Low temperature and hardening effects on photosynthetic apparatus efficiency and survival of forage grass varieties

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Freezing tolerance is essential for perennial plants and ability to adapt to extreme temperature is crucial for their survival in many environments. Freezing tolerance of hardened and unhardened plants of Dactylis glomerata and Lolium perenne varieties was probed by their quantum photosynthetic efficiency using the chlorophyll fluorescence technique. Quantum yield of photosystem II (PSII) electron transport (Φ<sub>PSII</sub>), maximal (F<sub>m</sub>’) and steady-state (F<sub>s</sub>) chlorophyll fluorescence yields of light-adapted samples were measured. Φ<sub>PSII</sub> depended on developmental phase, temperature and hardening process. A clear decline in PSII activity, especially after –10°C application was observed. Plant hardening during emergence phase had a positive impact on PSII activity, especially after –5°C application. After 72 h of –5°C temperature treatment, hardened plants showed quicker recovery of their photosynthetic apparatus (0.527–0.697) as compared to unhardened ones (0.224–0.330). Stress temperature of –10°C caused irreversible changes of photosynthetic apparatus of hardened and unhardened plants independently of growth phases (0.003–0.014). Φ<sub>PSII</sub> and F<sub>m</sub>’ parameters were strongly correlated with shoots survival under stress. Our results suggest that perennial plants’ hardening allows them to survive low temperatures due inter alia enhancing their photosynthetic machinery performance.