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The responses of photosystem II (PSII) activity of samara and leaf of Siberian elm (Ulmus pumila L.) to chilling (5°C) and freezing (-5°C & -15°C) temperature and their recovery were investigated. There was little difference in Fv/Fm between samara and leaf. Leaf showed more efficient photosynthesis (PIABS) and electron transport (fEo & ?o) than samara. Chilling and freezing had adverse impact on PSII function in both samara and leaf. Low temperature stress decreased photochemical efficiency (Fv/Fm & PIABS) and electron transport activity (?o, fEo & ETo/RC) in both leaf and samara. PSII of samara was more tolerant to low temperature stress than leaf. Photochemical efficiency (Fv/Fm) for chilled (at 5°C) or mild frozen (at - 5°C) samara or leaf showed a recovery trend. Severe freezing stress (-15°C) led to drastic and irreversible injuries to PSII in samara and leaf. The fact that samara has photosynthetic activity and is more tolerant to low temperature than leaf is of great ecological significance for seed development, population establishment and the northern distribution limit of Siberian elm in northern hemisphere. [ABSTRACT FROM AUTHOR]