PHOTOSYNTHESIS IN COLD-ACCLIMATED LEAVES OF PLANTS WITH VARIOUS DEGREES OF FREEZING TOLERANCE

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire differe nt degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and - after dete rmination of their frost resistance (LT(50)) - the net photosynthetic rate at an ambient CO2 of 33 Pa (P(n)33), the dependencies of P-n to l ight and to CO2 and the room temperature chlorophyll N. fluorescence w ere recorded under optimal conditions. Herbaceous plants acquired free zing tolerances to temperatures between -10 and -15 degrees C when har dened at temperatures around 0 degrees C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence afte r frost hardening. In non-senescing leaves P(n)33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around -10 degrees C Brassica survived down to -24 degree s C, but P(n)33 was almost abolished as a result of disturbances in th e chloroplasts. After transferring the plants to 20/15 degrees C P(n)3 3 recovered completely within a few days. Woody plants hardened at tem peratures around 0 degrees C tolerated -15 to -36 degrees C: P(n)33 wa s reduced by 25 to 60% and hardly recovered at 20/15 degrees C. Harden ing at -10 degrees C induced a tolerance of -32 to < -80 degrees C. P( n)33 was almost totally blocked, but at 20/15 degrees C it returned to the values of the plants hardened at 0 degrees C within a few days. I n woody plants disturbances were invariably localized in the chloropla sts. Thus, conifers, and especially Pinus cembra, can survive much low er temperatures than herbaceous plants and, at the same level of freez ing tolerance, exhibit appreciably less restriction in relative P(n)33 .