Lettuce (Lactuca sativa L. ‘Empire’) seeds (achenes) were primed in aerated solutions of polyethylene glycol 8000 (−1.56 MPa) for 24 hr at 18°C and dried to 6% water content. Control (untreated) and primed seeds then were commercially coated in a clay-based material (Royal Sluis Splitkote). Priming prevented the occurrence of thermodormancy in seeds imbibed at temperatures between 32° and 37° and accelerated the rate of germination at all temperatures tested. Coating delayed germination slightly and reduced the maximum germination temperature of control seeds. Primed seeds exhibited thermodormancy when tested immediately after coating, but they soon regained their capacity for high-temperature germination. When tested after 5 months of storage at 5°, primed seeds germinated at temperatures up to 37° regardless of coating. Osmotically primed seeds retained the capacity for germination at high temperature after seed coating and storage, providing an effective means of improving stand establishment of direct-seeded lettuce in hot weather.
Lettuce (Lactuca sativa L. ‘Empire’) seeds (achenes) were given an osmotic priming treatment (24 hr in aerated —1.5 MPa polyethylene glycol (PEG 8000) solution at 18°C in the light) which alleviated thermodormancy in laboratory tests. The seeds then were coated commercially for precision planting. Additional seeds also received a proprietary treatment for enhancing high temperature germination (Royal Sluis Split-kote D). In field trials in the Imperial Valley of California, where the soil temperature exceeded 35°C for the first 11 hr of imbibition under sprinkler irrigation, total emergence of untreated seeds after 6 days was between 18% and 21%, whereas that of primed and Splitkote D seeds ranged from 46% to 69%. Uniformity and rate of emergence were also greater for the primed seeds, with 91 % of the final emergence occurring by the 3rd day, as compared to only 70% for the control. Seed priming prior to coating can be an effective method of improving lettuce stand establishment under high temperature conditions.