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The alterations in structure and function of photosystem II (PS II) during the senescence of primary leaves of wheat seedlings have been compared with the changes induced by ultraviolet-B (UV-B) radiation in the presence or absence of photosynthetically active radiation (PAR). The results indicated that the senescence-induced loss in pigment content, thylakoid membrane integrity and carotenoid-to-chlorophyll (Car-to-Chl) energy transfer efficiency was intensified by exposure to UV-B radiation. Different parameters for the measurement of PS II activity, such as Chl a fluorescence, O2-evolution and thermoluminescence intensity, were altered during senescence and these alterations were furthered by UV-B irradiation. The damage of photosynthetic apparatus by UV-B exposure in the presence of PAR was less than the damage in absence of PAR. The activation of molecular defense mechanisms could be a factor in the alleviation of UV-B damage in the presence of PAR.

Response of senescing leaves of wheat seedlings to ultraviolet A (UVA) radiation (365 nm) has been examined. The results indicate that senescence-induced disorganization of thylakoid membrane, decline in carotenoid-to-chlorophyll energy transfer, and enhancement of lipid peroxidation are furthered by radiation. The senescence-induced decline in photochemical activity of photosystem II further declines on irradiation. UVA does not specifically alter any site other than those damaged by senescence.

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Cymbopogon distans (Steud.) Wats. (l), unlike other species of the series Citrati, has linear-to-filiform leaf blades and a shallowly concave lower glume of the sessile spikelet (1). Both of the varieties of C . distam reported in the present investigation grow wild in the Himalayan region. In our earlier communication, we had reported the chemical composition of C . distans (2), a species growing around Nainital. We have now identified its major constituent, Cl,H,40, as a-oxobisabolene. The second chemotype of C . distans was collected from Munsiyari, and the results presented herein account for ca. 80% ofthe oil obtained from chis chemotype. Its major constituents were observed to be citral, geranyl acetate, and geraniol; however, aoxobisabolene was not detected in this variety. RESULTS AND DISCUSSIONThe physico-chemical properties and chemical composition of the oils isolated from two morphologically identical varieties of C . distans varied markedly.C . DZSTANS (Nainital collection).-The oil (0.6% by weight offresh aerial part) had a specific gravity of 0.801, an acid value of 1.15, an ester value of 19.6 (ester value after acetylation 80.95), and a carbonyl value of 5.

In clusterbean leaves UV-B radiation caused a reduction in contents of chlorophylls and carotenoids and in the efficiency of photosystem 2 photochemistry. The degree of damage was reduced when UV-A accompanied the UV-B radiation. This indicates the counteracting effect of UV-A radiation against UV-B-induced impairment.

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