CHANGING ENVIRONMENTAL-EFFECTS ON FROST HARDINESS OF SCOTS PINE DURING DEHARDENING

The effects of raised temperature and extended photoperiod on the deha rdening of quiescent and winter-hardy Scots pine saplings were examine d in an open-top-chamber experiment. The saplings were exposed during winter to natural, square-curve fluctuating (between 1 and 11 degrees C with a 14 d interval), and constant (6 degrees C) temperatures with a natural and an extended (17 h) photoperiod. Frost hardiness of needl es was determined by controlled freezing tests and visual damage scori ng. The constant 6 degrees C temperature treatment caused a gradual de hardening of needles whereas under fluctuating temperatures the level of frost hardiness fluctuated. Trees exposed to extended photoperiods were less hardy than under natural photoperiods after the initiation o f shoot elongation, but before this there were no clear differences in frost hardiness between different photoperiodic treatments. The resul ts indicate that the Best hardening competence of Scots pine changes d uring quiescence. (C) 1997 Annals of Botany Company.