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 sunlit, controlled-environment chambers at two daytime [CO2] of 350 (ambient) and 700 (elevated) μmol mol−1. Dry bulb day/night maximum/minimum air temperatures, 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 °C for the ambient-CO2 plants, and at 28/18, 32/22, 36/26, 40/30, 44/34 and 48/38 °C 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 predicted that air temperatures could rise as much as 4-6 °C within the 21st century with a doubling of atmospheric [CO2]. Leaf photosynthesis measured at growth [CO2] and temperature was greater for elevated-CO2 plants and was highest at 32/ 22 °C, but markedly declined at temperatures above 40/30 °C. Growth temperatures from 28/18 to 40/30 °C had little effect on midday total activity and protein content of Rubisco, while higher temperatures substantially reduced them. Conversely, midday Rubisco rbcS transcript abundance declined with increasing temperatures from 28/18 to 48/38 °C. Elevated-CO2 plants exceeded the ambient-CO2 plants in most aspects of carbohydrate metabolism. Under elevated [CO2], midday activities of ADPG pyrophosphorylase and sucrose-P synthase and invertase paralleled net increases in starch and sucrose contents, respectively. They were highest at 36/26-40/30 °C, but declined at higher or lower growth temperatures. Thus, in the absence of other climatic stresses, soybean photosynthesis and carbohydrate metabolism would perform well under rising atmospheric [CO2] and temperature predicted for the 21st century.