| The photosynthesis of plants is the foundation of their life activities,and it has significant impacts on plant growth,metabolism,and environmental adaptation.Plant photosynthetic pigments are closely related to plant respiration and are widely used to reflect plant growth,health status,and environmental adaptation capacity.Currently,the detection of plant photosynthetic pigments mainly relies on hyperspectral reflectance,but measuring reflected light requires a certain angle,which may reduce the stability of monitoring these pigments.The methods for monitoring plant photosynthetic respiration mainly rely on gas exchange systems,which are typically complex in structure,expensive,and cumbersome to use.Therefore,there is a need for a fast,non-destructive,and low-cost detection method to accurately measure the net photosynthetic respiration rate of plant leaves in a portable instrument,in order to assess plant growth and metabolic activities.When plant leaves are exposed to light,some photons are absorbed by the plant for plant photosynthesis,while some photons are absorbed by pigments such as chlorophyll and undergo non-radiative energy transfer,resulting in fluorescence.Fluorescence can be used to detect the content of photosynthetic functional pigments in plants,while the intensity and wavelength of the fluorescence spectrum can reflect the efficiency of photosynthesis.In this paper,the excitation and emission characteristics of plant leaf pigments are considered simultaneously,a three-dimensional fluorescence spectral index is established,the excitation and emission sensitive bands of pigment inversion are analyzed and validated;the fluorescence of chlorophyll a and b of plant leaves is separated by Gaussian model;and the model of net photosynthetic rate is established by combining the separated chlorophyll a and b fluorescence emission coefficients,and the portable and low-cost net photosynthetic rate detection instrument of plant leaves is completed.This paper presents the results of this study.The research content and results of this paper include the following aspects:(1)This study analyzed the sensitivity of plant leaf photosynthetic pigments to fluorescence and conducted fluorescence analysis and content inversion of photosynthetic pigments based on three-dimensional fluorescence spectra.The main research contents include:(1)Establishing a three-dimensional fluorescence spectral index(3D-FSI)by combining information from different spectral bands to enhance the information contained in reflectance spectral data,including fluorescence difference index(FDI),fluorescence reciprocal difference index(FRDI),fluorescence ratio index(FRI),and fluorescence normalized index(FNI);(2)Analyzing the fluorescence characteristics of photosynthetic pigments and photoprotective pigments in plant leaves based on three-dimensional fluorescence spectra,and establishing the correlation between excitation and emission characteristics of different pigments with their absorption characteristics;(3)Calculating and analyzing the emission-sensitive wavelengths for the inversion of five pigments,including chlorophyll a,chlorophyll b,anthocyanin,carotenoid,and flavonoid,using single band and three-dimensional fluorescence spectral indices;(4)Modeling and validating chlorophyll a,chlorophyll b,and carotenoid,which have highly sensitive emission bands.The research results showed that under single band analysis,all pigments had no sensitive wavelength for pigment inversion.The use of three-dimensional fluorescence spectral indices significantly improved the correlation.For chlorophyll a,the best inversion effect was achieved with FNI composed of 686nm and 788nm under 430nm excitation wavelength,with an RMSE of 2.54(μg/cm~2);for chlorophyll b,the best inversion effect was achieved with FRDI composed of 670nm and 765nm under 630nm excitation wavelength,with an RMSE of 1.51(μg/cm~2);for carotenoid,the best inversion effect was achieved with FRI composed of 748nm and 717nm under 470nm excitation wavelength,with an RMSE of 3.7(μg/cm~2).Anthocyanin and flavonoid still showed no significant inversion effect.The conclusions of this study lay the foundation for subsequent chlorophyll a and chlorophyll b fluorescence separation.(2)In order to investigate the relationship between chlorophyll a and b fluorescence and net photosynthetic rate,a Gaussian model was used to separate the fluorescence spectra of chlorophyll a and b.The main contents include:(1)Analysis of the structure and spectral characteristics of chlorophyll a and chlorophyll b,exploring the saturation phenomenon of chlorophyll in the process of fluorescence emission,and analyzing the feasibility of chlorophyll a and b spectra;(2)Construction of Gaussian models for chlorophyll a and chlorophyll fluorescence spectra,obtaining the optimal values of chlorophyll a and b fluorescence parameters that fit the chlorophyll fluorescence using particle swarm algorithm,obtaining the fluorescence emission coefficients of chlorophyll a and b under 405nm and 445nm excitation conditions,and verifying the consistency between fitted fluorescence and measured fluorescence.(3)By comparing the measured fluorescence of chlorophyll with the fluorescence emission coefficients of chlorophyll a and b under 405nm and445nm excitation conditions,the content of simulated chlorophyll a and b was obtained using optimization algorithms,and compared with the measured values of chlorophyll a and b for further verification of the fluorescence emission coefficients of chlorophyll a and b.The research results show that under the excitation conditions of 405nm and 445nm,the emission characteristics of chlorophyll a and b are very similar.The two emission peaks of chlorophyll a are at 692nm,753nm and 691nm,754nm,respectively,while the two emission peaks of chlorophyll b are at 685nm,742nm and 686nm,743nm,respectively.The fitting to the measured spectra is good,with the minimum RRMSE reaching 6%.The fitted content of chlorophyll a and b compared with the measured content has a minimum RMSE of 2.8(μg/cm~2)and 2.0(μg/cm~2),respectively.This provides a theoretical basis for the development of subsequent instruments.(3)The research focuses on the cost-effective development of a measurement instrument for net photosynthesis rate in plant leaves.The consistency between the integrated values of continuous spectra and characteristic wavelength ranges in the model was analyzed.The simulated integration value with a bandwidth of 16nm was found to be closest to the measured integration value,with R~2reaching 0.9306 and0.7957 for the central wavelength ranges of 690nm and 740nm,respectively.By fitting the fluorescence spectra of chlorophyll a and b using the calculated fluorescence emission coefficients,a model was established to correlate the separated chlorophyll a fluorescence characteristic emission wavelength(691nm)with net photosynthesis rate,achieving an R~2of 0.8339.The portable net photosynthesis rate measurement instrument consists of a fluorescence excitation module(LED light with a central wavelength of 445nm),a fluorescence spectral information acquisition module,a housing module,and a result display module.Through verification experiments,the accuracy of net photosynthesis and respiration detection for low-concentration chlorophyll leaves can reach 81.8%.Therefore,the instrument has the capability to accurately measure net photosynthesis rate.The pigment fluorescence inversion based on three-dimensional fluorescence spectral indices and the portable net photosynthesis rate detection instrument provides a new method for plant information detection.It has the potential to utilize the fluorescence characteristics of plant photosynthetic functional pigments for rapid and effective assessment of plant growth status. |
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