Research On Nondestructive Testing Method Of Plant Leaf Water State Based On Terahertz Spectroscopy

Water is the most abundant and irreplaceable substance in plants,which plays a vital role in life activities.Plant growth and development have specific requirements for water content,and too much or too little will affect the normal physiological and biochemical processes of plants The water sensitivity and low radiation of terahertz spectrum make it a favorable tool for the detection of plant water content,and then used for the detection and analysis of plant health.However,the existing terahertz spectroscopy is still blank in the field of plant free water and bound water content analysis,which limits the application of terahertz spectroscopy in the field of plant physiological state analysis.In this paper,the effective relationship between water and leaves is simplified by using the terahertz theory.Finally,the linear absorption model combined with terahertz spectral imaging technology was used to establish the quantitative analysis scheme of leaf water content.The correlation between the scheme and the standard scheme was about 0.94.Then,by combining with binary scanning imaging technology,the quantitative distribution map of leaf water content is drawn,and the visual analysis of water content of spatially heterogeneous substances is realized.This method provides a feasible scheme for remote quantitative detection of water content and provides technical support for fixed-point irrigation,but it can not fully meet the requirements of leaf physiological state monitoring.The water in leaves can be subdivided into two different existing states: free water and combined water,and their content and proportion are also closely related to the health status of plants.Therefore,it is necessary to further realize the quantitative analysis of free water and bound water respectively.Since bound water is a water molecule combined with other solid substances,it cannot exist alone under natural conditions.Here,we first indirectly obtain the optical properties of the bound water through the saturated sugar solution.Assuming the saturated sucrose solution contains only sugar solid molecules and bound water molecules,we can calculate the absorption coefficient of the bound water through the effective absorption model,and take this as the input term to establish the content relationship of leaf free water,bound water and dry matter,The numerical values of the three are fitted by effective particle swarm optimization algorithm.At the same time,the correctness of the results of terahertz spectrum is verified by the traditional Abbe refraction method.The analysis results of various structural leaves also further confirmed the inter species universality of the method described in this paper.At the same time,the inflection points of free water and bound water content in the process of leaf dehydration were found,indicating that terahertz technology is expected to reflect the internal relationship between bound water / free water ratio and plant activity.On this basis,taking the typical abiotic stress of low temperature stress as an example,the physiological state of leaves was detected and analyzed by terahertz spectroscopy.The results showed that the absorption coefficient of leaves showed a downward trend after cold stress,while the coefficient of leaves showed an abnormal upward trend after freeze stress.The analysis of water state based on terahertz spectrum shows that the abnormal spectral response after freezing stress comes from the abnormal increase of leaf water volume ratio and free water volume ratio,which is also verified by traditional methods.More physiological data analysis showed that the difference of terahertz spectral response caused by cold stress and freezing stress came from the changes of leaf water environment and cell damage.These analyses confirmed the rationality and scientificity of leaf relative absorption coefficient as an index to evaluate plant low temperature stress.The response time of this index is earlier than the change of blade morphology,which is expected to be used for early low temperature damage early warning.The terahertz spectrum analysis scheme based on the effective medium theory proposed in this paper can be effectively applied to the analysis of water content in leaves of many kinds of plants,locate and quantitatively display the spatial distribution state of plant water content,and provide reliable quantitative results of free water and combined water content in leaves.As a means of terahertz spectrum data analysis,it digs the deep relationship between spectral characteristics and plant water state,expands the analysis range of terahertz spectrum,makes it provide more plant water state information,and provides technical support for terahertz spectrum technology in plant physiological state analysis.