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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 CO 2 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 CO 2 . Will C 3 grasses remain nutritionally superior to C 4 grasses under elevated CO 2 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 CO 2 . In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C 3 grasses, levels of fructan respond most strongly to elevated CO 2 . Five C 3 and five C 4 grass species were grown from seed in outdoor opentop chambers at ambient (370 ppm) or elevated (740 ppm) CO 2 for 2 months. As expected, a significant increase in sugars, starch and fructan in the C 3 grasses under elevated CO 2 was associated with a significant reduction in their protein levels, while protein levels in most C 4 grasses were little affected by elevated CO 2 . 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 CO 2 , 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 CO 2 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.

Plants synthesize a diversity of tannin structures but little is known about whether these different types have different oxidative activities in herbivores. Oxidative activities of hydrolyzable and condensed tannins were compared at pH 10 with two methods: EPR spectrometry was used to quantify semiquinone radicals in anoxic conditions and a spectrophotometric assay was used to measure the rate of browning of phenolics oxidized in ambient oxygen conditions. A little-studied group of hydrolyzable tannins (ellagitannins) contained the most active tannins examined, forming high concentrations of semiquinone radicals and browning at the highest rates. On average, galloyl glucoses and high-molecular-weight gallotannins had intermediate to low oxidative activities. Condensed tannins generally formed low levels of semiquinone radicals and browned most slowly. The results suggest that ellagitannin-rich plants have active oxidative defenses against herbivores, such as caterpillars, whereas the opposite may hold true for plants that contain predominantly condensed tannins or high-molecular-weight gallotannins.

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