Photosynthesis in frost‐hardened and frost‐stressed leaves of Hedera helix L.

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Abstract
The purpose of this study was to determine the respective extents to which winter reduction of photosynthetic capacity in ivy (Hedera helix L.) is caused by direct frost injury to the photosynthetic apparatus and by preceding protoplasmic changes connected with the acquisition of frost tolerance. Potted juvenile ivy plants were placed in the open under natural weather conditions whilst others were hardened under controlled conditions and subjected to the desired frost stress.Low non‐freezing temperatures induced frost tolerance in ivy leaves down to about – 12°C (50% injury = TL50) without impairing net photosynthetic rate as measured under standard conditions (20°C, light saturation, natural CO2 level; = Standard‐Fn. Only if the leaves froze (below − 3°C to −4°C) was a reversible inhibition of Standard‐Fn observed. As long as the temperatures did not fall below approximately −8°C the inhibition was small and Standard‐Fn reached about 80–90% of the control. In this case the stomatal opening narrowed, giving a poorer supply of CO2 to the mesophyll cells. Maximal frost tolerance (TL5O−20°C to −24°C) developed only with severe frosts below about − 10°C. After such frosts, Standard‐Fn was reduced to less than 20% of the control. The dependence of the rate of net photosynthesis on the internal CO2 concentration showed a lower initial slope, thus indicating disturbances of chloroplast functions. However, neither in outdoor plants nor in those artificially frosted at – 20°C could there be found an appreciable inhibition of the electron transport capacity from H2O to dichlorophenol indophenol or of ribulose bisphosphate carboxylase.If intact, severely frosted ivy plants were then held at higher temperatures (20/15°C), Standard‐Fn recovered completely in approximately 10 d. Furthermore, following a frost period with temperatures down to −12°C, mild weather caused a distinct improvement in Standard‐Fn in outdoor plants, and there was no loss of maximum frost tolerance. Thus it can be concluded that the inhibition of Standard‐Fn after severe frosts is not due to the development of maximal frost tolerance, but rather may be attributed to frost damage to the photosynthetic apparatus.

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