Comparative Physiological Responses of Two Bougainvillea Cultivars to Shade Conditions

The growth and landscape application of Bougainvillea, a widely cultivated ornamental plant, are often influenced by light conditions. The effects of shade on two Bougainvillea cultivars, ‘Imperial Delight (ID)’ and ‘San Diego Red (SDR)’, cultivated under full sunlight (CK) and 50% shade (S) were investigated. Under shading, both cultivars presented decreased leaf thickness; increased chlorophyll a, b and total contents; and reduced light compensation points (LCPs) and dark respiration rates (Rds), reflecting morphological and photosynthetic acclimatization. Decreased superoxide dismutase (SOD) activity and malondialdehyde (MDA) contents indicated attenuated membrane lipid peroxidation. In ‘ID’, shading significantly reduced the leaf, bract and branch dimensions, and decreased the soluble sugar and protein contents, and the apparent quantum efficiency (AQY), reflecting reduced growth potential. In contrast, ‘SDR’ exhibited significant increases in leaf and bract dimensions and branch length under shade, with relatively small reductions in the soluble protein content and AQY, indicating less growth suppression. Integrated multivariate and correlation analyses revealed overall differences in physiological responses to shading, identifying seven representative indicators related mainly to the leaf phenotype and chlorophyll content. In summary, coordinated physiological and metabolic regulation may underlie the cultivar-specific differences. Both cultivars exhibited a certain adaptability to shading, but ‘SDR’ performed better than ‘ID’, suggesting a possible advantage under shade. This study provides a valuable theoretical foundation for the shade-tolerant cultivation and breeding of Bougainvillea germplasms.