Effect of High CO2 Level on Growth and Nutritional Quality of Lettuce and Spinach

The rising atmospheric  CO₂ levels, in addition to being an important factor in driving the global climate change, can  directly affect plant growth and  development including its ability to provide nutritional sustenance to humans. The main objective of this study was to examine the effects of elevated CO₂ level on the growth and nutritional aspects consisting of protein, important minerals and phytochemicals in lettuce (Lactuca sativa, var. Black-Seeded Simpson)  and spinach (Spinacia oleracea, var. Bloomsdale Long Standing). Plants were grown in growth chambers at 18/20^oC (night/day) and 280-300 mmol.m^-2.s^-1 PAR with either 400 ppm (control) or 700 ppm  (elevated) CO₂. The net photosynthetic activity in lettuce was significantly higher  at elevated CO₂ level,  however, with no significant differences in spinach  between the two CO₂ levels. This was also reflected in an increased  biomass accumulation in lettuce at elevated CO₂ but not in spinach. However, in both lettuce and spinach, stomatal conductance and plant total leaf area were drastically reduced at elevated CO₂ level. Significant decrease in protein concentrations, more than 30% in lettuce shoots and 24% in spinach shoots, was noted at elevated CO₂ while carbon accumulation was not affected by higher CO₂ level. Shoots of both species had reduced levels of  phosphorus and potassium in response to high CO₂ level. In addition, concentrations of many micronutrients were drastically reduced; sulfur by 37%, manganese by 31%, zinc by 29% and copper by 21% at elevated CO₂ in lettuce shoots. The results on the total phenolic content and antioxidant capacity in lettuce and spinach shoots were variable with regard to increasing CO₂ level. Significant increase in the total phenolic content and antioxidant capacity was observed at elevated CO₂ in lettuce shoots. The total phenolic content in lettuce shoots increased by more than 2-fold in response to elevated CO₂. However, the elevated CO₂ level did not have any effect either on the total phenolic content or the antioxidant capacity in spinach shoots. The study also characterizes the changes in the accumulation of individual phenolic compounds in lettuce and spinach shoots in response to increasing CO₂level.

Partially funded by Kansas Agricultural Experiment Station.