LEAF PHOSPHATE STATUS AND PHOTOSYNTHESIS IN-VIVO - CHANGES IN SUGAR PHOSPHATES, ADENYLATES AND NICOTINAMIDE NUCLEOTIDES DURING PHOTOSYNTHETIC INDUCTION IN SUGAR-BEET

Sugar beets (Beta vulgaris L. cv. F58-554H1) were cultured hydroponica lly in growth chambers. Leaf orthophosphate levels were varied nutriti onally. The effect of decreased leaf phosphate (low-P) status was dete rmined on the rate of photosynthesis (P-N) and on pool sizes of leaf r ibulose-1,5-bisphosphate (RuBP), 3-phosphoglycerate (PGA), triose phos phate (triose-P), fructose-1,6-bisphosphate (FBP), fructose-6-phosphat e (F6P), glucose-6-phosphate (G6P), adenylates and nicotinamide nucleo tides during photosynthetic induction (measured at 0, 1.5, 5 and 30 mi n from irradiation). P-N reached 50 % of its final value in 4 min in c ontrol leaves and 10 min in low-P leaves. Hence the increase in the le ngth of induction period in low-P leaves was most likely due to a slow build-up in RuBP: ATP, NADPH, PGA, and FBP all reached high levels in 5 min at which time RuBP was half and P-N 16 % of their eventual valu es at 30 min. The slow-build-up of RuBP did not appear to be due to in sufficient ATP and NADPH for the conversion of PGA to triose-P; rather , low-P seemed to limit photosynthetic induction somewhere in the sequ ence of reactions between triose-P and RuBP formation.