Influence of Cultivation Under Shading Conditions on Lignin Biosynthesis and Characteristics of Oenanthe javanica and Oenanthe linearis Plants

Oenanthe javanica (LY) and Oenanthe linearis (SQ), collectively known as water dropwort, are popular aquatic vegetables in China. Their blanching cultivation produces tender, etiolated tissues with reduced lignin content and improved sensory qualities. To clarify the effects of shading cultivation on lignin synthesis and accumulation in these two cultivars, this study investigated their shading responses in terms of morphological traits, physiological indices, enzyme activities, cellular structure, and lignin synthesis-related gene expression levels. The results showed significant differences between the two cultivars during the 24-day shading treatment. Compared with ‘SQ’, ‘LY’ exhibited upright growth and marked elongation of new petioles, while old petioles were significantly decreased. The entire plant turned light yellow or white, conferring commercial value typical of blanched water dropwort. During the initial 0–16 d of shading, lignin content was higher in ‘SQ’; however, by day 24, it was 92.26 mg·g−1 lower in ‘SQ’ than in ‘LY’. In terms of enzyme activity, shading generally decreased the activities of PAL, CAD, and C4H, while increasing 4CL activity. Notably, shading reduced POD activity in ‘SQ’ but increased it in ‘LY’. Histological observation revealed that shading led to a gradual loosening of xylem cell arrangement in water dropwort; furthermore, the number and size of xylem cells in ‘LY’ were significantly larger than those in ‘SQ’. At the molecular level, shading significantly downregulated the expression of OlPAL, Ol4CL, OlCCR, OlCCoAOMT, and OlCAD1 in ‘SQ’, a trend that correlated with the observed decrease in lignin content and thus appears to be a primary cause of altered lignin accumulation. In ‘LY’, the expression level of OjPAL2 decreased, showing a positive correlation with both PAL enzyme activity and lignin content, suggesting it acts as a key regulator of lignin synthesis under these conditions. In conclusion, compared with ‘SQ’, ‘LY’ exhibits a higher degree of lignification but possesses stronger resistance to shading stress, making it more suitable for producing high-quality etiolated water dropwort.