Stomatal acclimation over a subambient to elevated CO2 gradient in a C3/C4 grassland

Authors: Maherali, H, CD Reid, HW Polley, HB Johnson, RB Jackson

We determined whether stomatal acclimation to [CO2] occurs in a C3/C4 grassland along a continuous gradient of [CO2] (200 to 550 mmol mol-1). Acclimation was assessed by measuring the response of stomatal conductance (gs) to a range of CO2 (a gs:Ca curve) at each growth [CO2] in the third and fourth growing seasons of the treatment. The slope of the gs:Ca response curves for Solanum dimidiatum (C3 perennial forb) did not vary across elevated [CO2], but increased significantly at growth [CO2] lower than current ambient, indicating that stomatal acclimation occurred below a threshold [CO2]. The substantial increase in gs at subambient [CO2] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (Vcmax; r2 = 0.27) and was not associated with CO2 saturated photosynthesis (Amax). The response of gs to Ca did not vary with growth [CO2] in Bromus japonicus (C3 annual grass) or Bothriochloa ischaemum (C4 perennial grass). Stomatal density, which increased with rising [CO2] in both C3 species, was not correlated with gs. Larger stomatal size at subambient [CO2], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modeling studies could improve the ability to predict changes in ecosystem water fluxes and water availability with rising CO2 and to understand their magnitudes relative to the past.
pce02.pdf