The response of lettuce to environmental stresses is cultivar specific: role of phenolic acids and flavonoids

There is an increasing interest for the inclusion of fresh fruits and vegetables in the human diet, mainly due to the health benefits associated with their consumption. Several epidemiological studies have revealed that diets rich in fruits and vegetables are associated with a reduced risk of cardiovascular diseases and various types of cancer. These beneficial effects are believed to be due to phytochemicals such as phenolic compounds, which can protect the key biological constituents of the cell. Having defense functions, their concentrations can increase when plants are subjected to environmental stresses. This practice represents a great potential for enhancing the health promoting properties of food products. The aim of this work was to study the changes in the composition of phenolic acids and flavonoids in two differently pigmented lettuce cvs when submitted to high light (700 PAR) or elevated C02 (700 ppm) or to a combination of both. The two cvs were very different for the contents and composition in phenolic acids and flavonoids, with the red one showing the higher values. In response to both high light and elevated C02, total flavonoids and phenolic acids increased mainly due to increases in quercetin, quercetin-3-0-glucuronide, chicoric acid and chlorogenic acid, these changes being cv. specific. Cyanidin also reached a higher value in the red cv. following stress. When high light was applied together with elevated C02, flavonoids of the green cv. were subjected to a much higher increase than when high light or elevated C02 were applied separately. The red cv. showed a different trend with elevated C02 not having a positive effect on the amounts of phenolic acids. These results suggest that the application of high light or elevated C02, alone or in combination, can induce the production of some phenolics increasing the health benefits of plant products such as lettuce. As each treatment alters in a distinct way the composition of phenolics and each phenolic compound has different nutraceutical properties, the choice of the treatment can be done on the base of the desirable compound to be enhanced.