XANTHOPHYLL CYCLE AND LIGHT STRESS IN NATURE - UNIFORM RESPONSE TO EXCESS DIRECT SUNLIGHT AMONG HIGHER-PLANT SPECIES

Photosystem II (PS II) efficiency, nonphotochemical fluorescence quenc hing, and xanthophyll cycle composition were determined in situ in the natural environment at midday in (i) a range of differently angled su n leaves of Euonymus kiautschovicus Loesener and (ii) in sun leaves of a wide range of different plant species, including trees, shrubs, and herbs. Very different degrees of light stress were experienced by the se leaves (i) in response to different levels of incident photon flux densities at similar photosynthetic capacities among Euonymus leaves a nd (ii) as a result of very different photosynthetic capacities among species at similar incident photon flux densities (that were equivalen t to full sunlight). For Euonymus as well as the interspecific compari son all data fell on one single, close relationship for changes in int rinsic PSII efficiency, nonphotochemical fluorescence quenching, or th e levels of zeaxanthin + antheraxanthin in leaves, respectively, as a function of the actual level of light stress. Thus, the same conversio n state of the xanthophyll cycle and the same level of energy dissipat ion were observed for a given degree of light stress independent of sp ecies or conditions causing the light stress. Since all increases in t hermal energy dissipation were associated with increases in the levels of zeaxanthin + antheraxanthin in these leaves, there was thus no ind ication of any form of xanthophyll cycle-independent energy dissipatio n in any of the twenty-four species or varieties of plants examined in their natural environment. It is also concluded that transient diurna l changes in intrinsic PSII efficiency in nature are caused by changes in the efficiency with which excitation energy is delivered from the antennae to PSII centers, and are thus likely to be purely photoprotec tive. Consequently, the possibility of quantifying the allocation of a bsorbed light into PSII photochemistry versus energy dissipation in th e antennae from changes in intrinsic PSII efficiency is explored.