Integrated Physiological, Morphological, and Transcriptomic Analyses Reveal Salt Adaptation in Silverleaf Sunflower (Helianthus argophyllus Torrey and Gray)

Silverleaf sunflower (Helianthus argophyllus Torrey and Gray) is an important salt-tolerant wild species with potential for improving cultivated sunflower, but its tolerance mechanisms remain unclear. Here, we compared the morphological, physiological, and transcriptomic responses of H. annuus and H. argophyllus under salt stress. H. argophyllus showed higher salt tolerance, which was accompanied by reduced membrane damage and better maintenance of photosynthetic performance under salt stress. This enhanced tolerance may be associated with lower stomatal density, dense trichomes, and a thick wax coating. The reference genome of cultivated sunflower (Helianthus annuus) provides limited insight into H. argophyllus because of substantial genomic variance between the two genotypes. Therefore, we generated a high-quality reference transcriptome of H. argophyllus using Iso-Seq long-read sequencing, identifying 50,153 unique genes and capturing over 91% of the gene repertoire. In total, 205 H. argophyllus-specific genes and 475 “fusion genes” were identified in the H. argophyllus transcriptome. Transcriptome profiling of leaves and roots under control and salt-stress conditions revealed upregulation of H. argophyllus-specific aquaporin genes. Commonly upregulated genes in both tissues were enriched in salt stress-responsive pathways. Genes upregulated only in the roots were more closely related to primary responses to abiotic stresses, while genes showing opposite regulation between roots and leaves reflected tissue-specific responses to salt stress. This study provides physiological, morphological, and transcriptomic insights into H. argophyllus salt tolerance and offers valuable resources for sunflower research and breeding.