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As part of a program to select maritime pine ( Pinus pinaster  Ait.) genotypes for resistance to low winter temperatures, we examined variation in photosystem II activity by chlorophyll fluorescence. Populations and families within populations from contrasting climates were tested during two consecutive winters through two progeny trials, one located at a continental and xeric site and one at a mesic site with Atlantic influence. We also obtained the LT 50 , or the temperature that causes 50% damage, by controlled freezing and the subsequent analysis of chlorophyll fluorescence in needles and stems that were collected from populations at the continental trial site.    P. pinaster  showed sensitivity to winter stress at the continental site, during the colder winter. The combination of low temperatures, high solar irradiation and low precipitation caused sustained decreases in maximal photochemical efficiency ( F v  / F m  ), quantum yield of non-cyclic electron transport (Φ PSII ) and photochemical quenching ( qP ). The variation in photochemical parameters was larger among families than among populations, and population differences appeared only under the harshest conditions at the continental site. As expected, the environmental effects ( winter  and  site ) on the photochemical parameters were much larger than the genotypic effects ( population  or  family ). LT 50  was closely related to the minimum winter temperatures of the population's range. The dark-adapted  F v  / F m   ratio discriminated clearly between interior and coastal populations.  In conclusion, variations in  F v  / F m  , Φ PSII ,  qP  and non-photochemical quenching ( NPQ ) in response to winter stress were primarily due to the differences between the winter conditions and the sites and secondarily due to the differences among families and their interactions with the environment. Populations from continental climates showed higher frost tolerance (LT 50 ) than coastal populations that typically experience mild winters. Therefore, LT 50 , as estimated by  F v  / F m  , is a reliable indicator of frost tolerance among  P. pinaster  populations.

Intraspecific Variation in Pinus Pinaster PSII Photochemical Efficiency in Response to Winter Stress and Freezing Temperatures