Performance of Camellia oleifera Seedlings Under Alkali Stress Improved by Spraying with Types of Exogenous Biostimulants

Exogenous biostimulants (EB) are crucial for reducing abiotic stress in plants. It is currently unclear how EB such as melatonin (MT), betaine (BA), and salicylic acid (SA) regulate the stress in Camellia oleifera seedlings under alkali stress (XP). This study demonstrates the moderating effect of SA (0.5, 1, and 2 mmol/L), BA (0.2, 0.4, and 0.8 g/L), and MT (200, 400, and 800 μmol/L) on the relative chlorophyll content, photosynthetic parameters, chlorophyll fluorescence parameters, osmoregulatory substances, and antioxidant enzymes in C. oleifera seedlings under XP. The results showed that spraying different types and different concentrations of EB under alkali stress had a certain alleviating effect on the phenotype of C. oleifera seedlings. Whether 7 or 15 days after the application of EB, the relative chlorophyll content (SPAD) and the degree of yellowish-green in the control group were different from those in the other 10 treatment groups, but the difference in brightness was not significant. As far as the malondialdehyde (MDA) content is concerned, the SA2, BA3, MT2, and MT3 treatment groups can significantly reduce the MDA content on the 7th day of EB application. The electrolytic leakage (EL) is also significantly reduced by MT2 and MT3. It was found that treatment groups SA3 and MT2 could improve the photosynthetic parameters of C. oleifera seedlings to different degrees on the 7th day of EB application. On the 15th day of EB application, treatment groups SA1, SA3, BA1, and BA2 all increased the photosynthetic rate of C. oleifera compared to the XP treatment group, but other treatments did not increase. At the same time, the results showed that the fluorescence parameters of the seedlings showed different degrees of improvement under different EB spraying conditions. Under alkali stress, soluble proteins (SP) and soluble sugars (SS) increased in the XP group, but it was found that the SA3, BA3, and MT2 treatment groups could reduce the content of osmoregulatory substances both on the 7th and 15th days of EB application. In terms of proline (Pro) content, BA1, BA2, and MT2 treatment groups could reduce Pro content on the 7th and 15th days of EB spraying, respectively. As for the antioxidant enzymes, the SA2, BA3, MT2, and MT3 treatment groups could basically increase the activity of antioxidant enzymes and further reduce oxidative damage on the 7th day of application of EB. According to the comprehensive results of the membership function, whether on the 7th or 15th day of EB spraying, the MT2 treatment group has the best overall mitigation effect of the three EB applications, ranking in the top three. This study will help to improve the scientific understanding of C. oleifera’s alkali resistance and interaction with EB while filling the knowledge gap on the physiological response to oleofylline stress.