Water is pervasively involved in the life cycle of seeds. Water in the environment, either as a vapor or liquid, directly affects seed moisture status. This article is devoted to the study of seed moisture status in postharvest events. Two topics are discussed: imbibitional chilling injury and upgrading of primed seeds. Imbibitional chilling injury is a physiological disorder that occurs in large-seeded legumes as well as other important agronomic seeds. Imbibitional chilling injury has been shown to reduce the survival rate of seedlings. Surviving seedlings have less emergence force per seedling and require a longer period to generate maximum force. Rapid hydration has been shown to induce injury at a particular seed moisture level. Methods of regulating the hydration rate were explored to alleviate chilling injury in snap beans (Phaseolus vulgaris L.) Plant breeding lines with the semihard seed characteristic delayed the onset of imbibition when the initial moisture level was low (8%). Coating seeds with polymeric films to complement the permeable testa retarded the imbibition rates. Both approaches alleviated chilling injury and improved seedling establishment under stressful conditions. Seed priming is a technique for elevation of seed moisture content before sowing. Primed seeds generally emerge more quickly than nonprimed seeds, especially under stressful environmental conditions. An additional merit of this technique is that it gives access to seeds with elevated moisture content. Various approaches may be employed to condition seeds after priming, but before redesiccation. Discarding the low-density fractions of primed tomato and lettuce seeds improved the percentage of germination compared with nonprimed seeds. Physiological mechanisms are presented to explain the association of density with seed viability in lettuce (Lactuca sativa L.).

The Ellis-Roberts seed viability equation is used to predict seed survival after storage at specified temperatures and moisture contents. Seed priming, which can break dormancy and accelerate germination, can also reduce seed storage life. Because primed seeds were not used in developing the Ellis-Roberts equation, the reciprocal nature of specific seed moisture content (MC, fresh weight basis) and temperatures that applies to nonprimed lettuce (Lactuca sativa L.) seeds may not apply to primed seeds. To determine how priming affects lettuce seeds in relation to the viability equation, an experiment was conducted using two cultivars, ‘Big Ben’ and ‘Parris Island Cos’. Seeds primed in polyethylene glycol 8000 (–1.45 MPa, 24 h at 15 °C) and nonprimed seeds were first adjusted to 6% and 9% moisture contents and then stored at 48 and 38 °C for up to 30 days, respectively. These storage conditions (6% MC and 48 °C; 9% MC and 38 °C) were predicted by the viability equation to result in equal longevities. Subsequent viability assays at 20 °C revealed that nonprimed seeds in both storage environments exhibited similar losses in viability over time, thus validating the Ellis-Roberts equation and the use of these conditions to apply different but equal aging stress. Primed seeds of both cultivars deteriorated faster than nonprimed seeds as expected. However, primed seeds did exhibit different rates of deterioration between the storage environments. Primed seeds stored at 9% MC and 38 °C deteriorated faster than primed seeds stored at 6% MC and 48 °C. The rate of decline in probit viability percentage was three times greater in primed ‘Big Ben’ seeds stored at 9% MC and 38 °C than for those stored at 6% MC and 48 °C (–1.34 versus –0.26 probits per day, respectively). ‘Parris Island Cos’ seeds stored at 9% MC and 38 °C had twice the rate of deterioration that those stored at 6% MC and 48 °C (–1.19 and –0.49 probits per day, respectively). The results indicate that primed lettuce seeds were more sensitive to the adverse effects of higher seed MC than were nonprimed seeds during storage at elevated temperatures.

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The impermeable seed‐coat trait in soybean [Glycine max (L.) Merr.) is of interest to researchers because impermeable seeds retain viability better than permeable seeds. The objective of this study was to determine how water stress during seedfllling may influence impermeable seed expression. A 2‐yr experiment was conducted outdoors with potted plants of two soybean genotypes that carried the impermeable seed‐coat trait. Starting at the R6 growth stage, one‐half of the plants were water stressed by imposing successive wilting cycles that consisted of withholding water until all plants in the treatment were wilted and then immediately rewatering until the soil was saturated. The control plants were watered daily. At maturity, seeds from water‐stressed plants were 15 to 33% smaller by weight, 8 to 22% smaller by volume, had a lower percentage of ruptured seed coats, a higher percentage seed coat, and 6 to 37% more impermeable seed (after 72 h of soaking) than well‐watered control plants over both years. The higher impermeable seed percentage (after 72 h of soaking) in the water‐stressed treatment was significantly correlated (r = −0.92**, significant at the 0.01 probability level) with the incidence of ruptured seed coats when data were combined over genotypes and years. Scanning electron microscopy suggested that ruptured seed coats were caused by the cotyledons expanding against the seed coat. The incidence of ruptured seed coat was positively associated with seed weight (r = 0.73*, significant at the 0.05 probability level), seed volume (r = 0.83*), and seed width (r = 0.92**). These data indicate that high soil moisture availability during seedfill will reduce impermeable seed expression by disrupting seed‐coat integrity.

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The Ellis-Roberts seed viability equation is accepted as an accurate predictor of seed longevity over a range of storage temperatures and moisture contents (MC). One application of this model is to identify different combinations of seed MC and temperature that can result in similar seed storage lifetimes. The present study was conducted to determine whether the reduction in storage life of primed seeds is consistent with the predictions of the Ellis-Roberts equation across a range of seed MC and temperatures. Seeds of two lettuce (Lactuca sativa L.) varieties (a romaine and a crisp-head type) were primed with polyethylene glycol and along with untreated seeds were adjusted to two different MCs (6% and 9%) and then aged in two different temperature environments (48°C and 38°C, respectively). Control seeds in both storage environments conformed to the Ellis-Roberts equation. Primed seeds aged faster than non-primed seeds but also exhibited different rates of viability loss between the storage environments. Primed seeds stored at 6% MC and 48°C aged slower than those stored at 9% MC and 38°C. These data suggest that priming causes lettuce seed to have a heightened sensitivity to the adverse effects of moisture during storage. To further investigate this observation, the moisture absorption properties of primed and non-primed seeds from both varieties were examined by equilibration over saturated NaCl solutions in sealed containers at 20°C. Primed seeds tended to have higher MC than non-primed seeds, although differences were seldom significant. Thus, an alteration of the RH/MC relationship for primed seeds could not be confirmed as a cause for the differential aging rates between equivalent environments. In conclusion, the predictable effects of MC and temperature on nonprimed lettuce seeds as described by the Ellis-Roberts equation may not hold true for primed seeds.

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