Concentrations of soluble sugars in guard cells in detached, sonicated epidermis from Vicia faba leaves were analyzed quantitatively by high performance liquid chromatography to determine the extent to which sugars could contribute to changes in the osmotic potentials of guard cells during stomatal opening. Stomata were illuminated over a period of 4 hours with saturating levels of red or blue light, or a combination of red and blue light. When stomata were irradiated for 3 hours with red light (50 micromoles per square meter per second) in a solution of 5 millimolar KCI and 0.1 millimolar CaC12, stomatal apertures increased a net maximum of 6.7 micrometers and the concentration of total soluble sugar was 289 femtomoles per guard cell (70% sucrose, 30% fructose). In an identical solution, 2. Light-induced stomatal opening is caused by the accumulation of both inorganic and organic solutes in guard cells. There is wide agreement that potassium ions (K+) and malic acid are the major solutes that produce the turgor pressure that drives stomatal opening (8,13,20,23,38). Nevertheless, several lines of evidence suggest that soluble sugars may also be significant as osmotica in guard cells of open stomata. noted that when stomata in detached leaf epidermis of Commelina communis were induced to open with light, rates ofK+ and malate accumulation could not account for all of the osmotica necessary to support the rate of stomatal opening in the early stages when apertures were '10 ,um. Outlaw and Manchester (18) removed epidermis from leaves of Vicia faba in which stomata were either open or closed and showed that stomatal opening to 10 ,m was accompanied by an increase of 420 to 720 fmol of hexose sugars and 140 fmol of sucrose per guard cell. Furthermore, they demonstrated the sugar content of these cells to be inversely proportional to the amount of starch present, although they did not identify whether sugars were produced by degradation of guard cell starch or were from other endogenous or exogenous sources. Based on these results, MacRobbie (12) has suggested that soluble sugars might account for the additional osmoticum necessary to support the rate of stomatal opening observed in Commelina.Experiments with whole leaves (3), detached leaf epidermis (17,27,34), and guard cell protoplasts (1,28,30) suggest that there are two photoreceptor systems in guard cells, one for blue light and the other for red light. Both systems can activate proton extrusion from the guard cell plasma membrane (1,28,30). In solutions containing KCI, blue light-induced stomatal opening in detached epidermis of V. faba saturates at 25 umol m-2 s-', is inhibited completely by KCN, and is inhibited partially by DCMU at intensities 225 umol m-2 s-' (27). In a similar solution, red light-induced opening saturates at 50 ,umol m-2 s-', is insensitive to KCN, and is completely inhibited by DCMU (27). In previous time course experiments (34), qualitative histological analysis of guard cells of www.plantphysiol.org on May 10, 2018 -Published ...