Reduced Root Elongation of Lupinus angustifoliusL. by High pH is not Due to Decreased Membrane Integrity of Cortical Cells or Low Proton Production by the Roots

Abstract

Root elongation and cell expansion were decreased markedly by pH≥6.0 compared to pH 5–5.5 in Lupinus angustifoliusbut only slightly in Lupinus pilosusand Pisum sativum. We tested whether poor root growth of L. angustifoliusat 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. pilosusand P. sativumin solution culture. Root elongation rates of L. angustifoliusexposed 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. angustifoliusor 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. angustifoliusat high pH. Increasing buffer concentration in the external solution decreased the rate of root elongation more in L. angustifoliusthan in L. pilosusand P. sativum. It is suggested that the arrested root elongation in L. angustifoliusby 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.