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 soybean, cv. 'Clark', were examined during early development (7-20 days after 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 temperatures (20, 25, 30, and 35 degrees C) to determine if future climatic change (temperature or CO2 concentration) could alter the ratio of carbon lost by dark respiration to that gained via photosynthesis. Although whole-plant respiration increased with short-term increases in the measurement temperature, respiration acclimated to increasing growth temperature. Respiration, on a dry weight basis, was either unchanged or lower for the elevated CO2 grown plants, relative to ambient CO2 concentration, over the range of growth temperatures. Levels of both starch and sucrose increased with elevated CO2 concentration, but no interaction between CO2 and growth temperature was observed. Relative growth rate increased with elevated CO2 concentration up to a growth temperature of 35 degrees C. The ratio of respiration to photosynthesis rate over a 24-h period during early development was not altered over the growth temperatures (20-35 degrees C) and was consistently less at the elevated relative to the ambient CO2 concentration. The current experiment does not support the proposition that global increases in carbon dioxide and temperature will increase the ratio of respiration to photosynthesis; rather, the data suggest that some plant species may continue to act as a sink for carbon even if carbon dioxide and temperature increase simultaneously.