In-season variable split-nitrogen application and water levels impact agronomic and grain quality traits in sorghum



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Grain sorghum (Sorghum bicolor, L.) is an important crop, which is widely used as food, forage, fodder and biofuel. Despite its natural adaption to resource-poor and stressful environments, increasing yield potential of sorghum under more favorable conditions holds promise. To meet the global increase in demand for gluten-free and antioxidant rich sorghum-based food, improving grain quality parameters especially protein-content and -digestibility of sorghum is critical. Nitrogen (N) is the most important nutrient for crops, having a dynamic impact on all growth, yield, and grain-quality-determining processes. Thus, increasing N use efficiency (NUE) in sorghum would provide opportunities to achieve higher yield and better-quality grain. This dissertation aims to assess the impacts and interactions of different levels of irrigation and the application of in-season split N on the nitrogen uptake, grain quality and amino acid composition in grain sorghum. To address these objectives, three seasons of field experiments from 2020 to 2022 were conducted in the states of Kansas and Texas. The experiments conducted provide valuable insights into the impact of irrigation amount and nitrogen application on the growth, yield, nutrient and amino acid composition in commercially available sorghum hybrids. Where, opportunities to increase grain protein content were identified while using split N applications, with panicle initiation identified as a critical developmental stage. Also, high-throughput phenotyping methods like remotely sensed vegetative indices NDVI and NDRE, have the capabilities to predict grain yield and protein, respectively. Enhanced grain yield and quality opportunities were found within the bottom portions of the panicle, where grain number and protein were lower than that of the upper portions. The need for balancing stay-green and senescence was recognized. Further, these studies were successful in showing that as the total protein increased, the kafirin portion of the protein vitreous endosperm of the grains increased, whereas the albumin-globulin levels decreased. Increased application of N had a significant effect on the concentration of 15 of the 22 amino acids analyzed. Results showed that although all amino acids increase with increased protein content, they do not increase proportionally, resulting in some key amino acids, like lysine, diluted. An optimal rate with different timings of N was confirmed at 92 kg N ha-1 applied equally across three growth stages including planting, panicle initiation, and booting for maximum protein and amino acid concentration. Moreover, interactions of irrigation, nitrogen, and hybrid was found for chlorophyll concentration at grain fill, shoot biomass, nitrogen content of leaves and panicles, and thousand grain weight. Importantly, this study was successful in finding that grain yield is significantly increased under split nitrogen application when compared to basal application. Also, under split N application with increased irrigation, the shoot (post grain harvest) of the sorghum plant can store over 50 kg N ha-1, providing more insight on the sustainability of the crop. An analysis of the complete amino acid profile of 11 commercial hybrids across different irrigation and N levels revealed that the combination of these treatments had differential impacts on the composition of multiple amino acids. Findings from these studies provide valuable insights into the impact of irrigation amount and nitrogen application on the growth, yield, nutrient and amino acid composition in commercially available sorghum hybrids.



NUE, Sustainability, Nitrogen, Sorghum, Protein, Amino Acid, Grain Quality