K⁺ status and ABA affect both exudation rate and hydraulic conductivity in sunflower roots

Abstract

Twenty‐day‐old sunflower plants ( Helianthus annuus L. cv. Sun‐Gro 380) grown in nutrient solutions with different KCl levels were used to study the effects of K + status of the root and of abcisic acid (ABA) on the exudation rate (J v ), the hydraulic conductivity of the root (L p ), the fluxes of exuded K + and Na + (J K and J Na ), and the gradient of osmotic pressure between the xylem and the external medium. J v and L p increased in direct proportion to the K + starvation of the root. Also addition of ABA (4 µ M ) at the onset of exudation in the external medium made J v and L p rise, and this effect also increased with the degree of K + starvation. Similarly, K + starvation and ABA promoted both the flux of exuded Na + and the accumulation of Na + in the root. We suggest that ABA acts as a regulating signal for the radial transport of water across the root, and that potassium may be an effector of this mechanism.