Leaf photosynthetic responses of C3 and C4 grass species to temperature and carbon dioxide.
Alterations in the atmospheric CO2 concentration ([CO2]) and temperature pose differential impact on the photosynthetic characteristics of native and invasive species of rangelands. An experiment was conducted in sunlit controlled environment chambers to quantify the interactive effects of elevated [CO2] and temperature on native rangeland C3 grass (Elymus virginicus L.) and native C4 grass (Andropogon gerardii Vitman) and an invasive C4 grass (Imperata cylindrica L.) species. Six treatments comprised of two CO2 levels of 360 (ambient) and 720 (elevated) ppm and three day/night temperature of low (20/12 °C), optimum (30/22 °C) and high (40/32 ºC). Compared to ambient, elevated [CO2] increased leaf Pn (40% to 150%) of native C3 species over all the temperatures. In contrast only a small increase (8 to 27%) was observed in the C4 species. However, the response of native species to temperature was based on their adaptation, where the C3 species, a cool-season grass, was very sensitive to high temperature, whereas the C4 species, a warm season grass was more tolerant to temperature changes. When growth temperature increased from 20 to 40 ºC, Pn was reduced by 91 and 23% at ambient [CO2] and by 84 and 9% at elevated [CO2] in the native C3 and C4 species respectively. In contrast, increased temperature from 20 to 40 ºC increased Pn of invasive C4 species by 18 and 85% at ambient and elevated [CO2], respectively. The leaf Pn of invasive C4 species was favored less by low temperature (-23%) than by high temperatures (+18%) at elevated CO2. Therefore, the invasive species would pose a serious threat to rangelands to changes projected in the climate, where the C3 species fail to survive and its prolific growth habit would also hamper the survival of the native C4 species in a warmer climate.