Developmental changes of plant in the regulation of photosynthate distribution of leaves were studied in hydroponically cultivated rice by the (CO2)-C-14 tracer technique and analysis of the activity of the regulatory enzymes, sucrose phosphate synthase (SPS), phosphoenolpyruvate carboxylase (PEPC), and pyruvate kinase (PK). The distribution of primary photosynthates into sugars, amino acids, organic acids, sugar phosphates, proteins, and polysaccharides was determined by column chromatography. The relative primary photosynthate distribution to the sugar phosphate fraction was significantly larger in the 5(th) leaf than in the 6(th) one. Correspondingly, the V-max of PEPC was significantly higher in the 5(th) than in the 6(th) leaf, while no significant differences between leaves were detected in the other enzymes. As a consequence, the ratio of the Vmax of SPS and PEPC was lower in the 5(th) than in the 6(th) leaf. As the 5(th) leaf develops before panicle initiation in rice, it predominantly supports vegetative growth, while the 6(th) leaf develops after panicle initiation and thus contributes mainly to reproductive growth. We conclude that the physiological properties of each leaf are regulated developmentally. When the 6(th) leaf became fully expanded (corresponding to the panicle initiation stage of plant), the distribution pattern of C-14 was transiently changed in the 5(th) leaf, indicating that individual organs that are mainly involved in vegetative development are affected to some extent by the whole-plant-level physiological transformation that occurs at the transition from the vegetative to the reproductive stage.
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