| To investigate the effects of triacontanol (TA) on carbon and nitrogen metabolism and endogenous hormones in sainfoin (Onobrychis viciifolia) seedlings under drought stress, a sand culture method with nutrient solution was employed. Using Gansu sainfoin (local vatiety, GS), new line P1 (drought\|resistant), and line 2049 (drought\|sensitive) as test materials, treatments were applied based on the pre\|screened drought\|sensitive concentration of -1.0 MPa PEG-6000 and the optimal TA concentration (10.0 μmol·L-1). The treatments included Hoagland nutrient solution (CK), -1.0 MPa PEG-6000 drought stress treatment (T1), and drought stress with 10.0 μmol·L-1 TA spray treatment (T2). Carbon and nitrogen metabolism indicators and endogenous hormone contents in the aboveground and underground parts of sainfoin seedlings were measured. The results showed that PEG-6000 inhibited nitrate nitrogen, ammonium nitrogen, free amino acids, auxin, zeatin, and gibberellin contents in both the aboveground and underground parts of the tested sainfoin. However, the values of these indicators increased under T2 treatment. Compared with T1, the T2 treatment increased nitrate nitrogen in the aboveground parts of GS, P1, and 2049 by 1.1%-3.0%, ammonium nitrogen by 1.3%-10.4%, free amino acids by 3.6%-15.0%, auxin by 13.3%-20.2%, zeatin by 14.6%-24.4%, and gibberellin by 5.3%-11.5%. In the underground parts, the corresponding increases were 1.9%-7.1%, 1.5%-4.2%, 0.8%-4.6%, 4.0%-7.7%, 11.9%-14.7%, and 4.0%-14.3%, respectively. Under T1 treatment, soluble protein, soluble sugar, starch, sucrose, fructose, glucose, abscisic acid content, and the C/N ratio in both the aboveground and underground parts of the tested sainfoin increased compared with CK, but decreased after T2 treatment. Compared with T1, the T2 treatment resulted in decreases in the aboveground parts of GS, P1, and 2049 as follows: soluble protein by 2.6%-9.8%, soluble sugar by 11.3%-41.4%, starch by 13.6%-59.8%, sucrose by 2.3%-38.6%, fructose by 4.1%-7.8%, glucose by 27.7%-31.5%, abscisic acid content by 12.2%-15.4%, and C/N ratio by 37.4%-41.9%. In the underground parts, the corresponding decreases were 5.8%-7.4%, 34.0%-42.8%, 13.6%-53.1%, 27.8%-34.7%, 4.8%-11.3%, 15.9%-32.2%, 8.6%-11.1%, and 23.0%-48.0%, respectively. The mitigation effect of TA spray followed the order: 2049 > P1 > GS. These findings indicate that TA application helps maintain carbohydrate synthesis and the balance of plant growth hormones in sainfoin under drought stress, thereby alleviating the adverse effects of drought on its growth and development. |