Jorge Silva

Effects of water stress on lipid metabolism in cotton leaves

Abstract After 2, 10 and 24 hr labelling with [1- 14 C] acetate, radioactivity incorporated into the lipids of cotton leaves is mainly found in phosphatidylcholine, phosphatidylglycerol and neutral lipids. Galactolipids are slowly synthesized and after 24 hr, account for only 10% of the total radioactivity. Under water stress, a marked decrease of precursor incorporation into leaf lipids occurs, particularly in phosphatidylcholine and galactolipids. Relative incorporation into neutral lipids, on the contrary, increases. Water deficits provoke an inhibition of the fatty acid desaturation, resulting in a sharp decrease of linoleic and linolenic acid biosynthesis. The decrease in unsaturated fatty acid biosynthesis occurs in all lipid classes, but is most pronounced in the galactolipid fractions. In the drought-resistant cotton variety (Mocosinho), the variations in lipid and fatty acid metabolism under water stress are less pronounced than in the drought-sensitive variety (Reba), and this attests a greater stability of the membrane system.

The role of membrane lipids in drought resistance of plants.

SummaryIn greenhouse as well as field-grown plants of Vigna unguiculata and Gossypium hirsutum submitted to drought, leaf membrane lipids show a series of modifications: 1 - A decrease in polar lipid content under severe water stress in sensitive and resistant varieties, due essentially to a decrease in monogalactosyl-diacylglycerol (MGDG) content.2 - A decrease in the degree of unsaturation of fatty acid in sensitive varieties, due to a degradation of linolenic acid (18:3).These modifications result from a slowing down of lipid biosynthesis and from an acceleration of degradative enzymatic processes.Comparison between lipid and fatty acid compositions of drought-susceptible and drought-tolerant varieties from various plant species, suggest that the linolenic acid content of the leaf membranes is related to a lack of ability of cells to withstand low water potentials. Clearly, plants having a low linolenic acid content from MGDG are more resistant than the others. This may be related to the susceptibility...

The effect of a time-delay in a predator-prey model

Abstract A time lag in the death rate of predators caused by starvation (negative numerical response) can destabilize the equilibrium point and give rise to a stable limit cycle. The oscillations grow with the time lag and can become so wide that predators become extinct. The behaviors produced by a small delay are, depending on the predation rate, either stable coexistence or extinction of both species. The model studied is therefore able to generate all predator-prey possible outcomes. The limit cycle appears even when the equilibrium point is (0, 0). The time lag can thus make persistent a system that otherwise would become extinct.

Effect of water deficit on cell permeability and on chloroplast integrity

SummaryThe study of physiological mechanisms involved in drought tolerance at the cellular level could lead to the characterization of tolerance parameters useful in crop breedings. In this study, we analyse in the case of Vigna and Phaseolus leaves, the effect of water deficit on cell permeability and chloroplast integrity.Chloroplasts were isolated from control and stressed plants. Thylakoid and stroma proteins were extracted from the isolated chloroplasts and analysed by electrophoresis.The results showed that water deficit induced at the cell level an increase in electrolyte and Pi leakages from leaf discs treated with PEG and rehydrated, according to the protoplasmic tolerance of the varieties. Concerning chloroplasts, we observed in response to drought: • an increase in the number of disorganized chloroplasts; • a decrease in the photosynthetic electron transfer; • a decrease in protein and chlorophyll contents of the chloroplasts; • modifications of protein patterns of thylakoids; • an inhibition o...

In vitro photodynamic lipid peroxidation of total lipophilic extracts from leaves of bean plants

In vitro lipid peroxidation, induced by reactive oxygen species photochemically generated, was carried out on purified alpha-linolenic acid (18:3(n - 3)) and on bean leaf total lipophilic extracts. The photosensitizer used was meso-tetraphenylporphine. The time-course of the reaction was evaluated by ultraviolet (UV) spectra analysis. The 18:3(n - 3) photoperoxidation kinetics comprised three steps: monohydroperoxidation, characterized by the appearance of conjugated diene patterns; dihydroperoxidation characterized by the appearance of conjugated triene patterns, oxidative cleavage of the mono- and dihydroperoxides, characterized by the disappearance of conjugated patterns. In contrast, for hydrated plant total lipophilic extract photoperoxidation, conjugated pattern appearance was slow and the maximum plateau was not obtained. In order to explain plant extract behaviour, we tested the effects of beta-carotene and quercetin (important components of the chloroplast membrane) on the time-course of 18:3(n - 3) photoperoxidation. The first step was inhibited by beta-carotene implying that type II photoperoxidation involving singlet oxygen (1O2) was predominant. Whilst the two last steps were inhibited by quercetin implying that type I photoperoxidation involving free radicals, prevailed. Since 18:3(n - 3) foliar content decreased under water deficit, we tested the behaviour of total lipophilic extracts from droughted plant leaves in presence of reactive oxygen species. In the case of droughted plants, the maximum of conjugated diene patterns was attained later than for hydrated plants, suggesting that drought affects the resistance capability of total lipophilic extracts from bean leaves to lipid photoperoxidation.