Study On The Mechanism Of Shading In Summer And Autumn Affecting Tea Quality

Shading in summer and autumn is one of the important agronomic measures to improve tea quality.With the rapid development of the tea industry,the transition from yield to quality of tea is an urgent problem to be solved in China.The tea plant is a kind of important foliar economic crop.It is of great practical significance to study how shading can improve the quality of tea.In this study,a two-year field experiment and a pot experiment were conducted to explore the effects of shading on the physiological characteristics,metabolic network and related gene expression of tea leaves and roots.In the first stage,’Xiangfeicui’ and ’Jinxuan’,two tea varieties with large differences in the polyphenol/amino acid ratio,were selected for field experiments.Environmental indicators,biochemical compositions,chlorophyll contents and chloroplast ultrastructure were examined in this study.Based on the results of field experiments,shading treatment was performed in pots.Non-targeted metabolomics technology was used to explore the effects of different duration treatments and shading among different varieties on the metabolism of tea leaves,and to detect the gene expression of enzymes related to quality metabolites.In the final stage,according to the good results of the previous period,in order to explore whether the effect of shading on tea quality is universal,the changes of leaf color,biochemical components and sensory evaluation were repeated on multiple tea varieties and multiple tea types.The main findings of this study are as follows:(1)Through the screening of different shading degrees and shade durations,it was found that 4 days and 12 days of shading with 95% shading degree can effectively reduce the polyphenol/amino acid ratio of the leaves of Xiangfeicui and Jinxuan,and improve the freshness of tea leaves.Based on this result,this study conducted field trials and found that shading improved light intensity,canopy temperature,canopy moisture,soil temperature,and soil moisture.Shading increased the content of photosynthetic pigments in leaves,and promoted the significant enrichment of plasmids and thylakoids in chloroplasts.Shading decreased the contents of tea polyphenols,soluble sugars and flavones in leaves of two tea varieties,and increased the contents of free amino acids,theanine and caffeine,but had no significant effect on the water extract content.In addition,12-day shading treatment significantly decreased the contents of DL-C,EC,EGC,ECG,GCG,and EGCG in leaves of the two cultivars.The sensory evaluation results showed that the shaded green tea scored significantly higher than the control green tea,indicating that shading can effectively improve the quality of green tea in summer and autumn.(2)In the pot experiment,the differential metabolic pathways between the two varieties in the shading stage were mainly enriched in the biosynthesis and metabolism of amino acids,starch and sucrose,glycerides and flavonoids.Among them,the levels of(+)-C,(-)-EC and α-Farnesene in Xiangfeicui leaves were 16.49,27.81 and 14.68 times higher than those in Jinxuan leaves after 4 days of shading.The differential metabolites of Xiangfeicui during different shade periods were mainly enriched in fructose,mannose,and phenylalanine biosynthesis and metabolism,among which the levels of L-Asparagine,L-Arginine and L-Phenylalanine increased by 100.14,17.80,and 12.11 times respectively after 12 days of shading.The differential metabolites of Jinxuan varieties during different shade periods were mainly enriched in galactose,valine,flavonoids and flavonol biosynthesis and metabolism.Among them,the levels of L-Arginine,L-NG-monomethylarginine and L-Asparagine increased by 71.80,39.48 and 20.52 times respectively after 12 days of shading.The expression of most genes related to flavonoid synthesis in the leaves of both cultivars was suppressed by shading,including PAL,4CL,CHS,F3’H,LAR,but the glutamine synthase gene in leaves(Cs GS1.1)and the theanine biosynthesis enzyme gene(Cs TSⅠ)in roots increased significantly under shading treatment.(3)The combined analysis of metabolites in tea leaves and roots showed that during the shading period between two varieties,α-Farnesene,L-Phenylalanine,(+)-C,(-)-EC in leaves had a significant correlation between D-Proline,Phosphorylcholine,L-Phenylalanine and other substances in roots.During the relighting stage between two varieties,the substances such as(+)-C,(-)-EC,EGCG,L-Malic acid in leaves were significantly correlated with N-α-acetyl-L-arginine,L-Apple acid,D-Tagatose and L-Sorbose in roots.At different shading stages in Xiangfeicui,Stachyose,L-Sorbose,7-Methylxanthine,α-Farnesene and other substances in leaves were significantly correlated with Uracil,Theobromine,L-Sorbose,L-Malic acid and other substances in roots.At different shading stages in Jinxuan,L-Homoserine,Isoquercetin,Indole acetic acid and D-Mannose in leaves had significant correlations Uracil,Sucrose,L-Sorbose,Jasmonolide,Isoquercetin and other substances in roots.(4)The combined analysis of the leaf and root metabolic pathways showed that the metabolites of Xiangfeicui leaves in the glycolysis and pentose phosphate pathways in sugar metabolism including UDP-glucose and α-D-glucose-1P.The relative levels of them decreased significantly under shading,and the levels of substances such as Sucrose in the roots of Xiangfeicui had the same trend.Most substances in lipid metabolism were only detected in the roots,and the levels of lipid metabolites in the roots of Xiangfeicui were significantly higher than those of Jinxuan after 4 days of shading,including γ-Linolenic acid,Linoleic acid,and fatty acids.The levels of flavonoid metabolism in both varieties showed a trend of increasing and then decreasing,and the metabolites in Jinxuan were higher than those in Xiangfeicui,including(+)-C,(-)-EC and EGCG.In nitrogen metabolism,the levels of alkaloids in leaves of Xiangfeicui,such as Adenine,decreased significantly under shading,while the levels of most metabolites in leaves of Jinxuan increased.Most of the amino acids had similar changes under shading.4-day shading increased the amino acid levels in leaves and roots,and 12-day shading made the level of amino acids reach the highest value,including L-Asparagine,L-Glutamic acid,L-Phenylalanine.(5)Several tea varieties were selected to process green tea,white tea and black tea after shading.According to the results of biochemical components and sensory evaluation,this study found that ’Baojinghuangjincha NO.1’,’Xiangfeicui’,’Jinxuan’and ’Bixiangzao’ had differences in component content under the shading.And the sensory evaluation results were consistent with the quality component analysis.Finally,it was shown that the pre-harvest treatment of 95% shading for 4 days,combined with the withering treatment for a certain period of time,can promote the quality improvement of the green tea,white tea and black tea,which was suitable for many varieties.The main reason for the quality improvement is that shading improves the freshness of the tea leaves and reduces the bitterness.