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 (F₇₆₀) 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 F₇₆₀ yield (F₇₆₀,y, defined as F760/(PAR_(in)×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 PAR_(in), 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.