Brassinosteroids improve the redox state of wheat florets under low-nitrogen stress and alleviate degeneration

Reducing nitrogen application rates can mitigate issues such as environmental degradation and resource wastage. However, it can also exacerbate problems such as wheat floret degeneration, leading to reduced yields. Therefore, investigating wheat floret degeneration mechanisms under low-nitrogen stress and identifying mitigation measures are conducive to achieving high yields and sustainable development. To investigate the physiological mechanism of how low-nitrogen stress affects wheat floret degradation and whether exogenous brassinosteroids (BRs) can alleviate this stress, experiments were designed with treatments of three nitrogen application rates (N0, no nitrogen application; N1, 120 kg ha–1 pure nitrogen; N2, 240 kg ha–1 pure nitrogen) and exogenous spraying (N0CK, no nitrogen with water spraying; N0BR, no nitrogen with 24-epibrassinolide (an active brassinosteroid) spraying; N1, 120 kg ha–1 pure nitrogen with water spraying). The results indicated that low-nitrogen stress generated a large amount of reactive oxygen species. Although wheat spikes synthesized flavonoids to combat oxidative stress, their energy metabolism (glycolysis and tricarboxylic acid cycle) and ascorbate-glutathione cycle were inhibited, which kept the reactive oxygen levels elevated within the spike, induced cell death and exacerbated floret degeneration. Furthermore, brassinosteroids played a role in regulating wheat floret degeneration under low-nitrogen stress. Exogenous foliar spraying of 24-epibrassinolide promoted energy metabolism and the ascorbate-glutathione cycle within the spike, which enhanced the energy charge and effectively mitigated a portion of the reactive oxygen induced by low-nitrogen stress, thereby alleviating the floret degeneration caused by low-nitrogen stress. In summary, low-nitrogen stress disrupts the redox homeostasis of wheat spikes, leading to floret degeneration, while brassinosteroids alleviate floret degeneration by improving the redox state of wheat spikes. This study provides theoretical support for balancing the contradiction between high yields and sustainable development and will be beneficial for the application of low nitrogen in production.