Leaf Water Content and Hormone Effects on Ribonuclease Activity

In barley (Hordeum vulgare) leaves in which the water balance was not hampered, kinetin and abscisic acid effected the well documented decrease and increase, respectively, in RNase activity. When the plants were exposed to water shortage, leaf-water saturation deficit increased steadily, with kinetin enhancing and abscisic acid retarding the rise. Under drought, the pattern of hormonal effects was inverted, with kinetin enhancing RNase activity over and above the activity assayed in abscisic acid-treated leaves. A very close relationship between RNase activity and water saturation deficit was found and significantly, it was maintained irrespective of the hormonal treatment, which in itself markedly modified leaf-water saturation deficit. The inverted effects of kinetin and of abscisic acid on RNase activity under conditions of water shortage were interpreted as resulting primarily from the effects of these hormones on leaf-water. It is suggested that under conditions of increased water deficiency in the plant, cell-water supersedes hormonal regulation in effecting RNase activity.