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 min 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. (1990a, 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.