The Effect of Air Humidity on Photosynthesis of Ryegrass and White Clover at Three Temperatures

Abstract

Increasing leaf-air vapour pressure deficit (VPD) decreased the stomatal conductance and the photosynthetic rate of leaves of ryegrass (Lolium perenne L.) and white clover (Trifolhim repens L.) at light saturation and at lower irradiance. In ryegrass both conductance and photosynthesis, and in clover photosynthesis, decreased less with increasing VPD in low irradiance than they did at an irradiance which saturated photosynthesis. In ryegrass, relative to their values at 10 mb, photosynthesis and conductance decreased less with increasing VPD at 25 °C than at 20 or 16·5 °C. In white clover, relative conductance (but not photosynthesis) was less reduced at 25 than at 16·5 °C Measurements of VPD of air in the leaf canopy of a field-grown crop are combined with the observed responses of photosynthesis to VPD and temperature in a model. This shows that high VPD is likely to depress photosynthesis significantly and that, during a typical day, the rate of light saturated photosynthesis may remain fairly steady, because the depression of photosynthesis due to rising VPD is offset by the stimulation due to rising temperature