Photosynthesis in cold acclimated leaves of plants with various degrees of freezing tolerance

Abstract

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire different degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and – after determination of their frost resistance (LT50) – the net photosynthetic rate at an ambient CO2 of 33 Pa (Pn33), the dependencies of Pn to light and to CO2 and the room temperature chlorophyll a fluorescence were recorded under optimal conditions.Herbaceous plants acquired freezing tolerances to temperatures between −10 and −15°C when hardened at temperatures around 0°C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence after frost hardening. In non‐senescing leaves Pn33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around −10°C Brassica survived down to −24°C, but Pn33 was almost abolished as a result of disturbances in the chloroplasts. After transferring the plants to 20/15°C Pn33 recovered completely within a few days. Woody plants hardened at temperatures around 0°C tolerated – 15 to −36°C: Pn33 was reduced by 25 to 60% and hardly recovered at 20/15°C. Hardening at −10°C induced a tolerance of −32 to <−80°C. Pn33 was almost totally blocked, but at 20/15°C it returned to the values of the plants hardened at 0°C within a few days. In woody plants disturbances were invariably localized in the chloroplasts.Thus, conifers, and especially Pinus cembra, can survive much lower temperatures than herbaceous plants and, at the same level of freezing tolerance, exhibit appreciably less restriction in relative Pn33.