Lettuce Germplasm in Humid Subtropical Environments Tolerant to Postharvest Development of Pink Rib Disorder

Pink rib discoloration or pinking in the midribs of lettuce (Lactuca sativa) leaves is a stress-induced disorder that leads to crop loss worldwide. Maintaining recommended field and postharvest conditions reduces its incidence but does not eliminate the issue. During the past decade, research has identified the tolerance of this disorder among lettuce types and cultivars grown in cooler climates. However, tolerance to pink rib among lettuce types grown in humid subtropical climates is unknown; therefore, it is necessary to screen lettuce germplasm under these growing conditions. During this study, diverse lettuce accessions were planted for early-season, mid-season, and late-season harvests over two seasons in Belle Glade, FL, USA. Harvested midribs were wounded to induce pink rib, stored for 6 to 9 days at 5 °C and >95% relative humidity, and rated for severity using a 5-point subjective scale. Genotype × environment interactions were evaluated to understand the environmental factors that favor the development of pink rib during storage and between planting seasons. Pink rib severity increased during storage, with the highest increase observed after 3 to 4 days in both seasons. After 9 days of storage, lettuce accessions with the least pink rib for each leaf type were identified. The lowest pink rib ratings after 9 days of storage were “moderate” (rating of 3) for crisphead, Latin, and romaine, “slight” (rating of 2) for butterhead types, and “none” (rating of 1) for leaf types. Additionally, pink rib severity increased among accessions during the late spring season harvest when field temperatures were higher and daylight hours were extended. The lettuce germplasm with low susceptibility to pink rib is promising to breed lettuce lines for future research.