Chlorophyll fluorescence analysis was conducted to evaluate why the frost-hardened winter rye is more resistant to photoinhibition of photosynthesis at 5 ®C than the non-hardened winter rye. Under a similar irradiance, the leaves of frost-hardened rye were able to keep a larger fraction of photosystem 2 reaction centres open than lhose of non-hardened rye. This ability of the frost-hardened rye fully accounts for its higher resistance to photoinhibition.