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Climate change is expected to increase drought frequency and intensity which will threaten plant growth and survival. In such fluctuating environments, perennial plants respond with hydraulic and biomass adjustments, resulting in either tolerant or avoidant strategies. Plants' response to stress relies on their phenotypic plasticity. The goal of this study was to explore physiology of young Populus nigra in the context of a time-limited and progressive water deficit in regard to their growth and stress response strategies. Fourteen French 1-year-old black poplar genotypes, geographically contrasted, were subjected to withholding water during 8 days until severe water stress. Water fluxes (i.e. leaf water potentials and stomatal conductance) were analyzed together with growth (i.e. radial and longitudinal branch growth, leaf senescence and leaf production). Phenotypic plasticity was calculated for each trait and response strategies to drought were deciphered for each genotype. Black poplar genotypes permanently were dealing with a continuum of adjusted water fluxes and growth between two extreme strategies, tolerance and avoidance. Branch growth, leaf number and leaf hydraulic potential traits had contrasted plasticities, allowing genotype characterization. The most tolerant genotype to water deficit, which maintained growth, had the lowest global phenotypic plasticity. Conversely, the most sensitive and avoidant genotype ceased growth until the season's end, had the highest plasticity level. All the remaining black poplar genotypes were close to avoidance with average levels of traits plasticity. These results underpinned the role of plasticity in black poplar response to drought and calls for its wider use into research on plants' responses to stress.

International audience; A global DNA methylation and proteomics approach was used to investigate somatic embryo maturation in hybrid larch. Each developmental step during somatic embryogenesis was associated with a distinct and significantly different global DNA methylation level: from 45.8% mC for undifferentiated somatic embryos (1-week proliferation) to 61.5% mC for immature somatic embryos (1-week maturation), while maturation was associated with a decrease in DNA methylation to 53.4% for mature cotyledonary somatic embryos (8-weeks maturation). The presence of 5-azacytidine (hypo-methylating agent) or hydroxyurea (hyper-methylating agent) in the maturation medium altered the global DNA methylation status of the embryogenic cultures, and significantly reduced both their relative growth rate and embryogenic potential, suggesting an important role for DNA methylation in embryogenesis. Maturation was also assessed by examining changes in the total protein profile. Storage proteins, identified as legumin- and vicilin-like, appeared at the precotyledonary stage. In the proteomic study, total soluble proteins were extracted from embryos after 1 and 8 weeks of maturation, and separated by two-dimensional gel electrophoresis. There were 147 spots which showed significant differences between the stages of maturation; they were found to be involved mainly in primary metabolism and the stabilization of the resulting metabolites. This indicated that the somatic embryo was still metabolically active at 8 weeks of maturation. This is the first report of analyses of global DNA methylation (including the effects of hyper- and hypo-treatments) and proteome during somatic embryogenesis in hybrid larch, and thus provides novel insights into maturation of conifer somatic embryos.

International audience; The maturation of somatic embryogenesis of hybrid larch is an essential step for plantlet production. ABA controls not only synchronicity of maturation, but has important consequences on eventual viability of the plantlet. To gain understanding of the role of this plant growth regulator during the maturation process of hybrid larch somatic embryos, we studied the incorporation of [ H]‐(±)‐abscisic acid [tritiated (±)‐ABA] over 6 weeks of culture. Results showed a rapid incorporation of label into the tissues directly in contact with the culture medium. Accumulation of tritiated (±)‐ABA occurred mainly in the maturing somatic embryos found at the periphery of the embryogenic mass but not in direct contact with the culture medium. Tritiated (±)‐ABA was mainly conjugated as a glucose ester form. Rates of tritium incorporation indicated a significant build‐up in tritiated (±)‐ABA metabolization at the third week of culture. The weekly measurement of labelling in the culture medium over 6 weeks showed no localized exhaustion of tritiated (±)‐ABA in positions where the embryogenic masses were placed in Petri dishes. The calculated ABA internal content of the maturing somatic embryos was similar to published ELISA measurements of ABA. This result suggests an absence of endogenous ABA synthesis by somatic embryos of hybrid larch during maturation.