Canopy structure explains the relationship between photosynthesis and sun-induced chlorophyll fluorescence in crops

Dechant, B., Ryu, Y., Badgley, G., Zeng, Y., Berry, J.A., Zhang, Y., Goulas, Y., Li, Z., Zhang, Q., Kang, M., Li, J., & Moya, I. (2020). Canopy structure explains the relationship between photosynthesis and sun-induced chlorophyll fluorescence in crops. Remote Sensing of Environment, 241, 111733

 

Sun-induced chlorophyll fluorescence (SIF) has been emerging as a direct indicator of canopy photosynthesis. Photons that are intercepted by chlorophylls have three pathways: photochemistry, non-photochemical quenching (NPQ), and fluorescence. In remote sensing perspective, it is very difficult to detect photochemistry and NPQ which is not optically visible. On the other hand, fluorescence is emitted radiation by plants, which can be detected from optical sensors. Owing to the technological advancements, it has proven that SIF can be detected from space. It opened new era to monitor global photosynthesis, and multiple satellite missions for SIF have been launched, or will be launched in coming years. So it is very timely to ask this question: what’s the relationship between SIF and canopy photosynthesis?

Figure 1: Overview of SIF related variables

 

SIF which can be detected from remote sensing is composed of three components (Fig 1). First is absorbed visible light by vegetation (APAR), and the second is fluorescence yield (PhiF), and the third is escape ratio (fesc) which determines how much emitted SIF escapes the canopy towards the sky. The relative contributions of three components to the SIF signal have not been fully explored, which is required to understand the relationships between SIF and canopy photosynthesis.

In this study, we used a unique set of field datasets including SIF, canopy photosynthesis, APAR and fesc from three crop sites. We tested which SIF related variables predict canopy photosynthesis best. The result was surprising to us. Rather than physiological information (APAR*PhiF in Figure 2), we found that APAR*fesc (the structural information of canopy) explained the variations of canopy photosynthesis in all three sites. Furthermore, we found that APAR*fesc, not SIF, predicted canopy photosynthesis better. We discussed it is due to the inherent higher noise in SIF while APAR and fesc have higher signal to noise.



                                                                                                                  Figure 2. Relationships between canopy photosynthesis (GPP) and SIF related variables

Those findings highlight that canopy structural information dominates SIF signal, and the relationships between SIF and canopy photosynthesis is largely modulated by canopy structure. We are extending similar analysis across a range of diverse ecosystems. This study was published in Remote Sensing of Environment.

 

Prof Youngryel Ryu’s group has been measuring and modeling SIF across scales. Please visit his lab webpage for further information: http://environment.snu.ac.kr/