Copyright © Physiologia Plantarum 2001 PHYSIOLOGIA PLANTARUM 113: 499 – 506. 2001 Printed in Ireland —all rights resered ISSN 0031-9317 Susceptibility to low-temperature photoinhibition and the acquisition of freezing tolerance in winter and spring wheat: The role of growth temperature and irradiance Tessa H. Pocock a , Vaughan Hurry b , Leonid V. Savitch a,c and Norman P. A. Huner a, * a Department of Plant Sciences, The Uniersity of Western Ontario, London, Ontario, N6A 5B7, Canada b Umea ˚ Plant Sciences Centre, Department of Plant Physiology, Umea ˚ Uniersity, S -901 87 Umea ˚ , Sweden c Agriculture and Agri -Food Canada, Eastern Cereal and Oilseed Research Centre (ECORC), Ottawa, Ontario, K1A 0C6, Canada *Corresponding author, e -mail: nhuner@julian.uwo.ca Received 20 April 2001 temperature per se. The role of excitation pressure is discussed. Five winter and five spring wheat (Triticum aestium L.) We assessed the correlation between susceptibility to low-tem- cultivars were grown under either control conditions (20°C/250 perature photoinhibition, maximum ribulose 1,5-bisphosphate photosynthetic photon flux density (PPFD) [mol m -2 s -1 ]), high irradiance (20°C/800 PPFD) or at low temperature (either carboxylase-oxygenase (EC 4.1.1.39) and NADP-dependent 5°C/250 PPFD or 5°C/50 PPFD). To eliminate any potential malate dehydrogenase (EC 1.1.1.82) activities, chlorophyll and protein concentrations and freezing tolerance determined by bias, the wheat cultivars were arbitrarily chosen without any electrolyte leakage. Susceptibility to photoinhibition is the only previous knowledge of their freezing tolerance or photosyn- thetic competence. We show that the differential susceptibilities parameter examined that is strongly and negatively correlated with freezing tolerance. We suggest that the assessment of to photoinhibition exhibited between spring and winter wheat cultivars, as assessed by chlorophyll fluorescence cannot be susceptibility to photoinhibition may be a useful predictor of explained on the basis of either growth irradiance or low growth freezing tolerance and field survival of cereals. Under laboratory conditions, cold-hardy plants, including winter cereals, are typically grown under the controlled environmental conditions of low temperatures (0 – 5°C) and moderate irradiance (250 mol m -2 s -1 ) to cold-acclimate and subsequently attain freezing tolerance. However, high or even moderate-to-low irradiances in combination with low temperatures are conditions conducive to photoinhibi- tion (O  quist et al. 1993a). Photoinhibition is defined as the light-dependent reduction in photosynthetic efficiency which can be a consequence of either the irreversible light-induced inactivation of photosystem II (PSII) reaction centres or the reversible down-regulation of PSII through non-radiative dissipation of excess light (Osmond 1994, Hideg and Murata 1997). Chlorophyll a fluorescence parameters provide a sen- sitive, non-invasive probe to measure the extent of photoinhibition. Introduction Winter wheat (Triticum aestium L.) has the ability to maximize its freezing tolerance through the process of cold acclimation, allowing it to survive the winter in tem- perate climates. In contrast, spring wheat exhibits limited freezing tolerance even though their genomes contain the genes associated with freezing tolerance in cereals, such as the wheat wcs120 gene (Sarhan et al. 1997). Thus, a comparison of winter and spring wheat cultivars has provided a useful approach to elucidate the mechanisms of cold acclimation and freezing tolerance in cereals (Fowler and Carles 1979). However, the process of cold acclimation in cereals is complex and is not only dependent upon growth temperature and the developmental stage of the plant prior to the freezing event but is also dependent upon growth irradiance (Dexter et al. 1933, Gray et al. 1997). Abbreiations – Fv/Fm, maximum PSII photochemical efficiency; k, growth coefficient; NADP-MDH, NADP-dependent malate dehydroge- nase; PPFD, photosynthetic photon flux density (mol m -2 s -1 ); PSII, photosystem II; Rubisco, ribulose 1,5-bisphosphate carboxylase-oxy- genase; T EL50 , lethal temperature at which 50% of electrolytes are leached out of leaf segments. Physiol. Plant. 113, 2001 499