Stomatal acclimation over a subambient to elevated CO 2 gradient in a C 3 /C 4 grassland

An investigation to determine whether stomatal acclimation to [CO 2 ] occurred in C 3 /C 4 grassland plants grown across a range of [CO 2 ] (200–550 µ mol 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 CO 2 (a g s – C i curve) at each growth [CO 2 ] in the third and fourth growing seasons of the treatment. The g s – C i response curves for Solanum dimidiatum (C 3 perennial forb) differed significantly across [CO 2 ] treatments, suggesting that stomatal acclimation had occurred. Evidence of non–linear stomatal acclimation to [CO 2 ] in this species was also found as maximum g s ( g smax ; g s measured at the lowest C i ) increased with decreasing growth [CO 2 ] only below 400 µ mol mol −1 . The substantial increase in g s at subambient [CO 2 ] for S. dimidiatum was weakly correlated with the maximum velocity of carboxylation ( V cmax ; r 2 = 0·27) and was not associated with CO 2 saturated photosynthesis ( A max ). The response of g s to C i did not vary with growth [CO 2 ] in Bromus japonicus (C 3 annual grass) or Bothriochloa ischaemum (C 4 perennial grass), suggesting that stomatal acclimation had not occurred in these species. Stomatal density, which increased with rising [CO 2 ] in both C 3 species, was not correlated with g s . Larger stomatal size at subambient [CO 2 ], 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 CO 2 and to understand their magnitudes relative to the past.