L' Annee biologique最新文献

Plant adaptations to environmental conditions

In response to extreme conditions, plants exhibit a variety of adaptations that are important in the maintenance of plant community. Adaptation may be defined as heritable modifications in physiological or developmental attributes that improve the fitness of an organism under conditions of its environment. Adaptations are selected by three types of constraint: climatic, edaphic and biotic. Water act as a key element in the plant distribution and many morphophysiological traits are conditioned by water, directly or indirectly. Plant have developed a wide variety of mechanisms for surviving in habitats exposed to episodic or prolonged periods of frost. The extreme altitude and latitude reached by plants are revealing of the capacity of some plants to resist to cold. The photosynthetic apparatus of plants is capable of optimally efficient absorption and utilization of visible radiation because light availability vary considerably in intensity, duration, and space. Nutrient-poor environments or toxic soils have a well adapted plant community like carnivorous plants in peat swamp land and halophytes in salted soils. In habitats exposed to stress, the interplay of numerous stressors restricts the area on which a particular plant species can survive. Morphological and biochemical traits as been selected to withstand constraints under given conditions and thus plants possess various adaptations. Global changes in the atmospheric composition and climate are able to affect vegetation dynamics and could induce changes in plant distribution. Adaptation will therefore be a key element in the survival of the plant community.

Programmed cell death (PCD) in plants has been the subject of controversy for a long time. There is no more doubts nowadays of the occurrence of such a phenomenon during normal plant development or during the plant responses to pathogens. There are some analogies between plant programmed cell death and animal cell's apoptosis, although plant PCD may occur through different morphological patterns. The initial molecular mechanisms leading to PCD during plant normal development and defense responses might be common but with various morphological manifestations depending upon the plant needs.

Antifeeding agents are mainly organic compounds issued from the metabolic pathways of plants that interrupt food intake by phytophagous insects. These substances can be toxic but generally do not directly kill the insect; it will die by inanition. Antifeeding agents are recognized by taste receptors located on the maxilla and epipharynx. In caterpillars, the two medial and lateral sensilla, the main organs involved in chemoreception, are located on the galea. They are uniporous at their apical extremity and contain four bipolar gustatory neurones, and another one that is a mecanoreceptor. As for olfactory organs, the dendritic parts of these neurones are soaked in a sensory liquor that transports chemical stimuli from outside to the membranous receptors. Antifeeding agents belong to various classes of chemicals and it is not possible to correlate their biological activity with common structural features. Main representatives are terpenoids (drimanes, clerodanes, limonoids, agarofurans) and steroids (phytoecdysteroids). Antifeeding activities can be evaluated by the amount of treated food eaten by a caterpillar in comparison with the reference. Antifeeding agents can be classified by their feeding ratio 50: FR₅₀ or by their antifeedant index 50: AFI₅₀. These values must be calculated for each studied species, each one showing different sensitivity for the same compound. Responses of gustatory neurones to a stimulation with an antifeeding agent can be recorded at the extracellular level by electrophysiological techniques, derived from the pioneer work of Hodgson. When stimulated, these neurones respond by characteristic spikes that depend of the recognition capacity of the neurone, and many hypotheses have been formulated to explain the modalities of this recognition. The most commonly accepted is that antifeeding agents selectively stimulate a special gustatory cell: the deterrent cell. Activation of this cell is often associated with simultaneous inhibition of other gustatory cells involved in perception of phagostimulants like sugars and free aminoacids. The occurrence of the deterrent cell has been demonstrated for many phytophagous caterpillars and can explain repellence for the non-host plants. Phytoecdysteroids are secondary metabolites widely spread in the plant kingdom and it has been hypothesized that they act as defensive substances against herbivorous. Two categories of caterpillars in relation to their sensitivity to the substance have been proposed: the tolerant ones that accept relatively large amounts of phytoecdysteroids in their diet, and the sensitive ones that reject diet with even low amounts. According to this classification, polyphagous species must be tolerant, and at the opposite monophagous species sensitive (diet breadth hypothesis). To verify this hypothesis, the antifeeding activity of three phytoecdysteroids (ecdysone: E, 20-hydroxyecdysone: 20E, and ponasterone: A ponA)