Postharvest lipidomics of lettuce leaves

Lipidomics is a new method in postharvest studies. RNA-seq data on lettuce leaves revealed important changes in gene expression linked to lipid metabolism. Here, we report a global lipid analysis in lettuce leaves over 14 d of storage, in dark cold conditions, with or without an initial light treatment. Our LC-MS analyses of 171 lipid compounds, among which digalactosyldiacylglycerol, triacylglycerol, and phosphatidic acid, revealed variations in response to the postharvest conditions. Our results suggest the reallocation of phospholipid fatty acids to galactolipids through storage time, which may be associated to lettuce capacities to withstand storage without rapid chlorophyll degradation in contrast to other leafy vegetables. The unsaturation level of fatty acids increased over storage time. Intermittent light treatment at the beginning of storage stimulated phosphatidic acid accumulation, concomitantly with upregulation of phospholipase D gene expression. We also observed an increase in triacylglycerol content after light treatment. Usual indicators of membrane damage, electrolyte leakage, and malondialdehyde, were unchanged during storage. Results are discussed in the light of known changes in postharvest life of lettuces and other leafy vegetables.