Enzymic potential for fructose 6-phosphate phosphorylation by guard cells and by palisade cells in leaves of the broad bean Vicia faba L.

Guard cells and palisade cells were dissected from freeze-dried leaflets of the broad bean, Vicia faba L. Individual cell samples (6-12 ng) were assayed for ATP-dependent and pyrophosphate-dependent phosphofructokinases. The assay indicator, NADH loss, was monitored in real time in oil droplets with a computer-driven microfluorometer. On a protein basis, both activities were 10-fold higher in guard cells than in palisade cells, indicating (i) elevated carbon metabolism in guard cells to meet demands for energy and carbon skeletons required during stomatal opening, and (ii) parallel glycolytic pathways in guard cells, one responsive to the potent regulatory metabolite fructose 2,6-bisphosphate and the other not. Future work will be devoted to clarifying the roles of the cytosolic and chloroplastic compartments in guard cells.