Direct Measurements of Turgor Pressure Potentials of Guard Cells, I.

Measurements were made of pressures applied in subsidiary andguard cells which caused closure and opening of stomatal pores.These pressures were lower than would be expected from plasmolyticallydetermined osmotic potentials of guard cell saps. The pressuresneeded in guard cells to open almost closed stomata were practicallythe same as those required to close open stomata when appliedin adjacent subsidiary cells. Completely closed stomata couldnot be opened by this technique. The implications for the understandingof the mechanism of guard cell deformation, the Spannungsphase,and the pressure potentials of guard cells are discussed.