Effects of High Atmospheric CO2 and Sink Size on Rates of Photosynthesis of a Soybean Cultivar

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Abstract
The effect of sink strength on photosynthetic rates under conditions of long-term exposure to high CO2 was investigated in soybean. Soybean plants (Merr. cv. Fiskeby V) were grown in growth chambers containing 350 .mu.l CO2/l air until pod set. At that time, plants were trimmed to 3 trifoliolate leaves and either 21 pods (high sink treatment) or 6 pods (low sink treatment). Trimmed plants were either left in 350 .mu.l CO2/l of air or placed in 1000 .mu.l CO2/l of air (high CO2 treatment) until pod maturity. Whole plant net photosynthetic rates of all plants were measured twice weekly, both at 350 .mu.l CO2/l of air and 1000 .mu.l CO2/l of air. Plants were harvested at this time for dry weight measurements. Photosynthetic rates of high sink plants at both measurement CO2 concentrations were consistently higher than those of low sink plants and those of plants given the 350 .mu.l CO2/l of air treatment were higher at both measurement CO2 concentrations than those of plants given the 1000 .mu.l CO2/l of air treatment. When plants were measured under treatment CO2 levels; rates were higher in the 1000 .mu.l plants than 350 .mu.l CO2 plants. Dry weights of all plant parts were higher in the 1000 .mu.l CO2/l air treatment than in the 350 .mu.l CO2/l air treatment and were higher in the low sink than in the high sink treatments.