EFFECT OF DROUGHT ON PHOTODYNAMIC PEROXIDATION OF LEAF TOTAL LIPOPHILIC EXTRACTS

In vitro lipid peroxidation, induced by photochemically generated reac tive oxygen species, was carried out either on purified cc-linolenic a cid(18:3n - 3)(1.32 mM) or on total lipophilic extracts of leaf(standa rdized at 1.32 mM for 18:3n - 3 content). Three cultivars of plants di ffering in their sensitivity to water deficit were used: Vigna unguicu lata L. cv. EPACE (drought-tolerant), Vigna unguiculata L. cv. IT-83D (moderately drought-tolerant) and Phaseolus vulgaris L. cv. Carioquinh a (drought-sensitive). The time-course of 18:3n - 3 photoperoxidation was comprised of three phases: (i) monohydroperoxidation, characterize d by formation of conjugated diene patterns, (ii) secondary oxidation, characterized by formation of conjugated triene patterns, (iii) oxida tive 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 ph ases were observed on photoperoxidation of total lipophilic extracts f rom leaves of well-hydrated plants. When each cultivar was subjected t o drought treatment, the conjugated diene pattern maximum was attained earlier than for well-hydrated plants, suggesting that drought affect s the resistance capacity of total lipophilic extracts from leaves to photoperoxidation. The more severe the drought, the more susceptible t he 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 d iscussed in relation with the chlorophyll a/b and carotenoids/18:3n - 3 molar ratios in the photoperoxidation apparatus. This work is the fi rst attempt to correlate plant drought tolerance and the sensitivity o f their leaf total lipophilic extracts to lipid peroxidation.