Comparison of Morphological, Physiological, and Related Gene Expression Responses to Drought Stress in Five <i>Camellia vietnamensis</i> Cultivars

The main production area of <i>Camellia vietnamensis</i> (<i>C. vietnamensis</i>) 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 <i>C. vietnamensis</i> industry. An exploration of the key adaptation mechanism of <i>C. vietnamensis</i> 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 <i>C. vietnamensis</i> 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 <i>C. vietnamensis</i> leaves. Compared with the control, the drought damage index, malondialdehyde, relative electrical conductivity, soluble protein, soluble sugar and proline contents of the five <i>C. vietnamensis</i> 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 (<i>Cv4CL1</i>, <i>CvCAD1</i>, <i>CvCAD2</i>, <i>CvPOX1</i>, <i>CvPOX2</i>, <i>CvPOX3</i>) were upregulated. Based on the results of the drought tolerance coefficients, principal component analysis, and hierarchical cluster analysis, we classified five <i>C. vietnamensis</i> 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 <i>C. vietnamensis</i> and the selection of drought-resistant cultivars.