Abstract
Effects of shading and soil moisture on formation of the idioblasts containing calcium oxalate raphides in petioles of three taro cultivars, Egu-imo, Takenoko-imo and Mizu-imo were investigated. The densities of the idioblasts containing calcium oxalate raphides, which are the cause of acridity in taro, decreased in proportion to the amount of shading in all cultivars. The frequencies of idioblasts with raphides under 100 and 60% shading decreased to about 30 and 60-78% in comparison to those in full sun. In relation to soil moisture content, the petioles of taro plants grown in moist soil contained less idioblasts than petioles of grown in a relatively drier soil. Shading and soil moisture resulted in large differences in idioblast densities in 'Egu-imo' and 'Takenoko-imo' but not in 'Mizuimo'. Under similar environmental stresses, petioles of young, upper leaves produced more idioblasts than those at lower nodal positions. These results indicate that the amount of raphides formed in the idioblasts is a sensitive response of taro plants to external environmental factors, in this case, the degree of shading and soil moisture content. Apparently, light and dry soil conditions promote the production of oxalic acid in taro petiole and the uptake of calcium ions by the corm and/or roots.
