Dynamic changes of canopy-scale mesophyll conductance to CO₂ diffusion of sunflower as affected by CO₂ concentration...

Leaf-level measurements have shown that mesophyll conductance (g(m)) can vary rapidly in response to CO₂ and other environmental factors, but similar studies at the canopy-scale are missing. Here, we report the effect of short-term variation of CO₂ concentration on canopy-scale g(m) and other CO₂ exchange parameters of sunflower (Helianthus annuus L.) stands in the presence and absence of abscisic acid (ABA) in their nutrient solution. g(m) was estimated from gas exchange and on-line carbon isotope discrimination (Δ(obs)) in a ¹³CO₂/¹²CO₂) gas exchange mesocosm. The isotopic contribution of (photo)respiration to stand-scale Δ(obs) was determined with the experimental approach of Tcherkez et al. Without ABA, short-term exposures to different CO₂ concentrations (C(a) 100 to 900 µmol mol⁻¹) had little effect on canopy-scale g(m) . But, addition of ABA strongly altered the CO₂-response: g(m) was high (approx. 0.5 mol CO₂ m⁻² s⁻¹) at C(a) <200 µmol mol⁻¹ and decreased to <0.1 mol CO₂ m⁻² s⁻¹ at C(a) >400 µmol mol⁻¹. In the absence of ABA, the contribution of (photo)respiration to stand-scale Δ(obs) was high at low C(a) (7.2 ‰) and decreased to <2 ‰ at C(a) > 400 µmol mol⁻¹. Treatment with ABA halved this effect at all C(a) .