Screening the USDA Lettuce Germplasm Collection for Rapid Germination Rate Under Cold Conditions

Quick emergence of seedlings has long been the goal of agronomists and horticulturists in order to maximize uniform emergence and weed competitiveness in direct-seeded crops. Exposure of seed to low temperatures after seeding can increase the time for emergence, which leads to increased days to maturity as well as increased potential for weed pressure. For a crop that is densely planted for early harvest, such as baby-leaf salad greens, rapid and uniform seed emergence is essential. Soil temperature during the shoulder seasons in Northwest Washington averages 10°C in spring and 12°C in fall. Rapid germination and growth under cool soil conditions enable growers to produce lettuce (Lactuca sativa) in the extended season and can increase economic competitiveness of the crop in regions outside its current range of production. The USDA National Plant Germplasm System (NPGS) lettuce collection has not been screened for rapid germination under cold conditions. In this study, 103 lettuce accessions [53 cos (romaine) and 50 leaf-type] from the NPGS collection, maintained at the Western Region Plant Introduction Station, Pullman, WA, were evaluated for germination at 5°C. Three replicates of 50 seeds each per accession were placed randomly in a germination chamber on a 12-12 day-night cycle (fluorescent light) and screened for percent germination after 7 and 10 days. There was a significant difference among accessions (P < 0.001) in percent seeds germinated at 5°C after 7 and 10 days. Overall, an average of 60% of seeds germinated after 7 d and 96% germinated after 10 d. Cos types had a significantly higher percent germination after 7 d than leaf types on average (62% and 58%, respectively; P = 0.0107). 4 of the 5 accessions with the highest percentage of germinated seedlings were cos types: PI 140394 (cos), PI 171665-1 (cos), PI 169509-4 (cos), PI 278072 (cos), and PI 344367-1 (leaf). The seed of accessions tested was either black-brown or white. Accessions with black-brown seed were found to have a significantly higher percent germination after 7 days at 5°C than white-seeded accessions (61% and 59%, respectively; p = 0.0097). Accessions with shortest germination time under cold temperatures have the potential to be used for the development of lettuce cultivars better suited to low-temperature extended season production. Field trials under cold soil temperatures should be performed to further assess the ability of these accessions to germinate rapidly in cold soils.