41.
3.1 IN T R O D U C T IO N 3 . 1 .1 G eneral aim
T h e e x p e rim e n ts r e p o r te d in t h i s c h a p t e r w e re c a r r i e d o u t so as to gain an u n d e r s t a n d in g o f th e e f fe c ts o f v a r io u s p h o s p h o r u s regimes on th e re la tio n between g r o w t h and t h e u p ta k e and d i s t r i b u t i o n o f p h o s p h o r u s b y th e w h eat p l a n t . In one e x p e r im e n t g r o w t h up to a n th e s is was v a r ie d in d e p e n d e n t l y o f t h e s u p p l y o f P b y v a r y i n g a ir t e m p e r a t u r e . 3 . 1 . 2 P h o s p h o ru s regimes I t is w i d e l y be lie ve d t h a t ce re a ls s u ch as w h e a t a c q u ir e enough p h o s p h o r u s d u r i n g th e f i r s t f o u r weeks o f g r o w t h to p ro d u c e maximum y i e l d s . T h is b e lie f a ris e s fro m s e v e ra l e a r ly s tu d ie s c a r r ie d o u t in c u l t u r e medium at g la s s h o u s e t e m p e r a t u r e s . G e ric k e (1924) made th e fo llo w in g s ta te m e n t: " I t a p p e a rs , t h e r e f o r e , t h a t w h ile p h o s p h o r u s is needed in th e e a r ly g r o w t h p e rio d o f w h e a t, i t is n o t o n ly u se le ss, b u t r e l a t i v e l y h a r m f u l , i f p r e s e n t in p h y s i o l o g ic a ll y a v a ila b le fo rm in th e c u l t u r e media f o r th e l a t t e r g r o w t h p e r io d "
In d e e d , G e ric k e 's s ta te m e n t was based on o b s e r v a tio n s o f la rg e d i f f e r e n c e s . T h e p la n ts g r o w n w i t h p h o s p h o r u s f o r o n l y f o u r weeks w e re 125 cm ta ll and p r o d u c e d 700 g o f f r e s h m a tte r w h ile th e p la n ts s u p p lie d w ith p h o s p h o r u s f o r 12 weeks w e re o n l y 90 cm ta ll and w e ig h te d o n l y 480 g . T h e se d i f f e r e n c e s may have been due to an im balance betw e en th e m ajor n u t r i e n t s o r d e f ic ie n c y o f essen tial elements in t h e n u t r i e n t s s o lu tio n used to g r o w t h th e p l a n t s .
B r e n c h le y (1929) fo u n d t h a t b a r le y s u p p lie d w ith p h o s p h o r u s f o r s ix weeks o r lo n g e r p r o d u c e d maximum n u m b e r o f t i l l e r s , ears and
4 2.
g ra in and more d r y w e ig h t. B o a t w r i g h t and V ie ts (1966) f o u n d t h a t wneat s u p p lie d w ith 2 mM P s o lu tio n c h a n g e d w e e k ly f o r t h e f i r s t 5 weeks p r o d u c e d th e maximum d r y m a tte r and g r a i n . Chapman and Keay (1971) showed t h a t w h e a t p la n ts g ro w n in sand re s p o n d e d to p h o s p h o ru s s u p p lie d up to ear em ergence (2 mM P s o lu tio n a p p lie d to th e sand e v e r y t h i r d d a y ) .
S u tto n et al. (1983) r e p o r te d t h a t a s u p p ly o f p h o s p h o r u s ( 1 . 4 mM P in a e ra te d n u t r i e n t s o l u ti o n ) fro m g e r m in a tio n to n o d in g was s u f f i c i e n t to p ro d u c e maximum d r y m a tt e r , w h ile f o r maximum g r a in y ie ld p h o s p h o r u s needed to be s u p p lie d u n t il th e mealy r ip e stage o f g r a in d e v e lo p m e n t, h o w e v e r , re e x a m in a tio n o f th e data in t h a t p a p e r s u g g e s ts t h a t g r a in y ie ld was maximal i f p h o s p h o r u s was o n ly s u p p lie d to h e a d in g .
