Drought stress is considered the main obstacle restricting Camellia vietnamensis Huang (C. vietnamensis) yield. Hainan is the southernmost distribution region of C. vietnamensis in China and experiences a drought period annually. To study the drought-stress-response mechanism of C. vietnamensis, we treated seedlings of drought-tolerant (HD1) and drought-sensitive (WH1) cultivars with PEG-6000 (PEG) to simulate drought stress and compared the physiology and transcriptome of their leaves at 0 d, 3 d and 6 d posttreatment. Under drought stress, the growth of C. vietnamensis was inhibited, the relative water content (RWC) of leaves decreased and the contents of malondialdehyde (MDA), antioxidant enzyme activities, osmotic regulatory substances and secondary metabolites increased. Compared with those of WH1, the leaf RWC, osmotic-regulation substance content (proline, soluble protein and soluble sugar) and antioxidant enzyme activity (superoxide dismutase, peroxidase and catalase) of HD1 were significantly increased, while the relative electrical conductivity and MDA content were significantly decreased. Compared with WH1, 2812, 2070 and 919, differentially expressed genes (DEGs) were detected in HD1 0 d, 3 d and 6 d posttreatment, respectively, and the number of DEGs increased with increasing treatment time. The detected DEGs are involved in the drought stress response of C. vietnamensis mainly through plant-hormone signal transduction and lignin and flavonoid biosynthesis pathways. Drought stress significantly activated the expression of several lignin and flavonoid biosynthesis genes in HD1. Moreover, total flavonoid and total polyphenol contents in HD1 were significantly increased, suggesting that the accumulation of flavonoids may be a key factor in the drought stress response of C. vietnamensis. Additionally, 191 DEGs were associated with coding transcription factors (TFs). This study provides insight into the molecular mechanism of the drought stress response of C. vietnamensis and provides a theoretical basis for the development and cultivation of new drought-resistant cultivars.

The main production area of Camellia vietnamensis (C. vietnamensis) is in the low mountain and hilly areas of southern China. The low survival rate of seedlings caused by drought is one of the main obstacles restricting the development of the C. vietnamensis industry. An exploration of the key adaptation mechanism of C. vietnamensis to drought stress is important in order to improve its drought resistance. We conducted a study on the morphological, physiological, biochemical, and drought resistance-related genes of five C. vietnamensis cultivars grown in Hainan province under varying degrees of drought stress. The results indicate that drought stress can lead to a decrease in the relative water content and photosynthetic capacity of C. vietnamensis leaves. Compared with the control, the drought damage index, malondialdehyde, relative electrical conductivity, soluble protein, soluble sugar and proline contents of the five C. vietnamensis cultivars increased with drought-stress duration and degree. With increasing drought-stress intensity, the activity of antioxidant enzymes and the content of related metabolites (total polyphenols, total flavonoids, tea saponins) gradually increased, and the expression levels of phenylpropanoid pathway-related genes (Cv4CL1, CvCAD1, CvCAD2, CvPOX1, CvPOX2, CvPOX3) were upregulated. Based on the results of the drought tolerance coefficients, principal component analysis, and hierarchical cluster analysis, we classified five C. vietnamensis cultivars into drought-tolerant cultivars (‘Haida 1’); moderately drought-tolerant cultivars (‘Haida 4’ and ‘Wanhai 4’); and drought-sensitive cultivars (‘Wanhai 3’ and ‘Wanhai 1’). The results of this study provide a theoretical basis for the promotion and cultivation of C. vietnamensis and the selection of drought-resistant cultivars.