REDUCED ROOT ELONGATION OF LUPINUS-ANGUSTIFOLIUS L BY HIGH PH IS NOT DUE TO DECREASED MEMBRANE INTEGRITY OF CORTICAL-CELLS OR LOW PROTON PRODUCTION BY THE ROOTS

Root elongation and cell expansion were decreased markedly by pH great er than or equal to 6.0 compared to pH 5-5.5 in Lupinus angustifolius but only slightly in Lupinus pilosus and Pisum sativum. We tested whet her poor root growth of L. angustifolius at high pH correlates with de creased proton extrusion or increased membrane permeability by compari ng effects of pH on intact and excised roots of L. angustifolius, L. p ilosus and P. sativum in solution culture. Root elongation rates of L. angustifolius exposed to pH 6.5-8.0 were much decreased, yet a pH of 7.5 neither decreased the membrane potential nor increased the permeab ility of Na+ relative to K+ in cortical cells of either L. angustifoli us or P. sativum. There was no correlation between low net proton effl ux and decreased rate of root elongation; in all three species, net pr oton efflux by both intact and excised roots in solution was lower at pH 5.0-5.3 than at pH 6.5-6.7. Exposing shoots to light increased acid ification of the external solution by the roots, but did not restore a rapid root elongation of L. angustifolius at high pH. Increasing buff er concentration in the external solution decreased the rate of root e longation more in L. angustifolius than in L. pilosus and P. sativum. It is suggested that the arrested root elongation in L. angustifolius by high pH does not result from an inability to extrude protons to the external solution or an impaired membrane permeability in the cortex, but may be related to a failure to acidify the apoplast. (C) 1996 Ann als of Botany Company