Paper Reviewed
Pérez-López, U., Miranda-Apodaca, J., Muñoz-Rueda, A. and Mena-Petite, A. 2015. Interacting effects of high light and elevated CO2 on the nutraceutical quality of two differently pigmented Lactuca sativa cultivars (Blonde of Paris Batavia and Oak Leaf). Scientia Horticulturae 191: 38-48.

Enhancing crop nutritional value has long been a goal of the agricultural industry. Growing plants under less than optimal conditions for a short period of time generally increases their oxidative stress. To counter such stress, plants will usually increase their antioxidant metabolism which, in turn, elevates the presence of various antioxidant compounds in their tissues, compounds that are of great nutritional value from a human point of view.

However, stress-induced nutritional benefits often come at a price, including a reduction in plant growth and yield, making it unproductive and costly to implement these practices in the real world. But what if there was a way to harness such benefits without sacrificing crop biomass? What if there was a way for society to have its proverbial cake and eat it too? An intriguing paper published in the journal Scientia Horticulturae explains just how this incredible objective can be accomplished, involving lettuce, light stress, and atmospheric CO2.

According to Pérez-López et al. (2015), the authors of this work, "few studies have utilized light and elevated CO2 levels in combination to enhance a crop's nutraceutical value." Thus, the team of four Spanish researchers set out to conduct just such an experiment involving two lettuce cultivars, Blonde of Paris Badavia (a green-leaf lettuce) and Oak Leaf (a red-leaf lettuce). In so doing, they grew the lettuce cultivars from seed in controlled environment chambers at either ambient (400 ppm) or enriched (700 ppm) CO2 for a period of 35 days, after which they supplied a subset of the two treatments with either normal (400 µmol photons m^-2 s^-1) or high light (700 µmol photons m^-2 s^-1) conditions for 4 days to simulate high-light stress. Thereafter they conducted a series of analyses to report growth and nutritional characteristics of the cultivars under these varying growth conditions. And what did those analyses reveal?

As shown in Figure 1, high light intensity treatment had no effect upon the dry weight of red lettuce, whereas it actually increased the growth of the green-leaf cultivar. Additionally, Pérez-López et al. report that both cultivars experienced increased growth in the high light intensity treatment under elevated CO2 conditions as compared with the ambient light and CO2 treatment. Specifically, there was a 68 percent enhancement in the dry weight of Oak leaf lettuce and a 91 percent increase for Blonde of Paris Badavia.

Figure 1. Effects of high light stress (normal = 400 µmol photons m^-2 s^-1, stressed = 700 µmol photons m^-2 s^-1) and CO2 (ambient = 400 ppm, elevated = 700 ppm) on the growth (left panel) and antioxidant capacity (right panel) of Blonde of Paris Batavia green-leaf lettuce and Oak Leaf red-lettuce. Adapted from Pérez-López et al. (2015).

With respect to antioxidant capacity, under ambient CO2 conditions, high light stress increased this parameter in red lettuce by 65 percent, whereas it remained unchanged in green lettuce compared to control conditions. Under elevated CO2 conditions, however, the antioxidant capacity was increased by 54 percent in the green lettuce cultivar and a whopping 302 percent in the red cultivar compared to control. Pérez-López et al. also report that "the concentrations of minerals (except Fe and Mg), glutathione and ascorbate remained constant compared with the high light and ambient CO2 conditions, which indicated that these components and biomass accumulated at comparable rates." They also found evidence suggesting "a relief of oxidative stress" at elevated CO2 levels. And in consideration of all of the above, the team of five researchers conclude that "the nutritional quality of lettuce can be improved in practice: however, the choice of the best cultivation practice depends on the cultivar of lettuce and the attributes that are targeted for improvement."