Effect of Abscisic Acid on Glucose Metabolism in Guard Cells of Vicia faba L.

The effects of abscisic acid (ABA) on the formation of osmoticallyactive solutes and on cell wall synthesis in guard cells wereexamined using sonicated abaxial epidermal strips of Vicia fabaL. incubated with ¹⁴C-glucose at pH 4 and 6. Radioactivity wasincorporated mainly into malate,sucrose, starch and cell-wallfractions. ¹⁴C- Glucose uptake by the guard cells was reducedwhen 1 µm ABA was added. Malate formation, which was moreactive at pH 6 than at 4, was inhibited by ABA at pH 6, butnot at pH 4. Conversion of ¹⁴C-glucose into ¹⁴C-sucrose wasstimulated by ABA at both pH values. Release of radio activesolutes (composed mainly of glucose and malate)from the guardcells into the medium was more active at pH 6 than at pH 4.ABA stimulated there lease at both pH values. Turnover of starchwas more remarkable when the pH value was 6. ABA inhibited thesynthesis of starch, but did not affect its degradation. Cell-wallsynthesis inthe guard cells was also inhibited by ABA, the inhibitionrate being greater at pH 4 than at pH 6.These results suggestthat ABA may have two different actions on stomatal movement:to changethe metabolic activities in the guard cells so as tolower the concentration of osmotically active solutes, and tochange the mechanical properties of cell walls by modulatingcell-wall metabolism. (Received September 7, 1987; Accepted November 30, 1987)