Abstract: Bentazon sensitive lethal (bsl) mutants can be applied in hybrid rice (Oryza sativa L.) breeding for improving seed production and quality. This study was to elucidate the physiological mechanism of bentazon tolerance in rice. Tolerant rice varieties (W6154s and Norin 8) as well as their corresponding mutants, sensitive to bentazon, were employed in this study. Plant net photosynthetic rate (P_n), chlorophyll fluorescence, and the level of superoxide radical ( ) as well as the contents of chlorophyll (Chl) and malondialdehyde (MDA), were analyzed for both tolerant and sensitive rice plants treated with bentazon. After treatment, the two sensitive mutants showed a significant reduction in Pn at 0.5 h. A continuous decrease of Chl contents was found over the first 3 d whereas a significant increase of MDA contents was noticed on the 3rd day and thereafter. Analysis of chlorophyll fluorescence revealed a bentazon-induced increase in the proportion of the reduced state of QA. In the early stage after bentazon treatment, wild types and their mutants showed no significant difference in the alteration of Pn and chlorophyll fluorescence. While these two parameters then increased progressively in both wild types and remained low in the mutants. A significant generation of was found over the 5 d period in the mutants. Both wild types and mutants contained the same level of bentazon after 2 h of treatment. Bentazon content dropped to barely detectable amount in the wild type varieties at 1 d. However, the mutants retained a substantial amount of the herbicide after 5 d. It is proposed that the herbicide might inhibit rice photophythesis and accumulation of oxidative stress with the treatment of bentazon in both lethal mutants. The damaging effect on PS II system can be significantly alleviated in the wild type varieties due to a higher rate of catabolism of the herbicide.

Key words: Oryza sativa, Bentazon, Sensitive lethal mutant, Tolerance, Physiological research