CAROTENOIDS AND PHOTOSYSTEM-II CHARACTERISTICS OF UPPER AND LOWER HALVES OF LEAVES ACCLIMATED TO HIGH LIGHT

Acclimation of the leaves or stems of four succulent species to differ ent light environments and to the light gradient across high light-acc limated tissues was examined through measurements of chlorophyll fluor escence and characterisation of the pigment composition of the thylako id membranes. Whereas the total amounts of light striking the upper (s un-exposed) and lower (self-shaded) surfaces were quite different, res ulting in a much smaller pool of the xanthophyll cycle carotenoids in the lower halves of high light-acclimated tissues, the conversion stat e of the xanthophyll cycle (the degree to which violaxanthin is conver ted to antheraxanthin and zeaxanthin) was similar throughout the tissu es during exposure to natural sunlight. Under full sunlight, less than 25% of the light absorbed by the upper surface was utilised through p hotosynthesis, with the majority of the remaining excitation energy be ing dissipated thermally. In contrast, a considerably greater fraction of the light absorbed by the lower surface was utilised in photosynth esis, ranging from one-third to more than two-thirds of the total ener gy absorbed.