Elevated Ammonia Increases Water and Nitrogen Use Efficiencies by Raising Net Photosynthesis and Transpiration Rate in Winter Wheat

This study investigated the effect of atmospheric ammonia (NH₃) on physiological traits and water- and nitrogen- (N) use efficiencies in wheat seedlings exposed to two N rates viz. control (−N) and recommended N dose (+N). Plants of two cultivars viz. high N efficient (Xiaoyan No. 6 (X6)) and low N efficient cultivar (Changhan No. 58 (C58)) were exposed to ambient (control, 0 nL L⁻¹) and elevated level of NH₃ (1000±40 nL L⁻¹) in open-top chambers. As compared with ambient NH₃, elevated NH₃ increased the rate of photosynthesis by 34.02 and 41.75%, transpiration rate by 21.59 and 20.05%, and instantaneous transpiration efficiency by 42.35 and 18.70% (averaged for both N levels) in X6 and C58, respectively. The mean increase in photosynthetic, transpiration, and instantaneous transpiration efficiency as the result of NH₃ enrichment for −N and +N treatments (averaged for both genotypes) was 40.49 and 43.39%, 34.02 and 41.75%, and 61.19 and 28.84% respectively, over ambient NH₃. An increase in water use efficiency, nitrogen apparent recovery fraction, and root N-uptake efficiency in the range of 32–144.59% was recorded for elevated NH₃ treatment compared with ambient NH₃. As compared with ambient NH₃, elevated NH₃ decreased overall N-use efficiency by 20.0 and 47.61% in X6 and 15.62 and 52.77% in C58 under −N and +N, respectively. Plant exposure to elevated NH₃ increased the photosynthetic and transpiration rates, water use efficiency, and Nr and root N-uptake efficiency while decreasing overall N-use efficiency as compared with ambient NH₃.