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Taxonomic Survey for the Presence of Ribulose-1,5-Bisphosphate Carboxylase Activity in Guard Cells ¹

Department of Biological Science (Unit I), Florida State University, Tallahassee, Florida 32306

Guard cell pairs were dissected from freeze-dried leaves of plants representing 15 families, including monocots, dicots, and pteridophytes. All three major photosynthetic carbon pathways (C₂, C₄, and Crassulacean acid metabolism) were represented. These individual guard cell pairs were assayed quantitatively for ribulose-1,5-bisphosphate carboxylase specific activity. Assay sensitivity averaged 0.08 picomoles of ribulose-P₂ dependent P-glycerate formation (i.e. 100-fold more sensitive than required to detect the activity present in a single Vicia faba mesophyll cell). The calculated specific activities for guard cells and mesophyll cells averaged 4 and 472 millimoles per kilogram dry weight per hour, respectively. For all species surveyed, (a) the enzyme activity calculated for guard cells was below the detection limit of the assay, or (b) the specific activity (weight or cell basis) calculated for guard cells was less than 1% of the specific activity calculated for adjacent mesophyll cells. Based on this survey, the generalization is made that the photosynthetic carbon reduction pathway is absent, or virtually so, in guard cell chloroplasts.

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