The Impacts of Growth and Environmental Parameters on Solar-Induced Chlorophyll Fluorescence at Seasonal and Diurnal Scales

Leizhen Liu,Shasha Liu,Jianjun Wu,Yanguo Teng,Xinyi Han,Jianhua Yang,Wenhui Zhao

doi:10.3390/rs11172002

Abstract

Solar-induced chlorophyll fluorescence (SIF) is considered to be a potential indicator of photosynthesis. However, the impact of growth and environmental parameters on SIF at different time-scales remains unclear, which has greatly restricted the application of SIF in detecting photosynthesis variations. Thus, in this study, the impact of growth and environmental parameters on SIF was thoroughly clarified. Here, continuous time series of canopy SIF (760 nm, F760) over wheat and maize was measured based on an automated spectroscopy system. Meanwhile, field measurements of growth and environmental parameters were also collected using commercial-grade devices. Relationships of these parameters with F760, apparent SIF (F760/solar radiance, AF760), and SIF yield (F760/canopy radiance of 685 nm, Fy760) were analyzed using principal component analysis (PCA) and Pearson correlation to reveal their impacts on SIF. Results showed that F760 at seasonal and diurnal scales were mainly driven by solar radiation (SWR), leaf area index (LAI), chlorophyll content (Chl), mean leaf inclination angle (MTA), and relative water content (RWC). Other environmental parameters, including air temperature (Ta), relative humidity (Rh), vapor pressure deficit (VPD), and soil moisture (SM), contribute less to the variation of seasonal or diurnal F760. AF760 and Fy760 are likely to be less dependent on Ta, Rh, and VPD due to the removal of the impact from SWR, but an enhanced relationship of AF760 (and Fy760) with SM was observed, particularly under water stress. Compared with F760, wheat AF760 was better correlated to LAI and RWC as expected, while maize AF760 did not show an enhanced relationship with all growth parameters, probably due to its complicated canopy structure. The relationship of wheat Fy760 with canopy structure parameters was further reduced, except for maize measurements. Furthermore, SM-induced water stress and phenological stages should be taken into consideration when we interpret the seasonal and diurnal patterns of SIF since they were closely related to photosynthesis and plant growth (e.g., LAI in our study). To our knowledge, this is the first exploration of the impacts of growth and environmental parameters on SIF based on continuous ground measurements, not only at a seasonal scale but also at a diurnal scale. Our results could provide deep insight into the variation of SIF signals and also promote the further application of SIF in the health assessments of terrestrial ecosystems.

Highlights

Solar-induced chlorophyll fluorescence (SIF) is an optical signal that is emitted by chlorophyll a molecules when absorbed solar energy exceeds the requirements of the photochemical reaction [1]
The consistent decreasing trend of F760 with gradient irrigation treatments suggested that water stress may yield lower F760 values and suggested that soil moisture was another factor affecting the emission of SIF
Based on the analysis over wheat measurements, we found that F760, AF760, and Fy760 at noon time were mainly affected by leaf area index (LAI) (r values: 0.56, 0.53, and 0.54, Figure 8a)

Summary

Introduction

Solar-induced chlorophyll fluorescence (SIF) is an optical signal that is emitted by chlorophyll a molecules when absorbed solar energy exceeds the requirements of the photochemical reaction [1]. In order to overcome this problem, the in-filling method using two dark bands in telluric oxygen (O2) absorption lines (O2-A, 760 nm and O2-B, 687 nm) has commonly been used for SIF retrieval because of the more comparable signal of SIF relative to reflected flux in these two O2 bands [3,4]. These two O2 bands are close to the terrestrial SIF spectrum peaks centered at ≈685 and ≈740 nm, known as red and far-red fluorescence [5]. The O2-A band has largely been used to estimate SIF from tower [7,8,9], aircraft [10,11], and satellite platforms [12,13,14,15]

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