PREMISE Boreal and northern temperate forest trees possess finely tuned mechanisms of dormancy, which match bud phenology with local seasonality. After winter dormancy, the accumulation of chilling degree days (CDD) required for rest completion before the accumulation of growing degree days (GDD) during quiescence is an important step in the transition to spring bud flush. While bud flush timing is known to be genetically variable within species, few studies have investigated variation among genotypes from different climates in response to variable chilling duration. METHODS We performed a controlled environment study using dormant cuttings from 10 genotypes of Populus balsamifera, representing a broad latitudinal gradient (43-58°N). We exposed cuttings to varying amounts of chilling (0-10 weeks) and monitored subsequent GDD to bud flush at a constant forcing temperature. RESULTS Chilling duration strongly accelerated bud flush timing, with increasing CDD resulting in fewer GDD to flush. Genotypic variation for bud flush was significant and stratified by latitude, with southern genotypes requiring more GDD to flush than northern genotypes. The latitudinal cline was pronounced under minimal chilling, whereas genotypic variation in GDD to bud flush converged as CDD increased. CONCLUSIONS We demonstrate that increased chilling lessens GDD to bud flush in a genotype-specific manner. Our results emphasize that latitudinal clines in bud flush reflect a critical genotype-by-environment interaction, whereby differences in bud flush between southern vs. northern genotypes depend on chilling. Our results suggest selection has shaped chilling requirements and depth of rest as an adaptive strategy to avoid precocious flush in climates with midwinter warming.

中文翻译：

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芽潮物候的纬度变化反映了香脂杨、香脂杨中低温需求的遗传变异

前提 北方和北温带林木具有精细调节的休眠机制，可将芽物候与当地季节性相匹配。冬季休眠后，休眠期在生长度日（GDD）积累之前完成休息所需的寒冷度日（CDD）的积累是向春芽冲洗过渡的重要步骤。虽然已知萌芽时间在物种内是遗传可变的，但很少有研究调查不同气候下基因型对可变冷却持续时间的变化。方法 我们使用来自 10 种代表宽纬度梯度 (43-58°N) 的杨树基因型的休眠插条进行了受控环境研究。我们将插条暴露于不同程度的冷却（0-10 周），并监测随后的 GDD 以在恒定的强制温度下发芽。结果 冷却持续时间大大加快了芽冲洗时间，随着 CDD 的增加，需要冲洗的 GDD 减少。芽冲洗的基因型变异是显着的，并按纬度分层，南方基因型比北方基因型需要更多的 GDD 来冲洗。纬度倾斜在最低限度的冷却下明显，而随着 CDD 的增加，GDD 到芽潮的基因型变异收敛。结论 我们证明，增加的冷却减少了 GDD 以基因型特异性方式出现的芽潮。我们的研究结果强调，芽潮中的纬度克隆反映了一种关键的基因型与环境的相互作用，即南部与南部之间芽潮的差异。北方基因型取决于寒冷。我们的结果表明，选择已将寒冷要求和休息深度塑造为一种适应性策略，以避免在仲冬变暖的气候中出现早熟。