GROWTH AND PHOTOSYNTHETIC RESPONSE OF 3 SOYBEAN CULTIVARS TO SIMULTANEOUS INCREASES IN GROWTH TEMPERATURE AND CO2

Three soybean (Glycine max L. Merr.) cultivars (Maple Glen, Clark and CNS) were exposed to three CO2 concentrations (370, 555 and 740 mu mol mol(-1)) and three growth temperatures (20/15 degrees, 25/20 degrees and 31/26 degrees C, day/night) to determine intraspecific differences in single leaf/whole plant photosynthesis, growth and partitioning, p henology and final biomass. Based on known carboxylation kinetics, a s ynergistic effect between temperature and CO2 on growth and photosynth esis was predicted since elevated CO2 increases photosynthesis by redu cing photorespiration and photorespiration increases with temperature. Increasing CO2 concentrations resulted in a stimulation of single lea f photosynthesis for 40-60 days after emergence (DAE) at 20/15 degrees C in all cultivars and for Maple Glen and CNS at all temperatures. Fo r Clark, however, the onset of flowering at warmer temperatures coinci ded with the loss of stimulation in single leaf photosynthesis at elev ated CO2 concentrations. Despite the season-long stimulation of single leaf photosynthesis, elevated CO2 concentrations did not increase who le plant photosynthesis except at the highest growth temperature in Ma ple Glen and CNS, and there was no synergistic effect on final biomass . Instead, the stimulatory effect of CO2 on growth was delayed by high er temperatures. Data from this experiment suggest that: (1) intraspec ific variation could be used to select for optimum soybean cultivars w ith future climate change; and (2) the relationship between temperatur e and CO2 concentration may be expressed differently at the leaf and w hole plant levels and may not solely reflect known changes in carboxyl ation kinetics.