Influence of daily carbon dioxide exposure duration and root environment on soybean response to elevated carbon dioxide

Little is known about effects of daily CO2 enrichment duration and root environment on plant response to elevated CO2. Two experiments were performed,vith Essex soybean (Glycine max L, Merr,) in open-top field chambers to address these questions. In one experiment, effects of 12 and 24 h d(-1) exposures to double-ambient CO2 were compared for plants grown in 14 L pots that were either insulated to moderate soil temperature or not insulated. Although never significant statistically, trends at some growth stages suggested that nighttime CO2 enrichment contributed to growth and yield. Plants grew and yielded more in insulated than noninsulated pots, but there were no significant CO2 enrichment X insulation interactions. In the second experiment, response to approximately 1.3, 1.6, and 1.9 times ambient CO2 was compared for plants grown in the ground or 14 L pots. Enhancement of photosynthesis, growth, and yield by CO2 enrichment was similar in pots and in the ground. Linear responses to different CO2 concentrations were significant for all yield components in both root environments, whereas quadratic responses were significant for plants in pots but not for plants in the ground. Tests of proportionality of response for yield components shelved no evidence of significant differences between plants in pots and in the ground except weight per 100 seeds. Seed yield enhancement at 1.9 times ambient CO2 was 36% for plants in pots and 33% for plants in the ground. Overall, proportional response of soybean to CO2 enrichment was relatively uniform in spite of large differences in baseline growth and yield.