Ion-specific mechanisms of osmoregulation in bean mesophyll cells

Transient kinetics of net H+, K+, Ca2+, and Cl- fluxes were measured non-in vasively, using an ion-selective microelectrode technique, for bean (Vicia faba L.) leaf mesophyll in response to 150 mM mannitol treatment. In a para llel set of experiments, changes in the plasma membrane potential and the t otal proline content in leaves were monitored. Regardless of the ionic comp osition of the bath solution, hyperosmotic stress caused a significant incr ease in the K+ and Cl- uptake into mesophyll cells. At the same time, no si gnificant proline changes were observed for at least 16 h after the onset o f stress, Experiments with inhibitors suggested that potassium inward recti fier (KIR) channels, exhibiting mechanosensitive properties and acting as p rimary receptors of osmotic stress, are likely to be involved. Due to the c oupling by membrane potential, changes in K+ and Cl- transport may modify a ctivity of the plasma membrane H+-pump. Such coupling may also be responsib le for the mannitol-induced oscillations (period of about 4 min) in net ion fluxes observed in 90% of plants, Calculations show that influx of K+ and Cl- observed in response to hyperosmotic treatment may provide an adequate osmotic adjustment in bean mesophyll, which suggests that the activity of t he plasma membrane transporters for these ions should be targeted to improv e osmotolerance, at least in this crop.