POTASSIUM HOMEOSTASIS IN VACUOLATE PLANT-CELLS

Plant cells contain two major pools of K+, one in the vacuole and one in the cytosol. The behavior of K+ concentrations in these pools is fu ndamental to understanding the way this nutrient affects plant growth. Triple-barreled microelectrodes have been used to obtain the first fu lly quantitative measurements of the changes in K+ activity (a(K)) in the vacuole and cytosol of barley (Hordeum vulgare L.) root cells grow n in different K+ concentrations. The electrodes incorporate a pH-sele ctive barrel allowing each measurement to be assigned to either the cy tosol or vacuole. The measurements revealed that vacuolar a(K) decline d linearly with decreases in tissue K+ concentration, whereas cytosoli c aa initially remained constant in both epidermal and cortical cells but then declined at different rates in each cell type. An unexpected finding was that cytoplasmic pH declined in parallel with cytosolic a( K), but acidification of the cytosol with butyrate did not reveal any short-term link between these two parameters. These measurements show the very different responses of the vacuolar and cytosolic K+ pools to changes in K+ availability and also show that cytosolic K+ homeostasi s differs quantitatively in different cell types. The data have been u sed in thermodynamic calculations to predict the need for, and likely mechanisms of, active K+ transport into the vacuole and cytosol. The d irection of active K+ transport at the vacuolar membrane changes with tissue K+ status.