Photosyntesis was similar to that of unstressed
plants when the stressed plant leaves were treated with elevated
CO2. This demonstrates that a rise in the external CO2 concentration
can completely compensate the physical leaf resistances
imposed by a short-term non-lethal water deficit, thus enabling
the 390 WS/750 plants to exhibit the same Pn level as plants grown
with good water availability. That result of Pn suggests that water
stress affected the CO2 diffusion from the atmosphere to the carboxylation
site, while the carboxylation capacity of the mesophyll
remained unaffected, confirming that physical resistance of gas
transfer explains almost exclusively the reduction in Pn atthe initial
stage of water deficit