Single guard cell recordings in intact plants: light-induced hyperpolarization of the plasma membrane

Guard cells are electrically isolated from other plant cells and therefore offer the unique possibility to conduct current- and voltage-clamp recordin gs on single cells in an intact plant. Guard cells in their natural environ ment were impaled with double-barreled electrodes and found to exhibit thre e physiological states. A minority of cells were classified as far-depolari zed cells. These cells exhibited positive membrane potentials and were domi nated by the activity of voltage-dependent anion channels. All other cells displayed both outward and inward rectifying K+-channel activity. These cel ls were either depolarized or hyperpolarized, with average membrane potenti als of -41 mV (SD 16) and -112 mV (SD 19), respectively. Depolarized guard cells extrude K+ through outward rectifying channels, while K+ is taken up via inward rectifying channels in hyperpolarized cells. Upon a light/dark t ransition, guard cells that were hyperpolarized in the light switched to th e depolarized state. The depolarization was accompanied by a 35 pA decrease in pump current and an increase in the conductance of inward rectifying ch annels. Both an increase in pump current and a decrease in the conductance of the inward rectifier were triggered by blue light, while red light was i neffective. From these studies we conclude that light modulates plasma memb rane transport through large membrane potential changes, reversing the K+-e fflux via outward rectifying channels to a K+-influx via inward rectifying channels.