We studied the molecular genetics of dormancy-related traits in an F₂ family of poplar (Populus) hybrids derived from a cross between a northern genotype of black cottonwood and a southern genotype of eastern cottonwood by mapping quantitative trait loci (QTLs) and candidate genes. Dormancy-related traits included timing of vegetative bud set, fall frost damage, chilling response, timing of vegetative bud flush, and winter survival under field conditions, as well as photoperiodic responses (timing of bud set and number of new leaves) in a warm greenhouse under either a uniform 8-h photoperiod or a naturally shortening photoperiod in the fall. QTL analyses were conducted using a linkage map consisting of AFLP, microsatellite, and candidate gene markers. The candidate genes were chosen because of their potential roles in either photoperiodic perception (PHYB1, PHYB2) or abscisic acid signal transduction (ABI1B, ABI1D, ABI3). Significant QTLs were detected for all dormancy-related traits, except for winter survival, which had a relatively low heritability compared with the other traits. Interestingly, half of the field bud set QTLs did not map near photoperiodic QTLs. This is consistent with the moderate genetic correlation between these traits (0.53 to 0.60) and suggests that genetic differences in photoperiodic responses play only a modest role in explaining genetic differences in the timing of bud set under field conditions. Except for ABI1D, each of the candidate genes tested mapped near one or more of the dormancy-related QTLs. We conclude that molecular markers and QTL analyses can be used to study the genetics of dormancy-related traits, to design more effective breeding programs, and to provide new insights into tree physiology.

Nomenclature: Black cottonwood, Populus trichocarpa Torr. & Gray; eastern cottonwood, Populus deltoides Bartr. Ex Marsh.