Guard Cell Regulation of Stomatal Response: Chloroplast Function and Abscisic Acid Metabolism in Guard Cells of Commelina Communis and Vicia Faba

136 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1983.Techniques were evaluated for obtaining viable, purified guard cells by sonication of epidermal peels of Vicia faba L. and by treatment at pH 4 of epidermal peels of Commelina communis L. The guard cells were free of mesophyll cells and chloroplasts and of living epidermal cells as shown by fluorescence microscopy and uptake of neutral red. The guard cells of Vicia were 98% free of non-guard cell chlorophyll. These cells were used to measure the light-induced electrochromic absorbance changes indicative of the electrical component of the high energy state in guard cell chloroplasts. The guard cells exhibited DCMU-sensitive photosystem 2 activity, DBMIB-sensitive electron transport and DCCD-sensitive relaxation of the electric field associated with photophosphorylation. These results indicate that guard cell chloroplasts, although reported to lack Calvin-Benson cycle enzymes, are photosynthetically competent with respect to electron transport and photophosphorylation.Metabolism of abscisic acid was investigated in guard cells of Vicia and Commelina and in mesophyll cells of both species. Metabolites were extracted and separated by thin layer chromatography after incubation of guard cells in buffer containing G-('3)H-abscisic acid. Controls showed no metabolism of abscisic acid by bacteria or extracellular enzymes. Guard cells of Commelina metabolized abscisic acid to phaseic acid, dihydrophaseic acid and alkali-labile conjugates. Guard cells of Vicia formed only the conjugates. Mesophyll cells of Commelina accumulated dihydrophaseic acid while mesophyll cells of Vicia accumulated phaseic acid. The demonstration of abscisic acid metabolism by guard cells establishes a mechanism for conversion of abscisic acid, known to affect stomatal aperture, to inactive metabolites within the cells controlling stomatal opening. This process could play a role in the regulation of stomatal conductance. Synthesis and metabolism of abscisic acid in the stomatal complex may resolve the apparent discrepancy between accumulation of abscisic acid in leaves and stomatal closure.The demonstrated photosynthetic competence and capacity to catabolize abscisic acid support the concept that metabolic responses of guard cells directly to the environment, in parallel with mesophyll responses, may be responsible for the tight coupling observed between carbon assimilation and stomatal conductance.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD