Document Type : Research Paper
Authors
Agriculture, Plant Production, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, Iran
10.22092/sbj.2026.373894.299
Abstract
Background and Objectives: Soybean (Glycine max L.) is widely recognized as one of the most important crops within the legume family, and it is undeniably one of the main sources of protein and vegetable oil in the world, and plays an exceptionally important role in human nutrition, animal feed, and the food industry. Despite the immense economic and nutritional importance of this plant, the occurrence of environmental stresses, especially flooding or waterlogging, can significantly reduce its overall growth and performance. Flooding disrupts root activity as well as water and nutrient absorption by severely reducing the available oxygen levels in the root zone, and consequently, it negatively affects the overall nutritional status of the plant by reducing root nodule activity and disrupting the vital process of biological nitrogen fixation. Therefore, the strategic use of symbiotic nitrogen-fixing bacteria, especially Rhizobium species, can be considered as one of the most effective biological solutions to reduce the adverse effects of this particular stress. Given the inherent genetic differences between soybean cultivars in their natural tolerance to flooding and their specific response to bacterial inoculation, identifying highly tolerant and efficient cultivars in terms of symbiosis with Rhizobium is of particular importance. Accordingly, this study was specifically conducted to evaluate the effect of waterlogging stress and Rhizobium inoculation on yield, yield components, and biological nitrogen fixation in selected soybean cultivars.
Materials and Methods: This research was conducted during the course of the 2022-2023 crop year as a controlled pot experiment at the Gorgan University of Agricultural Sciences and Natural Resources. The experiment was carried out as a factorial arrangement structured within a completely randomized design with three replications. The experimental treatments included four distinct soybean cultivars, specifically including Sari, Saman, Ketul, and Amir; two levels of bacterial inoculation, specifically including active inoculation and no inoculation; and two levels of waterlogging stress, specifically including no flooding and flooding for a consecutive period of 15 days. The seeds designated for the inoculation treatment were inoculated with Bradyrhizobium japonicum bacteria before the sowing process. Waterlogging stress was applied at the beginning of the flowering stage (referred to as the R1 developmental stage). At the end of the applied stress period, physiological traits including chlorophyll a, chlorophyll b, and carotenoids; traits related to nitrogen nutrition status and biological nitrogen fixation including leaf nitrogen content, as well as the number and diameter of root nodules; as well as yield components including the total number and weight of pods, the number of seeds per pod, and the final grain yield were meticulously measured and compared. After collecting the experimental data, they were statistically analyzed using an analysis of variance (ANOVA) procedure and comparing the means of the treatments.
Results: The analytical results showed that waterlogging stress and bacterial inoculation, as well as their mutual interaction with the cultivar, had significantly different effects on the measured physiological traits, nitrogen fixation, and the ultimate soybean yield. Under normal, non-stress conditions, Rhizobium inoculation increased the overall content of chlorophyll a and b in the majority of the evaluated cultivars; so that the highest proportional increase in chlorophyll a was observed in the Sari cultivar, and the highest increase in chlorophyll b was observed in the Saman cultivar. Also, bacterial inoculation successfully increased the content of carotenoids in all of the tested cultivars. Waterlogging stress substantially reduced photosynthetic pigments and leaf nitrogen, but the severity of this decrease was notably different across the different cultivars. Bacterial inoculation under waterlogging conditions was able to compensate for a significant part of the reduction in photosynthetic pigments. In terms of traits related to nitrogen fixation, Rhizobium inoculation increased the number of root nodules per plant under non-stress conditions in the Ketul and Amir cultivars from 12 to 28 and from 18 to 31 nodules, respectively. Under waterlogging conditions, the evaluated cultivars Ketul and Amir showed a more favorable response to the bacterial inoculation by successfully maintaining a higher number of functional root nodules. The detailed study of yield components also showed that the effect of inoculation on the number of pods was different in the different cultivars, and the response of the cultivars to waterlogging conditions was not the same across the board. Under non-stress conditions, bacterial inoculation increased the grain yield of the cultivar Sari from 28.6 to 68.4 g/plant; while a decrease in yield was observed in the cultivar Ketul. Also, under waterlogging conditions, bacterial inoculation increased the grain yield from 29.5 to 55.4 g/plant, indicating a clear and partial reduction in the overall stress-induced damage.
Conclusion: In general, waterlogging stress negatively affected the overall soybean yield by disrupting the essential photosynthetic activity, leaf nitrogen status, and the total number of active root nodules; however, the intensity of the physiological stress response and the degree of benefit derived from the bacterial inoculation varied among the tested cultivars. Bradyrhizobium japonicum inoculation helped to successfully maintain the optimum nitrogen nutrition status and effectively reduce the negative effects of waterlogging stress by improving some key physiological traits and increasing the overall capacity for nodulation. Among the cultivars that were studied, the Sari cultivar showed better performance in terms of maintaining photosynthetic pigments and sustaining final grain yield, and the Ketul and Amir cultivars showed better performance in terms of nodulation efficiency and positive response to the applied bacterial inoculation. Therefore, selecting an appropriate cultivar along with using Bradyrhizobium japonicum inoculation can be highly recommended as an effective and practical strategy to increase soybean tolerance to waterlogging stress and to significantly improve production sustainability under environmental conditions that are susceptible to this type of stress.
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