THE INFLUENCE OF INCREASING GROWTH TEMPERATURE AND CO2 CONCENTRATION ON THE RATIO OF RESPIRATION TO PHOTOSYNTHESIS IN SOYBEAN SEEDLINGS

Using controlled environmental growth chambers, whole plants of soybea n, cv. 'Clark', were examined during early development (7-20 days afte r sowing) at both ambient (approximate to 350 mu L L-1) and elevated ( approximate to 700 mu L L-1) carbon dioxide and a range of air tempera tures (20, 25, 30, and 35 degrees C) to determine if future climatic c hange (temperature or CO2 concentration) could alter the ratio of carb on lost by dark respiration to that gained via photosynthesis. Althoug h whole-plant respiration increased with short-term increases in the m easurement temperature, respiration acclimated to increasing growth te mperature. Respiration, on a dry weight basis, was either unchanged or lower for the elevated CO2 grown plants, relative to ambient CO2 conc entration, over the range of growth temperatures. Levels of both starc h and sucrose increased with elevated CO2 concentration, but no intera ction between CO2 and growth temperature was observed. Relative growth rate increased with elevated CO2 concentration up to a growth tempera ture of 35 degrees C. The ratio of respiration to photosynthesis rate over a 24-h period during early development was not altered over the g rowth temperatures (20-35 degrees C) and was consistently less at the elevated relative to the ambient CO2 concentration. The current experi ment does not support the proposition that global increases in carbon dioxide and temperature will increase the ratio of respiration to phot osynthesis; rather, the data suggest that some plant species may conti nue to act as a sink for carbon even if carbon dioxide and temperature increase simultaneously.