Monitoring Heat Stress Of Crops Using Sun-Induced Chlorophyll Fluorescence Data

In parallel with global climate change,the frequency and amplitude of extreme climatic events are increasing,which will result in the decline of crops production.Remote sensing has been acting as an effective tool for monitoring heat stress on crops at the large scale,which mainly based on reflectance derived vegetation indices(VIs)..However,VIs can only reflect "greenness" of vegetation.At the early stage of heat stress,even if vegetation photosynthesis capacity has already declined,the canopy structure may remain unchanged.VIs is unable to reflect the influence of high temperature on crops at this stage.Sun-induced chlorophyll fluorescence(SIF)is tighly coupued with photosynthes process and has been proved as the effective probe of photosynthes.It is sensitive to changes in the photosynthesis rate induced by heat stress.Remotely sensed SIF can be a novel pathway to monitor heat stress on crops.However,the capacity of SIF to monitor the influence of heat stress on crops has not been thoroughtly investigated.This study was devoted to invegatge the ponetial of SIF in monitoring the impacts of heat stress in 2010 on wheat in the Indo-Gangetic Plains,India,and the influences of heat stress in 2016 on paddy rice in Jurong county,Eastern China.The datasets employed in this study mainly include satellite SIF data,optical remote sensing data(AVHRR,MODIS,and MERIS),meteorological data,flux data,crop yield data,and so on.The ability of SIF and VIs to monitor heat stress on wheat and rice was assessed,through comparing the changes of SIF and VIs at different stages of heat stress,namely the initial,peak,and recession stages.To remedy the limitiation of currently widely used fraction of absorbed photosynthetical active radiation fAPARcanopy does not differentiate the difference between photosynthetical and non-photosynthetical components of canopy,the fraction of absorbed photosynthetical active radiation by green photosynthetic part fAPARgreen was estimated to isolate the contribution of physiological parameters to SIF,which was employed to monitor the physiological response of crops to heat stress.The main findings of this research are as follows:(1)VIs show lagged response to heat stressThe spatial and temporal characteristics of heat stress in the growing season of wheat in 2010 over the Indo-Gangetic Plain were analyzed using the meteorological data.The heat stress in this region continued about 3 months from February to May in 2010.The heat stress was the most severe in April,with monthy mean temperature being 3.5℃ higher than the multiyear average during 2007-2014.Starting from February,heat stress firstly occurred in the northwestern part of this region and gradually extended toward the southeast.In April,the entire region was hit by heat stress.On the basis of comparing the performance of GIMMS NDVI,MODIS EVI in monitoring this heat stress,it was found that NDVI and EVI showed 32±16 days time lag in the response to heat stress.The spatial patterns of decreases in NDVI and EVI showed significant discprepancy from those of heat stress.(2)SIF outperformances VIs in monitoring heat stressSIF responded to heat stress 16 days earlier than NDVI and EVI did.During the middle and ending periods of the heat stress,SIF showed a larger magnitude of decrease than VIs,indicating the higher sensitivity of SIF to heat stress in comparison with VIs.In addition,SIF show a better relationship with wheat yield in the study region than VIs,and was able to capture the reduction of wheat yield induced by heat stress in 2010.In order to isolate the contribution of physiological part from SIF,MODIS fPAR was used to calculate SIF yield(SIFyieid),which can be a proxy of the physiological statue of vegetation.Relative to MOODIS fPAR,which indicates the structural condition of canopies,SIFyield showed more sensitive response to heat stress.Owing to the inclusion of both structural and physiological information,SIF had more powerful ability to monitor heat stress than NDVI and EVI,which were mainly related to structure of canopy.(3)SIFyield-green was able to monitor the changes of the variations of crops physiological statue due to the heat stressThe Wide Dynamic Range Vegetation Index(WDRVI)was employed to estimate the fAPARgreen.Validation results showed that fAPARgreen was able to improve the estimation of gross primary productivity(GPP).With fAPARgreen,the errors of estimated GPP decreased by 13%for maize and 17%for rice,respectively,in comparion with estimates based on fAPARcanopy.In addition,fAPARgreen was more sensitive to heat stress than fAPARcanopy.Compared with SIFyield-canopy estimated by fAPARcanopy,SIFyield-green estimated from fAPARgreen has better performance in monitoring heat stress.It showed an earlier and more pronounced response to heat stress.Furthermore,during the early stage of heat stress,when the crops canopy structure remains unchanged,SIFyield-green was able to capture the decline in physiological functioning of crops due to the heat stress.