Halotolerant plant growth-promoting bacteria Enterobacter sp. Av16 and Acinetobacter sp. Av23 enhance seed germination and seedling photosynthesis of Apocynum pictum under salt stress

Apocynum pictum Schrenk, a halophyte, is commonly used as a traditional Chinese medicine, tea, and fiber crop. To improve the growth of A. pictum in saline soil, its responses to halotolerant plant growth-promoting bacteria (PGPB) were investigated at germination and during early growth stages. Inoculation with either Enterobacter sp. Av16 or Acinetobacter sp. Av23 significantly improved seed germination percentage and alleviated the adverse effects of salinity on seedling growth of A. pictum. Under salt stress, PGPB increased leaf area and improved photosynthetic pigments, including chlorophyll a+b and carotenoids, as well as intercellular carbon dioxide (Ci) and transpiration rate (Tr). More importantly, PGPB alleviated salt-induced damage to the photosynthetic apparatus by stabilizing the photosystems and optimizing electron transport processes. This was evidenced by increases in the density of reaction centers per cross-section (RC/CSm) and the efficiencies of electron transfer to photosystem I (δRo and ΦRo). Consequently, PGPB improved chlorophyll fluorescence and key photosynthetic parameters, including the maximum quantum yield (ΦPo), performance index on absorption basis (PIabs), overall performance index (PItotal), and net photosynthetic rate (Pn). Furthermore, PGPB activated antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), reducing the accumulation of reactive oxygen species (ROS) in A. pictum. In summary, PGPB enhanced A. pictum seed germination and photosynthetic capacity by stabilizing photosystems, improving stomatal gas exchange, and mitigating oxidative stress under salt stress. These findings highlight the potential of PGPB inoculation as a sustainable strategy to enhance salt resilience in A. pictum.