Frost-resistance and lipid composition of cold-hardened needles of Chilean conifers

Seasonal changes in the frost-resistance (LT50) of needles of three species of Chilean Podocarpaceae were studied. At the time of the greatest (winter) and lowest (summer) frost-resistance the lipid composition (phospholipids, galactolipids and fatty acids) of the needles was determined. Total polar lipid content was generally increased at the time of full hardiness, phospho- or galactolipids being involved in this increase. Most, but not all of the individual polar lipids fractions was enriched in linolenic acid in winter. The correlation between the LT50 of the needles and their content of linolenic acid was high (r = -0.77, P < 0.01). The unsaturation ratio of fatty acids was frequently associated with higher frost resistance in Podocarpus saligna and in P. nubigena. In the most resistant Saxegothaea conspicua only the phosphatidylcholine fraction showed a higher degree of unsaturation at this time. Artificial hardening (0 degrees for 12 or 24 hr) generally caused an increase of this ratio in this species. The effect of artificial hardening on the degree of frost resistance and on lipid accumulation was more efficient in summer than in winter. LT50 ranges in winter (-4.3 to -9.0 degrees) coincide with those reported for conifers and angiosperms from the Southern Hemisphere but not with those from the Northern Hemisphere. This supports the hypothesis that very frost-hardy trees that withstand temperatures lower than -20 degrees in the Northern Hemisphere seem not to have evolved in the Southern Hemisphere, because the mild oceanic winters did not provide the stimulus. The preferential accumulation of linolenic acid in all the lipid classes in winter seems to be a generalised mechanism of frost-resistance of extremely frost-hardy plants, while in the less hardy species of the Southern Hemisphere this occurred only in some lipid fractions.