# Leaf-Level Spectral Fluorescence Measurements: Comparing Methodologies for Broadleaves and Needles
## 5 Conclusions  
The interpretation of solar-induced fluorescence (SIF) acquired from towers, drones, aircrafts, or satellites, depends largely on our capacity to characterize and model the impact that different factors exert on the observed results. Variability in amplitude and shape of the leaf-level ChIF spectra appears across species, stress factors, and canopy light gradients. In order to link signals acquired at different scales, we need to develop standardized protocols to measure leaf-level ChIF properties across different types of leaves. In this study, we measured leaf-level ChIF spectra using methods representing notable differences in measurement geometry. We tested three species and compared the impact of needle arrangement on the resulting ChLF spectra. We found that the shape (but not the amplitude) of the leaf-level ChLF spectra was very robust in response to changes in illumination/viewing geometry across OC and FW methods. The IS method, despite being a standard for reflectance and transmittance measurements, did not appear to be a good solution to measure ChLF spectra, as it tended to underestimate the red ChLF wavelengths. We also presented a simple method to correct for baseline effects and emphasized its potential importance under certain conditions. Furthermore, we recommend the use of needle mats--which presented good replicability across FW and OC--as a reasonable compromise and reproducible method to measure temporal changes in ChLF spectra of needle-like leaves until better solutions become available and keeping in mind the potential underestimation of red ChLF due to re-absorption.  
The ideal protocol is always the one that best adjusts to its purpose and application. The FW method makes it possible to measure leaf ChLF properties both in situ as well as in the laboratory, where incoming PAR can be standardized for easier comparisons. The OC method instead provides the possibility of measuring leaf ChLF properties at both ambient and 77 K temperatures, but cannot be applied to leaves in situ. Other methods can be developed to serve still other purposes. The key point is to keep measurement setups consistent across experiments, as well as thoroughly documented for potential future use in meta-analysis, so that ChLF variation can be attributed to biochemical, physiological, and structural factors.  
Supplementary materials are available online at [[http://www.mdpi.com/2072-4292/11/5/532/s1](http://www.mdpi.com/2072-4292/11/5/532/s1)]([http://www.mdpi.com/2072-4292/11/5/532/s1](http://www.mdpi.com/2072-4292/11/5/532/s1)).  
Conceptualization, P.A.R. and A.P.-C.; Data curation, P.A.R.; Formal analysis, P.A.R.; Funding acquisition, A.P.-C.; Investigation, P.A.R.; Methodology, P.A.R., J.A., L.A. and A.P.-C.; Supervision, J.A. and A.P.-C.; Writing - original draft, P.A.R. and A.P.-C.; Writing - review & editing, P.A.R., J.A., L.A. and A.P.-C.  
This research was supported by Academy of Finland FLUOSYNTHESIS project #293443 and #288039. [PERSON] acknowledges the Doctoral Programme in Atmospheric Sciences (ATM-DP, University of Helsinki) for financial support. [PERSON] is currently partially funded by AVANFLEX project (Advanced Products for the FLEX mission), n\({}^{\circ}\) ESP2016-79503-C2-1-P, Ministry of Economy and Competitiveness, Spain.  
We would like to thank [PERSON], from the Division of Pharmaceutical Chemistry and Technology, Faculty of Pharmacy, University of Helsinki, Finland for assistance with the 3D printing of the optical chamber, and [PERSON] from University of Helsinki Language Services for help with language revision. We would also like to express our gratitude to the anonymous reviewers for their detailed and constructive comments which significantly improved our manuscript.  
The authors declare no conflict of interest.