Midday depression of photosynthesis in a tropical tree; vapour pressure difference and leaf temperature effects separated

Abstract

The reduction in photosynthetic rate during hot midday hours can be substantial in sun exposed leaves in a tropical rainforest. Stomatal closure caused by the accompanying increase in vapor pressure difference between intercellular spaces and external atmosphere (D W) is one possible reason. A direct effect of high temperature on metabolism is another. In this study we separated both factors and analyzed the high temperature effect in more detail. Leaves of young plants of Eperua grandiflora, a common canopy tree on sandy soils in the Guianas (SA) were measured in a growth chamber. A decreasing D W either at constant or increasing temperature caused a decrease in intercellular CO2 concentration (pi), indicating a negative stomatal effect on photosynthetic rate. A constant relative humidity with temperature increasing from 28 through 33 to 38° C resulted in a constant pi, but photosynthesis decreased by 30%. This direct effect of high temperature was of similar magnitude as the indirect effect caused by increasing D W. Mitochondrial respiration in the light and the difference in CO2 concentration between intercellular spaces and the site of carboxylation in the chloroplasts were independent of temperature in the range of 28 to 38° C. Hence, these two factors did not contribute to the high temperature effect. Electron transport and carboxylation capacities increased substantially in the same temperature range from 28 to 33° C. This compensated for the increase in oxygenation of ribulose-bisphosphate and photorespiration caused by the temperature increase, resulting in no net effect on photosynthesis. No substantial increase in the capacities was found from 33 to 38° C. The further increase in oxygenation and photorespiration was thus not compensated in this temperature interval, which resulted in the observed decrease in net photosynthesis.