Among ecological factors, light has a major influence on the life of land plants. Within natural plant consortia, variations in light intensity (irradiance) and quality (wavelength) are two direct consequences of plant stratification, experienced not only by plant individuals growing at different layers, but even by leaves of the same tree located in different positions of the crown. Two conditions can easily be described, i.e. the sun condition and the shade condition. With respect to full sunlight, the shade condition is characterized by low irradiance, but is also enriched in the far red spectral range, because most light has already been absorbed by the upper canopy, especially the red component. Between these two extremes, there is a gradient of light regimes across the plant consortium. Fluctuations in irradiance (clouds, leaf movements) increase the complexity of description of the light regime to which plants are exposed. A series of processes occur in plants to ensure photosynthesis and prevent photodamage, leading to photoacclimation. Clearly, the problems linked to light energy management depend on the position of a leaf/ plant in the consortium. Different mechanisms have been described at the molecular level to explain the functional flexibility of plants with respect to light availability. Some of them occur after exposure of a plant to stable light regimes for hours to days (long term responses, LTR) and are due to modulations of gene expression. The thylakoid membrane hosts in fact two photochemical centres, i.e. photosystem I (PSI) and II (PSII), working in series and served each by a complex for light harvesting (LHCI and LHCII).
Last date updated on June, 2014