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Improving the productivity of cropping systems in terms of irrigation water use and nitrogen (N) fertilizer and exploring the associated effective physiological traits are priorities mostly in water-limited areas. Therefore, this field experiment was conducted on a clay loam soil (thermic family of Typic Haplargids) in central Iran with the three planting dates of 2016 (June 30), early 2017 (June 10), and late 2017 (July 11). Three forage sorghum genotypes including SF002, SF001, and Pegah, three grain sorghum genotypes of MGS5, GS24, GS28, and one common corn hybrid were grown under two irrigation regimes (55% and 85% of the maximum allowable depletion – MAD) as well as two N levels (0 and 112.5 kg N ha−1 in the form of urea, 46% N). The results showed considerable genetic variation among the sorghum genotypes in terms of yield. Due to water-limited conditions, the grain and biomass yields of the corn hybrid were decreased more those of sorghum genotypes. However, higher potential sorghum genotypes recorded total dry biomass (shoot biomass and grain yield) values similar to those of corn under both normal and deficit irrigation regimes. On the other hand, the grain share of the total biomass in the corn hybrid was higher in the two irrigation regimes. Under deficit irrigation, the use efficiency values of irrigation water (IWUEb) and N fertilizer (NUEb) for the biomass yield in Pegah and GS24 were higher than those in the corn hybrid. However, IWUEg and NUEg in corn were significantly higher under both irrigation regimes compared to those recorded for even the high-yield potential genotypes of the grain and forage sorghum. The positive effects of N application on the plants declined under water-limited stress, but the negative effects of water deficit stress were reduced with N application, while dry matter and grain yield increased as a consequence of the increase in the maximum leaf area index, chlorophyll a, chlorophyll b, and carotenoid contents, as well as the enhanced antioxidant activities of catalase, ascorbate peroxidase, and peroxidase enzymes. It can be concluded that the corn-based planting system is superior to sorghum even under low irrigation conditions, and N supply could moderate the negative effects of water shortage stress on plant growth. |
