PHOTOPERIODIC RESPONSES OF A NORTHERN AND SOUTHERN ECOTYPE OF BLACK COTTONWOOD

Photoperiod is an important signal controlling the onset of dormancy in perennial plants. Short days typically induce growth cessation, the initiation of cold acclimation, the formation of a terminal bud, bud dormancy and other adaptive responses. Photoperiodic ecotypes have evolved in many species with large latitudinal distributions. The photoperiodic responses of two northern (53 degrees 35' and 53 degrees 50'N) and two southern (34 degrees 10' and 40 degrees 32'N) genotypes of black cottonwood (Populus trichocarpa Ton. and Gray) were characterized by growing trees under a range of photoperiods in the greenhouse and growth chamber. Short days induced bud set in both ecotypes, resulting in trees with fewer leaves and less height growth than trees grown under long days. Short days also enhanced anthocyanin accumulation in the northern ecotype and decreased branching of the southernmost genotype. Two aspects of the photoperiodic response were evaluated for each trait; critical photoperiod, which was defined as the longest photoperiod that elicited a short-day response, and photoperiodic sensitivity, which was defined as the change in response per unit change in photoperiod. For each of the traits analyzed, the northern ecotype had a longer critical photoperiod and greater photoperiodic sensitivity than did the southern ecotype. The short critical photoperiod and reduced photoperiodic sensitivity of the southern ecotype resulted in a significant delay in bud set compared to that of the northern ecotype, even under a 9-h photoperiod. Typically, photoperiodic ecotypes have been characterized as having different critical photoperiods. Ecotypic differences in photoperiodic sensitivity, however, indicate that differences in the photoperiodic response curves cannot be completely described by the critical photoperiod alone. These results also suggest that the critical photoperiod, photoperiodic sensitivity and speed of bud set have a common physiological basis. Bud set occurred earlier in the northern ecotype primarily because bud scale leaves were initiated earlier. For one of the northern genotypes, leaf primordia that were initiated under long days subsequently differentiated into bud scale leaves after the trees were transferred to a 9-h photoperiod. This demonstrates that;primordia initiated under long days are not necessarily committed to becoming foliage leaves. The response to photoperiod did not differ appreciably between the greenhouse and growth chamber conditions that were tested.