Zeaxanthin and the Heat Dissipation of Excess Light Energy in Nerium oleander Exposed to a Combination of High Light and Water Stress

Abstract

Upon termination of watering of plants of Nerium oleander exposed to high light, photochemical efficiency became reduced as leaf water content decreased. Evidence is presented that this type of photoinhibition reflects to a substantial degree radiationless dissipation of excitation energy, probably mediated by the carotenoid zeaxanthin. During the imposition of water stress, the zeaxanthin content of leaves increased at the expense of violaxanthin and a-carotene as a water deficit developed over a period of several days. The increase in zeaxanthin content was linearly related to an increase in the rate of radiationless energy dissipation in the antenna chlorophyll as calculated from the characteristics of chlorophyll a fluorescence measured with a pulse amplitude modulated fluorometer at room temperature. The increase in the rate of radiationless dissipation was also linearly related to a decrease in PSII photochemical efficiency as indicated by the ratio of variable to maximum fluorescence. Leaves of well-watered shade plants of N. oleander exposed to strong light showed a similar increase in zeaxanthin content as sun leaves of the same species subjected to drought in strong light. Shade leaves possessed the same capacity as sun leaves to form zeaxanthin at the expense of both violaxanthin and jcarotene. The resistance of this species to the destructive effects of excess light appears to be related to interconversions between 13-carotene and the three carotenoids of the xanthophyll cycle.Prolonged exposure of leaves to light levels where the excitation energy exceeds the capacity for orderly dissipation by the photosynthetic system can lead to photoinhibition. Photoinhibition is characterized by a sustained decrease in the efficiency of photon utilization by PSII photochemistry and is observed (a) when shade leaves are abruptly exposed to bright light which greatly exceeds the PFD2 experienced by the leaves during their development and (b) when sun leaves acclimated to natural sunlight are exposed to additional environmental stresses water deficit or unfavorably high or low temperatures (for a review see Ref. 22). Under these conditions, a given light level which previously was nonexcessive becomes excessive, because the utilization of energy through photosynthesis is decreased by the additional stress treatment. The mechanism by which the photon yield of photosynthesis is lowered in these two cases is probably not the same (3,8). Shade leaves exposed to bright light show evidence of damage to PSII reaction centers, whereas sun leaves exposed to a combination of high light levels and water stress (Nerium oleander) or salinity stress (various species of mangroves) do not. Rather, in these sun leaves, the decrease in photochemical efficiency of PSII can to a large extent be accounted for by a strong increase in thermal deactivation of excitation energy in the antenna Chl (3, 8). Excitation energy is thereby diverted away from the photosynthetic reaction centers and is no longer available for photochemis...

Stoichiometry of Photosystem I, Photosystem II, and Phycobilisomes in the Red Alga Porphyridium cruentum as a Function of Growth Irradiance

“…The lack of any correlation of the amount of zeaxanthin with Chl content, thylakoid area, or growth irradiance argues against an antenna function. It has recently been proposed that zeaxanthin is involved in the radiationless dissipation of excess light energy (7). Photoprotection may be its role in P. cruentum as well.…”

Seasonal changes in the photosynthetic performance of two evergreen Nothofagus species in south central Chile

“…When a combination of light and water stress occurs, Nerium oleander shows high dissipation of light energy by an accumulation of Z (Demmig et al 1988). However, in Lavandula stoechas growing under field conditions, the xanthophylls cycle is induced by light irrespective of the water status (Munné-Bosch & Alegre 2000).…”

Xanthophyll Cycle Pigment Localization and Dynamics during Exposure to Low Temperatures and Light Stress inVinca major1

“…Under various photoinhibitory conditions large quantities of the xanthophyll cycle pigments Z and A have been found to be retained in leaves for extended periods after darkening (Demmig et al, 1988;Adams et al, 1995;Verhoeven et al, 1996;Demmig-Adams et al, 1998). The xanthophyll cycle pigments Z and A are formed from V when light is excessive, and they are involved in a photoprotective process whereby excess absorbed excitation energy is dissipated thermally in the light-harvesting antennae of PSII (Demmig-Adams and Adams, 1996; Eskling et al, 1997; Gilmore, 1997).…”