Aims Endophyte is a new microbial resource, which has a complex micro-ecological relationship with plants. In this paper, the antagonistic effect of endophytic fungi on the growth of salt-tolerant plants was confirmed by adding endophytic fungi to the plants and testing the physiological characteristics and biomass of the seedlings.
Methods Two salt-tolerant endophytic fungi Botryosphaeria sp. Z1 (T3), Hypoxylon sp. Y6 (T4), and their mixture (T2) were cultured in solid fermentation and added to potted soil planted with seedlings of Casuarina equisetifolia at different salinity (mass fraction: 0, 3‰, 6‰ and 9‰), which were treated with an added fungi of seedlings. The addition of microbiological medium (T1) and no added microbiological medium (CK) were treated as two experimental controls, which was to confirm the effect of the fungi, but not the microbiological medium. Samples were taken at 15 day intervals, four times, for a total of 60 days, and the relative electronic conductivity (REC), malondialdehyde (MDA) content, soluble sugar (SS) content, soluble protein (SP) content, proline (PRO) content, superoxide dismutase (SOD) activity, peroxidase (POD) activity, catalase (CAT) activity, reactive oxygen (H2O2) content of leaves and the biomass of seedling were determined each time.
Important findings We found that the biomass of seedlings treated by fungal strain was significantly increased under salt stress, and the physiological characteristics were significantly affected by soil salinity, time and fungal species. At 3‰ salinity, the contents of SS and SP, and the activity of SOD and CAT were higher in T2 treatment than in CK and T1 after 15 days of treatment, but the REC was significantly lower in T2, T3 and T4 treatments during the stress period. At 9‰ salinity, T2 treatment significantly reduced the REC and the contents of H2O2 and MDA, but significantly increased the content of SS and the activity of SOD and CAT. Especially, SP content and POD activity were also significantly increased in T2 treatment after 60 days. A stepwise regression was used to model the relationship between physiological characteristics and biomass of seedlings treated with the strains, and the path analysis showed that REC, POD activity and SS content were the main physiological factors affecting biomass. In conclusion, the effects of different fungi on the content of osmoregulatory substances, oxidase activity and biomass of seedlings were different, and the antagonistic effect of the mixed fungi was the most obviously. Thus, the relationship between osmotic regulatory substances, antioxidant enzymes, biomass and endophytic fungi of C. equisetifolia under salt stress was clarified, which provided a basic for the further study on the stress resistant engineering fungi for C. equisetifolia.