Effect of enhanced solar UVB (280-320nm) radiation on secondary pigment synthesis in some plants
Abstract
The investigation was carried out to find out whether enhanced UVB radiation increases the synthesis of secondary metabolites in some plants grown under tropical region. Like tropical plant temperate plant also grown successfully in the tropical region. Tropical plants are thought to have an inherent resistance to abiotic stress. The abiotic stress that enhances the ability of the plant by increasing the content of the secondary bioactive components. While the plants exposed to enhanced UVB radiation in the field condition, the synthesis of secondary pigments like flavonoids and anthocyanin level varied among species. In radish and beet - root the syn thesis of flavonoid and anthocyanin the content was increased, whereas, in the onion and carrot, the level of synthesis of secondary pigment was less. But compared to ambient plants the UVB the irradiated the plant has more secondary pigment. So it proves that the UVB radiation increases the synthesis of secondary pigment content by activating the defence mechanism and to protect plants against UVB damage.
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Introduction
Due to the depletion of the stratospheric ozone layer increased the ultraviolet radiation in the biosphere, especially in the range of UV--B(280-320nm) is increasing in the earth's surface. Such increases were very high in the low latitude region when compared to the high latitude region. The increases in solar UV-B radiation have a great impact on the agricultural crops in the tropical region. The effect of UV-B on plants includes inhibited growth, morphology ,Chl biosynthesis, photosynthetic activity, electron transport, damage the DNA, protein, lipid membrane, decrease biomass of plants and increases in the level of secondary pigment synthesis in plants.(1,2).Tropical plants reside in an environment which possesses a naturally high amount environmental stress. When the plant exposed to enhanced UVB radiation continuously means the plants to synthesis secondary or protective pigment by defence mechanism. Naturally tropical plant synthesis, high amount photoprotective pigment compares to temperate plants. When they exposed to the addition of UV-B radiation means it increases the synthesis of secondary metabolites. The continuous exposure plants to UV-B radiation increase photobleaching of chlorophyll pigment, it affects photosynthetic activity and other physiological processes. Such damages are reduced in some level by these protective pigments like anthocyanin and flavonoids.(3)
Flavonoids are the class of secondary phenolics with significant antioxidant and chelating properties widespread in various plants. Accumulation of the UV-B absorbing pigments one of the ways by which plants alleviate the harmful effect of UV-B light (4). Lie et al 1995 observed that the largest UV-B absorbing compound in barley plants. It has been shown that the photo-induced accumulation of these flavanoids precedes by n some enzyme phenylpropanoid biosynthetic pathway such as phenylalanine ammonialyase and chalcone synthase of flavanoids biosynthetic pathway(6,7) . However, the flavonoid concentration may be higher (8) in tropical species growing at high elevated regions.
Anthocyanin is another important secondary metabolites it accumulated in leaf tissue.
Many studies have shown that very small amount of UV-B radiance differences have a large effect on the accumulation of anthocyanin and flavonoids. The Anthocyanin also, act as UV-B attenuators in protecting the cellular components against radiation damage. High UV-B radiation is known to increase anthocyanin production is several crop species (9,10). DrummHerrel and Mohr (1981) have demonstrated that anthocyanin synthesis in sorghum mesocotyls the involved interaction between UV-A, UV-B and phytochrome photoreceptors . UV-B induced anthocyanin production has also been reported in mustard hypocotyls, corn, wheat, and rye coleoptiles (12Wellmann, 1982). Although this pigment production represents a specific UV-B effect, anthocyanin biosynthesis may not be particularly adapted since it has little absorption in the UV-B waveband. The effect of UVB on plant physiological has been widely studied. But limited studies only focus on the synthesis of secondary metabolites in plants grown for agriculture purpose. In the present work, an attempt has been made to study the effects of enhanced solar UV-B radiation on the temperate vegetative crops, which are successfully grown in the tropical regions and are also economically important. After screening a wide range of plants with underground storage plants for their UV-B induced responses, there is a need to analyze synthesis of secondary metabolites. In Daucus carota L. (Carrot), Raphanus sativus L. (radish) Allium cepa L (onion) and Beta vulgaris L (beet -root) a comparative investigation has been carried out to understand the impact of ambient solar radiation and UV-B enhanced solar radiation (20% above the ambient UV-B level).
