Studies on Plant Response to Rainfall

(VI) Wilting Phenomenon after Rainfall Exposure

Abstract

Wilting of kidney bean leaves was examined after rainfall and after artificial rainfall (mist) in a growth chamber (20°C, 6 klux).
On rainy days, kidney bean leaves exposed to rainfall for 20 hours were wilted during the subsequent several hours.
Exposure to mist in the growth chamber caused wilting in kidney bean and sweet potato leaves. The degree of wilting was weak after 1-2 days of exposure to mist, but that after exposure to mist for 3 days or more was strong. The degree of wilting in all the plants reached their maxima one hour after the end of mist exposure irrespective of duration of exposure, and decreased gradually after that. The plants misted for 1-2 days recovered from leaf wilting after 3-4 hours, but those exposed to mist for 3 days or more still showed some weak wilting or waving of leaves even after 24 hours. The degree of wilting was greater in young leaves than in older ones.
Wetting of only the shoot by the exposure to mist had a similar effect on wilting as that when the vermiculite in the pot as well as the shoot was allowed to be wetted by the mist. The use of deionized water also had a similar effect. Therefore, the effect of mist is probably caused by wetting of the shoot (mainly, leaf), and not by the high moisture in the rhizosphere or by any substances contained in the tap water.
Transpiration of leaves after mist treatment increased markedly. Notingly, the greatest transpiration occurred during the first hour (without mist) after the end of mist treatment, and the transpiration rate in light was about 3 times higher than that of the non-misted leaves. The rate in darkness also had a similar tendency, but was somewhat lower.
Leaves of kidney bean and sweet potato detached from the misted plants had a higher drying rate than that of the non-misted leaves. After 24 hours at 20°C and 65-70% in relative humidity, the weight of the leaves detached from misted plants decreased to 10% of the initial weight, but 50% in the non-misted leaves. The following hypothesis may explain the wilting phenomenon occurring after rainfall.
The structure on the leaf surface is injured by wetting of rain water, and the permeability of water in the leaf is increased, so that transpiration is increased greatly. This increase of transpiration destroys the water balance between uptake by root and loss from leaf, and a water deficit occurs in leaf, resulting in the wilting of the leaves.