Phenotypic plasticity of bud set process in a P. nigra full-sib family.

Riassunto: Phenological process of bud set is of paramount importance for local adaptation and permits the trees of the temperate zones to survive cold winters. Even if photoperiod is widely accepted to be the main environmental signal for bud set in poplars, the timing of bud formation is also influenced by other factors such as temperature, nutrition and drought. The characterization of growth cessation in different European populations is an important tool for the identification and selection of candidate genotypes for conservation strategies and environmental restoration. Fourteen european native metapopulations, spanning 12° of latitude, were sampled in 4 different countries and about 820 genotypes were planted in a common garden study in Northern Italy (Savigliano; 44°36'N, 07°37'E). A randomized block design was defined for the establishment of the experimental plantation and six complete blocks were used with one replicated genotype randomly assigned to each block. The phenological study has been realized on the basis of a protocol designed to monitor the six phenological phases of bud set in black poplar. The innovative approach offers the possibility of dissecting the process into phases, durations and subprocesses to characterize their relative contributions. Data showed that night length is the crucial factor in the onset of growth cessation in poplar as shown by the strong latitudinal cline (1.5 days/lat of difference between accessions). The different phases taken into account are characterized by the higher genetic contribution in the total phenotypic variance and it decreases during the process (from the onset of growth cessation to the completely formed bud). Duration of traits appears to be highly influenced by temperature, especially the duration of bud formation (subprocess 1), which might be particularly important in the adaptation to its short term fluctuations. Bud set resulted as a constant process from the onset of growth cessation to the end of bud maturation. Subprocess 1 differs only by a few days among the different populations with the northern genotypes completing this duration stage slightly earlier than the southern ones.