J. Vieira Da Silva

A predator-prey model with satiation and intraspecific competition

Abstract We present a new predator-prey model where, except for the prey growth, assumed to be logistic, we endeavor to give some behavioral justification to all elements of the predator-prey interaction. The functional response takes account of predator satiation and predator competition. It is supported by some experimental evidence. We distinguish two contributions to the numerical response: the positive part, proportional to the functional response, is the birth rate of predators; the negative part is the death rate due to hunger. Two outcomes are possible. If the prey are unable to grow fast enough to replace the amount killed by the predators, both species become extinct. In the opposite case, both populations stabilize at a constant population. At this equilibrium level, the prey are not abundant enough to satiate the predators. The predation rate that allows the highest predator population is one half of the ideal prey growth rate. A higher exploitation rate can allow higher populations only temporarily. Evolved predator behavior, reguges for the prey, or other mechanisms can explain this regulation. Two more population behaviors (cycles and predator extinction) can be obtained with a time-lag in one of the responses. This is shown in a separate paper. The model is structurally stable. It can thus withstand small environmental perturbations.

Enzymatic Breakdown of Polar Lipids in Cotton Leaves Under Water Stress

Water stress provokes a decrease in leaf polar lipid content (Pham Thi et al.1982). This decrease is due to an inhibition of the biosynthesis from 14C-acetate (Pham Thi et al. 1985), but certainly also to an acceleration of degradative phenomena. The enzymes responsible for polar lipid catabolism in plants have received little attention (Galliard et al.1974). This paper describes the enzymatic systems acting on MGDG and PC breakdown and their evolution under water stress.

Influence de la sécheresse sur l'ultrastructure mitochondriale chez le cotonnier — Quelques implications métaboliques

Summary Ultrastructural studies of Cotton leaves show that under water stress, mitochondria are, along with chloroplasts, the most susceptible cell organelles. The damage are, in time order: Swelling of cristae, loss of polyribosomes, local breakdown of the external membrane. Under severe water stress, mitochondria appear like empty bags. The development is much more slow in Gossypium anomalum , a drought-resistant species, than in G. hirsutum , a sensitive one. These ultrastructural modifications are discussed in connection with nitrogen metabolism and photorespiratory metabolism. The role of membrane stability in water stress tolerance is also assessed.

Sur la reprise au cours de la rehydratation, du point de compensation de CO2, de la photorespiration et de l'ultrastructure, chez deux especes de cotonnier

Abstract Recovery during rehydration of CO 2 compensation point, photorespiration and ultrastructure in two cotton species With two cotton species, one drought-resistant, the other not, the evolution of CO 2 compensation point, photorespiration and leaf ultrastructure were followed after rehydration. The amount and speed of recovery depend on the intensity and duration of the previous osmotic treatment. Recovery is much easier in the resistant species. After recovery from a small water stress, net photosynthetic activity can be higher than the control. With severe stress, however, metabolic and structural alterations increase on rehydration. In general metabolic recovery parallels repair of structural membranes.

Photosynthesis and Allocation

The long summer drought that characterizes mediterranean-climatic regions places severe constraints on the photosynthetic potential of the resident plants. In this section a variety of mechanisms are described that characteristic evergreen plants inhabiting these regions have evolved to adapt to the drought. These include such physiological mechanisms as osmoregulation in the case of Olea to midday stomatal closure in the case of Arbutus. Allocation to root biomass is also an important adaptive mode for enhancing drought tolerance by certain species. Differential allocation patterns apparently explain dissimilar productivities of Mediterranean pines which have similar photosynthetic capacities.

Annual Variation of Photosynthesis of the Olive Tree Under Different Watering Conditions and Related to Chlorophyll Accumulation

Olive trees were irrigated since planted either with drinking or with brackish water. The amount of irrigation applied covered 50%, 75% and 100% of potential evapotranspiration (PET at Tunis: 1400 mm; mean rainfall 4–50 mm). The results show that the olive tree has high photosynthetic capacity (compared with other species) but that it is rather affected by extreme watering conditions and by salinity. Excess of water (compared with needs) induces the same effects as lack of water or salinity and decreases photosynthetic capacity and chlorophyll accumulation. Nevertheless the effects of excess of water and salinity are not additiva Lack of water reduces photosynthesis more than does salinity. Also the effect of reduced irrigation on stomatal density is different of that of salinity.

Effects of Low Temperature on Lipid and Fatty Acid Composition of Sorghum Roots and Shoots

When exposed to chilling, but non-freezing temperatures, plant cell membranes undergo a number of modifications in their lipid and fatty acid properties (see Lyons et al.1979). The most currently observed changes are an accumulation of phospholipids, particularly PC, and an increase in the level of fatty acid unsaturation. Sorghum are tropical plants, which show a chilling sensitivity differing from one species to another. Thus, Sorghum bicolor is sensitive to chilling, while Sorghum halepense could resist to temperatures lower than 5°C (Bagnall 1979).

Photosynthesis under Osmotic Stress. A Possible Cause of the Osmotic Stress Induced Susceptibility to Photoinhibition

It has been proposed that drought by restricting carbon assimilation by CO2 depletion (stomatal closure) or by biochemical alterations in the chloroplast could predispose the photosynthetic apparatus to photoinhibition. However experimental data have shown contradictory results depending upon the considered species : in Nerium oleander (1) and in low light adapted willow (2), they indicated a susceptibility to photoinhibition during drought while those made in sunflower (3) and in cotton (4) demonstrated that water stress does not modify photosynthetic apparatus susceptibility to photoinhibition. This discrepancy questions the underlying causes of this susceptibility and suggests that the water stress induced slowdown of carbon metabolism is not the main factor that predisposes the plants to photoinhibition during drought.