Cupric sulfate pentahydrate (CuSO4·5H2O) has been proposed for use in Hawaii as a molluscicide to control golden apple snail (Pomacea canaliculata Lamarck) infestations of taro [Colocasia esculenta (L.) Schott]. Two hydroponic, greenhouse studies were conducted to determine the effects of solution Cu2+ levels on taro growth, the solution Cu2+ toxicity threshold, and useful diagnostic indicators of Cu toxicity. In the first experiment, taro cultivars Lehua maoli and Pololu were grown at nine levels of Cu2+ ranging from 0.5 to 25.0 μm. In the second experiment, `Lehua maoli' was grown at six levels of Cu2+ ranging from 0.25 to 2.5 μm. Significant (P ≤ 0.05) toxic effects included reduced dry matter production, leaf area, and root length: root dry weight ratio, and both impaired photosynthesis and a generalized reduction of cation accumulation in leaf blade tissue. The solution Cu2+ toxicity threshold (based on 90% of relative total dry weight) for young taro plants was 1.2 μm. Because Cu does not accumulate in leaf blade tissues with increasing solution Cu2+ levels, leaf Cu concentration cannot be used as a diagnostic indicator of Cu toxicity in taro.
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