To evaluate the effects of leaf wetting on midday depression of photosynthesis in horticultural crops, we analyzed leaf gas exchange (transpiration rate, stomatal conductance, photosynthetic rate) of tomato plants under two different treatments (the Wet treatment and No-wet treatment as control). The gas exchange was measured at 11:00 (the time at which leaf gas exchange was assumed to be active) and 14:00 (the time at which leaf gas exchange was assumed to be inactive due to midday depression) on the clear days of November 2 to 7, 2016. The gas exchange measurements in the Wet treatment were conducted just after droplets on the leaf surface had evaporated. In the No-wet treatment, transpiration rate, stomatal conductance, and photosynthetic rate at 14:00 were decreased compared to those at 11:00. This suggests that midday depression occurred due to stomatal closure induced by excessive transpirational water loss. In contrast, in the Wet treatment, there was no such depression of leaf gas exchange, suggesting that leaf wetting might contribute to maintaining stomatal aperture through improving leaf water status. Thus, leaf wetting could avoid midday depression of photosynthesis in tomato plants.
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