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

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.