THE ROLE OF TEMPERATURE IN DETERMINING THE STIMULATION OF CO2 ASSIMILATION AT ELEVATED CARBON-DIOXIDE CONCENTRATION IN SOYBEAN SEEDLINGS

Soybean (Glycine max cv. Clark) was gown at both ambient (ca 350 mu mo l mol(-1)) and elevated (ca 700 mu mol mol(-1)) CO2 concentration at 5 growth temperatures (constant day/night temperatures of 20, 25, 30, 3 5 and 40 degrees C) for 17-22 days after sowing to determine the inter action between temperature and CO2 concentration on photosynthesis (me asured as A, the rate of CO2 assimilation per unit leaf area) at both the single leaf and whole plant level. Single leaves of soybean demons trated increasingly greater stimulation of A at elevated CO2 as temper ature increased from 25 to 35 degrees C (i.e. optimal growth rates). A t 40 degrees C, primary leaves failed to develop and plants eventually died. in contrast, for both whole plant A and total biomass productio n, increasing temperature resulted in less stimulation by elevated CO2 concentration. For whole plants, increased CO2 stimulated leaf area m ore as growth temperature increased. Differences between the response of A to elevated CO2 for single leaves and whole plants may be related to increased self-shading experienced by whole plants at elevated CO2 as temperature increased. Results from the present study suggest that self-shading could limit the response of CO2 assimilation rate and th e growth response of soybean plants if temperature and CO2 increase co ncurrently, and illustrate that light may be an important consideratio n in predicting the relative stimulation of photosynthesis by elevated CO2 at the whole plant level.