Drying Rates following Priming Affect Temperature Sensitivity of Germination and Longevity of Lettuce Seeds

Seed priming (controlled hydration followed by drying) is used to alleviate high temperature inhibition of germination and improve seedling emergence of lettuce (Lactuca sativa L.) and other species. However, seed priming can also reduce the longevity of seeds during dry storage. Alternative drying methods [i.e., slow drying or moisture content reduction (MCR) before drying] can extend seed longevity compared to conventional rapid drying procedures after priming. Three postpriming drying treatments were tested on ‘Conquistador’ and ‘Genecorp Green’ lettuce seeds: rapid drying, slow drying and MCR (10% fresh weight loss, then held at 100% relative humidity (RH) for 6 hours, followed by rapid drying). The effects of the postpriming treatments on seed quality and longevity were compared based upon standard germination tests, germination rates, thermogradient table tests, controlled deterioration (CD) tests, and headspace volatiles analysis. The latter may be correlated with seed longevity as release of volatiles (e.g., acetaldehyde, ethanol) is associated with lipid peroxidation. While neither slow drying nor MCR before drying restored lettuce seed longevity to that of the control (not primed) seeds, the MCR method generally gave better results in both cultivars compared to rapid drying. Among the CD test conditions used, 50 °C and 75% RH gave the most consistent results for estimating potential longevity. Headspace volatile emissions from both control and primed lettuce seeds were very low and were not well correlated with seed longevity. Alternative postpriming drying regimes can extend seed longevity while retaining the beneficial effects of priming. Seed priming is a technique to improve the germination behavior of seed, inducing faster and more uniform germination over broader temperature ranges and breaking dormancies inherent in certain species (e.g., thermoinhibition of lettuce, Lactuca sativa L.) (McDonald, 1999). During priming, seeds are hydrated to a level below that needed for radicle emergence, allowing pregerminative metabolism to proceed, and then dried to the original moisture content (McDonald, 2000). However, these procedures often result in a reduction of seed longevity in storage compared to nonprimed seed (Chojnowski et al., 1997; Maude et al., 1994; McDonald, 1999; Tarquis and Bradford, 1992). Postpriming treatments using alternative drying methods (e.g., slow drying or seed moisture content reduction before drying) have extended seed longevity compared to conventional rapid drying methods after priming in species such as celery (Apium graveolens L.) (Coolbear et al., 1991), foxglove (Digitalis purpurea L.) (Hay and Probert, 1995), pepper (Capsicum annuum L.) and pansy (Viola ×wittrockiana) (Bruggink et al., 1999), and tomato (Lycopersicon esculentum Mill.) (Gurusinghe et al., 2002). To evaluate the potential benefits of postpriming treatments to increase the shelf-life of primed lettuce seed, the quality and longevity of primed seeds dried using different methods were analyzed and compared based upon several germination and vigor tests: standard germination test, germination time course tests, thermogradient table test, and controlled deterioration (CD) tests. Emission of volatiles from seeds was also assayed to evaluate its use as a seed aging test (Wilson and McDonald, 1986). Materials and Methods Seeds and priming treatments. Seeds of ‘Conquistador’ and ‘Genecorp Green’ lettuce cultivars were obtained from Seminis Vegetable Seeds (Oxnard, Calif.). ‘Annie’ and ‘Red Line’ cultivars were also evaluated but did not respond favorably to the priming protocol utilized (data not shown; see Schwember, 2003). ‘Conquistador’ is a Romaine-type cultivar that can be sensitive to rapid aging; therefore special care is needed in priming and in seed storage. It exhibits a higher upper temperature limit as compared to standard Romaine cultivars even without priming, but priming is still applied to improve emergence uniformity. ‘Genecorp Green’ is a selection of ‘Waldmann’s Green’, both photoand thermosensitive, but responding favorably to priming. (J. de Vries, Seminis Vegetable Seeds, personal communication). The same priming procedure was conducted for all the treatments: seeds were osmoprimed in glass bubblers containing an aerated –1.25 MPa solution of PEG 8000 for 24 h under continuous light. Seeds were then rinsed briefly with water after incubation and surface water was removed by suction in a Buchner funnel. The postpriming treatments were as follows. Rapid drying. Primed seeds were dried for 4 h at 32 °C and 25% to 30% relative humidity (RH) in a ventilated oven, then were transferred to a sealed chamber (with internal fan for air circulation) that contained saturated CaCl 2 solution to maintain 33% RH at 15 °C for 48 h. Slow drying. Primed seeds were dried at 20 °C for 24 h in a sealed chamber over 5% w/v glycerol (99% RH, –1.4 MPa), then were transferred and equilibrated at 33% RH and 15 °C as described above. Moisture content reduction (MCR). Primed seeds were weighed and spread over germination paper in a fume hood to allow their total fresh weight to decrease by about 10% (e.g., 10 g of wet seeds were air-dried until the total weight reached 9 g), then were placed on a screen in a sealed chamber over distilled water (100% RH) for 6 h at 20 °C before subsequent rapid drying and transfer to 33% RH at 15 °C as described above. Seed samples collected at various times were oven-dried at 130 °C for 1.5 h and their moisture contents (MC) were determined on a fresh weight basis. The final MC of seeds following the three postpriming treatments was 5% to 6%. Germination tests and seedling evaluations. For germination tests, lettuce seeds were plated in 8.8-cm-diameter petri dishes over single circular germination blotters wetted with distilled water (8 to 9 mL). Each dish contained 25 seeds and there were three replicates per treatment. The lettuce seeds were incubated under continuous fluorescent light at 20 °C, and after 5 to 6 d were scored as normal or abnormal seedlings using the ISTA rules (International Seed Testing Association, 1999). In separate tests under the same conditions, germination (radicle emergence) time courses were evaluated for both cultivars for each treatment. Thermogradient table test. A temperature gradient ranging from 16 to 18 °C to 34 to 36 °C (2 °C increments) with 10 locations and one blotter per location was established using a temperature gradient table (Seed Processing Holland B.V., Enkhuizen, The Netherlands). As each germination location on the thermogradient table spanned 2 °C, the temperature locations are referred to by their midpoints. Thirty-three seeds were distributed over each blotter, and the percentage of normal seedlings per blotter was assessed 72 h after planting. The results were replicated in three separate experiments. Controlled deterioration tests (CD tests). Seeds for CD tests were adjusted to higher MC by incubation over saturated NaCl solution (75% RH) for 48 h at 2.5 °C in a sealed HORTSCIENCE 40(3):778–781. 2005. Received for publication 6 Aug. 2004. Accepted for publication 12 Oct. 2004. This research was funded in part by Incotec International B.V., by the Western Regional Seed Physiology Research Group and by CSREES Regional Research Project W-168. We thank Incotec for assistance with the temperature gradient experiments and Seminis Vegetable Seeds for providing the lettuce seeds. Frederick J. Sundstrom and Carlos F. Quiros provided constructive comments on the manuscript. To whom reprint requests should be addressed; e-mail [email protected].