Phenotypic variation of bud set in a Populus nigra full-sib family (POP5). POP5 grown in two experimental sites in Italy: Cavallermaggiore (CV) and Viterbo (VT). Genotype means for the progenies in CV (a) and VT (b) are plotted in grey lines. Full-sib family mean is indicated by red lines, the female parent '58-861' by blue lines and the male parent 'Poli' by a green line. Horizontal dashed lines indicate the two critical points of bud set process, which correspond to the onset of growth cessation (date2.5) and the transition from shoot to bud (date1.5). (c) Phenotypic variation for all phenological stages, and (d) subprocesses observed in CV (white boxes) and VT (grey boxes) using block-adjusted genotype means expressed as cumulative night length (CNL). The parent '58-861' is indicated by blue "+", and the parent 'Poli' by green "*". The significance level of differences between the two parents for each trait is indicated as: ns, non-significant; *, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001.