Differential Response of Taro (Colocasia esculenta L. Schott) Cultivars to Aluminum Toxicity

The differential response of taro to Al-toxicity was evaluated using four taro cultivars (Akado, Bun-long, Kai-Uliuli; and Lehua Maoli), grown in hydroponic solution at three initial levels of Al (0, 440, and 1330 μM). Cultivar Lehua Maoli was found to be Al-tolerant at 1330 μM Al compared to cultivars Akado, Bun-long, and Kai-Uliuli, based on its higher dry weights of leaf blade, petiole, and root, as well as its larger leaf areas and longer root lengths. Increasing Al levels in solution significantly depressed total oxalate concentrations in roots; however, cv. Lehua Maoli maintained the highest level of oxalates at 1330 μM Al. To determine the effects of varying levels of Al and Ca on growth, nutrient composition, and oxalate formation, two taro cultivars (Lehua Maoli and Bun-long) were grown in hydroponic solution with varying levels of Ca (50, 100, and 1000 μM) and Al (0, 148, and 890 μM). Solution levels of 890 μM Al and 1000 μM Ca provided the best separation of growth parameters between cultivars, in which Al-tolerant cv. Lehua Maoli had significantly greater root, petiole, and leaf blade dry weights compared to cv. Bun-long. Increasing Al levels significantly increased Al concentrations in roots, petioles, and leaf blades. As found earlier, Al-tolerant cv. Lehua Maoli had a significantly greater Al concentration in roots and a significantly lower Al concentration in leaf blades at 890 μM Al and 1000 μM Ca compared to Alsensitive cv. Bun-long. Increasing Al levels in solution significantly decreased concentrations of insoluble oxalates and water soluble oxalate in roots. Aluminum-tolerant cv. Lehua Maoli had significantly greater concentrations of insoluble oxalates in roots at 890 μM Al and 1000 μM Ca. To confirm differential response of taro cultivars to excess Al in soil, Al-tolerant cv. Lehua Maoli and Al-sensitive cv. Bun-long were planted in a highly weathered Ultisol (Haiku series) at three pH levels 3.8, 4.2 and 5.2. A soil pH of 5.2, which was obtained by adding 600 mg Ca kg·1 as Ca (OH) 2 was used as a control. Increasing soil pH from 3.8 to 5.2 resulted in significant increase in dry weights of leaf blade, petiole, and root fresh weights. Cultivar Lehua Maoli had significantly greater dry weights of leaf blades, petioles, and root fresh weights than cv. Bun-long at pH 4.2 across all Ca levels. Thus, the use of the Haiku soil series at pH 4.2 and 5.2 is recommended for screening the taro germplasm for Al tolerance. One possible mechanism of Al tolerance in taro is the formation of a solid Al oxalate complex. To determine the possible formation of solid Aloxalate in taro, Al-tolerant cv. Lehua Maoli was grown in hydroponic solution at 1000 μM Ca and 2 levels of Al (0 and 890 μM). The whole root tissue as well as the isolated protoplasm were analyzed using: a) scanning electron microscope (SEM) energy dispersive X-ray flourescence analysis; b) oxalate decarboxylase enzyme; c) X-ray diffraction; and d) solid state NMR. The use of oxalate decarboxylase enzyme, an enzyme specific for oxalate, demonstrates the formation of Al-oxalate in the protoplasm of root tissues of Al-tolerant cv. Lehua Maoli, as shown by the increased soluble Al concentration of the protoplasm treated with enzyme compared to that of the control. This result indicates that Al was cleaved by oxalate decarboxylase enzyme from oxalate complexes and that Al tolerance in taro is related to the formation of Al-oxalate complex inside the cells