Genotypic difference in the influence of aluminum and low pH on ion flux, rhizospheric pH and ATPase activity between Tibetan wild and cultivated barley

Low-pH and Al3+ toxicity are the major factors causing the inhibition of root elongation and plant growth in acid soils. However, it is still unclear about the adaptive mechanisms for plant roots to cope with simultaneous low-pH and Al3+ toxicity in the acid soil. In this study, changes in root tip H+, K+, and Ca2+ fluxes and rhizospheric pH of three barley genotypes, including two Tibetan wild barley (XZ16, Al-tolerant; XZ61, Al-sensitive), and Al-tolerant cv Dayton, were investigated using microelectrode ion flux estimation (MIFE) technology. The results showed that Al-induced ion flux changes were mainly occurred at the elongation zone. Under low-pH treatment, at 24 h, the rhizosphere pH of low-pH tolerant XZ16 and Dayton were 0.7 and 0.3 units higher than low-pH sensitive XZ61, respectively. The H+ influx of tolerant XZ16 and Dayton was higher than that of sensitive XZ61 during the whole process of both low-pH and Al treatments. Furthermore, the Al stress significantly increased the Ca2+ efflux, inhibited the K+ efflux and ATPase activities, especially in the Al-sensitive XZ61. These results indicated that the higher low-pH tolerance in XZ16 was connected with the higher ability of H+ uptake and rhizospheric alkalization. The different mechanisms between low-pH and Al tolerance were probably tightly modulated by Al-induced increase of Ca2+ efflux and inhibition of ATPase activities.