|MadSci Network: Biochemistry|
Visible light does cause chemical damage to plants under certain conditions. There is the potential for damage any time the light energy absorbed by the photosynthetic apparatus exceeds the amount that can be used for photosynthesis. In part this damage is due to photooxidative damage, apparently initiated by the superoxide anion radical and singlet oxygen. These can produce oxidative damage to the photosynthetic pigments such as chlorophyll. The level of light necessary to cause damage will depend upon the previous level of light to which the leaf was adapted. A plant that is adapted to relatively shaded conditions may experience damage when exposed to full sunlight. The amount of damage may also be affected by other environmental stresses such as temperature (both low and high temperature stress), water stress, etc. In addition, plants have evolved means to protect themselves from this excess absorbed visible light. Antioxidants such as reduced ascorbate and reduced glutathione help prevent damage by superoxide, while the singlet state of oxygen is de-excited by carotenoid pigments. The xanthophyll cycle, the cyclic interconversion of three oxygenated carotenoid pigments is used to convert excess absorbed visible light into thermal energy that can be radiated away by the leaf. A good review of this topic may be found in the Annual Review of Plant Physiology and Plant Molecular Biology, vol 43, pages 599-626 (1992), by Demmig-Adams and Adams.
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