Interactive effects of CO₂ and nitrogen supply on growth and physiological traits of millet cultivars under drought stress

An experiment was conducted to evaluate the effects of irrigation regimes (irrigation after 40 and 75% depletion of available soil water) and nitrogen levels (0 and 85 mg N kg⁻¹ soil) on millet cultivars (Bastan and Pishahang) under two atmospheric CO₂ concentrations (390 ± 50 and 700 ± 50 µmol mol⁻¹). Evaluation of different biochemical traits at 50% flowering stage and measurement of dry matter weight at milk stage revealed that water shortage reduced shoot dry weight as the consequence of decreasing chlorophyll and carotenoid contents, leaf area, membrane stability index, and root dry weight but it increased not only the production of H₂O₂ but also MDA and antioxidant enzyme activities. Drought conditions with elevated levels of CO₂ and nitrogen supply, however, partly relaxed limitations observed under water shortage conditions so that H₂O₂ and MDA production decreased but chlorophyll content and leaf area increased, leading to reduced loss of shoot dry weight. Increased CO₂ had a greater effect on the reduction of ascorbate peroxidase and catalase in Bastan, while in Pishahang it had more effect on the reduction of malondialdehyde. Results showed that elevated CO₂ and nitrogen supply depended for their effects on both irrigation regime and the genotype grown.