Regulatory Effects of Melatonin on Hemerocallis fulva Growth, Soil Physicochemical Properties and Rhizosphere Bacterial Communities under Cadmium Stress

Abstract

Hemerocallis fulva (daylily), native to Asia, is valued for its ornamental, medicinal, and cultural uses. Cadmium (Cd) stress limits growth of daylily. This study evaluated effects of exogenous melatonin (MT) on daylily growth, physiology, soil properties, and rhizosphere bacterial communities under Cd stress. One-year-old seedlings were treated with Cd alone (20 mg·kg-1) or Cd combined with 20, 50, or 100 µM MT. 50 µM MT showed the strongest alleviating effect, increasing plant height by 63.36%, shoot fresh and dry weights by 59.90% and 72.04%, root fresh and dry weights by 72.63% and 68.72%, chlorophyll content by 46.15%, and soluble sugar content by 32.18%. It also increased SOD, CAT, and POD activities by 112.02%, 26.82%, and 69.91%, respectively, and reduced shoot Cd accumulation. Soil enzyme activities improved, with S-AKP, S-SC, S-CAT, and S-US increasing by 7.47%, 39.22%, 48.75%, and 13.86%, respectively. Bacterial richness was enhanced, and 50 µM MT yielded 9172 total ASVs and 7236 unique ASVs. The rhizosphere bacterial community composition changed, reflected in elevated abundances of Chlamydiota, Firmicutes A, Deinococcota and the genus Chryseobacterium. Network analysis indicated a more complex bacterial co-occurrence pattern under 50 µM MT treatment, and FAPROTAX predicted higher abundances of functions related to fermentation, xylan degradation, nitrite respiration, and photoheterotrophy. PLS-PM indicated that shoot Cd content and soil AK were associated with bacterial richness, while bacterial richness, S-SC activity, and shoot Cd content were associated with plant growth.