Combined effects of elevated CO2 and nitrogen fertilizers on plant physiological variables in two contrasting Azolla spp.

• Effect of elevated CO 2 and N-fertilizer was assessed in Azolla microphylla and A. pinnata under OTC. • Elevated CO 2 without N-fertilizer enhanced biochemical and physiological properties of Azolla. • N-fertilizer under ambient CO 2 reduced growth rate and abundance of cya and nifH genes in Azolla. • Interestingly, elevated CO 2 nullify the negative effect of high dose of N-fertilizer on growth rate, nitrogen fixation, photosynthesis and antioxidants in both Azolla species under OTC. • Higher values of these parameters were observed in Azolla microphylla than A. pinnata. Azolla is an aquatic fern and has multipurpose use in agriculture. Elevated carbon dioxide (eCO 2) and nitrogen (N) fertilizer are two important factors, influencing the growth, N-fixation and photosynthetic efficacy of Azolla. The combined effect of eCO 2 and N-fertilizer on growth and physiological parameters of Azolla is largely unknown particularly under CO 2 -maintained- open top chamber (OTC) condition. Therefore, the present study was conducted in OTC to assess these parameters under combined influence of eCO 2 [(550 ± 20 μmol mol−1 as compared to ambient CO 2 (aCO 2) i.e. 400 ± 10 μmol mol−1)] and higher dose of N-fertilizer (60 kg N ha−1, 50 % higher than recommended dose of N for duel cultivation of Azolla and rice i.e. 40 kg N ha−1) along with control (no N-fertilizer) in two Azolla species (Azolla microphylla and A. pinnata). The results indicated that growth, chlorophyll, carotenoids, cyanobiont heterocyst frequency, nitrate reductase, abundance of cya and nifH gene copy numbers were significantly higher under eCO 2 treatments without N-fertilizer, irrespective of Azolla species. The addition of N-fertilizers reduced all parameters except antioxidants in both aCO 2 and eCO 2. Interestingly, between two Azolla species, these parameters were observed higher in A. microphylla compared to A. pinnata. Treatments with aCO 2 separated from eCO 2 under influence of N-fertilizers, implied the significant interaction of CO 2 and N-fertilizers in both Azolla species. Moreover, two-way interactions of eCO 2 and higher N-fertilizer dose, showed a significant difference of all parameters in both Azolla species. Overall, the present study suggested that combination of eCO 2 and N-fertilizer, nullify the negative effect of higher dose of N-fertilizer on physiological variables in both Azolla species under CO 2 -maintained-OTC. [ABSTRACT FROM AUTHOR]