Effect Of Shading On Accumulation Of Chlorophylls In Tea Shoots And Related Mechanism

As understanding of health care function of tea, food stuffs containing tea or tea components become more and more popular. The ultramicro green tea powder, prepared using tender tea leaf, can not only be drinked directly, but also be used as food additives in processing tea food. In this way, it will improve food taste, as well as endow the food with natural green colour, so as to full use of tea. The ultramicro green tea powder grinded using superfine grinding technique requests tender raw materials with high level of chlorophylls, by which ultramicro green tea powder with characteristics of small size particles, smooth taste and bright green colour can be prepared. Chlorophylls are not only indispensable pigment for photosynthesis in tea plant, but also an important indicator of green tea color quality. Effects of light-shading on content of chlorophylls, tea polyphenols, amino acids, caffeine and total catechines in tea shoot were studied using dark light-shading net. The mechanism in accumulation and biosynthesis of chlorophylls in tea shoots under conditions of light-shading was also explored. This study will provide a reference for developing technique for production of high quality raw material of green tea powder.Effects of light-shading on accumulation of chlorophylls in tea shoots were investigated. The results showed that content of chlorophylls significantly raised under shading conditions. The optimum shading conditions were as follows:16-day shading at85%light-shading rate in spring season, and8-day shading at85%light-shading rate in summer season. The chlorophylls content in treatment of spring tea shoots was6961.95μg/g, with34.8%higher than control; and the content of chlorophylls in summer tea shoots reached to20195.85μg/g, being29.3%higher than control, which indicated that productive character of the tea shoots for green tea was improved.Changes of content of tea polyphenols, amino acids, caffeine and total catechines in tea shoots under shading conditions was also analyzed, and the results showed that the content of tea polyphenols and total catechines in16-day shading treatment at85%light-shading rate in spring decreased to143.4mg/g and122.8mg/g, being12.2%and19.4%lower than control respectively, but levels of amino acids and caffeine increased to39.3mg/g and24.3mg/g, being14.4%and 30.6%higher than control respectively; treatment of8-day shading at85%light-shading rate in summer made the content of tea polyphenols and total catechines drop to157.1mg/g and131.3mg/g, being9.1%and13.2%lower than control respectively, meanwhile, levels of amino acids and caffeine reached to29.1mg/g and26.7mg/g, with18.8%and17.1%higher than control respectively.The ultrastructure observation on tea leaves using transmission electron microscope indicated that the proportion of chloroplast volume in each leaf cell and the number of grana lamella increased, but the amount of starch grains were significantly reduced in light-shading conditions. The structure feature of chloroplasts under shading condition makes for accumulation of chlorophylls which attach to inner membranes of lamella.Determination of biosynthetic precursors of chlorophylls showed light-shading treatments decreased the levels of aminolevulinic acid, porphobilinogen and urogen Ⅲ but increased the levels of protoporphyrin Ⅸ, magnesium protoporphyrin Ⅸ and protochlorophyllide.Study on expression of partial genes involved in chlorophyll biosynthesis using RT-PCR method showed that the expression level of HEMA2, HEME1, HEMF, HEMG2and CHLI2genes was upregulated in three shading conditions, which increased the level of magnesium protoporphyrin IX. The level of PORA mRNA at75%light-shading rate and that of PORB mRNA at85%and98%light-shading rate increased compared with non-shading treatment. Expression of PORA or PORB gene encoding protochlorophyllide oxidoreductase will transfer from protochlorophyllide to chlorophyllide, and low level expression of PORA and PORB genes under non-shading condition suppressed this transformation. Weakened biosynthesis of direct chlorophyll precursor—chlorophyllide hindered chlorophyll biosynthesis, thus resulting in lower content of chlorophyll in non-shading treatment than light-shading treatment.