Calcium acetate induces calcium uptake and formation of calcium-oxalate crystals in isolated leaflets of Gleditsia triacanthos L.

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Abstract
During treatment of isolated, peeled leaflets of Gleditsia triacanthos with 0.5–2 mM [45Ca]acetate, saturation of the cell-wall free space with Ca2+ occurred within 10 min and was followed by a period of 6–10 h during which there was no significant Ca-uptake into the protoplast, but apoplastic Ca2+ was periodically released into the medium. Later, Ca2+ was absorbed for 3–4 d at rates of up to 2.2 μmol Ca2+·h-1·(g FW)-1 to final concentrations of 350 μmol Ca2+· (g FW)-1. The distribution of absorbed Ca2+ between cell wall, vacuole and Ca-oxalate crystals was determined during Ca-uptake. Wheras intact, cut leaflets deposited absorbed Ca2+ as Ca-oxalate in the crystal cells, peeled leaflets lacking crystal cells accumulated at least 40–50 μmol·(g FW)-1 soluble Ca2+ before the absorbed Ca2+ was precipitated as Ca-oxalate. These observations indicate that the mechanisms for the continuous uptake of Ca2+, the synthesis of oxalate and the precipitation of Ca2+ as Ca-oxalate are operational in the crystal cells of intact leaflets, but not in the mesophyll cells of peeled leaflets where they must be induced by exposure to Ca2+. The precipitation of absorbed Ca2+ as Ca-oxalate by the crystal cells of isolated Gleditsia leaflets illustrates the role of these cells in the excretion of surplus Ca2+ which enters normal, attached leaves with the transpiration stream. In addition to acetate, only Ca-lactate and Ca-carbonate lead to Ca-uptake, but at rates well below those observed with Ca-acetate. Other small organic anions (citrate, glycolate, glyoxalate, malate) and inorganic anions (chloride, nitrate, sulfate) did not permit Ca-uptake. Acetate-14C was rapidly absorbed during Ca-uptake, but less than 20% was incorporated into Ca-oxalate; the rest remained mostly in the soluble fraction or was metabolized to CO2. Acetate, as a permeable weak acid, may enable rapid Ca-uptake by stimulating proton extrusion at the plasmalemma and by serving as a counterion during Ca-accumulation in the vacuole, but is unlikely to function as the principal substrate for oxalate synthesis.