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Introduction
Sunflower achenes (single-seeded fruits) are usually dormant at harvest and germinate poorly (Cseresnyes, 1979; Corbineau et al., 1990) over a wide range of incubation temperatures. Depending on the genotype and maternal environment (Benech-Arnold, 2004; Bodrone et al., 2017), this initial primary dormant state can persist for several weeks or months (Brunick, 2007) and is a problem to the seed industry which needs non-dormant seeds to be processed and commercialized (Maiti et al., 2006), particularly for counter-season markets.
According to the classification proposed by Baskin and Baskin (2004), sunflower achenes display non-deep, physiological dormancy, which can result from the embryo being dormant itself ('embryo dormancy') and/or from 'coat-imposed dormancy' (Finch-Savage and Leubner-Metzger, 2006). The latter is the inhibition of germination by the structures that surround the embryo (i.e. the fruit 'envelopes'), which include the single-layered endosperm, the seed coat (which is dead at maturity) and the pericarp (Seiler, 1997; Szemruch et al., 2014). At harvest maturity, the sunflower embryo usually presents low or intermediate levels of dormancy, whereas the achene is deeply dormant as a result of strong 'envelope-imposed' dormancy. Even after embryo dormancy has vanished, it may take between a few weeks and several months of dry storage for envelope-imposed dormancy to disappear completely (Corbineau et al., 1990; Bianco et al., 1994; Domínguez et al., 2016; Bodrone et al., 2017).
Primary dormancy of freshly harvested seeds is reduced during dry storage and this results in the widening of the temperature range for germination and also in changes in sensitivity to hormones (Bewley, 1997). The rate at which primary dormancy is alleviated during dry storage is controlled by temperature in many different species (Probert, 2000), including sunflower (Bazin et al., 2011a,b). Typically, warmer storage temperatures accelerate dormancy release of many species during this after-ripening (AR) period (Baskin and Baskin, 1976, 1986; Foley, 1994; Allen et al., 1995; Bauer et al., 1998; Steadman et al., 2003a, b; Chantre et al., 2009), although this response can be affected by seed moisture content (MC) (Bazin et al., 2011a; Basbouss-Serhal et al., 2016). Seed MC changes according to air relative humidity and seed properties (Ellis et al., 1995). The fact that...