2019 ASHS Annual Conference

Fresh-cut lettuce (Lactuca sativa) contains nutrients important in human diet, but the product is highly perishable. Modified atmosphere packaging (MAP) with low oxygen (< 3%) is used to maintain shelf life of fresh-cut lettuce. Fresh-cut leaves of many cultivars deteriorate rapidly under MAP, which results in substantial losses and risks of propagation of human pathogens. The rate of deterioration of fresh-cut lettuce under MAP varies among cultivars and differences are highly inheritable. The mechanisms determining the rate of deterioration have not been identified, though a major determinant locus (qSL4) was detected on chromosome 4. We found that high respiration induced by low O₂ (< 1%) was highly associated with tissue deterioration, while CO₂ (0-14%) and ethylene (0-10 ppm) had no significant impact on the rate of deterioration of fresh-cut lettuce. Cellular respiration of cut leaves dramatically varied among cultivars when oxygen content was lower than 1% in the salad bag, but when oxygen content was close to 20%, the difference of deterioration rate between rapidly deteriorating (i.e., La Brillante: LaB) and slow deteriorating (Salinas 88: S88) cultivars significantly decreased. RNAseq analysis using S88 and LaB revealed that transcriptomic changes mainly occurred in the initial stages of deterioration rather than later. When comparing the lettuce in the very early stage of deterioration and non-deteriorated one, there were more up-regulated genes (1,837) in LaB than in S88 (1,185). A total of 567 genes were up-regulated in both cultivars. There were 1,735 genes down-regulated only in LaB, and 2,367 only in S88, and 1,060 in both cultivars. Two differentially expressed genes (encoding the disease resistance protein RPM1 and a glycosyltransferase) located in qSL4 region represent candidates potentially responsible for the rate of deterioration in fresh cut lettuce. Consistent with shelf life tests, genes associated with cellular respiration were expressed differently in LaB and S88 during storage. Genes involved in glycolysis such as two ATP consuming enzymes (fructokinase and hexokinase) and major components of electron transportation chain (ETC), including complex I and IV, and ATP synthase coding genes were up-regulated in LaB but down-regulated in S88. Two alternative oxidase genes that deliver electrons to ETC to reduce oxygen without generating ATP displayed significantly higher expression in S88. These results suggest that susceptibility of cultivars to low oxygen could determine their shelf life. Quality of slow deteriorating genotypes (e.g., S88) is maintained in low oxygen environments by reduced consumption of sugars (i.e., sucrose) and energy (i.e., ATP) and cellular activity, while rapidly deteriorating genotypes (e.g., LaB) act in the opposite manner.