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

Grasses with the C3 photosynthetic pathway are commonly considered to be more nutritious host plants than C4 grasses, but the nutritional quality of C3 grasses is also more greatly impacted by elevated atmospheric CO2 than is that of C4 grasses; C3 grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C4 grasses under elevated CO2. Will C3 grasses remain nutritionally superior to C4 grasses under elevated CO2 levels? We addressed this question by determining whether levels of protein in C3 grasses decline to similar levels as in C4 grasses, and whether total carbohydrate : protein ratios become similar in C3 and C4 grasses under elevated CO2. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C3 grasses, levels of fructan respond most strongly to elevated CO2. Five C3 and five C4 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 C3 grasses under elevated CO2 was associated with a significant reduction in their protein levels, while protein levels in most C4 grasses were little affected by elevated CO2. However, this differential response of the two types of grasses was insufficient to reduce protein in C3 grasses to the levels in C4 grasses. Although levels of fructan in the C3 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 C3 grasses. We conclude that C3 grasses will generally remain more nutritious than C4 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 C4 grasses.