I t a p p e a rs t h a t i t is n o t th e p e rio d f o r w h ic h p h o s p h o r u s is p r o v id e d t h a t is c r i t i c a l to g r o w t h and g r a i n d e v e lo p m e n t b u t th e o n to g e n y o f th e p la n t . H o w e ve r in all o f th e s e s tu d ie s o n l y one level o f p h o s p h o r u s was a p p lie d ( f o r v a r y i n g le n g th s o f tim e ) and th e t is s u e p h o s p h o r u s c o n c e n tr a tio n s w e re m o s tly h i g h e r th a n w o u ld be f o u n d in p la n ts in th e f i e l d . In E x p e rim e n t 1 below p la n ts w e re g r o w n w ith b o th th e s ta n d a r d n u t r i e n t s o lu tio n s u p p lie d in th e C a n b e r r a P h y to t r o n (1 mM P) and w i th th e same s o lu tio n m o d ifie d to 0 .2 5 mM P. T h e l a t t e r s o lu tio n , when a p p lie d f o r a s h o r t p e r io d , p r o d u c e d sm aller ( n o t s t u n t e d ) p la n ts w ith fe a t u r e s a k in to p la n ts g r o w n in th e fie l d ( in A u s t r a l i a ) . B y u s in g tw o p h o s p h o r u s le vels i t was p o ssib le to t e s t f o r in te r a c t io n e ffe c ts between th e level and th e tim e f o r w h ic h p h o s p h o r u s was a p p lie d .
4 3.
3 . 1 . 3 T e m p e r a t u r e r eg ime s
S u t t o n (1969) s t a t e d t h a t , t e m p e r a t u r e m u s t a f f e c t t h e r a t e of r e l e a s e of p h o s p h o r u s from t h e solid p h a s e into t h e soil solution and t h e r a t e of d i f f u s i o n and he q u o t e d s t u d i e s which r e p o r t e d t h a t i s ot op ic al l y d i l u t a b l e a nd anion e x c h a n g e r e s i n e x t r a c t a b l e p h o s p h o r u s
i n c r e a s e with t e m p e r a t u r e . He also n ot e d t h a t t h e e q ui l i b ri u m
c o n c e n t r a t i o n s of p h o s p h o r u s in t h e soil solut ion i n c r e a s e b y 1-2% with
e ac h d e g r e e r i s e in t e m p e r a t u r e . In c o n t r a s t Chien e t al. (1982)
c o n c l u d e d t h a t s o r p t i o n i n c r e a s e d b u t d e s o r p t i o n d e c r e a s e d with
i n c r e a s i n g t e m p e r a t u r e whe n b o t h s o r p t i o n a n d d e s o r p t i o n w e re
initially a t t h e same t e m p e r a t u r e . T h i s a g r e e d with e a r l i e r w o r k b y
Beaton a n d Read (1963) a nd B a r r o w (1974) a nd s u g g e s t s t h a t more
f e r t i l i z e r is r e q u i r e d to maintain a g i v e n level of soil solution
p h o s p h o r u s in a w a r m e r soil. B u t , like S i n g h a n d J o n e s (1 9 77 ) , Ch ie n e t al. ( 1982) f o u n d t h a t wh e n d e s o r p t i o n followed s o r p t i o n at 25°C t h e a m o u n t s of p h o s p h o r u s d e s o r b e d i n c r e a s e d with t e m p e r a t u r e . In t h i s c a s e l e ss f e r t i l i z e r would be r e q u i r e d b y p l a n t s g r o wn a t a h i g h e r t e m p e r a t u r e . S i ng h a nd J o n e s (1977) s t a t e d t h a t phys iologic al e f f e c t s of t e m p e r a t u r e on l e t t u c e p l a n t g r o w t h a n d c o n s e q u e n t
p h o s p h o r u s r e q u i r e m e n t p l a y e d only a minor role r e l a t i v e to s o r p t i o n . T h e c o n c e n t r a t i o n of p h o s p h o r u s in p l a n t t i s s u e r e p r e s e n t s a
b a l a n c e b e t w e e n d r y m a t t e r a c c um ul a ti on a n d p h o s p h o r u s u p t a k e .
Power e t al. ( 1964) f o u n d t h a t a t low soil t e m p e r a t u r e s ( 7 - 1 1 ° C ) t h e
u p t a k e of p h o s p h o r u s c o n t i n u e d a t a r e l a t i v e l y r a p i d r a t e a n d
i n c r e a s e d t h e t i s s u e c o n c e n t r a t i o n . At h i g h e r t e m p e r a t u r e s ( u p to
2 7 ° C ) d r y m a t t e r p r o d u c t i o n was limited b y t h e u p t a k e of
p h o s p h o r u s . On t h e o t h e r h a n d , Smika a n d Ellis (1971) f o u n d t h a t
4 4.
when t h e soil te m p e r a tu r e was v a r ie d fro m 3° to 10°C e i t h e r r a p i d l y