Plant Age Affects Wound-Induced Senescence in Lactuca Sativa L

In the present study we investigated the performance of dark-stored wounded leaf discs and pieces (to some extent mimicking fresh-cut product) of Lactuca sativa L. in relation to the physiological maturity at harvest. We used two related genotypes, i.e. a green (cv. Troubadour) and a red butterhead (cv. Teodore) differing in their pigment levels. For both genotypes, senescence of the wounded (fresh-cut) tissue prepared from leaves of younger plants was significantly delayed compared to wounded tissue prepared from the more mature plants. Shelf-life (time to unacceptable quality) of fresh-cut was doubled when plants were harvested one week before the commercial harvesting date. To explain differences in shelf-life between fresh-cut products prepared from plants harvested at different age, a number of physiological and nutritional parameters were determined at harvest. The red lettuce contained about two times more chlorophyll, carotenoids, and polyphenolic antioxidants than the green lettuce, but the shelf-life of both genotypes was about similar. Increasing the amount of pigments and polyphenols through application of LED light (with high percentage blue) during cultivation did not affect the shelf life of the wounded leaf tissue. The content of chlorophyll, carotenoids, phenolic compounds, as well as total antioxidant capacity was not affected by age of the plants for either genotype. The content of ascorbic acid decreased with maturation in the green lettuce but it was not affected by maturity in the red lettuce. This shows that there are no obvious signs of leaf senescence with age and the differences in pigments and antioxidants show no relation to the fresh-cut shelf life and thus cannot explain the effect of plant age on senescence of the wounded tissue. The net photosynthesis rate and carbohydrate reserves in the red lettuce were about half of that in the green lettuce but the shelf-life of both genotypes was similar. The net photosynthesis rate was not influenced by plant maturity in the green lettuce, whereas it decreased with maturation in the red lettuce. A decrease in sucrose and starch, and therefore the total content of carbohydrates with aging was observed in both genotypes. This effect was more pronounced in the red than in the green lettuce. There was no apparent relationship between the absolute levels of the total carbohydrates and the shelf-life of the fresh-cut product showing that carbohydrate reserves in itself cannot explain the effect of plant age on senescence of the wounded tissue. The leaves from older plants apparently have a reduced capability to cope with the stress from wounding. No simple one to one relationship emerged between any of the measured nutritional parameters, their change during maturation and the eventual shelf-life of the fresh-cut produce.