THE ROLE OF POTASSIUM CHANNELS IN THE TEMPERATURE CONTROL OF STOMATALAPERTURE

We used the patch-clamp technique to examine the effect of temperature (13-36 degrees C) on the depolarization-activated K channels (K-D cha nnels) and on the hyperpolarization-activated channels (K-H channels) in the plasma membrane of Vicia faba guard-cell protoplasts. The stead y-state whole-cell conductance of both K channel types increased with temperature up to 20 degrees C. However, whereas the whole-cell conduc tance of the K-H channels increased further and saturated at 28 degree s C, that of K-D channels decreased at higher temperatures. The unitar y conductance of both channel types increased with temperature like th e rate of diffusion in water (temperature quotient of approximately 1. 5), constituting the major contribution to the conductance increase in the whole cells. The mean number of available K-H channels was not af fected significantly by temperature, but the mean number of available K-D channels increased significantly between 13 and 20 degrees C and d eclined drastically above 20 degrees C. This decrease and the reduced steady-state voltage-dependent probability of opening of the K-D chann els above 28 degrees C (because of a shift of voltage dependence by +2 1 mV) account for the depression of the whole-cell K-D conductance at the higher temperatures. This may be a basic mechanism by which leaves of well-watered plants keep their stomata open during heat stress to promote cooling by transpiration.