a b s t r a c tThe lifecycle of the facultative biennial oilseed-crop evening primrose (Oenothera biennis) is a major constraint for its commercial production under different growing conditions, as a variable proportion of plants fails to flower during the first season and remains as vegetative rosettes (biennial behavior). The aim of this work was to understand how flowering is regulated in this species and to identify the main determinants of its biennial behavior.Different planting dates and manipulative treatments (seed vernalization, photoperiod extension and fertilization) were employed to analyze if: (i) biennial behavior occurs when obligate requirements for vernalization or photoperiod are not satisfied; and (ii) responses to these environmental cues depend on the size and/or growth rate of rosettes.Our results indicate that O. biennis has an obligate long-day requirement for flowering and a facultative vernalization response. There is no minimum size requirement for vernalization response (as very small seedlings responded to the vernalization treatment) and the rate of development toward flowering under inductive photoperiods was strongly affected by rosette's growth rate. The incidence of high temperatures just before the onset of reproduction is proposed as an inhibitory factor that prevents reproduction under otherwise photo-inductive conditions. This last factor would explain the high incidence of biennial behavior frequently observed in spring/summer sowing in this crop.

The objective of this work was to compare and characterize the response to nitrogen (N) supply of a textile and an oilseed cultivar of flax (Linum usitatissimum L.). The dynamics of growth, biomass partitioning, growth rates, and leaf area evolution and duration of the cultivars Omega (oilseed) and Diane (textile) were compared under three rates of N supply. Plants were grown in pots in the field with nil (N1), 2.5 (N2), and 5 g N per pot (N3); N2 and N3 doses were divided into five applications. Shoot biomass of N2 and N3 treatments was similar in both cultivars and significantly higher than in N1. Partitioning of biomass was unaffected by N treatment but showed contrasting patterns between cultivars. The oilseed cultivar produced 30 -42% more reproductive biomass and 18-30% less stem biomass than its textile counterpart. Differences between cultivars in these traits increased with increasing N rate. Maximum growth rates per plant concentrated around 15728Cd after sowing (fructification). Faster leaf senescence after flowering contributed to the lower leaf area and leaf biomass of the textile cultivar.The availability of N at different specific moments of the plant cycle would be useful to improve the growth of organs of economic interest. Results can also provide useful information to future nutrition models and breeding programs in textile and oilseed flax.

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