ABSTRACT
Carotenoids have long been associated with a role in the photoprotection of the photosynthetic apparatus. The three xanthophylls of the xanthophyll cycle, violaxanthin, antheraxanthin, and zeaxanthin, are the only carotenoids of the photosynthetic membrane that undergo very rapid and reversible changes in their concentration in response to light stress. Adverse effects on the photosynthetic apparatus are thought to arise when photosystem II (PSII) becomes highly reduced. The increase in the reduction state of PSII that results from inhibition of the energy dissipation process in the pigment bed leads to such adverse effects on the photosynthesis process. Zeaxanthin is involved in the dissipation of excess excitation energy in the pigment bed of the photosynthetic membranes of all higher plants. A comparison of sun and shade leaves reveals that sun leaves possess a greater capacity not only to utilize light through photosynthesis, but also to dissipate any excess excitation energy directly within the pigment bed.
