K+ uptake, H+-ATPase pumping activity and Ca2+ efflux mechanism are involved in drought tolerance of barley

Chemical signals play a significant role in improving plant water use efficiency under drought stress. Hydroponic and pot experiments were conducted using three barley genotypes to study genotypic differences in K+, Ca2+ and H+ fluxes and physiological and biochemical traits of drought tolerant Tibetan wild barley XZ5 and cv Tadmor and drought sensitive cv ZJU9 in response to drought. Transient and steady-state ion fluxes were measured by noninvasive ion-selective microelectrode MIFE technique. We showed that exogenous PEG (polyethylene glycol 6000) and mannitol and soil drought stress all resulted in an immediate K+ uptake from root epidermis and leaf mesophyll, with much more uptake in XZ5. Long-term drought stress are more detrimental to root K+ homeostasis, and the degree of K+ uptake differed due to severity of drought stress and was less presented in XZ5. Barley subjected to drought stress caused a large H+ efflux in root epidermis and H+ influx in leaf mesophyll, with significantly less alteration in XZ5 and Tadmor than in ZJU9. Meanwhile a dramatic Ca2+ efflux was observed in root epidermis and leaf mesophyll under drought stress. PEG and mannitol treatments induced marked increases in H+-K+- ATPase in XZ5 and Tadmor. Our results demonstrate that K+ uptake, Ca2+ efflux and leaf H+ influx/ alkalization of apoplastic pH could be a chemical signal in barley in response to drought stress, and that stimulated H+-K+-ATPase and K+ uptake, but less Ca2+ efflux and H+ alteration under drought, when concerning ionic mechanisms underlying drought tolerance, play an important role in drought tolerance in XZ5.