Interactive effect of drought stress and high planting density on flower/pod abscission of soybean

Flower/pod abscission in soybean substantially affects yield formation, and conditions such as drought and high planting density further induce extensive flower/pod abscission. Yet, the synergistic regulatory mechanisms under interactive stress remain unclear. In this study, the density-tolerant cultivar SN12 and the conventional cultivar SN28 were used as experimental materials to systematically investigate the regulatory mechanisms of flower/pod abscission in response to drought stress (D), high-density planting stress (M), and interactive stress of drought and high-density planting stress (MD). The results showed that stress treatments significantly increased the flower/pod abscission rate, with the interactive stress exerting the strongest effect. They also significantly reduced the net photosynthetic rate and transpiration rate, thereby decreasing carbon assimilation capacity in source tissues-especially in the conventional cultivar SN28. Soluble sugars, proline, and soluble proteins increased significantly, maintaining cellular osmotic potential and indicating a stress-induced reallocation of resources toward defense. Sugar metabolism analysis revealed that SN12 maintained floral carbon supply by upregulating SPS, SUS, and invertase activities, whereas SN28 suffered pronounced “sugar starvation”. Transcriptome analysis showed that interactive stress reshaped hormone signaling pathways; the upregulation of GH3 reduced active IAA levels, and Aux / IAA and SAUR genes exhibited opposite expression patterns between the two cultivars. Meanwhile, ethylene and ABA signaling were significantly activated in SN28. Concurrently, elevated β-Gal, PG, PME, and CE activities accelerated cell separation in the abscission zone. WGCNA revealed synergistic regulation of abscission by sugar metabolism and hormone signaling modules. In summary, interactive stress promoted flower/pod abscission via “reduced carbon supply from source tissues–sugar starvation of floral organs–hormonal imbalance”.