Calcium stimulation of glucose-6-phosphate dehydrogenase activity in shoot apices of Spinacia oleracea during floral evocation

The earliest biochemical marker of floral evocation in the shoot apex of S. oleracea is the doubling of the rate of glucose-6-phosphate dehydrogenase (G6PD) activity 12–15 h after transfer of 4-week-old plants from short days to continuous light i.e. 1–2 h after the leaves are raised to the floral state. Quantitative cytochemical analysis of G6PD activity in the vegetative apices showed that addition of 10⁻⁷ M Ca²⁺ to the cytochemical enzyme reaction medium for G6PD activity raises the rate of enzyme activity to that seen in the induced apices. Higher concentrations of Ca²⁺ result in G6PD inhibition in the vegetative apices and any added Ca²⁺ at concentrations of 10⁻⁷ M or higher inhibit the G6PD activity seen in both the induced apices and leaf primordia of both types of apex. The addition of EGTA abolishes the cytochemical reaction. The ability of the Ca²⁺ to activate the G6PD activity in addition to the incubation medium occurs during the periods of 8–11 h of continuous light, but is already lost by 12 h when no change is achieved by Ca²⁺ treatment. This can be interpreted as indicating a point in time close to the moment of floral evocation. A model is proposed in which Ca²⁺ is able to activate the inactivated-G6PD molecules in the vegetative apex through increased Ca²⁺ flux possibly through the action of plasmalemmal Ca²⁺-ATPase activity as part of the floral evocation process. © 1998 John Wiley & Sons, Ltd.