Exogenous Melatonin Effects on Drought-Stressed Longan Plants: Physiology and Transcriptome Insights

Drought stress severely constrains yield and quality stability in longan (Dimocarpus longan Lour.), an important medicine and food homology fruit in China. Melatonin (MT) shows potential for alleviating abiotic stress, but its mechanisms in drought-stressed longan remain unclear. Here, we investigated two cultivars (Shixia and Chuliang) under drought and exogenous MT treatments (CW, well-watered control: CM, exogenous MT application under well-watered control: DW, drought stress: DM, exogenous MT application under drought stress), revealing the following findings: (i) Drought treatment significantly reduced endogenous MT levels in both studied cultivars, and the reduction was reversed by exogenous foliar MT application. Specifically, under drought conditions, exogenous MT treatment increased endogenous MT content by 272.7% in Shixia and 53.6% in Chuliang, respectively. (ii) Drought and exogenous MT treatments modulated the activities of plant defense enzymes (superoxide dismutase, SOD: peroxidase, POD: phenylalanine ammonia lyase, PAL: and catalase, CAT) and the levels of related metabolites (malondialdehyde, MDA: proline, Pro). Across both cultivars, drought stress increased the activities of SOD, POD, and PAL, as well as the Pro content. Exogenous MT treatment, however, reduced the activities of SOD, POD, and PAL while increasing CAT activity and MDA content to some extent in both cultivars. Notably, the Pro content was significantly reduced in Shixia but significantly increased in Chuliang following exogenous MT application under drought stress. (iii) Drought and exogenous MT treatments regulated gene expression in longan cultivars. Relative to CW, 848, 3356, and 2447 differentially expressed genes (DEGs) were detected in CM, DW, and DM in Shixia, respectively. Relative to CW, 1349, 5260, and 5116 DEGs were identified in CM, DW, and DM in Chuliang. A gene ontology analysis indicated significant enrichment for abiotic stress defense and hormone-responsive processes. The KEGG pathway analysis showed significant enrichment in protein processing in the endoplasmic reticulum (ko04141), amino sugar and nucleotide sugar metabolism (ko00520), ascorbate and aldarate metabolism (ko00053), plant&ndash:pathogen interaction (ko04626), and starch and sucrose metabolism (ko00500). These findings provide physiological and transcriptomic insights into MT-regulated drought responses in longan, highlighting its potential for improving productivity in drought-prone regions.