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1 - A lot of sun, while metabolism is blocked by low temperatures increases the risk of phototoxicity.
2 - Risk of "sun burn": Snow melt in spring.
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Green plants need solar radiation, but at times, there may be too much of it. Radiation can be in excess of tolerable intensity
- when the photosynthetic machinery is not prepared to use any photons
- when plants are suddenly exposed to high radiation without enough time for acclimation
The worst case is the combination of the two, as can happen at sudden a release from snow cover.
Whenever leaf pigments absorb photons but have no use for them (e.g. at low temperatures, upper photograph) this energy needs to be dissipated without harming the whole machinery (e.g. by re-emitting the energy in the form of fluorescence or warmth), otherwise phototoxicity does occur. Mountain plants are perfectly equipped for such situations and therefore photodamage by visible light is a very rare phenomenon. In order to avoid such situations, rapidly emerging plants (as shown in the lower photograph) lack chlorophyll initially and are packed with protective compounds (see UV-B).
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