Abstract
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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Selected References
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