Physiological and Molecular Responses of <i>Apocynum venetum</i> L. (Apocynaceae) on Salt Stress

Soil salinization is a crucial factor that impacts plant distribution and growth. <i>Apocynum venetum</i>, an ornamental plant with medicinal value, has shown remarkable salt tolerance. However, the specific mechanisms through which <i>A. venetum</i> responds to salt stress are not yet fully understood. To address this gap, we conducted a study where 10-week-old <i>A. venetum</i> seedlings were subjected to salt stress by irrigating them with a nutrient solution containing varying concentrations of NaCl (0, 100, 200, and 350 mmol·L⁻¹). After the salt stress treatment, various growth indicators (such as plant height, root length, root fresh weight, root dry weight, leaf fresh weight, leaf dry weight, root water content, leaf water content, and root–leaf ratio) as well as physiological indicators (including SOD and CAT activities in both leaves and roots, soluble protein contents in leaves and roots, and chlorophyll and carotene contents in leaves) were determined. In addition, the gene expression profile of roots under salt stress was examined by transcriptome sequencing to explore the mechanism of salt response in <i>A. venetum</i>. Our results show that salt stress led to yellowing and wilting of <i>A. venetum</i> seedling leaves. Furthermore, the chlorophyll and carotenoid contents in the leaves of the 350 mmol·L⁻¹ NaCl-treated group were significantly reduced. Although the leaf and root biomass gradually decreased with an increase in the salt concentration, the root–leaf ratio exhibited a decreasing trend. NaCl stress also caused significant changes in physiological indices in the <i>A. venntum</i> leaves and roots. The activities of superoxide dismutase (SOD) and catalase (CAT) increased in both leaves and roots of the 100 mmol·L⁻¹ NaCl-treated group. The soluble protein content in both leaves and roots increased under the 200 mmol·L⁻¹ NaCl stress. To screen changes in root gene expression, transcriptome sequencing and qRT-PCR were performed. GO and KEGG enrichment analyses revealed that salt stress primarily affects carbohydrate metabolism, MAPK signaling transduction, phytohormone signaling pathways, glyoxylate and dicarboxylate metabolism, and other pathways. This study provides a novel understanding of the growth and physiological response of <i>A. venetum</i> leaf and root to NaCl stress, as well as the changes in the transcription levels in <i>A. venetum</i> root. The results serve as a reference for future research on salt-tolerant mechanisms and molecular breeding of <i>A. venetum.</i>