Ultrastructural, physiological and biochemical analyses of chlorate toxicity on rice seedlings

Chlorates are oxidizing substances considered phytotoxic to all green plant parts. In this study the physiological, biochemical and ultrastructural effects of chlorate treatments were studied in rice seedlings grown at two different temperatures (28 and 6degreesC). Visual analyses demonstrated that the chlorate treatment provoked severe damage to plant leaves only at 28 degreesC. The electron microscopy of the chloroplast revealed several changes in the organelle organization, whose integrity of outer membrane and the internal network of thylakoid membranes were almost completely lost. A high level of lipid peroxidation was observed in 28 degreesC-chlorate-treated leaves in comparison to plants exposed to 6 degreesC-chlorate treatment. Photochemical efficiency of photosynthesis (F-v/F-m) was drastically affected by the 28 degreesC-chlorate treatment but, at low temperature, chlorate-treated plants suffered a discrete reduction in F-v/F-m. Root structure and organization were analyzed by light microscopy (LM) and scanning electron microscopy (SEM). The 28 degreesC-chlorate treatment provoked severe damages to the exodermis, endodermis, cortex cells and root hairs. In the 6 degreesC-chlorate treatment, damage to root cells was also observed, but at a lower intensity. Nitrate reductase (NR) activity was analyzed and revealed the involvement of this enzyme in the chlorate action, even at a low temperature. (C) 2004 Elsevier Ireland Ltd. All rights reserved.