FROST HARDINESS GRADIENTS IN SHOOTS AND ROOTS OF PICEA-MARIANA SEEDLINGS

Frost hardiness of tissues along the length of the stem and the root was investigated in first-year black spruce (Picea mariana (Mill.) B.S.P,) seedlings. Frost hardiness of 1 cm long stem and root segments was evaluated based on Index of Injury, calculated from post-freezing electrolyte leakage. Frost hardiness was tested approximately weekly beginning seven weeks after seedlings were transferred from an 18 to a 10 h photoperiod, both at day/night temperatures of 26 degrees C/16 degrees C. Trees were transferred to temperatures of 10 degrees C day and 5 degrees C night at a 10 h photoperiod after a further 18 days. Frost hardiness was greater at the terminal bud and least at the root tips. Although shoots were generally more frost hardy than roots, differences in hardiness along the stem and root axes were gradual, rather than abruptly differing at the shoot-root interface. All tissues, including root tips, increased in frost hardiness after conditioning for 18 days under short photoperiods (10 h) and warm temperatures (26 degrees C/16 degrees C, day/night). Under cold temperatures (10 degrees C/5 degrees C, day/night) all tissues, excepting the root tips, tolerated -16 degrees C with little subsequent electrolyte leakage.