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Articles

Influence of Leaf Starch Concentration on CO₂ Assimilation in Soybean ¹

^a Department of Agronomy, Purdue University, West Lafayette, Indiana 47907

Net photosynthetic rate, CO₂ 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 CO₂ 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 µeinsteins/m²·sec (30,090 lux) at plant level. On the 21st day after seeding, plants were subjected for 12.5 hours to one of three CO₂ concentrations (50, 300, or 2000 µl/l) in an attempt to alter leaf carbohydrate levels. Following the CO₂ treatment, gas exchange measurements were made at a CO₂ concentration of 300 µl/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 CO₂ 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 CO₂/dm² leaf area·hr as starch concentration increased from 0.5 to 3 mg/cm² 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) CO₂ diffusion resistance. This suggests that starch accumulation may reduce net photosynthetic rate by impeding intracellular CO₂ transport.

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