Effect of drought on photodynamic peroxidation of leaf total lipophilic extracts

In vitro lipid peroxidation, induced by photochemically generated reactive oxygen species, was carried out either on purified alpha-linolenic acid (18:3n-3) (1.32mM) or on total lipophilic extracts of leaf (standardized at 1.32 mM for 18:3n-3 content). Three cultivars of plants differing in their sensitivity to water deficit were used: Vigna unguiculata L. cv. EPACE (drought-tolerant), Vigna unguiculata L. cv. IT-83D (moderately drought-tolerant) and Phaseolus vulgaris L. cv. Carioquinha (drought-sensitive). The time-course of 18:3n-3 photoperoxidation was comprised of three phases: (i) monohydroperoxidation, characterized by formation of conjugated diene patterns, (ii) secondary oxidation, characterized by formation of conjugated triene patterns, (iii) oxidative fragmentation of peroxidized compounds, characterized both by the disappearance of conjugated polyenic patterns and by the formation of thiobarbituric acid reacting substances. Only the first and second phases were observed on photoperoxidation of total lipophilic extracts from leaves of well-hydrated plants. When each cultivar was subjected to drought treatment, the conjugated diene pattern maximum was attained earlier than for well-hydrated plants, suggesting that drought affects the resistance capacity of total lipophilic extracts from leaves to photoperoxidation. The more severe the drought, the more susceptible the leaf extracts were to lipid peroxidation induced by reactive oxygen species. The more resistant to drought the plant was, the slower the rate of formation of conjugated polyenic patterns. These results are discussed in relation with the chlorophyll a/b and carotenoids/18:3n-3 molar ratios in the photoperoxidation apparatus. This work is the first attempt to correlate plant drought tolerance and the sensitivity of their leaf total lipophilic extracts to lipid peroxidation.