Response of sorghum genotypes to water deficit stress under different CO₂ and nitrogen levels

An open-top chamber experiment was conducted in the present study to investigate the growth and biochemical responses of six sorghum genotypes under two irrigation regimes (based on 40% and 75% soil-water depletion) and two N fertilizer levels (0 and 105 kgN ha^-1), at two atmospheric CO₂ conditions (390 ± 50 and 700 ± 50 μmol mol^-1). The results revealed that water limited stress decreased the plant dry weight by reducing the plant leaf area, SPAD value, Fv/Fm, leaf RWC and membrane stability index (MSI), while it increased the specific activity of APX, CAT and POX enzymes, DPPH, LPC, Phe, TSC, H₂O₂, MDA and EL. The lowest decrease of the plant dry weight due to limited water stress was observed in the GS5 genotype, which was followed by the lowest decrease in the leaf RWC and MSI; also, the highest increase was seen in APX, Phe and TSC, while the lowest one was recorded in EL. Elevated CO₂ improved DPPH and Phe under both control and water limited conditions, resulting in the decrease of APX, POX, H₂O₂ and EL, while a more pronounced effect was observed in the stress conditions. Furthermore, with the application of nitrogen, the reduction in DPPH, H₂O₂ and MDA values was greater under water limited stress rather than control conditions. It could be, therefore, concluded that the responses of sorghum genotypes to water deficit stress had interaction with atmospheric CO₂ concentrations and nitrogen levels; this could be considered in breeding programs as well as planting management of sorghum.