Abstract:

Objective: This study aims to explore the relationship between different leaf colors of Acer truncatum and the content and distribution of internal pigments, as well as the ultrastructure of mesophyll cells, to provide cytological basis for clarifying the internal mechanism of leaf coloration of A. truncatum, and provide a reference for revealing the color changing mechanism of red leaf plants. Method: A. truncatum leaves were divided into three categories of green, red and yellow leaves according to the leaf color. The contents of chlorophyll, carotenoid and anthocyanin in the leaves with different colors were determined, and the distribution of pigments in various tissues of the leaves was observed. According to the process of leaf color change, it was able to be divided into six categories: light green, green, yellow, reddish tip, full red and brown tip. The chloroplasts and other organelles of mesophyll cells were observed by using a transmission electron microscopy. Result: There were significant differences in pigment content and distribution, and ultrastructure among leaves with different colors. 1) The anthocyanin content of red leaves was significantly higher than that of yellow and green leaves (P<0.05), while the chlorophyll content was lower. The chlorophyll content of yellow leaves was the lowest, the carotenoid content was significantly (P<0.05) higher than that of red leaves, and the ratio of carotenoid to chlorophyll was significantly higher than that of green leaves and red leaves (P<0.05). The chlorophyll and carotenoid contents of green leaves were significantly higher than those of yellow and red leaves (P<0.05), and the anthocyanin content was significantly lower than that of red leaves (P<0.05), but there was no significant difference in anthocyanin content between green and yellow leaves. Correlation analysis showed that the ratio of pigment content in A. truncatum leaves was negatively correlated with chlorophyll content. 2） There were no pigments in the upper and lower epidermic cells. Anthocyanin was mainly distributed in the palisade tissue. The anthocyanin distribution was prominent in red leaves and less in green leaves and yellow leaves. The distribution of carotenoids was prominent in the yellow leaves. Anthocyanin in yellow leaves was also distributed in spongy tissue. 3） Chloroplasts in young green leaves were still developing, and those in green leaves were mature and had starch grains. After the leaves turned red, the ribosomes in the chloroplasts reduced, and residues of the chloroplast disassembly appeared in fully red leaves. Chloroplasts were damaged in the yellow leaves, and there were osmiophilic granules in the cells. There were lots of osmiophilic granules in tip-brown leaves, and some cells were hollow. Conclusion: There is no structural variation in the coloration of A. truncatum leaves. The color change is caused by the decrease of chlorophyll and carotenoid content during the reddening process of leaves, especially a rapid decrease in chlorophyll content. At the same time, the anthocyanin content increases, and the ratio of anthocyanin to chlorophyll and carotenoid increases. Anthocyanins in the leaves are mainly distributed in the palisade tissue, and synthetized before leaf coloration. With the increase of the anthocyanin content, leaves eventually exhibit color. In the process of leaf coloration, the chloroplasts gradually disintegrate, and some senescence features occur. The colored leaves in A. trumcatum are senescent colored leaves. Yellow leaves are in poor nutritional condition, and their color changes from green to yellow, and cannot turn red. Yellow leaves age faster than red leaves.

Key words: Acer trumcatum, content of pigments, pigments distribution, ultrastructure