Re: Does visible light cause any chemical damage to plants?

     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.