Transport of Metabolites between the Mesophyll and Epidermis of Commelina cyanea R. Br.

The concentrations of 10 major metabolites in the epidermis were one-third to one-half of those in the mesophyll when the stomata were open. I4CO2 was supplied to the leaf and the radioactivity contained in glucose, sucrose, sugar phosphates, malate, glycine, serine and alanine, and the ethanolinsoluble HC1-hydrolysable fraction was followed for 2 h in both the mesophyll and epidermis. The net fluxes from mesophyll to epidermis were in the range 0.7-4 nmol m-2 s-I when the leaf was actively photosynthesizing and fell to near zero or became negative with no net photosynthesis. Fluxes were largely independent of concentration differences, which did not vary greatly from initial values. Although the amounts reaching the epidermis greatly exceeded those fixed from C02 in situ, available evidence suggests that the epidermis may have the capacity to supply its own carbon needs; the carbon imported no doubt supplies extra substrates and energy and may influence the flux of K+ ions through the guard-cell membranes. Efflux curves indicated that one-half to three-quarters of the soluble substances in the epidermis diffused readily from it but only about 10% of those in the mesophyll diffused rapidly.