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

Both salt stress and high carbon dioxide (CO2) 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 CO2 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 CO2 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) CO2. 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 (Δ15N). 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 Δ15N correlated with the NUR and with the nitrate reductase activity. Under mild salt stress, N metabolism was better maintained under elevated CO2 levels than under ambient CO2 levels, since NUR, NTR, photosynthetic nitrogen use efficiency, and nitrate reductase activity were less affected, yielding lower Δ15N and higher growth. In addition, growth was negatively correlated with Δ15N indicating that the Δ15N determination may allow the estimation of barley growth. As a consequence of all these results, barley plants subjected to elevated CO2 levels will likely overcome mild saline conditions because of their capacity to maintain efficiency in N metabolism.

► We analyse the effect of salinity on nitrogen metabolism in barley and the possible effects provoked by elevated CO2 levels.
► Salt stress reduced the nitrogen uptake and translocation rates and nitrate reductase activity.
► Nitrogen isotope discrimination, protein concentration, photosynthetic nitrogen use efficiency, and growth decreased by salinity.
► Under mild salt stress, nitrogen metabolism was better maintained under elevated CO2 than under ambient CO2 levels.