Carbon dioxide enrichment moderates salinity-induced effects on nitrogen acquisition and assimilation and their impact on growth in barley plants

Abstract Both salt stress and high carbon dioxide (CO 2 ) levels can affect plant nitrogen (N) metabolism by acting in parallel, decreasing N metabolism; or acting in opposite directions, with salt stress decreasing N metabolism and elevated CO 2 levels enhancing it. The objective of this work was to analyse the effect of salinity on N acquisition, distribution, and assimilation, the consequences of these effects on growth in barley ( Hordeum vulgare L., cv. Iranis), and the possible effects on these processes provoked by elevated CO 2 levels. Several steps of N metabolism were studied in H. vulgare plants exposed to 0, 80, 160, or 240 mM NaCl under ambient (350 μmol mol −1 ) or elevated (700 μmol mol −1 ) CO 2 . Salt stress reduced the N uptake (NUR) and translocation (NTR) rates and nitrate reductase (EC 1.7.1.1) activity, altering plant N isotope discrimination (Δ 15 N). Although salt stress increased glutamine synthetase (EC 6.3.1.2) activity, N and protein content, and photosynthetic nitrogen use efficiency decreased. The decrease in nitrate reductase activity was related to decreases in NUR and NTR, while Δ 15 N correlated with the NUR and with the nitrate reductase activity. Under mild salt stress, N metabolism was better maintained under elevated CO 2 levels than under ambient CO 2 levels, since NUR, NTR, photosynthetic nitrogen use efficiency, and nitrate reductase activity were less affected, yielding lower Δ 15 N and higher growth. In addition, growth was negatively correlated with Δ 15 N indicating that the Δ 15 N determination may allow the estimation of barley growth. As a consequence of all these results, barley plants subjected to elevated CO 2 levels will likely overcome mild saline conditions because of their capacity to maintain efficiency in N metabolism.