Ke. Burr et al., HEAT TOLERANCE, COLD-HARDINESS, AND BUD DORMANCY RELATIONSHIPS IN SEEDLINGS OF SELECTED CONIFERS, Journal of the American Society for Horticultural Science, 118(6), 1993, pp. 840-844
Greenhouse-cultured, container-grown seedlings of interior Douglas fir
[Pseudotsuga menziesii var. glauca (Beissn.) France], Engelmann spruc
e [Picea engelmannii (Parry) Engelm.], and ponderosa pine (Pinus ponde
rosa var. scopulorum Engelm.) were acclimated and deacclimated to cold
in growth chambers over 19 weeks. Heat tolerance and cola hardiness o
f needles, and bud dormancy, were measured weekly. Heat tolerance of D
ouglas fir and Engelmann spruce needles increased with development thr
ough the first complete annual cycle: new needles on actively growing
plants; mature needles, not cold-hardy, on dormant plants; cold-hardy
needles on dormant and quiescent plants; and mature, needles, not cold
-hardy, on actively growing plants. Heat tolerance of ponderosa pine n
eedles differed in two respects. New needles had an intermediate toler
ance level to heat, and fully cold-hardy needles were the least tolera
nt. Thus, the physiological changes that conferred cold hardiness were
not associated with greater heat tolerance in all the conifers tested
. In none of these species did the timing of changes in heat tolerance
coincide consistently with changes in cold hardiness or bud dormancy.