HEAT TOLERANCE, COLD-HARDINESS, AND BUD DORMANCY RELATIONSHIPS IN SEEDLINGS OF SELECTED CONIFERS

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.