Influence of Leaf Starch Concentration on CO(2) Assimilation in Soybean

Net photosynthetic rate, CO(2) compensation concentration, and starch and soluble sugar concentrations were measured in soybean (Glycine max [L.] Merrill) leaves in an attempt to evaluate the effect of carbohydrate concentration on rate of CO(2) assimilation.Plants were grown in a controlled environment room at 23.5 C, 50% relative humidity, 16-hour photoperiod, and quantum flux (400-700 nm) of 510 mueinsteins/m(2).sec (30,090 lux) at plant level. On the 21st day after seeding, plants were subjected for 12.5 hours to one of three CO(2) concentrations (50, 300, or 2000 mul/l) in an attempt to alter leaf carbohydrate levels. Following the CO(2) treatment, gas exchange measurements were made at a CO(2) concentration of 300 mul/l on the lowermost trifoliolate leaf. Immediately after measurement, the leaf was removed and stored at -20 C until carbohydrate analyses were performed.Increasing the CO(2) concentration for 12.5 hours significantly increased leaf starch concentration but not soluble sugar concentration. There was a strong negative correlation between net photosynthetic rate and starch concentration. Net photosynthetic rate declined from approximately 38 to 22 mg CO(2)/dm(2) leaf area.hr as starch concentration increased from 0.5 to 3 mg/cm(2) leaf area. Carbohydrate concentrations had no effect on compensation concentration.The decrease in net photosynthetic rate as starch concentration increased resulted from an increase in mesophyll (liquid phase) CO(2) diffusion resistance. This suggests that starch accumulation may reduce net photosynthetic rate by impeding intracellular CO(2) transport.