Stress by extreme radiation

Pinus cembra
1 - Pinus cembra

Evergreen Pinus cembra has a hard time in late winter, when exposed to bright sun with zero photosynthetic activity due to cold temperatures. However, a suite of physiological-biochemical changes associated with winter dormancy protects plants. A visible indication of this adjustment is the yellow appearance of needles. Other species as for instance Larix decidua (larch, in the background) avoid such problems by shedding their leaves in autumn. With this strategy, they can grow in the coldest place on earth, in sub-polar western Siberia.

1 - A lot of sun, while metabolism is blocked by low temperatures increases the risk of phototoxicity (Central Alps).
Rumex alpinus
2 - Rumex alpinus emerging from snow in spring.
2 - Risk of "sun burn": Snow melt in spring (Central Alps).
Nototriche turritella
3 - Nototriche turritella, here at 4200 m in the Bolivian altiplano resists extremely intense solar radiation and 8 months without rain (total c. 300 mm a-1) by narrow, highly pubescent (reflective) leaves and a massive and deep tap root (picture taken during the rainy season in February).
3 - Combination of high radiation and drought (Bolivian Altiplano, 4200 m).

Green plants need solar radiation, but at times, there may be too much of it. Radiation can be in excess of tolerable intensity

  1. when the photosynthetic machinery is not prepared to use any photons
  2. 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).