Photosynthesis of Desiccating Leaves of Poplar

The 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.