Accelerated root growth induced by nitrate deficit is related to apoplast acidification

Abstract

We measured the rate of growth, osmotic pressure, hydraulic conductance, longitudinal and transverse extensibility of barley (Hordeum vulgare L.) roots in Knop solution with nitrate and at substitution of NO3− with Cl−. During the first three days after NO3− removal, root growth acceleration was related to the increase in their longitudinal extensibility. It was shown that root exposure to buffer with pH 4.5 and also activation of H+ pump with naphthyl acetate imitated changes in extensibility induced by NO3− deficit. Earlier, we have demonstrated medium acidification near root surface and calculated its expected level (pH 4.5). This permits a supposition that the cause for changes in extensibility and root growth acceleration at NO3− deficit was apoplast acidification, evidently related to the ceasing of NO3− symport with H+ and activation of the plasmalemmal H+ pump. ABA did not affect root extensibility at pH 4.5; however, at pH 6.0, it was similar to the action of diethylstilbestrol, an inhibitor of H+ pump, and opposite to the action of NO3− deficit. Thus, the absence of ABA effects on root growth, in spite of its accumulation at NO3− deficit, could be explained by apoplast acidification as well.