Physiological and Cellular Ultrastructural Responses of <i>Isatis indigotica</i> Fort. under Salt Stress

This study aimed to analyze the effects of salt stress on the growth physiology and plant-cell ultrastructure of <i>Isatis indigotica</i> Fort. (<i>I. indigotica</i>) to evaluate its adaptability under salt stress. The effects of different concentrations of salt (NaCl; 0, 25, and 300 mmol·L⁻¹) on the agronomic traits, activities of related enzymes, ion balance, and mesophyll-cell ultrastructure of <i>I. indigotica</i> were studied in a controlled pot experiment. Results showed that compared with those of the control group, the aerial-part fresh weight, underground fresh weight, tiller number, root length, root diameter, plant height, and leaf area of salt-stressed <i>I. indigotica</i> increased at 25 mmol·L⁻¹ and then decreased at 300 mmol·L⁻¹. The changes in levels of superoxide dismutase, peroxidase, ascorbate peroxidase, and catalase showed a similar trend, with significant differences compared with control group. Salt stress altered the ion balance of <i>I. indigotica</i>, resulting in a significant increase in Na⁺ content and a significant decrease in K⁺ content. The contents of Ca²⁺ and Mg²⁺ changed to varying degrees. The analysis of the microstructure of the root showed that under salt treatment, the epidermal cells of the root significantly thickened and the diameter of the xylem decreased. The results of ultrastructural analysis of mesophylls showed that salt stress can cause cell-membrane contraction, cell-gap enlargement, disorder in the structures of chloroplasts and mitochondria, and an increase in the number of osmiophilic particles. These changes were aggravated by the increase in NaCl concentration. This study reveals the response of <i>I. indigotica</i> to salt stress and provides a basis for further study on the salt-tolerance mechanism of <i>I. indigotica</i>.