| Solar-induced chlorophyll fluorescence(SIF)is an important light dissipation pathway in process of vegetation photosynthesis,which can effectively indicate the photosynthetic state and has been widely used in estimation of vegetation gross primary productivity(GPP).Most of the current SIF-based GPP estimation models have obvious limitations,usually under non-stressed natural light conditions,based on empirical relationships and model simulations,or only considering the difference in vegetation functional types between C3 and C4,and assuming photosynthetic light reaction and carbon reaction are coordinated.However,many studies have found that even with the same vegetation type,SIF-GPP coupling relationship varies with environmental conditions.The current SIF-based GPP estimates do not consider the potential impact of environmental factors,especially in the future when the global climate environment is complex and volatile,relying on empirical linear relationships will lead to large deviations in GPP estimation based on SIF.Therefore,environmental factors have become the main source of uncertainty in SIF-based GPP estimation at different temporal and spatial scales.Base on long-term tower-based and satellite observation data,with environmental factors(light,temperature,moisture)as control variables,the study about differences of SIF and GPP in response to environment in different vegetation types or different geographic regions is of great significance to explore the structural and physiological mechanism between SIF and GPP,as well as the remote sensing GPP estimation based on SIF at different temporal and spatial scale.This study focuses on the influence of environmental factors on the coupling relationship between SIF and GPP,a scientific issue involving plant photosynthetic physiology,canopy radiative transfer,and ecology.The main conclusions of this paper are as follows:(1)A set of experimental datasets for the study if the coupling relationship between multi-scale SIF and GPP was established,and fluorescence dynamical experiments were carried out at the leaf scale;long-term spectral observations were caried out at the tower-based scale;and the collection and processing of global SIF and GPP remote sensing products have been carried out at the satellite scale,providing strong support for theoretical verification and model construction.(2)The effect of canopy escape probability on the physiological correlation between SIF and GPP was explored.The spectral invariance theory and the recollisional probability model were used to analyzed the parameter sensitivity of the SIF escape probability model in the red and far-red bands.Downscaling of canopy directional SIF to the photosystem level can better study the physiological correlation mechanism between SIF and GPP.In addition,there are differences in the structural and physiological information contained in different bands of SIFs,so it is difficult to track the changes of GPP in both structural and physiological aspects at the same time with a single bang of SIFs.(3)There are differences in the response of SIF and GPP to light conditions.The fluorescence quantum yield(ΦF)has a weak response to photosynthetically active radiation(PAR),while the light energy utilization rate(LUEp)significantly decreased with the increase of PAR,so the value of LUEp/ΦF decreases with increasing PAR.A model of estimating GPP by SIF considering light conditions was estimated,which significantly improved the GPP estimation accuracy based on SIF in the red and far-red bands of winter wheat and summer maize(for the red band,R2 increased by 0.05,and RMSE decreased by 0.5 umol CO2 m-2 s-1;for the far-red band,R2 increased by 0.04 and RMSE decrease by 0.24 umol C02 m-2 s-1).(4)Low temperature inhibited both fluorescence and photosynthetic capacity,and there were differences in the responses of winter wheat SIF and GPP to low temperature.During overwintering,low temperature stress led to a significant decrease in LUEp,followed by a decrease in ΦF in the 688 nm,while ΦF in the 760 nm was relatively stable.Therefore,the red-light band SIF can better track the changes of GPP under low temperature stress.On this basis,a model for estimating GPP by SIF considering the influence of low temperature is established,which improved the accuracy of GPP estimation(or the red band,R2 increased by 0.05,and RMSE decreased by 0.32 umol CO2 m-2 s-1;for the far-red band,R2 increased by 0.12 and RMSE decrease by 0.71 umol CO2 m-2 s-1)(5)The responses of SIF and GPP to water stress were different.Under drought conditions,both LUEp and ΦF showed a significant downward trend,and ΦF at 688 nm could better track the response of LUEp to drought stress,while the response of ΦF at 760 nm to drought stress relatively weak.Therefore,the GPP estimation model based on SIF in far-red band needs to consider the influence of water stress.The GPP estimation accuracy of the SIF in the far-red band considering the influence of water stress has been improved,with R2 increased from 0.64 to 0.70,and RMSE decreased from 4.80 umol CO2 m-2 s-1 to 4.03 umol CO2 m-2 s-1.The main innovative contributions of this study include:(1)The current research on GPP estimation based on SIF generally ignored the influence of light conditions,and it is not clear that the response differences of SIF and GPP to light conditions in different vegetation functional types at different bands are not clear.Based on leaf and canopy observation experiments,this paper quantitatively evaluates the response law of SIF and GPP to PAR,and estimated an improved model for SIF to estimate GPP considering light conditions.(2)Temperature is an extremely important environmental factor for vegetation growth and development,and it is unclear whether the relationship between SIF and GPP is affected by low temperature.Combined with the experimental observation data of the whole growth cycle of winter wheat,this paper analyzed the effect of low temperature on the relationship between SIF and GPP in winter wheat.It was found that low temperature inhibited fluorescence and photosynthesis at the same time,but the decrease of LUEp was larger than that of ΦF,and an improved model of GPP estimation based on SIF coupled with the influence of low temperature was estimated.(3)There will be a mismatch between light and carbon reactions under water stress conditions,and the impact on the coupling relationship between SIF and GPP is not clear.Based on the tower based experimental observation data,combined with canopy conductance model and crop water stress index(CWSI),this paper explored the impact of water stress on the relationship between SIF and GPP,and found that the impact of water stress on GPP was greater than that of far-red band SIF.The estimation of GPP based on SIF was improved by considering the effects of water stress. |
