Temperature-Dependent Water and Ion Transport Properties of Barley and Sorghum Roots : II. Effects of Abscisic Acid.

Water flux through excised roots (J(v)) is determined by root hydraulic conductance (L(p)) and the ion flux to the xylem (J(i)) that generates an osmotic gradient to drive water movement. These properties of roots are strongly temperature dependent. Abscisic acid (ABA) can influence J(v) by altering L(p), J(i), or both. The effects of root temperature on responses to ABA were determined in two species differing in their temperature tolerances. In excised barley (Hordeum vulgare L.) roots, J(v) was maximum at 25 degrees C; 10 micromolar ABA enhanced J(v), primarily by increasing L(p), at all temperatures tested (15-40 degrees C). In sorghum (Sorghum bicolor L.) roots, J(v) peaked at 35 degrees C; ABA reduced this optimum temperature for J(v) to 25 degrees C by increasing L(p) at low temperatures and severely inhibiting J(i) (dominated by fluxes of K(+) and NO(3) (-)) at warm temperatures. The inhibition of K(+) flux by ABA at high temperature was mostly independent of external K(+) availability, implying an effect of ABA on ion release into the xylem. In sorghum, ABA enhanced water flux through roots at nonchilling low temperatures but at the expense of tolerance of warm temperatures. These effects imply that ABA may shift the thermal tolerance range of roots of this heat-tolerant species toward cooler temperatures.