Phosphate Transport and Apoplastic Phosphate Homeostasis in Barley Leaves

Levels of apoplastic inorganic phosphate (P_i) in leaves andP_i-transport activities of mesophyll cells were measured insitu in control and P_i-deficient plants. When detached leaveswere fed a solution that contained 10 mM P_i, the apoplasticP_i levels, as measured by an infiltration method, remained almostconstant. When the leaves were immersed in pure water, the apoplasticP_i level gradually decreased. With 50 mM P_i in the feeding solution,the level increased dramatically. The apoplastic P_i levels inP_i-deficient leaves were somewhat, but not very much lower thanthose in controls. When the immersion medium was changed topure water 60 mm after feeding with 10 mM P_i, the apoplasticP_i levels started to decrease and then returned to the initiallevel. It is suggested that intracellular P_i may be transportedback to the apoplast to maintain the apoplastic P_i levels ata constant value. Changes in cytoplasmic pH were measured during feeding of P_ito the leaves by use of the pH-sensitive fluorescent dye, pyranineafter Yin et al. (l990a, b). On feeding of P_i the cytoplasmicpH decreased in P_i-deficient plants as a result of co-transportof P_i and protons in situ. After removal of P_i from the immersionmedium, the cytoplasmic pH returned to the original value. ³ Present address: Institute für Biochemische Pflanzenpathologie,GSF-München, D-8042 Neuherberg, Germany.