Leaf ion homeostasis and plasma membrane H+-ATPase activity in Vicia faba change after extra calcium and potassium supply under salinity

Salt stress in plants impacts apoplastic ion activities and cytosolic ionic homeostasis. The ameliorating effects exerted by calcium or potassium on compartmentation of ions in leaves under salinity are not fully understood. To clarify how calcium or potassium supply could ameliorate ion homeostasis and ATPase activities under salinity, 5 mM CaSO4 or 10 mM K2SO4 were added with, or without, 100 mM NaCl for 7 d and 21 d to Vicia faba grown in hydroponics. The apoplastic pH was detected with Oregon Green dextran dye in intact second-uppermost leaves by microscopy-based ratio imaging. The cytosolic Ca2+, Na+, K+ activities and pH were detected in protoplasts loaded with the acetoxy methyl-esters of Fura-2, SBFI, PBFI and BCECF, respectively, using epi-fluorescence microscopy. Furthermore, total Ca2+, Na+, K+ concentrations and growth parameters were investigated. The ATPase hydrolyzing activity increased with time, but decreased after long salinity treatment. The activity largely increased in calcium-treated plants, but was depressed in potassium-treated plants after 7 d. The calcium supply increased Vmax, and the ATPase activity increased with salinity in a non-competitive way for 7 d and 21 d. The potassium supply instead decreased activity competitively with Na+, after 21 d of salinity, with different effects on Km and Vmax. The confirmed higher ATPase activity was related with apoplast acidification, cytosol alkalinization and low cytosolic [Na+], and thus, might be an explanation why extra calcium improved shoot and leaf growth.