Copper Toxicity in Rice

Long-term experiments (30 days) were carried out with rice in solution culture containing varying concentrations of Cu in an attempt to examine the threshold toxic concentration of this metal and its effects on Mn and Fe concentration in plant tissues. Cu concentration by rice roots (Oryza sativa L.) consisted of two distinct accumulation phases: a slow phase, in solution containing up to 0.01 mg/L, and a rapid phase above this concentration. In both phases the net absorption rate, as measured by Cu concentration in the roots, increased with metal concentration in the culture solution, but the increase was much faster in the second phase. Total Cu tissue concentration showed little variation between normal tissue and that clearly showing initial symptoms of the metal toxicity. Cu toxicity, as expressed by reduced root length, appeared to be a direct result of the accumulation of excess Cu in total tissues, the average concentration being 35.1 μg/g (dw) Cu at threshold toxicity. With respect to the effects of excess Cu on Mn and Fe concentrations, it was observed that while Mn and Fe concentrations in the shoot decreased at solutions of Cu concentrations above 0.05 mg/L, in the root these metals showed different patterns, with Fe concentration decreasing above 0.05 mg/L Cu and Mn concentration showing no defined trend. It is suggested that Cu affects Mn and Fe net translocation by different mechanisms, having the X coefficient for Mn and Fe values of 0.995 and 0.581, respectively.