| Tea is one of the three largest non-alcoholic beverages in the world.It is deeply loved by people because of its special flavor and health-care function of high nutritional value.As one of China’s main cash crops,tea has brought huge benefits to the country.Recently,the problem of excessive heavy metal lead in tea has become more and more serious,which has greatly affected the quality of tea and severely restricted the tea industry.In this study,two tea varieties,Yingshuang and Wuniuzao,were used as the research objects,and the lead aerosol stress was applied to tea by simulating air pollution.The lead transfer pathways of tea seedlings under lead stress and the microscopic morphological changes of tea were studied.The effects of lead stress on the physiological and biochemical indexes of the leaves of the two varieties were analyzed.Quantitative detection models of the physiological and biochemical indexes of tea were established based on the visible and near infrared spectroscopy technology,the rapid and non-destructive detection of the physiological and biochemical indexes of tea under lead stress was realized.The main research contents are as follows:(1)The lead transfer pathway of tea seedlings under lead stress and the microscopic morphological changes of tea were studied.The lead content in different organs(roots,stems,leaves)of the two varieties under lead stress was measured by inductively coupled plasma mass spectrometer.The results showed that the stress concentration and time would lead to a significant increase in the lead content in leaves and stems(p<0.05),and it was found that the lead content in leaves and stems showed a very significant correlation(p<0.01).It can be seen that under aerosol stress,lead accumulates in large amounts in leaves.In order to reduce the toxic effects of leaves,the defense mechanism of tea plays a role in transferring lead to the lower part.From the scanning electron micrographs of the leaves,it can be seen that the stomata of the stress group were severely damaged,and many white crystals appeared inside the stomata.It is speculated that the stomata is an important way for heavy metal lead to enter the tea seedling leaves.It could be seen through transmission electron microscopy that the ultrastructures of the two varieties are affected to a certain extent.There were more starch grains,chloroplast structure deformation,thylakoid dissolution and disorder,nuclear membrane blur,nucleolar disappearance,and cell wall.And the edge of the cell membrane was fuzzy,and the overall structure of the cell was irregular;there were two nucleoli in the nucleus of the frost-free variety,and the nucleoli was abnormal,and there were many starch grains in the chloroplast.The difference in lead absorption,transport and leaf ultrastructure effects of the two varieties under aerosol lead stress indicates the difference in resistance of the two varieties under lead stress.(2)The changes of physiological and biochemical indexes of the two varieties under lead stress were studied.The results showed that most of the stress groups(low and high)chlorophyll a and chlorophyll b showed a trend of"increasing first and then decreasing",and with the increase of time,the pigment content began to decrease,indicating that the tea produced stress in the early stage of stress.The reaction promotes the synthesis of chlorophyll.The heavy metal lead breaks the chloroplast in the late stage of stress,and the synthesis of chlorophyll is inhibited.The starch content of Wuniuzao and Yingshuang in the stress group showed a trend of"decreasing first and then increasing".With the increase of stress time,the starch content also began to increase gradually,indicating that when the tea is under external stress,starch will accumulate.The sucrose content of the two varieties showed a trend of"increasing first and then decreasing"during the 20-40 day period in the stress group,and the content of glucose and fructose both showed the trend of"increasing first and then decreasing"during the 10-30 day period in the stress group,indicating that lead stress will affect the metabolism of tea.(3)Quantitative detection models of leaf physiological and biochemical indicators were established based on visible and near infrared spectroscopy.Spectral collection of leaves of two varieties treated differently at various times,using 4 pretreatment methods and original spectra of multivariate scattering correction(MSC),standard normal transformation(SNV),detrending(DT),standardization(NOR)and original data(ORI)established full-spectrum partial least square regression(PLSR)models for each indicator.On the basis of the optimal model,three methods of successive projections algorithm(SPA),competitive adaptive reweighted sampling algorithm(CARS)and the combination of the two algorithms(CARS-SPA)were used to select spectral variables,and PLSR models based on different characteristic wavelength selection methods were established,it could be found that the model effect based on the CARS-SPA algorithm was improved compared with the full spectrum,and most of them were better than the SPA and CARS algorithms,and the number of model variables was significantly reduced.Therefore,based on the CARS-SPA characteristic wavelength algorithm,three regression models of PLSR,principal component regression(PCR)and support vector regression(SVR)of physiological and biochemical indicators have been established.Through the comparison between the various models,the optimal detection model for each index was finally selected.The coefficient of determination(R_P~2)of the prediction set of chl-a,chl-b,starch,sucrose,glucose and fructose were 0.827,0.724,0.630,0.690,0.631 and 0.785,respectively.It provides a theoretical basis for the rapid and non-destructive real-time monitoring of leaf physiological and biochemical indexes in the future. |
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