Frost Hardening Decreases Efficiency
The "strategy" of winter-green plants such as conifers for coping with frost is to promote the extracellular freezing of tissue water. Photosynthesis ceases when the formation of ice takes place due to the severe dehydration and low temperatures. About 30% of the chlorophyll-binding antenna proteins of the chloroplasts of Scots pine are degraded during the course of frost hardening (pines appear somewhat yellow-green in winter). This can be seen as a protection of the photosynthetic membranes against light stress during sharp frosts (Hansen 2000). It results in a considerable restriction of photosynthetic function in the non-frozen state, however. Experiments in this regard on the C02 assimilation of pine needles have shown an about 80% reduction in potential photosynthetic capacity (see Chap, 1.1.3, Fig. 1.1.7). This limitation of photosynthesis does not necessarily have negative consequences, however, as respiration is also considerably restricted at lower temperatures (Hansen and Beck 1994).
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