 1 .3 1 x 10 X ^ w h e re X = % area s t i l l g re e n ( R ^ = 0 . 7 9 9 ) .

87

.

6 .3 RESULTS AND D ISC U S SIO N

6 .3 .1 E x p e rim e n t 1. Mid g r a i n f i l l i n g a p p lic a tio n s o f p h o s p h o r u s

T h e c o n tr o l and low P p l a n ts w e re s im ila r to th o s e d e s c r ib e d in C h a p t e r 4. In c o n t r o l p la n ts th e f o l i a r t r e a tm e n t s had no e f f e c t on leaf p h o to s y n th e s is , n i t r o g e n s t a t u s , g r a in d r y w e ig h t o r g r a i n p h o s p h o r u s ( T a b le 6 . 1 ) . T h e a p p lic a tio n s o f p h o s p h o r u s to th e fla g leaf more th a n balance d losses w h ic h o c c u r r e d betw e en 20 and 30 d a y s a f t e r a n th e s is . T h i s showed t h a t some p h o s p h o r u s e i t h e r d id n o t e n t e r t h e le a f, o r e n te r e d t h e le af and was n o t t r a n s lo c a te d . T h e actual p r o p o r t i o n a b s o rb e d was n o t m e a s u re d . O t h e r w o r k s u g g e s ts t h a t i t can v a r y fro m 13-45% ( A l s t o n , 1979) to 97% ( B a r e l and B l a c k , 1979a). U p ta k e and t r a n s lo c a ti o n are g r e a t e r in a hu m id a tm o s p h e re ( T h o r n e , 1958), w i t h some w e t t i n g a g e n ts ( K o o n tz and B i d d u l p h , 1958; Bouma, 196 9), when a h i g h e r c o n c e n tr a t io n o f p h o s p h o r u s is a p p lie d ( K o o n tz and B i d d u l p h , 1958; Ba rel and B la c k , 1979a), fro m more acid s o lu tio n s (B o u m a , 196 9 ), fro m some p h o s p h o r u s com pou nds ( B a r e l and B l a c k , 1979a), and b y c e r ta in p la n t species ( B a r e l and B l a c k , 1979a).

In low P p l a n ts fla g leaf p h o to s y n th e s is d e c lin e d fro m 8 .4 to less

_ p - i

t h a n 1 pmole CO^ m s in th e u n t r e a t e d p la n ts o r w h e re p h o s p h o r u s was a p p lie d to t h e e a r. T h e fla g leaf almost m a in ta in e d th e same r a te o f p h o to s y n th e s is when p h o s p h o r u s was a p p lie d to t h e leaf ( T a b le 6 . 1 ) . B o th th e p h o s p h o r u s and n i t r o g e n c o n c e n tr a t io n s in t h e t r e a te d f la g le a f w e re h i g h e r th a n in th e u n t r e a t e d o r ear t r e a te d p la n t s . Some o f th e p h o s p h o r u s was p o s s ib ly on th e s u r fa c e o f t h e t r e a te d le a f, b u t t h a t w h ic h was a b s o rb e d re d u c e d th e loss o f

8 8.

n i t r o g e n , as s e e n v i s u a l l y b y t h e loss of c h l o r o p h y l l , a n d t h e r a p i d d e c l i n e in p h o t o s y n t h e s i s in t h e u n t r e a t e d p l a n t s .

P l a n t s g i v e n foli ar p h o s p h o r u s via t h e flag leaf p r o d u c e d g r a i n s in c e n t r a l s p i k e l e t s which w e r e 3.4% h e a v i e r (P = .05) a t 30 d a y s a f t e r a n t h e s i s (10 d a y s a f t e r b e g i n n i n g t h e f oli ar t r e a t m e n t s ) . T h i s was l a r g e l y d u e to a lower n u m b e r of g r a i n s p e r e a r in t h e t r e a t e d

p l a n t s . T h e yield p e r e a r was not i n c r e a s e d b y t h e t r e a t m e n t s

( T a b l e 6 . 1 ) . T h e s i n g l e a p p l ic a t i o n of 400 pg P into t h e ro o t zone 21 d a y s a f t e r a n t h e s i s also r e d u c e d s e n e s c e n c e a p p a r e n t l y b y m a i n t a i n i n g t h e p h o s p h o r u s c o n c e n t r a t i o n s , a n d h e n c e n i t r o g e n a n d c h l o r o p h y l l in t h e flag leaf ( T a b l e 6 . 2 ) . T h e r e was no i n c r e a s e in g r a i n d r y w e i g h t as a r e s u l t of t h e d e l a y e d s e n e s c e n c e b u t a small i n c r e a s e in t h e g r a i n p h o s p h o r u s c o n c e n t r a t i o n following t h i s a p pl i c a t i on via t h e r o o t s . From t h e r e l a t i o n s h i p in F i g u r e 6 . 2 it is e v i d e n t t h a t t h e r a t e of assimilation of c a r b o n b y a leaf with onl y 30% of t h e s u r f a c e a r e a still g r e e n is low d u e to t h e small a r e a of a c t i v e t i s s u e a n d t h e low r a t e of

-2 -1

p h o t o s y n t h e s i s ( 2 - 6 pmole CC^ m s ).

6 . 3 . 2 E x p e r i m e n t 2. P r e a n d e a r l y p o s t a n t h e s i s a p p l i c a t i o n s of p h o s p h o r u s

P h o s p h o r u s a p p l ie d to t h e le ave s i n c r e a s e d t h e flag leaf a r e a

'I

d u r a t i o n b y 10% f o r t h e p e r io d from a n t h e s i s to 35 d a y s a f t e r

a n t h e s i s a n d b y 21% b e t w e e n 15 a nd 35 d a y s a f t e r a n t h e s i s . T h i s was d u e to d e l a y e d s e n e s c e n c e b e t w e e n 15 a nd 30 d a y s a f t e r a n t h e s i s . All flag leaf b l a d e s w e r e d e a d 35 d a y s a f t e r a n t h e s i s . A p p l i c a t i o n s to