Biophysical changes in the roots of Scots pine seedlings during cold acclimation and after frost damage

Root system health is a key factor for seedling quality and it is a prerequisite for the proper growth of seedlings after out-planting. If the seedlings are moved to freezer storage too early in the season, roots can be damaged and thereby the quality of seedlings declines. We aimed to develop a novel method to assess an appropriate time for moving the seedlings to freezer storage in the autumn and to detect possible root damage during overwintering. One-year-old containerized Scots pine seedlings were measured before, during and after freezer storage, and after a series of frost exposure tests. The electrical impedance spectra (from 4 kHz to 200 kHz) of roots changed during cold acclimatization. The impedance loss factor (δ) of roots at 50 kHz frequency decreased when the threshold of frost tolerance was exceeded in the freezing tests. Root hydraulic conductance (Kr) increased before and during the freezer storage. In the initial phase of cold acclimatization, Kr increased considerably after exposure to frost temperatures, indicating damage, but that effect disappeared with frost hardening. In regrowth tests, the largest number of new root tips was observed after exposure to −12 °C in the freezing tests just before and after freezer storage, and after exposure to −30 °C during the freezer storage. During the freezer storage, shoot growth declined at higher exposure temperatures than root tip formation, suggesting that roots and specifically root tips would not be the primary reason for declined shoot growth. We conclude that the biophysical measurements of roots are useful for assessing the condition of the root system for overwintering applications in tree seedling nurseries, and that roots tolerated lower temperatures than previously thought.