Carbon isotope discrimination and bundle sheath leakiness in three C4 subtypes grown under variable nitrogen, water and atmospheric CO2 supply

The changes in composition and productivity of semi-arid C-4 grassland, anticipated with rising atmospheric CO2, will depend on soil water and nutrient availability. The interactive effects of soil resource limitation and elevated CO2 on these grasses, furthermore, may vary among C-4 biochemical subtypes (NADP-ME, NAD-ME, PCK) that differ in bundle sheath leakiness (Phi)) responses to drought and nitrogen supply. To address C-4 subtype responses to soil resource gradients, the carbon isotope discrimination (Delta), bundle sheath leakiness (Phi), leaf gas exchange (A, g(s), c(1)/c(a) and above-ground biomass accumulation were measured on three dominant grasses of semi-desert grassland in south-eastern Arizona. Bouteloua curtipendula (PCK), Aristida glabrata (NADP-ME) and the non-native Eragrostis lehmanniana (NAD-ME) were grown in control led-environment chambers from seed under a complete, multi-factorial combination of present ambient (370 ppm) and elevated (690 PPM) CO2 concentration and under high and low water and nitrogen supply. E. lehmanniana (NAD-ME) had the highest photosynthetic rate (A) and lowest Phi compared to the other two grasses when grown under low nitrogen and water availability. However, favourable water and nitrogen supply and elevated atmospheric CO2 enhanced photosynthetic performance and above-ground biomass production of B. curtipendula (PCK) to a greater extent than in A. glabrata and E. lehmanniana. Contrary to previous studies, Phi and Delta in the NADP-ME subtype (A. glabrata) were most affected by changing environmental conditions compared to the other subtypes; deviations from the classic NADP-ME anatomy in Aristida could have accounted for this result. Overall, response of semi-arid grasslands to rising atmospheric CO2 may depend more on species-specific responses to drought and nitrogen limitation than on general C-4 subtype responses.