Dynamic changes of canopy-scale mesophyll conductance to CO2 diffusion of sunflower as affected by CO2 concentration and abscisic acidpce_2230

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