title: Temperature-dependent responses of the photosynthetic and chlorophyll fluorescence attributes of apple (Malus domestica) leaves during a sustained high temperature event.
abstract: The objective of this study was to follow changes in the temperature-dependent responses of photosynthesis and photosystem II performance in leaves of field-grown trees of Malus domestica (Borkh.) cv. 'Red Gala' before and after exposure to a long-term heat event occurring late in the growing season. Light-saturated photosynthesis was optimal at 25 °C before the heat event. The high temperatures caused a reduction in rates at low temperatures (15-20 °C) but increased rates at high temperatures (30-40 °C) and a shift in optimum to 30 °C. Rates at all temperatures increased after the heat event and the optimum shifted to 33 °C, indicative of some acclimation to the high temperatures occurring. Photosystem II attributes were all highly temperature-dependent. The operating quantum efficiency of PSII during the heat event declined, but mostly at high temperatures, partly because of decreased photochemical quenching but also from increased non-photochemical quenching. However, a further reduction in PSII operating efficiency occurred after the heat event subsided. Non-photochemical quenching had subsided, whereas photochemical quenching had increased in the post-heat event period and consistent with a greater fraction of open PSII reaction centres. What remained uncertain was why these effects on PSII performance appeared to have no effect on the process of light-saturated photosynthesis. However, the results provide an enhanced understanding of the impacts of sustained high temperatures on the photosynthetic process and its underlying reactions, notably photochemistry.
authors.affiliation: School of Agricultural and Wine Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW, Australia. Electronic address: dgreer@csu.edu.au.
authors.forename: Dennis H
authors.initials: DH
authors.lastname: Greer
mesh_terms: D000064:Acclimatization; D002734:Chlorophyll / Q000378:metabolism; D005453:Fluorescence; D006358:Hot Temperature; D008027:Light; D027845:Malus / Q000502:physiology* / Q000528:radiation effects; D010788:Photosynthesis; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D018515:Plant Leaves / Q000502:physiology / Q000528:radiation effects
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll
keywords: Chlorophyll fluorescence; Field measurements; Heat event; Photorespiration; Photosynthesis; Respiration; Temperature responses