The physiological responses of dimethyl sulfoxide (DMSO) exposure were investigated in rice (Oryza sativa L. cv. XZX 45) seedlings. The seedlings were hydroponically exposed to different concentrations of DMSO for 72 h. Results showed that a linear decrease in relative growth rate and water use efficiency was observed with rice seedlings with increasing DMSO concentrations. The estimation of cell death measured by Evans blue uptake also indicated DMSO-induced damage in root tissues. Negligible decrease in chlorophylls was noted, while significant reduction in carotenoids content was only observed at 13.54 mM DMSO. Although DMSO did not have any significant effect on protein content in roots, the protein content in shoots was significantly decreased in a dose-dependent manner. Proline content in both plant tissues was positively affected by DMSO exposure, responding an inverted U-shaped curve with DMSO concentrations. Results also showed that DMSO-induced accumulation of hydrogen peroxide (H 2 O 2 ) was evident in roots rather than shoots. DMSO did not result in any significant changes in superoxide dismutase and peroxidase activities as well as malondialdehyde content. Catalase (CAT) activity in both roots and shoots was quite sensitive to changes in DMSO treatments than other enzymes, suggesting that CAT may play central role in the detoxification of H 2 O 2 in rice seedlings under DMSO exposure. Results suggest that growth inhibition and cell death of rice seedlings caused by DMSO exposure were largely related to the accumulation of H 2 O 2 in plant tissues.

Dimethyl phthalate (DMP) is one of the most widely used plasticizers. Due to its anthropogenic inputs, DMP has become a serious contaminant in soils and water. Hydroponic experiments were conducted focusing on phytotoxic responses of rice seedlings (Oryza sativa L. cv. XZX 45) to 1,2-dimethyl phthalate. Relative growth rate (RGR) and water use efficiency (WUE) were measured as response endpoints to quantify the effective concentrations (ECs) of DMP to rice seedlings. As a comparison, acute phytotoxicity of dimethyl sulfoxide (DMSO) used as solvent media was also determined. Results showed that acute toxicity of DMP to rice seedlings was evident, but selected endpoints had different responses to DMP exposure. RGR of rice seedlings was more sensitive to change of DMP than WUE. EC-48 h values for 10, 20 and 50 % inhibition of the RGR were estimated to be 6.18, 38.41 and 329.41 lL for rice seedlings exposed to DMP, respectively, while smaller ECs were obtained for 96 h exposure.Results from phytotoxicity of DMSO demonstrated that the doses of DMSO used for dissolving DMP had a negligible effect on rice seedlings. In conclusion, DMP is problematic at relatively low concentrations for rice seedlings, and inhibitory effects are highly dependent on response endpoints and the duration of exposure period.

Hydroponic experiments were conducted with rice seedlings (Oryza sativa L. cv. ND1) exposed to methylene blue (MB) to investigate the accumulation of hydrogen peroxide (H2O2) and activities and gene expression of antioxidant enzymes. Results showed that a linear decrease in relative growth rate and water use efficiency was observed with rice seedlings with increasing MB concentrations. MB-induced accumulation of H2O2 was evident in both roots and shoots. The activities of antioxidative enzymes were analyzed and found to be different at different MB treatment concentrations. The activities of enzymes related to the ascorbate-glutathione cycle were more sensitive to MB treatments than other antioxidative enzymes. Transcript level, by real-time quantitative PCR, of antioxidative enzymes showed that the analyzed genes were differently expressed during different MB concentrations in both roots and shoots. The isoform analysis of superoxide dismutase (SOD) gene showed that the expressions of Cu/ZnSOD and MnSOD were relatively constant, where significant upregulation of FeSOD was observed with rice seedlings exposed to high MB concentrations. Furthermore, the expression of CAT, POD, and MDHAR genes responded biphasically to MB treatments by showing negligible changes at 1.56-15.63 μM MB and significant induction at 31.36-62.52 μM MB. The expression of GR, APX, and DHAR genes showed a remarkable induction to MB. Our results suggest that on transcription level, and in accordance with enzymatic responses, enzymes of GR, APX, and DHAR play central role in the H2O2 detoxification in rice seedlings under MB exposure.

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