Document: ALE-3-30-28

Effects of elevated CO₂, water supply and nitrogen nutrition on growth and photosynthesis in three C₄ grasses.

FRAVOLINI, A.* and D.G.WILLIAMS

University of Arizona, Tucson, AZ, USA ¹

Abstract:
Photosynthesis and growth responses of semi-arid C₄ grasslands to rising atmospheric CO₂ will depend on soil water and nutrient availability. Furthermore, these responses may vary among biochemical subtypes (NADP-ME, NAD-ME, PCK) that have discernable differences in C₄ bundle sheath leakiness in response to drought and nitrogen supply. To address C₄ subtype responses to soil resource gradients, we measured photosynthetic rates(A), stomatal conductance(g), the ratio of intercellular to ambient CO₂ concentration (c_i/c_a), leaf carbon isotope discrimination (), and above ground biomass accumulation in Eragrostis lehmanniana (NAD-ME), Aristida glabrata (NADP-ME) and Bouteloua curtipendula (PEP-CK), in a controlled environment chamber. These species have similar life history strategies and occupy the same habitats in semi-arid grasslands of southeastern Arizona. The three species were grown from seed under a complete, multi-factorial combination of present ambient (370ppm) and elevated (690 ppm) CO₂ concentration and under high and low water and nitrogen supply. Leaf-level photosynthetic rate in A. glabrata was relatively unchanged by CO₂ treatments, likely due to its NADP-ME biochemistry. Photosynthetic rates under elevated CO₂ increased in B. curtipendula and E. lehmanniana across all treatments, although responses were greatest in E. lehmanniana. Reduction in A was observed more in B. curtipendula, the PEP-CK type, by deficits of nitrogen compared to the other two grasses. In contrast, biomass of B. curtipendula increased most in response to elevated CO₂ when nitrogen and water were not limiting. Carbon isotope discrimination and c_i/c_a values will be used to estimate to help interpret changes in A and biomass accumulation under favorable and stressful conditions. These results suggest that responses of semi-arid grasslands to rising atmospheric CO₂ will depend on the interaction between the composition of C₄ grasses as it relates to biochemical subtype, drought, and soil fertility.

Keywords: C₄ grasses, gas exchange, leaf carbon isotope discrimination

This abstract is being presented at: 10:30 AM in session:
GAS EXCHANGE