Der Einfluß der Photoperiode auf das Austreiben, die Frosthärte und die Hitzeresistenz von Zirben und Alpenrosen

Abstract

From September, 1965 to January, 1967, phenological observations of new growth initiation and measurements of frost and heat resistance of Pinus cembra and Rhododendron ferrugineum were carried out monthly. Samples were taken from plants growing near timberline (on the Patscherkofel Mountain near Innsbruck, Austria at 2000 m. elev.) and from plants which were growing in a greenhouse with a constant temperature of 15 °C with the natural photoperiod or under various constant daylengths (8, 12 or 16 hr). 1. The plants in the greenhouse initiated new growth much earlier than the plants in nature, especially under long day conditions. Short day or 12-hour photoperiods inhibited the early initiation of new growth rather strongly. Also, under the natural photoperiod in the greenhouse, not all the buds broke, early; this was particularly true for P. cembra . These plants apparently were exhibiting a vernalization effect (Table 1). 2. The frost resistance (the lowest temperature at which the leaves or needles could withstand a 3-hour exposure without evident damage) changed considerably during the course of the year, particularly for plants in nature. The maximum resistance values in winter were − 43 °0 ( P. cembra tree), − 40 °C (P. cembra seedling), and −18 °C (R. ferruginum ). The minimum values in summer were − 6 °C, − 4 °C, and − 4 °C, respectively. The annual variation in frost resistance was reasonably synchronous with the prevailing temperature but even more closely correlated with the changing photoperiod during the year. The dominant influence of photoperiod on the variation in frost resistance for both species was evident from the fact that plants in the greenhouse with constant temperature behaved almost the same as plants in nature when under natural light conditions. However, when a constant photoperiod was applied, the annual variation in frost resistance resistance was greatly damped. Under a continuously long day photoperiod, the frost resistance dropped and remained at a low value. Under a short day regime, the plants achieved and maintained a high resistance against low temperatures throughout the course of these experiments. In the second year of the experiments, all annual variation in frost resistance essentially disappeared. This suggests that the endogenous rhythm, which together with temperature and photoperiod controlled the annual variation in frost resistance, has slowly faded out during the course of this first year under constant temperature and photoperiod. After the endogenous rhythm is dissipated because it is no longer regulated by environmental conditions, the frost resistance variation also deviates from this natural rhythm; it then oscillates more quickly (especially for P. cembra under an 8-hour day length). 3. The heat resistance (the highest temperature at which the leaves or needles could withstand a 30-minute exposure without apparent damage) also displayed an annual variation whose amplitnde, however, was much smaller than that of the variation in frost resistance. The amplitude was 6 °C ( P. cembra trees), 7 °C ( P. cembra seedlings) and 8 °C ( Rhododendron ferrugineum ) for these plants in nature. This variation in heat resistance closely paralleled that of frost resistance; i. e., the maximum and also the minimum values of frost and heat resistance occurred at the same time, An increase in heat resistance in summer was not demonstrable for these two species; apparently the natural habitat of these two species is too cool. Plants in the greenhause also exhibited an annual variation in heat resistance. Although, when p, cembra was placed under short day conditions, the heat resistance remained high, the effect of long days or a light interruption of the night did not seem to decrease the heat I esistance of this species, Rhododendron ferrugineum , on the other hand, reacted more clearly to long day conditions; the heat resistance was permanently decreased. Under natural day lengths or with short day conditions, the heat resistance variation was almost as pronounced as in nature, even during the second year. Therefore, it appears that the endogenous rhythm for the control of heat resistance is of greater significance than that for the control of frost resistance. This basic difference suggests that the internal processes and conditions of these two types of temperature resistance cannot be exactly the same. 4. These investigations show clearly for P. cembra and Rhododendron ferrugineum that not only temperature and endogenous rhythms are important in determining the annual course of variation in temperature resistance, but that photoperiod also plays a very significant role.