Nocturnal Transpiration in Wheat

Abstract

Over the night, stomata of wheat leaves took several hours to reach their most closed position and began to open some hours before dawn. The pattern and amount of night transpiration was changed by current vapour pressure deficit (VPD) but not by VPD or transpiration during the previous day. Mean night transpiration per unit VPD was unchanged by current VPD. Night transpiration of whole plants increased linearly with VPD though genotypes differed significantly in amount. The most profligate genotype transpired at 50 g m^-2 leaf h^-1 at a VPD of 30 mbar which was twice the rate of the most thrifty genotype.

Attempts were made to estimate the proportion of night transpiration occurring through the stomata and the cuticle by three methods: comparisons of stressed and unstressed leaves, wilting patterns of detached leaves, and transpiration rates of detached leaves in ABA solutions. The methods gave equivalent rankings of the genotypes and similar absolute values for the 'cuticular component', which contributed 13-50% of total night transpiration. We conclude that transpiration could exceed 0.5 mm per night in unstressed crops, though this would be considerably reduced by selection of genotypes with both low cuticular and low stomatal transpiration.