Heat tolerance of actively growing, bud-initiated, and dormant black spruce seedlings

Abstract

To determine the effect of stage of bud development on heat tolerance, overwintered black spruce seedlings (Piceamariana (Mill.) B.S.P.) were (i) maintained in a dormant state, (ii) allowed to grow new shoots under favorable greenhouse conditions, or (iii) induced to initiate bud scales using short-photoperiod treatment following a period of new shoot elongation. Seedlings of the three shoot types were then exposed for 10 min to temperatures of 25, 42, 45, 50, or 55 °C in an environmental test chamber. Damage to the current-year shoot was lower in bud-initiated seedlings than in active seedlings, particularly at 42 °C, when 0 and 46%, respectively, of the length of the main shoot was damaged. The current-year shoot growth was much more sensitive to heat stress than the lignified first-year shoot, and therefore dormant seedlings, which had only first-year shoot growth, were most heat tolerant, showing damage only at 50 and 55 °C.