Elevated CO2 reduces the drought effect on nitrogen metabolism in barley plants during drought and subsequent recovery

The objective of this study was to determine the response of nitrogen metabolism to drought and recovery upon rewatering in barley (Hordeum vulgare L.) plants under ambient (350 μmol mol−1) and elevated (700 μmol mol−1) CO2 conditions. Barley plants of the cv. Iranis were subjected to drought stress for 9, 13, or 16 days. The effects of drought under each CO2 condition were analysed at the end of each drought period, and recovery was analysed 3 days after rewatering 13-day droughted plants. Soil and plant water status, protein content, maximum (NRmax) and actual (NRact) nitrate reductase, glutamine synthetase (GS), and aminant (NADH-GDH) and deaminant (NAD-GDH) glutamate dehydrogenase activities were analysed. Elevated CO2 concentration led to reduced water consumption, delayed onset of drought stress, and improved plant water status. Moreover, in irrigated plants, elevated CO2 produced marked changes in plant nitrogen metabolism. Nitrate reduction and ammonia assimilation were higher at elevated than at ambient CO2, which in turn yielded higher protein content. Droughted plants showed changes in water status and in foliar nitrogen metabolism. Leaf water potential (Ψw) and nitrogen assimilation rates decreased after the onset of water deprivation. NRact and NRmax activity declined rapidly in response to drought. Similarly, drought decreased GS whereas NAD-GDH rose. Moreover, protein content fell dramatically in parallel with decreased leaf Ψw. In contrast, elevated CO2 reduced the water stress effect on both nitrate reduction and ammonia assimilation coincident with a less-steep decrease in Ψw. On the other hand, Ψw practically reached control levels after 3 days of rewatering. In parallel with the recovery of plant water status, nitrogen metabolism was also restored. Thus, both NRact and NRmax activities were restored to about 75–90% of control levels when water supply was restored; the GS activity reached 80–90% of control values; and GDH activities and protein content were similar to those of control plants. The recovery was always faster and slightly higher in plants grown under elevated CO2 conditions compared to those grown in ambient CO2, but midday Ψw dropped to similar values under both CO2 conditions. The results suggest that elevated CO2 improves nitrogen metabolism in droughted plants by maintaining better water status and enhanced photosynthesis performance, allowing superior nitrate reduction and ammonia assimilation. Ultimately, elevated CO2 mitigates many of the effects of drought on nitrogen metabolism and allows more rapid recovery following water stress.

Research highlights
► In irrigated plants, nitrate reduction and ammonia assimilation were higher at elevated than at ambient CO2, which in turn yielded higher protein content.
► NRact and NRmax activity declined rapidly in response to drought under ambient CO2. Similarly, drought decreased GS whereas NAD-GDH rose. Moreover, protein content fell dramatically in parallel with decreased leaf water potential.
► In contrast, elevated CO2 reduced the water stress effect on both nitrate reduction and ammonia assimilation coincident with a less-steep decrease in water potential.
► On the other hand, after re-irrigation, in parallel with the recovery of plant water status, nitrogen metabolism was also restored. The recovery was always faster and slightly higher in plants grown under elevated CO2 conditions compared to those grown in ambient CO2.
► The results suggest that elevated CO2 improves nitrogen metabolism in droughted plants by maintaining better water status and enhanced photosynthesis performance, allowing superior nitrate reduction and ammonia assimilation.