Salinity-induced inhibition of leaf elongation in maize is not mediated bychanges in cell wall acidification capacity

The physiological mechanisms underlying leaf growth inhibition under salt s tress are not fully understood. Apoplastic pH is considered to play an impo rtant role in cell wall loosening and tissue growth and ws demonstrated to be altered by several growth-limiting environmental condition. In this stud y we have evaluated the possibility that inhibition of maize (Zea mays) lea f elongation by salinity is mediated by changes in growing cell wall acidif ication capacity. The kinetics of extended apoplast pH changes by leaf tiss ue of known expansion rates and extent of growth reduction under stress was investigated tin vivo) and was found similar for non-stressed and salt-str essed tissues at all examined apoplast salinity levels (0.1, 5, 10, or 25 m M NaCl). A similar rate of spontaneous acidification for the salt and contr ol treatments was demonstrated also in in situ experiments. Unlike growing cells that acidified the external medium, mature nongrowing cells caused me dium alkalinization. The kinetics of pH changes by mature tissue was also u nchanged by salt stress. Fusicoccin, an enhancer of plasmalemma H+ ATPase a ctivity level, greatly stimulated elongation growth and acidification rate to a similar extent in the control and salt treatments. That the ability of ; the growing tissue to acidify the apoplast did not change under same salt stress conditions that induced inhibition of tissue elongation rate sugges ts that salinity does not inhibit cell growth by impairing the acidificatio n process or reducing the inherent capacity for cell wall acidification.