Varying Contributions of Drivers to the Relationship Between Canopy Photosynthesis and Far‐Red Sun‐Induced Fluorescence for Two Maize Sites at Different Temporal Scales

Abstract

Sun‐induced fluorescence (SIF) has been found to be strongly correlated with gross primary production (GPP) in a quasi‐linear pattern at the scales beyond leaves. However, the causes of the GPP:SIF relationship deviating from a linear pattern remain unclear. In the current study conducted at two maize sites in Nebraska in 2017 summer growing season, we investigated the relationship between GPP and SIF at 760 nm (F760) at two temporal scales and quantified the contributions of incoming photosynthetically active radiation (PARin), fraction of absorbed PAR (fPAR), light use efficiency (LUE), and F760 yield (F760,y, defined as F760/(PARin×fPAR)) to GPP and F760 variabilities to further understand the linearity and deviations in the GPP:F760 relationship. We found the following: (1) For individual growth stages when canopy structure and chlorophyll content were stable, GPP and F760 were strongly controlled by PARin, while LUE and F760,y had much lower contributions to the GPP:F760 relationship; during this period, LUE and F760,y had either a slightly negative or no clear relationship, which explained some deviations in the GPP:SIF relationship. (2) At the seasonal scale, the contribution of LUE to GPP variability as well as the contribution of F760,y to F760 variability increased and was comparable to the contribution of PARin; the LUE:F760,y relationship showed a strong linear relationship, which strengthened the linear GPP:F760 relationship. Both maize sites showed similar patterns. A framework was applied to estimate LUE at individual stages and as a result, significantly improved the GPP estimation, thus enhancing the SIF potential for inferring photosynthesis.

Quantifying high‐temperature stress on soybean canopy photosynthesis: The unique role of sun‐induced chlorophyll fluorescence

“…The high correlation between SIF yield and LUE from our result demonstrates that the SIF response to LUE can be greater than to APAR (Figures 2 and 3). This result itself may seem to contradict the previous findings from continuous crop SIF measurements where APAR explained the majority of the SIF variability (Miao et al, 2018(Miao et al, , 2020Yang et al, 2018) However, our result represents different variability from what was observed in earlier studies, given we measured SIF under relatively invariant solar radiation and under considerable variation of stress-induced photosynthetic depression. Largely invariable incident PAR resulted in much less APAR dependency in both SIF and GPP, and a sufficient variation in photosynthetic depression due to the experimental treatments amplified physiological information and its impact on SIF.…”

“…However, each measurement point represents the whole canopy response, which integrates leaf‐level responses over the area of 1 m 2 through the vertical layers of the canopy. This approach allowed us to more closely match the SIF and GPP measurement footprint, which has been a limitation in other SIF–GPP studies (Miao et al, 2018, 2020; Yang et al, ,2015, 2018)…”

“…We calibrated spectral radiance of the spectrometers using a calibrated light source (HL-CAL-2000, Ocean Optics) and a cross-calibration approach following earlier studies (Miao et al, 2018(Miao et al, , 2020. For the field data collection in 2018, we established eight plots that consist of four control and four treatment plots.…”

“…Largely invariable incident PAR resulted in much less APAR dependency in both SIF and GPP, and a sufficient variation in photosynthetic depression due to the experimental treatments amplified physiological information and its impact on SIF. Previous studies, on the other hand, collected data under more variable incident PAR and without environmental stress (Miao et al, 2018(Miao et al, , 2020Yang et al, 2018). The number of gas-exchange data samples was limited in our study due to practical difficulties associated with operating the heavy chamber in a dense soybean canopy (i.e., to minimize damage to the plants) and the size of the hightemperature plots.…”

Intermediate Aerosol Loading Enhances Photosynthetic Activity of Croplands

“…MODIS AOD was used to estimate the aerosol conditions in the NCP. Solar‐induced chlorophyll fluorescence (SIF), which has been suggested to be a direct proxy of ecosystem photosynthesis, especially for heavily managed agricultural ecosystems, was used to approximate ecosystem photosynthetic activity of the croplands (Guanter et al., 2014; Miao et al., 2018, 2020; Sun et al., 2017). We used the new long‐term GOME‐2A SIF data set with SIFTER v2 algorithm, which corrected the instrument degradation of GOME‐2A satellite (Kooreman et al., ; van Schaik et al., 2020).…”