C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2

Grasses with the C-3 photosynthetic pathway are commonly considered to be more nutritious host plants than C-4 grasses, but the nutritional quality of C-3 grasses is also more greatly impacted by elevated atmospheric CO2 than is that of C-4 grasses; C-3 grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C-4 grasses under elevated CO2. Will C-3 grasses remain nutritionally superior to C-4 grasses under elevated CO2 levels? We addressed this question by determining whether levels of protein in C-3 grasses decline to similar levels as in C-4 grasses, and whether total carbohydrate : protein ratios become similar in C-3 and C-4 grasses under elevated CO2. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C-3 grasses, levels of fructan respond most strongly to elevated CO2. Five C-3 and five C-4 grass species were grown from seed in outdoor open-top chambers at ambient (370 ppm) or elevated (740 ppm) CO2 for 2 months. As expected, a significant increase in sugars, starch and fructan in the C-3 grasses under elevated CO2 was associated with a significant reduction in their protein levels, while protein levels in most C-4 grasses were little affected by elevated CO2. However, this differential response of the two types of grasses was insufficient to reduce protein in C-3 grasses to the levels in C-4 grasses. Although levels of fructan in the C-3 grasses tripled under elevated CO2, the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C-3 grasses. We conclude that C-3 grasses will generally remain more nutritious than C-4 grasses at elevated CO2 concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C-4 grasses.