Influence of Periodic Fluctuations in Root Environment on Adaptation to Salinity in Sorghum bicolor

Abstract

In Sorghum bicolor (L.) Moench, a 3-week exposure to 75 mM NaCl induced the ability to grow at 300 mm NaCl, a lethal concentration for non-pretreated plants. This property was defined as adaptation to salinity. The chemical composition of the root medium was periodically modified during the 3-week maturation of the adaptation response. Two daily 1-h interruptions of the salinisation treatment were sufficient to prevent the initiation of the adaptation process, while two daily 1-h modifications of the external NaCl concentration only disturbed its completion. Moreover, the effect of a short interruption in the salinisation treatment depended on the time of day it occurred. The rate of growth was not directly influenced by the water content or the amount of Na⁺ ions accumulated in the shoot; it appears to be a function of the mode of response of the plant to salinity. It is concluded that the nature of the plant response to salinity is controlled by the roots, which display a circadian fluctuation in sensitivity to the ionic and osmotic components of the soil environment.