Interaction pattern (i.e. the genotype-specific environment-phenotype function) for the onset of growth cessation (date2.5). Data were recorded as day of the year (DOY) in a full-sib Populus nigra family (POP5) grown in different sites in Italy: Cavallermaggiore (CV), Viterbo (VT) and Savigliano (SAV). The red dots indicate the female parent '58-861' that is native to North of Italy. The green squares indicate the male parent 'Poli' that is native to South of Italy. The black open dots indicate the mean of the full-sib family. A set of seven genotypes from POP5 was grown and scored in SAV and the corresponding value is indicated by "*". Numbers above and below each line represent the effective night length (h) and the cumulative minimum temperature < 10°C (CMT10), respectively.