Effects of drought stress on chloroplast ultrastructure and physiological characteristics of Seriphidium transiliense seedlings

Drought stress is a common abiotic factor affecting plant growth and development. It has severely impacted the growth and development of Seriphidium transiliense with climate change and increasingly frequent extreme weather events. In this study, we conducted a continuous drought pot experiment on S. transiliense seedlings to analyze changes in chloroplast ultrastructure, reactive oxygen species (ROS), malondialdehyde (MDA), osmotic adjustment substances, antioxidant enzyme activity, and endogenous hormone content under drought stress. The results showed that as drought stress severity increased, chloroplast ultrastructure was progressively damaged: Granal lamellae became blurred, osmiophilic particles increased, starch granules initially increased and then decreased, and cell wall separation occurred. Under severe stress, chloroplasts disintegrated and ruptured. Physiological responses exhibited a significant (P < 0.05) increase in oxygen free radicals and MDA content in leaves and roots. The levels of proline (Pro), soluble sugars (SS), and antioxidant enzyme activity initially increased and then declined in both leaves and roots. However, soluble protein (SP) content followed a different pattern, initially increasing and then decreasing in leaves, while it gradually increased in roots. The endogenous hormone abscisic acid (ABA) initially increased and then decreased in both leaves and roots, whereas indole-3-acetic acid (IAA) showed a steady decline. In conclusion, as drought stress severity increases, S. transiliense seedlings accumulate osmotic adjustment substances, enhance antioxidant enzyme activity, and regulate endogenous hormone levels to mitigate oxidative stress and maintain cell membrane stability, thereby improving drought resistance.