Bidirectional sun-induced chlorophyll fluorescence emission is influenced by leaf structure and light scattering properties - A bottom-up approach

- When leaves are sunlit from above, Chl fluorescence is emitted upward and downward.
- Downward sun-induced fluorescence (SIF) is mainly emitted in the far-red/NIR.
- Leaf architecture plays a role in the partitioning between upward and downward SIF.
- Leaf scattering properties are linked to bidirectional leaf SIF.

Sun-induced chlorophyll fluorescence (SIF) at leaf level is emitted in both upward and downward directions in the red and far-red part of the spectrum (650-850 nm) when a leaf is illuminated from the upper leaf surface. Hence, total SIF is represented by the sum of the upward and downward emission components. Nevertheless, the downward component of leaf SIF is often not considered despite that downward fluorescence yield (↓FY) can amount up to 40% of the total fluorescence yield (FYtot). Downward SIF is mainly emitted in the far-red, since this part of fluoresced light is highly scattered within leaves, unlike red Chl fluorescence, which is mostly reabsorbed. While total FY can be quite different among distinct species, the relative partitioning between upward and downward fluorescence shows more similarities among different leaf types, especially in the far-red. It is shown that bidirectional SIF emission properties in the far-red can be attributed to the scattering properties of the leaf, whereby an equifacial leaf follows a different trend compared to bifacial leaves. This was done by comparing SIF data with simultaneously measured reflectance, transmittance and fluorescence data by means of the FluoWat leaf clip coupled with an ASD hyperspectral spectroradiometer. These results could further improve Chl fluorescence modeling at leaf level, and help to advance the interpretation of SIF at the canopy level.