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 spruce (Picea engelmannii (Parry) Engelm.), and ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) were acclimated and deacclimated to cold in growth chambers over 19 weeks. Heat tolerance and cold hardiness of needles, and bud dormancy, were measured weekly. Heat tolerance of Douglas fir and Engelmann spruce needles increased with development through 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 needles differed in two respects. New needles had an intermediate tolerance level to heat, and fully cold-hardy needles were the least tolerant. 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. The heat tolerance limits of actively growing seedlings of various temperate-zone conifers are between 45 and 60C (primary, direct heat injury; Levitt, 1980), depending on duration of expo- sure (Helgerson, 1990; Seidel, 1986; Shirley, 1936). Physiological changes that confer cold hardiness result in greater heat tolerance in certain species due to thermal hardening at high and low temperatures (Alexandrov, 1964; Levitt, 1980), and many conifers are thought to have greater heat tolerance when dormant (Jameson, 1961; Kayll, 1968). However, it is not clear when changes in heat tolerance occur in relation to other seasonally changing physi- ological attributes, because many of the data have been derived from infrequent sampling (Koppenaal and Colombo, 1988). If heat tolerance were correlated with changes in dormancy or cold hardiness, then the available data on these two characteristics could provide information about changes in heat tolerance, and a prediction of heat tolerance could be incorporated into decisions regarding nursery production, outplanting practices (Binder and Fielder, 1988; de Keijzer and Hermann, 1966), and the selection of genotypes for particular landscape sites (Pair, 1987; Peck and Wallner, 1982).