UV-B Radiation Stress in Rice

K.K.Vinod

Rice is grown mostly in tropical and subtropical countries. It is known that UV-B radiation is highest in tropical regions where rice is grown, because the stratospheric ozone layer in high latitudes, and solar angles are higher. The current information on rice is insufficient to conclude the potential risks of UV-B exposure to rice production. UV-B radiation would also have indirect effects on rice production through indirect effects on other components of rice ecosystem such as weeds, diseases and nitrogen-fixing cyanobacteria. Increased UV-B radiation induces a significant reduction in the total biomass in a number of rice cultivars, accompanied by a reduction in tiller number and photosynthetic capacity of plants. The prolonged exposure of UV-B light affects plant height, leaf area, dry weight, net assimilation rate and relative growth rate in some rice cultivars.

Evidence were shown for that UV-B mediates photoinduction of anthocyanin synthesis in seedlings of a cyanic rice cultivar, Purple puttu, which is associated with PAL biosynthesis. They observed that sunlight triggered the photoinduction of anthocyanin in shoots of purple puttu seedlings, whereas seedlings exposed to sunlight filtered through window glass showed little formation of anthocyanin. In addition to UV-B receptor the anthocyanin level was also modulated by phytochrome. However, the anthocyanin photoinduction was restricted to only a few cultivars of rice, indicating variability within the cultivars with respect to anthocyanin induction. 

The stresses imposed by UV-light irradiation can cause reactive oxygen species generation (ROS) such as O^2- and H₂O₂. Though H₂O₂ is an innocuous metabolite present in cells irradiation with UV-light breaks it down to extremely deleterious hydroxyl free radicals (OH^•). Since H₂O₂ can easily diffuse through cell membranes it is extremely deleterious to cellular constituents such as DNA. Studies have indicated that in vitro anthocyanins could act as effective antioxidants as anthocyanins prevent ascorbic acid (AsA) against metal induced oxidation by forming a stable AsA-metal-anthocyanin co-ordinate complex. Above complex protected AsA from H₂O₂ and OH^•, and also anthocyanins from damage.