Influence of temperature on bud flush phenology in Populus nigra : a comparison between two growing seasons

Plant phenology represents a serious ecological and evolutionary trade-off between survival and growth. Besides, it is responsive to global warming, especially spring bud flush that is predominantly controlled by temperature. The aim of this study was to improve the knowledge on the bud flush dynamics focusing on the influence of temperature on this process. A protocol was developed for black poplar (Populus nigra) to dissect the bud burst spring phenology in different stages, from the swelling of the buds to complete leaf formation, passing through three intermediate phases. A P. nigra full-sib family originating from a controlled cross between parental genotypes from contrasting climate was studied at the same location and in two different growing seasons. Cumulative average temperature above 10°C (CAT10) and cumulative minimum temperature below 5°C (CMT5) were used to describe phenotypic and genetic variance of the bud flush stages. Both growing seasons showed similar CAT10 values for the initial stage of bud flush, despite seven days of difference observed between the years. Southern parent was more sensible to temperature variation than the northern parent. Greater fluctuations in temperatures increased the extent of phenotypic variance among genotypes during the process than stable temperatures. Moderate to low values of heritability (H2<0.50) were observed for the different stages in the two years. The relative importance of year, genotype and genotype × year (G×Y) interaction effects on total phenotypic variation of bud flush stages was estimated. No significant G×Y interaction was observed. A large number of QTLs (28 and 22 on paternal and maternal maps, respectively) was found, corresponding to 9 genomic regions for each parent. QTLs were characterized by small or modest effect (average PVE=7.1%), without common genomic regions between parental maps. These results highlight the complex nature of the traits involved during bud flush and leaf elongation.