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

An investigation to determine whether stomatal acclimation to [CO₂] occurred in C₃/C₄ grassland plants grown across a range of [CO₂] (200–550 µmol mol^?1) in the field was carried out. Acclimation was assessed by measuring the response of stomatal conductance (g_s) to a range of intercellular CO₂ (a g_s–C_i curve) at each growth [CO₂] in the third and fourth growing seasons of the treatment. The g_s–C_i response curves for Solanum dimidiatum (C₃ perennial forb) differed significantly across [CO₂] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO₂] in this species was also found as maximum g_s (g_smax; g_s measured at the lowest C_i) increased with decreasing growth [CO₂] only below 400 µmol mol^?1. The substantial increase in g_s at subambient [CO₂] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation (V_cmax; r² = 0·27) and was not associated with CO₂ saturated photosynthesis (A_max). The response of g_s to C_i did not vary with growth [CO₂] in Bromus japonicus (C₃ annual grass) or Bothriochloa ischaemum (C₄ perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO₂] in both C₃ species, was not correlated with g_s. Larger stomatal size at subambient [CO₂], however, may be associated with stomatal acclimation in S. dimidiatum. Incorporating stomatal acclimation into modelling 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.