Grain filling is a pivotal determinant of wheat (Triticum aestivum L.) yield, relying on the efficient coordination of source–sink dynamics and assimilate partitioning. Jasmonic acid (JA) is widely involved in plant growth and development, and also plays an important role in resource allocation. However, it remains unclear if JA is involved in regulation of grain filling and assimilate transport in wheat. The objective of this study was to identify the roles of JA in regulating wheat grain filling and weight, by comparing cultivars, comparing superior and inferior grains of the same spikes, manipulating source–sink relationship by removing superior grains, and applying exogenous JA. The results showed the negative association between endogenous JA level and grain filling rate and final grain weight, irrespective of cultivars and source-sink treatments. JA increased assimilate retention in stems by reducing fructan mobilization and diminishing sucrose availability for grain development. In addition, JA suppressed sucrose unloading by reducing invertase (CWI and SAI) activities, and inhibited starch biosynthesis through reducing starch synthase (AGPase, GBSS and SBE) activities. These coordinated changes limited substrate sugar supply and starch accumulation, ultimately slowing grain filling rate. Overall, our findings reveal that JA changes the source–sink relationship, favoring carbon retention in vegetative tissues and reducing assimilate partitioning into grain.

The trade-off between yield and environmental effects caused by nitrogen fertilizer application is an important issue in wheat production. A reduction in fertile florets is one of the main reasons for the lower yields under low nitrogen application rates. Brassinosteroids (BRs) have been found to play a role in nitrogen-induced rice spikelet degeneration. However, whether BRs play a role in wheat floret development and the mechanisms involved are not clear. Therefore, a nitrogen gradient experiment and exogenous spraying experiment were conducted to investigate the role and mechanism of BRs in wheat floret development under low nitrogen stress. The results showed that as the nitrogen application decreased, the endogenous BRs content of the spikes decreased, photosynthesis weakened, and total carbon, soluble sugar and starch in the spikes decreased, leading to a reduction in the number of fertile florets. Under low nitrogen stress, exogenous spraying of 24-epibrassinolide promoted photosynthesis, and stimulated stem fructan hydrolysis and the utilization and storage of sucrose in spikes, which directed more carbohydrates to the spikes and increased the number of fertile florets. In conclusion, BRs mediate the effects of nitrogen fertilizer on wheat floret development, and under low nitrogen stress, foliar spraying of 24-epibrassinolide promotes the flow of carbohydrates from the stem to the spikes, alleviating wheat floret degeneration.

Drought stress at the booting stage causes severe floret degeneration and a decrease in grain number. Polyamines are involved in wheat floret development under drought stress, but the underlying physiological mechanisms are unclear. This study showed that drought-induced accumulation of reactive oxygen species led to wheat spikelet cell apoptosis and floret degeneration. Drought induced stomatal closure to reduce photosynthesis, then inhibited the activities of sucrose phosphate synthase, sucrose synthetase (cleavage direction) and ADP-glucose pyrophosphorylase in spikes and leaves, and soluble vacuolar invertase and cell wall invertase in spikes, thus providing a poor nutrient base for floret development. Exogenous spermidine application increased antioxidant enzyme activities and polyamine metabolism, promoted starch and sucrose metabolism, amino acid utilization and increased the levels of glycolytic and tricarboxylic acid cycle intermediates to mitigate oxidative damage and maintain energy homeostasis in the spike, thereby reducing floret degeneration and increasing grain number.

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.