The effect of different growth temperatures, of transfer between them, and of changes in outside temperature on frost hardiness and ATPase activity of Scotch pines (Pinus silvestris L.) were studied. Plants which had been grown in the greenhouse and transported to outside “remembered” something of their earlier growing circumstances during a long period. They differed from the plants continuously grown outside in the level of hardiness, although changes in temperature caused similar fluctuations in the hardiness of both groups. It is evident that two mechanisms are present, the one causing a long term determination of the level of frost hardiness obtainable, the other causing a short term adaption. The heavy particle ATPase activity as investigated below 20°C grouped itself according to the main temperature groups of the experiment. Above 30°C, the pretreatments had given rise to differences within the temperature groups. The level of the ATPase activity increased as the growing season came closer.

Changes in lipid and fatty acid composition of pine needle chloroplasts were determined upon transfer of the trees from outside (‐5°C) to 32°C. Within 7 1/2 hours after transfer, conversion of a portion of digalactosyldiglyceride into monogalactosyl diglyceride was observed. This portion consisted of a conjugate with extra long chain fatty acids (cerotic acid, 26: 0, and C26‐cyclo‐propane fatty acid). Only minor further changes in lipid and fatty acid composition were observed. Dehardening of the needles occurred within the same period. The data support the hypothesis that the degalactosidation reaction specifically depends on the fatty acid composition. The extra long chain fatty acids may link different sub‐units of the chloroplasts and may contribute to an additional layer of H‐bonded water on the membrane surface because of the increased exposure of the galactose groups.

Phospholipid (PL) and fatty acid composition of chloroplasts of pine needles (Pinus sylvestris L.) and apple bark tissue (Malus sylvestris Mill. cv. Golden Delicious) was determined in a series of experiments in which growth temperature and daylength were changed. Trees were exposed to 0 and 20°C and to daylength conditions of 9 and 14 h. All 16 possible combinations were investigated by transfer of the trees from the original condition to each of the other conditions. There was no direct relation between cold hardiness and PL composition in apple bark and pine chloroplasts, when temperature and/or daylength were changed. PL composition seemed to be strongly determined by the sequence of the imposed sets of daylength and temperature. The effect of these environmental factors on PL composition strongly differed from that for cold hardiness. The correlation between the levels of PL (and phosphatidylcholine) and cold hardiness, as reported in the literature, was also evident in this experiment, when treatments, presenting the normal seasonal order, were compared. It seems that the yearly cycle of temperature and daylength is important in determining the PL composition of apple bark and pine chloroplasts.

Cold hardiness of pine needles and apple bark was determined in a series of experiments. Plants were exposed to 0°C and 20°C and exposed to day length conditions of 9 and 14 h. All 16 possible combinations were investigated by transfer of the plants from the original condition to each of the other conditions. In order to interpret the results, it was necessary to distinguish between a single and a dual environmental factor change. In pine plants at 20°C, a change in two factors decreased cold hardiness contrary to a single change in temperature or day length. In plants at 0°C, the effects of a change in day length and temperature were additive. Differences in pine and apple are discussed in relation to natural conditions and to geographical distribution. It is suggested that for increased hardiness shortening of day length should precede a low temperature regime and that reversal of this order may upset the signal system, resulting in dehardening.