Developmental regulation of photosynthate distribution in leaves of rice

mRNA expression patterns of genes for metabolic key enzymes sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), pyruvate kinase, ribulose 1,5-bisphosphate carboxylase/oxygenase, glutamine synthetase 1, and glutatmine synthetase 2 were investigated in leaves of rice plants grown at two nitrogen (N) supplies (N-0.5, N-3.0). The relative gene expression patterns were similar in all leaves except for 9(th) leaf, in which mRNA levels were generally depressed. Though increased N supply prolonged the expression period of each mRNA, it did not affect the relative expression intensity of any mRNA in a given leaf. SPS V-max SPS limiting and PEPC activities, and carbon flow were exartimed. The ratio between PEPC activity and SPS V-max was higher in leaves developed at the vegetative growth stage (vegetative leaves: 5(th) and 7(th) leaves) than in leaves developed after the ear primordia formation stage (reproductive leaves: 9(th) and flag leaves). PEPC activity and SPS V-max decreased with declining leaf N content. After using (CO2)-C-14 the C-14 photosynthate distribution in the amino acid fraction was higher in vegetative than in reproductive leaves when compared for the same leaf N status. Thus, at high PEPC/SPS activities ratio, more C-14 photosynthate was distributed to the amino acid pool, whereas at higher SPS activity more C-14 was channelled into the saccharide fraction. Thus, leaf ontooeny was an important factor controlling photosyntliate distribution to the N- or C-pool, respectively, regardless of the leaf N status.