ABSTRACT
Relative humidity of the atmosphere is an important stress factor, both excess or deficit produces harmful effects. The guard cells function as humidity sensors which measure the difference in water potential inside and outside the leaf. Low relative humidity increases transpiration because of the larger vapor pressure difference. Humidity has a greater effect on potassium absorption than on the absorption of calcium, magnesium or manganese in rice plants. With increasing air humidity, the rate of photosynthesis increases with a concomitant decrease in the rate of respiration. Atmospheric drought caused by low humidity/high temperature results in stomata closure and increase in leaf abscisic acid (ABA) content. The unstressed model leaf is considered to be adapted to 100 percent humidity. Low relative humidity applied to four rice varieties grown in a normal soil increased stomata resistance, but did not affect the bulk root and stem base water potential, and ABA content. Under water stress conditions, the ABA/cytokinin ratio increases in the apoplast.
