Soybean photosynthesis, Rubisco and carbohydrate enzymes function at supraoptimal temperatures in elevated CO2

Soybean (Glycine max L. Merr. cv. Bragg) was grown season-long in eight sun lit. controlled-environment chambers at two daytime [CO2] of 350 (ambient) and 700 (elevated) mu mol mol(-1). Dry bulb day/night maximum/minimum air t emperatures, which followed a continuously and diurnally varying. near sine -wave control set point that operated between maximum (daytime, at 1500 EST ) and minimum (nighttime, at 0700 EST) values, were controlled at 28/18 and 40/30 degreesC for the ambient-CO2 plants, and at 28/18, 32/22, 36/26, 40/ 30, 44/34 and 48/38 degreesC for the elevated-CO2 plants. The objective was to assess the upper threshold tolerance of photosynthesis and carbohydrate metabolism with increasing temperatures at elevated [CO2], as it is predic ted that air temperatures could rise as much as 4-6 degreesC within the 21s t century with a doubling of atmospheric [CO2]. Leaf photosynthesis measure d at growth [CO2] and temperature was greater for elevated-CO2 plants and w as highest at 32/22 degreesC, but markedly declined at temperatures above 4 0/30 degreesC. Growth temperatures from 28/18 to 40/30 degreesC had little effect on midday total activity and protein content of Rubisco, while highe r temperatures substantially reduced them. Conversely, midday Rubisco rbcS transcript abundance declined with increasing temperatures from 28/18 to 48 /38 degreesC. Elevated-CO2 plants exceeded the ambient-CO2 plants in most a spects of carbohydrate metabolism. Under elevated [Co-2], midday activities of ADPG pyrophosphorylase and sucrose-P synthase and invertase paralleled net increases in starch and sucrose contents, respectively. They were highe st at 36/26-40/30 degreesC, but declined at higher or lower growth temperat ures. Thus, in the absence of other climatic stresses. soybean photosynthes is and carbohydrate metabolism would perform well under rising atmospheric [CO2] and temperature predicted for the 21st century.