Aija Ryyppö

The relation between growth cessation and frost hardening in Scots pines of different origins

The cessation of shoot elongation, diameter growth and needle elongation were compared with the initiation of frost hardening of the stems and needles in an 8-year-old provenance trial of Scots pine (Pinus sylvestris L.) established in central Finland. The saplings were of six different origins ranging from Estonia to northern Finland, forming a latitudinal gradient of ca. 10°N. The frost hardiness of the stems of current-year shoots was assessed by electrical impedance analysis and that of current-year needles by electrolyte leakage and visual scoring of damage. Artificial freezing tests were used in the assessments. The pattern of growth cessation (shoot and needle elongation, diameter growth) tended to follow the latitude of origin, i.e. growth ceased in the northernmost provenance first and in the southernmost one last. Both stems and needles of the northern provenances hardened earlier than the southern ones, but the differences in hardiness disappeared as hardening progressed. Growth cessation and initial hardening to –15°C were clearly correlated at the provenance level, indicating that growth must cease prior to hardening, and that earlier cessation of growth predicts earlier frost hardening of stems and needles. No differences in frost hardiness of stems were found at the provenance level at the end of the growing period in August. At that time, the frost hardiness of needles of the northernmost provenance was higher than that of other origins. Within the provenance, the stems were less hardy than the needles.

Cold Hardiness of Scots Pine ( Pinus sylvestris L.)

The natural range of Scots pine (Pinus sylvestris L.) is the widest among the pine species (Mirov 1967). It is found at latitudes from 70°N in northern Scandinavia to about 40°N in the south, in Turkey and Spain, and at longitudes from 10°W in the west, with a small population in Scotland (6°W) to 150°E in the east in Russia (Figure 1). At the eastern end of its range the northern distribution limit shifts south, reaching about 65°N at the Ural mountains. Scots pine typically grows at low altitudes, even close to sea level in western Europe, but the altitude of the growing site increases in more southerly locations. In north-eastern Europe and on the west-Siberian Lowlands the northern limit occasionally follows the southern limit of the permafrost zone (Sokolov et al. 1977). At the southern limit of its distribution in the Caucasus mountains in Turkey Scots pine grows at elevations above 800 m, even as high as 2500 m (Rubner 1960; Sarvas 1964; Boratynski 1991).

The electrolyte leakage method can be misleading for assessing the frost hardiness of roots

Abstract The relative electrolyte leakage (REL) method is often used to assess the frost hardiness of plant organs. However, testing the frost hardiness of roots with REL yields different results when the roots are frozen while remaining in the soil or in some other substrate than when REL follows a freezing test made on excised root segments. In this study, we show that the electrolytes may already leak out of injured roots during freeze/thaw treatment and during washing of the roots after the treatment. In this case, the conductivities measured after the freezing test and killing of the tissue both give lower readings than expected for injured samples, and hence the results of the REL are incorrect. We suggest that roots should be washed free of soil and placed in test tubes prior to the freezing test. The leakage of electrolytes from injured roots to soil may also limit the use of the REL method for measuring root viability.