24-Epibrassinolide and Methyl Jasmonate Promoted Seed Development of Styrax tonkinensis and Affected Seed Chemical Compositions, Especially Seed Lipid Metabolism

Due to the increasing demand for biofuel feedstock production, Styrax tonkinensis , as a woody oilseed tree species, has received much more attention. However, the effects of various plant growth regulators on its seed development haven’t been explored deeply. Here, we applied 24-epibrassinolide (EBL) and methyl jasmonate (MJ) on the whole trees, and the aim of our study was to explore their effects on seed development of S. tonkinensis as well as seed chemical compositions, especially seed lipid metabolism. The results showed that EBL and MJ promoted the seed development, which was reflected in the increase of fresh and dry weight, as well as transverse and longitudinal diameter. The concentration of crude fat (CF) exhibited an ‘increase–decrease-increase’ trend, and the peak appeared at 70 days after flowering. EBL (in concentration of 5 μM, EBL5) and MJ (in concentration of 200 μM, MJ200) had the most significant effect on the accumulation of CF and the CF content in single seed. In addition, the activities of enzymes related to fatty acid (FA) synthesis were also higher under the two treatments. 12 FAs were detected using gas chromatography-mass spectrometry, among which palmitic acid, oleic acid, and linoleic acid were the main components. EBL5, EBL10, and MJ200 favored the accumulation of soluble sugar, especially in the middle and late stages. The starch concentration in seeds was lower, but it significantly increased after the application of EBL5. Furthermore, our results demonstrated that EBL and MJ enhanced SOD and POD activity while decreased MDA content. We highlighted that EBL and MJ promoted seed development of S. tonkinensis , affected seed chemical compositions and contributed to the accumulation of CF and FA.