Quantitative genetics of bud phenology, frost damage, and winter survival in an F2 family of hybrid poplars

We studied the quantitative genetics of bud phenology, fall frost damage, and winter survival in an F-2 family (no. 822) of Populus hybrids derived from a cross between two full-sub F-1 hybrids (P. trichocarpa (Torr. gr GrayxP. deltoides Bartr.). Field traits studied included the timing of bud set (BSF) in Minnesota and Oregon, the timing of bud flush (BFF) in Oregon, as well as fall frost damage (FDF) and winter survival (WSF) in Minnesota. We conclude that Family 822 has substantial genetic variability for all field traits, BSF and BFF are under moderate to strong genetic control (H-i(2)=0.38-0.80), FDF and WSF are under low to moderate genetic control (H-i(2)=0.27-0.40), and late bud set is associated with increased frost damage and decreased winter survival. In a warm greenhouse, we measured the timing of bud set and the number of new leaves on trees growing under either an 8-h photoperiod (BSSD and NLSD) or a natural photoperiod (NP) from August to December (BSNP and NLNP). We found that BSSD, NLSD, and NLNP are under moderate genetic control (H-i(2)=0.53-0.70), but the heritability of BSNP could not be determined because few trees set bud in the warm greenhouse under the NP. By comparing results from the greenhouse experiments with results from the field, we conclude that the genetic correlation between BSSD and BSF (0.53-0.60) is relatively modest and that NPs in the fall are relatively ineffective at promoting bud set under warm greenhouse temperatures, although bud set readily occurred in the field. Although, low levels of light pollution in the greenhouse might have affected BSNP, results from both greenhouse and field experiments suggest that genetic differences in photoperiodic responses play a modest role in explaining genetic differences in the timing of bud set under natural field conditions. Therefore, genetic differences in responses to other environmental factors, such as temperature, deserve greater attention.