Abstract:

Objective: To investigate the relationship between the color Lab characteristic values and pigment content, as well as the microscopic and ultrastructure of mesophyll cells in Euonymus maackii leaves, and to clarify the color pattern of the leaves, thereby providing a theoretical and scientific basis for the phenological management and breeding of colored-leaf varieties of E. maackii. Method: This study employed an improved leaf color acquisition method to collect the Lab characteristic values of E. maackii leaves with different colors. The contents of chlorophyll, carotenoids, anthocyanins, and soluble sugars in leaves of E. maackii with different colors were determined. The microscopic structure and tissue hierarchy of E. maackii leaf paraffin sections were observed. The ultrastructure of mesophyll cells in E. maackii leaves, including chloroplasts, central vacuoles, and plastoglobules, was observed using transmission electron microscopy. Result: The leaves of E. maackii can be categorized into three color series: green, yellow, and red. There are significant differences in pigment content, microscopic, and ultrastructure among leaves of different colors. Leaves in the yellow series have higher brightness (L^* value) and chroma (C^* value). The degradation of chlorophyll and the accumulation differences of anthocyanins and carotenoids are the main reasons for the color differences in leaves. The study found that the contents of chlorophyll and carotenoids in green series leaves were significantly higher than those in yellow and red series leaves. The contents of soluble sugar and anthocyanins in red series leaves were significantly higher than those in yellow and green series leaves, which are the main reasons for the color differences. Comparative analysis of anatomical structures revealed significant differences in the thickness of palisade and spongy tissues among different leaves. The palisade-sponge ratio, by affecting the size of the pigment attachment surface, led to differences in pigment accumulation. Ultrastructural analysis of leaves showed that the organelle structures of mesophyll cells in red series leaves were relatively intact, while those in yellow series leaves were damaged. The disintegration of chloroplasts and vacuoles accelerated the degradation of chlorophyll. Through path analysis, the direct and indirect pathways affecting leaf color were further clarified: the brightness (L^* value) and chroma (C^* value) of leaves are directly or indirectly closely related to parameters such as chlorophyll b, leaf thickness, and spongy tissue thickness. The red-green hue value (a^* value) and yellow-blue hue value (b^* value) of leaves directly reflect the red and yellow colors of leaves. The main factors affecting the a^* and b^* values of leaves are the content of chlorophyll a, upper epidermis thickness, and spongy tissue thickness. Conclusion: The autumnal red and yellow color changes in the leaves of E. maackii are primarily determined by pigment regulation mechanisms and are also influenced by leaf structure. The increase in the brightness (L^* value) and chroma (C^* value) of leaves with different colors is mainly regulated by the reduction in chlorophyll b content and the increase in carotenoid content. Under the stress of low autumn temperatures, the diversity of E. maackii leaf structure leads to a variety of leaf colors. Leaves with restricted material synthesis, in order to preserve the plant’s nutrient reserves, gradually disintegrate their organelles and break down the contained pigments, causing nutrient reflux and thus lack anthocyanins, appearing yellow. On the other hand, leaves that are in good growth condition and still have strong material synthesis capabilities under low-temperature stress synthesize a large amount of anthocyanins, thereby appearing red.

Key words: Euonymus maackii, Lab characteristic value, pigment content, anatomical structure, color pattern