Arsenic absorption by rice (Oryza sativa, L.) in relation to the chemical form and concentration of arsenic added in nutrient solution was examined. A 4 x 3 x 2 factorial experiment was conducted with treatments consisting of four arsenic chemical forms [arsenite, As(Ill); arsenate, As(V); monomethyl arsenic acid, MMAA; and dimethyl arsenic acid, DMAA], three arsenic concentrations [0.05, 0.2, and 0.8 mg As L-J], and two cultivars [Lemont and Mercury] with a different degree of susceptibility to straighthead, a physiological disease attributed to arsenic toxicity. Two controls, one for each cultivar, were also included. Arsenic phytoavailability and phytotoxicity are determined primarily by the arsenic chemical form present. Application of DMAA increased total dry matter production. While application of As(V) did not affect plant growth, both As(Ill) and MMAA were phytotoxic to rice. Availability of arsenic to rice followed the trend: DMAA < As(V) < MMAA < As(III). Upon absorption, DMAA was readily translocated to the shoot. Arsenic(Ill), As(V), and MMAA accumulated in the roots. With increased arsenic application rates the arsenic shoot/root concentration decreased for the As(Ill) and As(V) treatments. Monomethyl arsenic acid (MMAA), however, was translocated to the shoot upon increased application. The observed differential absorption and translocation of arsenic chemical forms by rice is possibly responsible for the straighthead disorder attributed to arsenic.

Arsenic absorption by rice (Oryza sativa, L.) in relation to As chemical form present in soil solution was examined. Rice plants were grown in soil suspensions equilibrated under selected conditions of redox and pH, affecting arsenic solubility and speciation. A decrease in pH led to higher dissolved arsenic concentrations. When the soil redox potential dropped below 0 mV, most of the arsenic was present as As(Ill). Under more oxidizing conditions both As(Ill) and As(V) are present. Chemical speciation of As in the watersoluble fraction affected its phytoavailability. Most indigenous arsenic taken up by the plants remained in the root. Plant arsenic availability increased with increasing arsenic concentration in solution (lower soil pH) and with increasing amounts of soluble As(Ill) (lower soil redox). We also studied the uptake of monomethyl arsenic acid (MMAA), a widely used defoliant and herbicide, as affected by soil redox-pH condition. Amended MMAA was approximately two times more phytoavailable than the indigenous inorganic As forms and increased with decreasing pH and redox.

The effect of dimethylarsenic acid (DMAA) applied to the root on arsenic (As) uptake and concentration, net photosynthesis (Pn), and growth parameters of rice (Oryza sativa L. cv. 'Mercury') plants was studied. The experiment consisted of four treatments (0, 0.2, 0.8, and 1.6 mg As/L) with four replications in a completely randomized design. The DMAA was applied in nutrient solution as its sodium salt. The solution culture was changed every four days to avoid changes in the As chemical form. Arsenic uptake and concentration in shoot and root increased upon increased DMAA concentration in solution. Upon uptake, DMAA was readily translocated to the shoot. At the two higher rates of DMAA application (0.8 and 1.6 mg As/L), Pn and photosynthetic capacity were significantly decreased in response to tissue As concentration. Leaf area and dry matter production were also significantly reduced at the two higher rates of DMAA. At the lower rate (0.2 mg As/L) of DMAA application, there was no significant reduction in Pn or growth. Dimethylarsenic acid application did not affect nutrient allocation within the rice plant at concentration levels used in this study.

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