Direct Determination of Potassium Ion Accumulation in Guard Cells in Relation to Stomatal Opening in Light

Understanding the mechanism of stomatal opening in leaves is important because stomata are the avenues for CO, and H.,O diffusion. Stomata in most species open in the light when the guard cells that encompass the pores take up water and increase in turgor relative to adjacent epidermal cells (1). Thus stomatal opening is likely an osmotic phenomenon that depends upon the accumulation of solute in the guard cells, and during opening tllis accumulation has been reported to range from 0.: M to 0.5 M in various experiments (2). For over 100 years botanists have believed that the primlary solute accumulating in the light was soluble carbohydrate produced during photosynthesis, because guard cells possess chloroplasts and other epidermal cells do not. However careful quantitative work [reviewed in (3)] failed to provide strong support for this view.