Primed Lettuce Seeds Exhibit Increased Sensitivity to Moisture during Aging

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. INTRODUCTION Lettuce seeds are commercially primed to prevent the induction of thermoor photo-inhibition during the germination process. A secondary advantage of lettuce seed priming is a faster rate of radicle emergence across temperatures when seeds are planted. The combination of these advantages allows for more rapid and uniform field emergence, less variation in plant size, and thus a greater number of cartons per acre at harvest packout. A major disadvantage of priming lettuce seeds is a shorter shelf-life as compared to non-primed seeds. For example, priming protocols that increased germination rates were also shown to decrease longevity under controlled deterioration conditions (10% MC and 40°C) (Tarquis and Bradford, 1992). Primed lettuce seeds stored at 45°C and 50% relative humidity (RH) were reported to exhibit slower and less synchronous germination than the non-primed seeds in as little as 14 days of storage (Hacisalihoglu et al., 1999). Priming has also been shown to shorten seed longevity in many other species (Bruggink et al., 1999). We are not aware of any lettuce priming protocol that can eliminate temperature and/or photo-inhibition without resulting in a more rapid decrease Proc. IV IS on Seed, Transplant and Stand Establishment