Effect of UV-B Radiation on Leguminous Plants

Abstract

Sun is the primary source of energy for photosynthetic life on Earth. Ultraviolet-B (UV-B, 315-280 nm) radiation is a natural constituent of solar light reaching Earth's surface due to the thinning of stratospheric ozone (O-3) layer. Many studies have shown the harmful effects of UV-B on morphological, biochemical and physiological responses of plants. After the successful implementation of the Montreal protocol, this problem is now resolved up to some extent, as O-3 concentration is not showing a decreasing trend in the stratosphere. However, under recent climate changing scenarios, the presence of water vapors in the stratosphere could degrade the O-3 layer as suggested by recent reports. Beside, four new ozone depleting substances, previously not included under Montreal protocol, i.e. CFC-112a (CF2ClCCl3), CFC-112 (CFCl2CFCl2), CFC-113a (CF3CCl3) and HCFC-133a (CF3CH2Cl), have been detected in the atmosphere. This may led to more penetration of UV-B causing adverse effects on growth, physiology and yield of many agricultural crops in the future. This chapter presents an overview on the effects of ultraviolet-B (UV-B) radiation on leguminous plants. The findings were carried out under natural as well as artificial conditions from 0-50 kJ m(-2) day(-1) simulating about 0-62 % depletion in stratospheric O-3 layer. Most reports show the negative impact of UV-B on various parameters of legumes. Contrary to this, lower UV-B doses are beneficial in some cases. Effect of UV-B not only varied with different legumes, but also varied among the cultivars of same species. Significant reductions in total biomass, up to 93 %, photosynthesis, up to 90 %, and yield up to 62 % have been recorded in various studies. However, about 300 % increments have been noticed for UV-B absorbing compounds, which might be the protective mechanism adopted by the plants against UV-B. Earlier reviews on plants responses against UV-B are generally focussed on the above ground changes in plants. As legume-rhizobia-mycorrhiza are related symbiotically with legumes, so UV-B effect above ground on plants might be responsible for the below ground disturbances as UV-B is unable to penetrate the soil. Therefore in the present review, the above ground responses of legumes against UV-B is linked with the below ground changes. Studies reported reduction of about 62 % in nodulation, 78 % in nitrogenase activity, 31 % in nitrate reductase activity, 67 % in nitrite reductase activity and 76 % in leghaemoglobin content with various UV-B doses. On the other hand, UV-B exclusion studies have shown significant increments in parameters related to nitrogen metabolism. Singnificant negative impacts of UV-B were also reported on microbial biomass of rhizosphere.