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While nitrogen (N) derived from ammonium would be energetically less expensive than nitrate-derived N, the use of ammonium-based fertilizer is limited by the potential for toxicity symptoms. Nevertheless, previous studies have shown that exposure to elevated CO 2  favors ammonium assimilation in plants. However, little is known about the impact of different forms of N fertilizer on stomatal opening and their consequent effects on CO 2  and H 2 O diffusion in wheat plants exposed to ambient and elevated CO 2 . In this article, we have examined the response of the photosynthetic machinery of durum wheat ( Triticum durum , var. Amilcar) grown with different types of N fertilizer (NO 3  − , NH 4  + , and NH 4 NO 3 ) at 400 versus 700 ppm of CO 2 . Alongside gas exchange and photochemical parameters, the expression of genes involved in CO 2  ( PIP1.1  and  PIP2.3 ) and H 2 O ( TIP1 ) diffusion as well as key C and N primary metabolism enzymes and metabolites were studied. Our results show that at 400 ppm CO 2 , wheat plants fertilized with ammonium as the N source had stress symptoms and a strong reduction in stomatal conductance, which negatively affected photosynthetic rates. The higher levels of  PIP1.1  and  PIP2.3  expression in ammonium-fertilized plants at 400 ppm CO 2  might reflect the need to overcome limitations to the CO 2  supply to chloroplasts due to restrictions in stomatal conductance. This stomatal limitation might be associated with a strategy to reduce ammonium transport toward leaves. On the other hand, ammonium-fertilized plants at elevated CO 2  did not show stress symptoms, and no differences were detected in stomatal opening or water use efficiency (WUE). Moreover, similar gene expression of the aquaporins  TIP1 ,  PIP1.1,  and  PIP2.3  in ammonium-fertilized plants grown at 700 ppm compared to nitrate and ammonium nitrate plants would suggest that an adjustment in CO 2  and H 2 O diffusion is not required. Therefore, in the absence of a stress context triggered by elevated CO 2 , ammonium- and ammonium nitrate-fertilized plants were able to increase their photosynthetic rates, which were translated eventually into higher leaf protein content.

Differential Regulation of Stomatal Conductance as a Strategy to Cope With Ammonium Fertilizer Under Ambient Versus Elevated CO2