Effect of calcium concentration in growth medium on oxalate content and evaluation of the role of guttation in the regulation of oxalate in eddo

The effects of calcium concentrations in the growth medium on oxalate content of leaf blades, petioles and corms and the involvement of guttation in the regulation of oxalate homeostasis were investigated in eddo (Colocasia esculenta (L.) Schott var. antiquorum Hubbard and Rehder). The plants were grown hydroponically in solutions containing 0 mM calcium, 1 mM calcium nitrate (control), 15 mM calcium nitrate or 15 mM calcium chloride. Total oxalate content (soluble plus insoluble) of leaf blades, petioles and corms did not differ with the calcium concentration in solutions containing 1 mM or 15 mM calcium nitrate or 0 mM calcium. The soluble oxalate content of these parts decreased as the calcium concentration of the solution was increased. Solutions containing 15 mM calcium nitrate or 15 mM calcium chloride gave a significantly lower proportion of soluble oxalate content to total oxalate content in each part, especially in leaf blades than 0 mM calcium or 1 mM calcium nitrate. In contrast, a positive correlation was found between insoluble oxalate content and calcium concentration in the solution. These results demonstrate that high calcium concentrations in the growth medium reduce soluble oxalate content of the plant. Soluble oxalate was detected in eddo guttation fluid. Soluble oxalate content in this fluid (mg /mL) and the amount of soluble oxalate exuded by guttation (mg /leaf /night) were significantly lower in the solutions containing 15 mM calcium than in those containing 0 mM and 1 mM calcium. These results indicate that guttation may affect the concentration of soluble oxalate in the plant bodies although not strongly contributing to a decrease in soluble oxalate content in eddo grown under high calcium conditions.