Search

Recent development of sun‐induced chlorophyll fluorescence (SIF) technology is stimulating studies to remotely approximate canopy photosynthesis (measured as gross primary production, GPP). While multiple applications have advanced the empirical relationship between GPP and SIF, mechanistic understanding of this relationship is still limited. GPP:SIF relationship, using the standard light use efficiency framework, is determined by absorbed photosynthetically active radiation (APAR) and the relationship between photosynthetic light use efficiency (LUE) and fluorescence yield (SIFy). While previous studies have found that APAR is the dominant factor of the GPP:SIF relationship, the LUE:SIFyrelationship remains unclear. For a better understanding of the LUE:SIFyrelationship, we deployed a ground‐based system (FluoSpec2), with an eddy‐covariance flux tower at a soybean field in the Midwestern U.S. during the 2016 growing season to collect SIF and GPP data simultaneously. With the measurements categorized by plant growth stages, light conditions, and time scales, we confirmed that a strong positive GPP:SIF relationship was dominated by an even stronger linear SIF:APAR relationship. By normalizing both GPP and SIF by APAR, we found that under sunny conditions our soybean field exhibited a clear positive SIFy:APAR relationship and a weak negative LUE:SIFyrelationship, opposite to the positive LUE:SIFyrelationship reported previously in other ecosystems. Our study provides a first continuous SIF record over multiple growth stages for agricultural systems and reveals a distinctive pattern related to the LUE:SIFyrelationship compared with previous work. The observed positive relationship of SIFy:APAR at the soybean site provides new insights of the previous understanding on the SIF's physiological implications. Sun‐induced fluorescence was continuously measured at the canopy level across multiple growth stages in a soybean fieldThe positive relationship between SIF and GPP was dominated by a strong relationship between SIF and APARSIF yield was positively correlated with APAR and negatively correlated with LUE under stable sunny conditions

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 F760yield (F760,y, defined as F760/(PARin×fPAR)) to GPP and F760variabilities to further understand the linearity and deviations in the GPP:F760relationship. We found the following: (1) For individual growth stages when canopy structure and chlorophyll content were stable, GPP and F760were strongly controlled by PARin, while LUE and F760,yhad much lower contributions to the GPP:F760relationship; during this period, LUE and F760,yhad 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,yto F760variability increased and was comparable to the contribution of PARin; the LUE:F760,yrelationship showed a strong linear relationship, which strengthened the linear GPP:F760relationship. 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. Driver contributions to the relationship between sun‐induced fluorescence (SIF) and gross primary production vary with temporal scaleIncoming photosynthetically active radiation controls the variations of SIF and gross primary production at maize peak growth stageContributions of SIF yield and light use efficiency to SIF and gross primary production increase significantly over the growing season