We compared the sensitivity to cold stress, in terms of photosynthetic capacity and changes in chlorophyll fluorescence of photosystem 2 (PS2), of an evergreen and a deciduous oak species, which co-occur in the southeastern United States. We predicted that the evergreen species, Quercus virginiana, which must endure winter, is likely to have an inherently greater capacity for energy dissipation and to be less susceptible to chilling stress than the deciduous species, Quercus michauxii. Short-term cold stress in both species lead to greater than 50 % reduction in maximum photosynthetic rates, 60-70 % reduction in electron transport, and irreversible quenching of PS2 fluorescence. The kinetics of recovery in the dark after exposure to 1 h high irradiance (1000 µmol m-2 s-1) and chilling (5 °C) showed that the evergreen Q. virginiana exhibited more protective qE and less irreversible quenching (qI) than the deciduous Q. michauxii. The large qE which we observed in Q. virginiana suggests that the capacity for photoprotection at low temperatures is not induced by a long-term acclimation to cold but preexists in evergreen leaves. This capacity may contribute to the ability of this species to maintain leaves during the winter. and J. Cavender-Bares ... [et al.].
When a terrestňal plant is subjected to a mild drought stress, the leaf net CO2 uptake declines as a result of the stomatal closure. In contrast to previous beliefs the photosynthetic apparatus is vety resistant to dehydration. Particularly the relations between photochemistry, its regulation and leaf CO2 assimilation (f) are identical when F is changed either by increasing leaf water deficit or by decreasing the ambient CO2 concentration. When the stomata close in a leaf under water deficit the CO2 concentration within the leaf declines. As a result photorespiration is favoured, the photochemical yield of open photosystem 2 (PS 2) centres and the activity of some enzymes dechne (sucrose-phosphate synthase, nitráte reductase). The dechne of photochemical yield of PS 2 is consequential to an increase in thermal dissipation of the excitons trapped by PS 2 units. The dechne in CO2 concentration occurring in a desiccating leaf may trigger an integrated response of leaf metabohsm which still remains to be explored.