Frost hardening of Pinus radiata seedlings: effects of temperature on relative growth rate, carbon balance and carbohydrate concentration.

Pinus radiata (D. Don) seedlings were grown for 100 days at day/night temperatures of 10/1, 15/1, 20/1 and 25/1 °C, to determine whether temperatures above a threshold of 5 °C influence frost hardiness development. Relationships between hardening and relative growth rate, carbohydrate concentration and net carbon balance were also investigated. Seedlings hardened at a nearly constant rate in each treatment, although the rate of hardening was strongly temperature dependent. It increased as the temperature declined, but in a curvilinear fashion. Temperatures below 9.5 °C were effective in hardening the seedlings. During the daily temperature cycle, dehardening occurred at temperatures above the threshold, whereas hardening occurred at temperatures below the threshold. The net difference between the two processes determined the development of frost hardiness. The development of frost hardiness was negatively correlated with relative growth rate and positively correlated with the accumulation of starch and sugars. We conclude that frost hardening is a complex process that is causally linked to carbohydrate concentrations.