Abstract:

【Objective】 In order to provide a theoretical basis for revealing the mechanism of leaf color formation, the consistency of leaf physiological indicators of Eucommia ulmoides ‘Hongye’ at different periods of time was studied. 【Method】 Chlorophyll a, chlorophyll b, total chlorophyll, carotenoids, flavonoids and anthocyanin in five periods were tested and compared between E. ulmoides ‘Hongye’ and E. ulmoides ‘Xiaoye’. A CR2500 color difference meter (Konica Minolta, Japan) was used to measure brightness (L), redness (a^*) and yellowness (b^*) values. Color saturation (Chroma, C^*) and hue angle (h) were calculated using the a^* and b^* values, where C^*=(a^*2+b^*2)/2, h=arctan (b^*/a^*). The leaf color index (color index) was calculated as follows: color index=(180 - h)/(L+C^*). The key enzymes in the leaf color transition process and the anthocyanin synthesis and metabolism pathway, phenylalanine lyase (PAL) and chalcone isomerase (CHI) were extracted, and the enzyme activity was determined. The data were analyzed using SPSS 16.0. The analysis of variance was carried out to test the significance of differences within each period, and correlations among indicators were analyzed. 【Result】 ① Although the variation law of E. ulmoides ‘Xiaoye’ was similar to E. ulmoides ‘Hongye’, the initial value and variation range of each index of the two varieties were significantly different. The color index of E. ulmoides ‘Xiaoye’ was always in the yellow-green grade, while it was in the yellow-green grade only in the first and second stages for E. ulmoides ‘Hongye’. After that, the color was in the pink grade in the third and fourth stages, and the deep red grade in the fifth stage. ② The content of various pigments in the two varieties increased gradually. Among all the indicators, i.e. C_a (chlorophyll a content), C_b (chlorophyll b content), C_T (total chlorophyll content), C_Car (carotenoid content), C_f (flavonoid content) and C_A (anthocyanin content), the anthocyanin content played the key role in the process of reddening. The comparison of the content of each pigment in the fifth period (38 d) to the first period (6 d) indicated that the C_A in E. ulmoides‘Hongye’increased by 553.96% compared to that of E. ulmoides ‘Xiaoye’. ③ There was no significant difference in PAL enzyme activity in different time periods between the two varieties. The enzyme activity values of the two varieties in each period were similar. The CHI enzyme activity of E. ulmoides ‘Hongye’ increased linearly with the change in leaf color by 306.14% in the fifth period (38 d) compared to the first period (6d) and differed significantly among different stages. The CHI enzyme activity of E. ulmoides ‘Xiaoye’ increased gradually with leaf color change, although there was no significant difference in enzyme activity among different periods. ④ The L, a^* and b^* values were significantly correlated with C_Car, C_A and C_f, while it was not significantly correlated with C_a, C_b, C_T and C_a/C_b. CHI enzyme activity was correlated or significantly correlated with the L, a^* and b^* values, with C_Car, C_A and C_f of Eucommia leaves, while no significant correlation was found between the PAL and leaf color difference. 【Conclusion】 In the process of color changes in E. ulmoides ‘Hongye’ leaves, the change of C_A was much greater than that of chlorophyll content, and this was the direct cause of their color appearance. The high activity of the CHI enzyme in E. ulmoides ‘Hongye’ leaves played a key role in the process of typical leaf color transformation.

Key words: Eucommia ulmoides‘Hongye’, chalcone isomerase enzyme (CHI), leaf chromatic aberration, anthocyanin, leaf color