Research on the Response Mechanism of the Photosynthetic System of Panax ginseng Leaves to High-Temperature Stress

Ginseng is widely regarded as the “King of Herbs” in traditional Chinese medicine. In recent years, escalating global warming and intensified human activities have led to a continuous rise in environmental temperatures, posing a significant threat to ginseng cultivation in China. Therefore, understanding how high-temperature stress affects the photosynthetic performance of ginseng is essential for developing efficient and sustainable cultivation practices. In this study, four temperature regimes were established to systematically investigate the impact of elevated temperatures on the photosynthetic system of ginseng leaves: 25/16 °C (CK), 30/20 °C, 35/24 °C, and 40/28 °C (day/night). The results demonstrated that high-temperature stress significantly inhibited photosynthesis. Specifically, the activities of key chlorophyll biosynthesis enzymes—porphobilinogen deaminase and delta-aminolevulinate dehydratase—were markedly reduced, resulting in the accumulation of critical intermediates in the chlorophyll pathway, including protoporphyrinIX, Mg-protoporphyrinIX, and protochlorophyll. Chlorophyll synthesis was severely impaired as a result. Consequently, the contents of chlorophyll a, chlorophyll b, and carotenoids declined by 25.38%, 12.52%, and 54.63%, respectively, indicating substantial disruption of the photosynthetic pigment system. Anatomical observations revealed that high-temperatures induced stomatal closure, impairing stomata exchange and further reducing photosynthetic efficiency. Moreover, chloroplast ultrastructure was severely compromised, characterized by excessive accumulation of osmiophilic granules, disorganized and loosely stacked thylakoid membranes, and impaired capacity for light energy capture and conversion. This study provides theoretical insights into the response mechanisms of ginseng leaf photosynthesis under heat stress and establishes a scientific basis for enhancing thermotolerance through breeding programs and improved cultivation management strategies.