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Carbon Assimilation and Translocation in Soybean Leaves at Different Stages of Development

Light and Plant Growth Laboratory, Plant Physiology Institute, SEA, Federal Research, United States Department of Agriculture, Beltsville, Maryland 20705

Carbon assimilation, translocation, and associated biochemical characteristics of the second trifoliolate leaf (numbered acropetally) of chamber-grown soybean, Glycine max (L.) Merr., plants were studied at selected stages of leaf development during the period from 10 to 25 days postemergence. Leaves of uniform age were selected on the basis of leaf plastochron index (LPI).

The test leaf reached full expansion (A_max) and maximum CO₂ exchange rates on a leaf area basis at 17 days postemergence (LPI 4.1). Maximum carbon exchange rates per unit dry weight of lamina were attained several days earlier and declined as specific leaf weight increased. Chlorophyll and soluble protein continued to increase beyond the attainment of A_max, but were not accompanied by further increases in photosynthetic rates.

Much of the fixed carbon in leaves is partitioned between starch and sucrose. Starch content of leaves as a percentage of dry weight at the end of an 11-hour photoperiod was taken as an indication of the potential energy reserve accumulated by the leaf. Starch levels were the same regardless of leaf age during the period from 0.3 A_max to 7 days after attaining A_max. Respiratory and synthetic activity of leaves decreased considerably during the same period, suggesting that starch accumulation is not entirely controlled by the energy demands of the leaf.

Sucrose content increased steadily during leaf expansion and was accompanied by corresponding increases in sucrose phosphate synthetase (EC 2.4.1.14) activity and translocation rates. Sucrose phosphate synthetase may have an important regulatory role in photosynthate partitioning and translocation.