Photosynthesis of Desiccating Leaves of Poplar

Abstract

AbstractThe relation between photosynthesis and water content was investigated using detached leaves of Populus euramericana (Dode) Guinier cv. Robusta.The time course of photosynthesis was measured at different light intensities, at different CO2 contents of the air and at constant temperature during the desiccation of the leaves. The time course of decreasing water content was obtained from continuous measurement of water transpired from the leaves.A large reduction of light saturated (400 W × m−2) photosynthetic rates was observed with decreasing water contents between 78 and 64% (water potential between −14 and −24 atm (bar)). This reduction was much greater in air with 0.3 % CO2 than in air with 5 % CO2, indicating a significant influence of CO2 diffusion resistance on rate of photosynthesis. The reduction of the rate of light and CO2 saturated photosynthesis (at 400 W × m–2 and 5% CO2 in the air) is a measure of the inactivation of the photosynthetic enzyme system by desiccation. A proportional reduction of the light saturated and light limited rate of photosynthesis (for different H2O contents) was found, when measured in air containing a saturating amount of CO2 (5 %). The reduction of the light limited rate of photosynthesis (at 20 W × m−2) was the same at both CO2 levels.