Growth and nutritional quality improvement in two differently pigmented lettuce cultivars grown under elevated CO2 and/or salinity

The interest of improving yield and quality of vegetables has increased in the recent years due to their benefits on human health. The aim of this study was to investigate if salt stress and elevated CO2 applied alone or in combination can improve the growth and nutritional quality of two differently pigmented (green and red) Lactuca sativa (L.) cultivars. Seedlings grown under ambient (400 +/- 20 mu mol mol(-1)) or elevated (700 +/- 20 mu mol mol(-1)) CO2 concentration for 35 days were subsequently supplied with 0 or 200 mM NaCl for 4 days. Then, biomass production, antioxidant capacity and minerals, nitrates, carbohydrates, proteins and hydrophilic and lipophilic antioxidant concentrations were measured. Red-pigmented lettuce showed higher nutritional quality than green-pigmented lettuce due to higher concentrations of Ca, P, Zn, and higher concentrations of lipophilic (Chl-a, Chl-b, and carotenoids) and hydrophilic (reduced ascorbate, total phenolics, and anthocyanins) antioxidants. Under elevated CO2, both lettuce cultivars increased the uptake of almost all minerals to adjust to the higher growth rates, reaching similar concentrations to the ones detected under ambient CO2; only Mg and Fe were reduced. Furthermore, the antioxidant capacity, Chl-b and glutathione concentration increased in both cultivars. Under salt stress, the N and K concentrations decreased in both cultivars, while Ca, Mg, and P concentrations were also reduced in the red cultivar, probably due to a blockage in the uptake of these nutrients. Both lipophilic and hydrophilic antioxidant compounds increased in order to defend against the oxidative stress caused by an imbalance in ATP and NADPH production and consumption. The magnitude of the response was dependent on the cultivar. When salt stress was imposed under elevated CO2, each cultivar responded differently. The red cultivar seemed to gain a greater advantage from elevated CO2 than the green cultivar because it better adjusted both mineral uptake and antioxidant metabolism. We conclude that elevated CO2 alone or in combination with short environmental salt stress permits us to increase the nutritional quality (increasing the concentration of some minerals and antioxidants) of lettuce without yield losses or even increasing production; however, the choice of the best growing conditions is dependent on the attributes we wish to improve. (C) 2015 Elsevier B.V. All rights reserved.