Elevated CO2 concentration increased photosynthetic CO2 fix-
ation in sunflower primary leaves in comparison with ambient
CO2 concentrations (Fig. 3A), especially in young leaves (16 and
22 days). Stomatal conductance and transpiration rates in sun-
flower primary leaves decreased during leaf ontogeny, but these
parameters showed the highest values in the presence of ele-
vated CO2 concentrations, especially after 22 days (Fig. 3B and C).
The stomatal response to atmospheric changes has been exten-
sively studied on a wide variety of species, in which stomatal
conductance is usually reduced by elevated CO2 concentrations
(Long et al., 2004; Ainsworth and Rogers, 2007). Likewise, sto-
matal density decreases (Lake et al., 2002), thereby leading to a
decreased transpiration rate and an increased leaf temperature
(Long et al., 2004). Although stomata in most species close when
the CO2 concentration rises beyond certain levels, the response