Because high temperatures under field conditions are often associated with high water vapor pressure deficits, we hypothesized that decreased stomatal conductance at elevated CO2 may protect photosynthesis in C4 species from damage caused by high temperature stress. This hypothesis was tested using Zea mays L. grown in controlled environment chambers at both 380 and 560 μmol mol-1 [CO2]. Intact sections of leaves were exposed in the light to gradually increasing leaf temperatures from 35 to 45 oC, held at 45 oC for 15 minutes and then leaf temperature was gradually decreased again to 35 oC. During this cycle of temperature, leaves were kept at either the growth [CO2] or the opposite [CO2].
Results/Conclusions
Exposure of leaves to 45 oC for 15 minutes reduced photosynthesis by 30 to 60% within 15 minutes, and rates remained lower by 25 to 40% after leaf temperature was returned to 35 oC. Contrary to the hypothesis, the reduction in photosynthesis by this heat treatment was significantly less at the lower than at the higher [CO2], in both relative and absolute terms. It was primarily the [CO2] during the high temperature treatment which affected the amount of inhibition of photosynthesis. Stomatal conductance in all cases increased during the treatment time at 45 oC, while photosynthesis was decreasing. After returning the leaf temperature to 35 oC, stomatal conductance was the same or higher than the initial value at 35 oC, while sub-stomatal carbon dioxide concentrations were higher than the initial values. Also unexpectedly, stomatal conductances during the exposure to 45 oC leaf temperature did not differ with [CO2], although the usual differences in stomatal conductance with [CO2] occurred at 35 and 40 oC. Reducing the light level during high temperature treatments did not decrease the inhibition of photosynthesis, which makes it unlikely that photo-damage caused the lower photosynthesis after the high temperature treatment. Leaf water potentials during the high temperature treatment were not low enough to have directly caused the inhibition of photosynthesis. Photosynthesis rates returned to initial values within 24 h of the high temperature treatment. Reasons for the differences in response between [CO2] are not known, but elevated [CO2] did not protect photosynthesis from inhibition by high leaf temperatures in this case.