Desiccation tolerance is a seed trait of significant horticultural importance. For example, the ability to tolerate considerable levels of postharvest desiccation is one of the key factors for maintaining seed viability and extending shelf life during storage (Berjak and Pammenter, 2008; Ellis and Roberts, 1981; Roberts, 1973; Walters, 1998; Walters et al., 2010). Desiccation tolerance manifests during the seed developmental program and has been associated with patterns of maximal dry matter accumulation (i.e., physiological maturity), sharp declines in water content and water potential resulting from vascular separation between the seed and mother plant during the maturation drying phase, and the ability of subcellular components to recover upon rehydration from the stresses and strains imposed by drying (Kermode and Finch-Savage, 2002; Pérez et al., 2012; Walters et al., 2010).
Postharvest drying in equilibrium with low relative humidity (e.g., 20% RH) followed by storage at low temperatures (e.g., −20 °C) reduces the deterioration rate of seeds in storage such that seeds of some species are predicted to maintain viability for years to centuries. Longevities of this nature are expected of seeds held in germplasm repositories adhering to international standards for postharvest handling and storage (Food and Agriculture Organization of the United Nations, 2014). But, seed managers are not always able to provide or maintain ideal storage conditions due to financial, infrastructural, and labor constraints. This is significant since all stored seeds will eventually succumb during storage due to accumulated aging damage that cannot be resolved upon rehydration. Moreover, the level and rate of viability loss due to aging within a seed lot increases with increasing seed water content, RH, and storage temperature (Ellis and Roberts, 1981; Walters, 1998, 2015; Walters et al., 2010). Therefore, seeds will tend to have limited shelf life when stored without prior moisture content adjustments, in environments of fluctuating or persistently high RH and heat, or under conditions where only one or two important storage components (e.g., seed water content, RH, temperature) are controlled. Consequently, an important first step for seed managers is to understand the response of seeds to dehydration before storage (Food and Agriculture Organization of the United Nations, 2014). This is particularly true for nondomesticated wildflower species of horticultural importance, where a priori knowledge of storage behavior is almost always lacking (Kauth and Pérez, 2011; Milstein, 2005; Walters, 2015).
Similarly, seed vigor is a concept that attempts to reconcile the biochemical, physiological, and genetic properties of seed lots contributing to germination and establishment potential. Seed lots displaying high vigor are characterized by rapid and uniform germination to high percentages despite exposure to unfavorable storage or establishment conditions (Powell, 2006). It should be noted, however, that the sensitivity or tolerance to stress can be uncoupled from germination rate or uniformity. In other words, sensitivity to stress as displayed by a loss of germination ability (i.e., reduced germination percentage) can precede reductions in germination rate or uniformity. Nevertheless, seed aging is a main source of vigor loss. In fact, the relationship between seed deterioration and vigor is inverse (Powell, 2006; Seed Vigor Testing Committee, 2002).
In the present study, we examined the desiccation tolerance of mature, freshly harvested Gaillardia pulchella seeds to gain some perspective on the storage potential of this species. Additionally, we examined responses to aging stress in an attempt to understand vigor of fresh seeds. We selected G. pulchella because it is in high demand and producers seek more information related to the seed physiology of wildflowers (Kauth and Pérez, 2011). We ask: 1) how are seed–water relations characterized; 2) what is the relationship of seed–water relations to desiccation tolerance; 3) does germination response change following moderate to severe desiccation stress; and 4) how does aging stress influence seed vigor. We used water sorption isotherms, equilibrium RH drying techniques, and SSAA assays to address these questions.
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