Phosphate transport and apoplastic phosphate homeostasis in barley leaves

Levels of apoplastic inorganic phosphate (P_i) in leaves and P_i-transport activities of mesophyll cells were measured in situ in control and P_i-deficient plants. When detached leaves were fed a solution that contained 10 mM P_i, the apoplastic P_i levels, as measured by an infiltration method, remained almost constant. When the leaves were immersed in pure water, the apoplastic P_i level gradually decreased. With 50 mM P_i in the feeding solution, the level increased dramatically. The apoplastic P_i levels in P_i-deficient leaves were somewhat, but not very much lower than those in controls. When the immersion medium was changed to pure water 60 mm after feeding with 10 mM P_i, the apoplastic P_i levels started to decrease and then returned to the initial level. It is suggested that intracellular P_i may be transported back to the apoplast to maintain the apoplastic P_i levels at a constant value.Changes in cytoplasmic pH were measured during feeding of P_i to the leaves by use of the pH-sensitive fluorescent dye, pyranine after Yin et al. (l990a, b). On feeding of P_i the cytoplasmic pH decreased in P_i-deficient plants as a result of co-transport of P_i and protons in situ. After removal of P_i from the immersion medium, the cytoplasmic pH returned to the original value.