A whole-plant chamber system equipped with a transpiration sap flow meter was developed for measuring the transpiration rate even if leaves are wetted. A preliminary experiment in which dynamics of transpiration rate and/ or evaporation rate of wetted and non-wetted plants were measured and compared with each other demonstrated the validity of the measurement system. The system was then used to analyse leaf wetting effects on gas exchange of corn under slight water stress conditions of soil (a volumetric soil water content of 9.7%). Leaf wetting decreased vapour pressure in leaves by decreasing leaf temperature but it increased vapour pressure in the air; therefore, vapour pressure difference between leaves and air, as a driving force of transpiration, was significantly lower in wetted plant. As a result, transpiration rate decreased by 44% and leaf conductance as an index of stomatal aperture was increased by leaf wetting. Such increasing leaf conductance due to leaf wetting increased the photosynthetic rate by 30% and therefore it improved water use efficiency (2.4 times). These results suggest that morning leaf wetting due to night time dew formation may have an advantage in crop production in semi-arid regions.
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