THE RELATIONSHIP BETWEEN WATER-CONTENT AND FROST TOLERANCE IN SHOOTS OF HARDWOOD SEEDLINGS

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 (Betula alleghaniensis 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 (Quercus rubra L.), bur oak (Quercus macrocarpa 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 degrees C for SD versus -15.7 degrees 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 degrees 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 degrees C or lower