Salicylic acid modulates source-flow-sink coordination to stabilize carbon allocation and grain yield in waxy maize under heat stress

Heat stress (HT) during grain filling severely constrains maize yield formation by disrupting whole-plant carbon allocation. Although salicylic acid (SA) is known to enhance crop thermotolerance, its role in coordinating source-flow-sink carbon dynamics under HT remains poorly understood. In this study, two waxy maize hybrids were pre-sprayed with SA and subsequently exposed to 15 days of HT treatment during grain filling. HT reduced post-anthesis dry matter accumulation and translocation, inhibited sucrose metabolism in leaves and grains, down-regulated sugar transporter genes in the stem and grains, and consequently decreased 13 C allocation to developing grains and grain yield. SA reprogrammed source-flow-sink coordination under HT by simultaneously enhancing carbon supply, transport capacity, and sink utilization. At the source, SA sustained assimilate accumulation and maintained sucrose phosphate synthase, sucrose synthase, and invertase activities in leaves. Along the transport pathway, SA up-regulated key sucrose transporter genes in stems, thereby improving sucrose loading and long-distance translocation. At the sink, SA stabilized sucrose metabolism and transporter expression in grains, resulting in increased carbon partitioning to developing kernels. Partial least squares path modeling further identified coordinated regulation of sucrose metabolism in leaves and grains, together with sucrose transport in the stem and grains, as the main pathways through which SA mitigated heat-induced yield loss. Overall, exogenous SA alleviated the adverse effects of HT in waxy maize by reprogramming source-flow-sink carbon coordination, thereby stabilizing carbon allocation and maintaining yield under HT conditions.