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 greater 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 whether poor root growth of L. angustifolius at high pH correlates with decreased proton extrusion or increased membrane permeability by comparing effects of pH on intact and excised roots of L. angustifolius, L. pilosus 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 permeability of Na+ relative to K+ in cortical cells of either L. angustifolius or P. sativum. There was no correlation between low net proton efflux and decreased rate of root elongation; in all three species, net proton 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 acidification of the external solution by the roots, but did not restore a rapid root elongation of L. angustifolius at high pH. Increasing buffer concentration in the external solution decreased the rate of root elongation 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 Annals of Botany Company