Early Diagnosis Of Winter Wheat Under Water Stress Using Reflectance And Chlorophyll Fluorescence

Water stress is a common state of vegetation stress.In previous studies on vegetation water stress monitoring based on vegetation index,only few studies on early detection of vegetation water stress.Water stress is one of the most important environmental factors affecting plant photosynthesis,and chlorophyll fluorescence SIF（Solar-Induced chlorophyll Fluorescence）is considered to be a sensitive"probe"for detecting the photosynthetic function of vegetation affected by plant physiological state and environmental conditions.Chlorophyll fluorescence has great potential for early diagnosis of plant stress and photosynthetic function.This paper is based on the continuous diurnal observation test of winter wheat water stress conducted at Malan Farm in Xinji City,Hebei Province from 2017 to 2018.Combining the acquired data of winter wheat reflectance spectrum,physiological and biochemical parameters,fluorescence kinetic parameters and fluorescence spectrum,etc.Feasibility of chlorophyll fluorescence for early diagnosis of winter wheat under water stress.The main work and research conclusions of the paper are as follows:（1）Analysis of the spectral and physiological and biochemical response characteristics of winter wheat in different stages of water stress.In the later stage of water stress,compared with the normal growth of winter wheat,the leaf area index,chlorophyll content and leaf relative water content of water stress winter wheat were greatly reduced(ΔLAI_{late stage}=4.59,ΔSPAD_{late stage}=41.6,ΔRWC_{late stage}=20.2%).Water canopy reflectance spectrum of winter wheat in the effective photosynthetic radiation band（400-750 nm）and In the water absorption area（1420～1470nm）,the reflectance difference is large(_{late stage}=0.0678);while in the early stage of water stress,the difference between the physiological and biochemical parameters(ΔLAI_{early stage}=2.91,ΔSPAD_{early stage}=4.54,ΔRWC_{early stage}=1.97%)and the spectral characteristic band reflectance(_{early stage}=0.0236)of normal growth and water stress winter wheat is small;The model based on the reflection spectrum index cannot reliably monitor early stage of water stress,but it can provide good information such as canopy structure and chlorophyll content.（2）Analysis of the daily variation and correlation of fluorescence kinetic parameters and fluorescence spectrum in the early stage of water stress.Fluorescence kinetic parametersΦPSII and Fv/Fm can monitor the early water stress of winter wheat,but the data itself is inefficient to measure;the characteristic band of the fluorescence spectrum（The left peak of the SIF spectrum f685 and the right peak of the SIF spectrum f760）has a high correlation withΦPSII and Fv/Fm,which can be used as the basis for establishing a model of early water stress monitoring.（3）The SCOPE model simulation experiments perform global sensitivity analysis on the input variables that affect fluorescence generation.The contribution rate of Cab（chlorophyll a+b content）and LAI（Leaf Area Index）to fluorescence generation is 86.1%,which can be used as a theoretical basis for optimizing the final early water stress monitoring model.（4）Establishment and evaluation of early water stress diagnosis model for winter wheat.Select the most relevant model whit leaf relative water content,whichare f757/f688 and PRI₅₇₀/f₆₈₈ as the basic models,and then use the NDVIdev optimization model to obtain and verify the final winter wheat water stress diagnosis models:f757/（f688·NDVIdev）and PRI₅₇₀/（f688·NDVIdev）.