Highly Localized Responses to Salinity and Drought in Rice Roots

Drought and salt stress are first sensed by the root system of plants. Many physiological responses, including variation in stomatal conductance, are regulated by the stress hormone ABA which is generated in the root and sent up to the shoot which then synthesizes additional ABA to sustain the response. To address whether responses are systemic we have used a split root system, wherein roots are divided into two and each half treated independently to water, salt or drought. Four varieties were examined - the salt tolerant Pokkali, the drought tolerant ARB6 and two sensitive varieties Jaya and IR-20. ABA concentrations in the xylem sap increased dramatically after Day 1 in all four cultivars in response to stress on at least one side the of the split root system. The sensitive varieties appeared to derive much of their nutrition and fluid from the watered side when subjected to asymmetric conditions, whereas roots on the stressed side of tolerant varieties underwent anatomical and physiological modifications facilitating fluid uptake and maintenance of xylem sap flow under these conditions.