The relationship between water content and frost tolerance in shoots of hardwood seedlings

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Abstract
Two experiments were designed to test the relationship between water content and frost tolerance in stems of hardwood seedlings during the frost hardening process. In the first experiment, 3-month-old container-grown yellow birch (Betulaalleghaniensis Britton) seedlings were submitted to a decreasing temperature regime in a 8-h (short day, SD) or 16-h (long day, LD) photoperiod in growth chambers. In the second experiment, 3-month-old container-grown red oak (Quercusrubra L.), bur oak (Quercusmacrocarpa Michx.), and yellow birch seedlings were naturally cold hardened in a production polyhouse. To estimate frost tolerance, freezing tests were performed on whole seedlings (experiment 1) or on the upper 5 cm of the stem apex (experiment 2), and stem damage was visually estimated. Water content of the apex (5 cm) was expressed as the ratio of dry mass over fresh mass (DM/FM) for both experiments. Frost tolerance and DM/FM increased during the course of both experiments. In the growth-chamber experiment, SD seedlings hardened deeper than LD ones (−19.2 °C for SD versus −15.7 °C for LD), and DM/FM was higher for SD after 60 days of photoperiod treatment. In the polyhouse experiment, bur oak, yellow birch, and red oak reached a frost tolerance of −26.7, −25.1, and −20.4 °C, respectively, on November 2. The three species had DM/FM ratios of about 50% on November 2. For both experiments, DM/FM values higher than 45% corresponded to seedlings with a frost tolerance of −10 °C or lower.

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