Changing environmental effects on frost hardiness of Scots pine during dehardening

The effects of raised temperature and extended photoperiod on the dehardening of quiescent and winter-hardy Scots pine saplings were examined 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 needles was determined by controlled freezing tests and visual damage scoring. The constant 6 degrees C temperature treatment caused a gradual dehardening 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 of shoot elongation, but before this there were no clear differences in frost hardiness between different photoperiodic treatments. The results indicate that the Best hardening competence of Scots pine changes during quiescence. (C) 1997 Annals of Botany Company.