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

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 (F-760) at two temporal scales and quantified the contributions of incoming photosynthetically active radiation (PAR(in)), fraction of absorbed PAR (fPAR), light use efficiency (LUE), and F-760 yield (F-760,F-y, defined as F-760/(PAR(in)xfPAR)) to GPP and F-760 variabilities to further understand the linearity and deviations in the GPP:F-760 relationship. We found the following: (1) For individual growth stages when canopy structure and chlorophyll content were stable, GPP and F-760 were strongly controlled by PAR(in), while LUE and F-760,F-y had much lower contributions to the GPP:F-760 relationship; during this period, LUE and F-760,F-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 F-760,F-y to F-760 variability increased and was comparable to the contribution of PAR(in); the LUE:F-760,F-y relationship showed a strong linear relationship, which strengthened the linear GPP:F-760 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.