Effects of exogenous melatonin on photosynthesis physiology biochemistry of blue honeysuckle seedlings under saline-alkali stress

Saline-alkali stress severely restricts the growth and cultivation of blue honeysuckle (Lonicera caerulea L.). Exogenous melatonin (MT) regulates plant stress resistance, but its alleviation mechanism in this species remains unclear. This study analyzed the effects of 0, 25, 100, and 200 µmol/L MT on blue honeysuckle seedlings under saline-alkali stress. Results showed 100 µmol/L MT exerted the optimal effect: it mitigated photoinhibition by increasing PSII’s maximum photochemical efficiency (Fv/Fm) and optimizing light energy distribution; enhanced antioxidant defense via upregulating SOD, POD, and CAT activities while reducing MDA content; regulated proline and soluble sugar accumulation to maintain osmotic balance; and activated stress response networks by upregulating NAC/WRKY transcription factors and auxin/ABA-related genes, as well as SNAT/ASMT pathway genes for endogenous MT synthesis. Notably, 200 µmol/L MT induced inhibitory effects via oxidative feedback. In conclusion, 100 µmol/L exogenous MT significantly improves blue honeysuckle’s saline-alkali tolerance through synergistic regulation of photosynthetic stability, antioxidant defense, and transcriptional reprogramming, providing a theoretical basis for saline-alkali land cultivation and MT agricultural application.