Organic acid and potassium accumulation in guard cells during stomatal opening

Leaflets of Vicia faba L. with either open or closed stomata were quick-frozen and freeze-dried. Individual guard cell pairs and pure samples of palisade parenchyma, spongy parenchyma, and epidermis lacking guard cells were dissected from the leaflets, weighed, and assayed for organic acids or K(+).K(+) was measured by a new enzymatic method. In guard cells of open stomata, as compared to closed stomata, K(+) was 2- to 4-fold higher, malic acid 6-fold higher, and citric acid 3-fold higher. Both aspartic and glutamic acids were also higher, but the amounts present were low compared to malic and citric acids. Isocitric acid was significantly higher in one experiment, but not in another. Glyceric acid was not increased. Succinic acid was too low to detect by the method used; but in guard cells of open stomata the concentration must have been less than 2% of that of malic acid. Malic acid was higher in the palisade parenchyma from the leaflet with open stomata. The ion balance shows that malic and citric acids provide much of the counter ion for the K(+) taken up during stomatal opening.