UV-irradiation enhances rice allelopathic potential in rhizosphere soil

Ultraviolet-B radiation is rising continuously due to stratospheric ozone depletion over temperate latitudes. This study investigated effects of UV exposure on rice allelopathic potentials. For this purpose, two rice (Oryza sativa L.) cultivars BR-41 (high allelopathic = able to inhibit neighboring plants in native environment) and Huajingxian (low allelopathic = not able to inhibit neighboring plants, which were suppressed by higher allelopathic plants) were exposed to UV. Rhizosphere soils and leaf leachates bioassay were used to evaluate allelopathic potentials against receiver plants (barnyard grass and lettuce). These bioassays showed significant inhibition in lettuce and barnyard growth after UV in both rice cultivars. Interestingly, Huajingxian, which did not exhibit allelopathic potential in absence of UV showed significant inhibition after UV exposure. Phenolics, enzymes activities and genes responsible for biosynthesis of allelopathic compounds were examined after UV exposure. Phenolic compounds accumulated in rice leaves were quantified through HPLC analysis. They were significantly higher in BR-41 leaves after UV exposure. Enzyme activities (PAL and C4H) were significantly higher after UV exposure. The relative transcripts of genes (OsPAL and OsCYC1) responsible for biosynthesis of allelopathic compounds were also significantly higher after UV exposure. These results suggest that enhanced UV-irradiation levels due to ozone depletion may increase rice allelopathic potentials on paddy weeds.