Physiological Mechanisms on Severe Water Deficits Called "Acute Wilt Diseases" in Crop Plants

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It was observed that leaves of sunflower plants grown in a phytotron (natural light) wilted severely and died leaving the leaf color still green at the soil water potential where leaves of the plants grown outdoors did not wilt severely. Such phenomenon observed in sunflower grown in the phytotron is thought to be one of so-called "acute severe wilting" phenomena in many crop plants.It was clarified that such "acute severe wilting" of sunflower occurred in the plants grown under the conditions with high air humidity by comparing the degrees of wilting and death of leaves among the plants grown under various conditions.Leaf water potential decreased considerably in the plants grown in the phytotron compared with the plants grown outdoors under deficient soil moisture. There were no differences in leaf osmotic potential and the bulk modulus of elasticity between the plants grown in the phytotron and the plants grown outdoors. Leaf diffusive conductance of the plants grown outdoors decrea … More sed markedly as leaf water potential decreased from about ー 6 bars and became very small at leaf water potential of about - 15 bars. On the other hand, leaf diffusive conductance of the plants grown in the phytotron did not decrease markedly until leaf water potential decreased to about - 15 bars. Leaf conductance of the plants grown in the phytotron was still higher than that of the plants grown outdoors at leaf water potential lower than about ー 16 bars. When ABA solusion was sprayed on the leaf of the plants grown in the phytotron, diffusive conductance of the leaf of which water potential was higher than about - 14 bars decreased markedly.From the results mentioned above, it was clarified that the leaves of sunflower grown under the conditions with high air humidity still continue to lose water through transpiration at low leaf water potential since their cuticle resistance is relatively low and stomata do not close much at the low leaf water potential, which might be the reason why the leaves of sunflower grown under the conditions with high air humidity die rapidly due to severe water deficits under deficient soil moisture. Less