# Leaf-Level Spectral Fluorescence Measurements: Comparing Methodologies for Broadleaves and Needles
); [PERSON]] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; [EMAIL_ADDRESS] (J.A. ); [PERSON]] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; [EMAIL_ADDRESS] (J.A. ); [PERSON]] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; [EMAIL_ADDRESS] (J.A. ); [PERSON]ESS] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; [EMAIL_ADDRESS] (J.A. ); [PERSON]] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; [EMAIL_ADDRESS] (J.A. ); [PERSON]ESS] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; [EMAIL_ADDRESS] (J.A. ); [PERSON]] (A.P.-C.)1 Optics of Photosynthesis Laboratory, Institute for Atmospheric and Earth System Research/Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, 00014 Helsinki, Finland; jon.aetherton@helsinki. remetotely-sensed SIF [10; 11]. Fortunately, the complete spectrum is measurable at the leaf-level, which facilitates the investigation of the connection between different ChlF spectral bands and physiological leaf functionality. At the leaf scale, the shape of the ChlF spectra is characterized mainly by two peaks, located around 685-690 (red peak) and 740-750 nm (far-red peak) [12; 13; 14]. Variations in peak amplitude can affect the red/far-red ratio (i.e., F685/F740) and depend on: (a) the dynamic response of leaves to environmental and stress factors (e.g., to temperature [15] or water stress [16]); (b) light absorption and scattering effects within the leaf, associated with variation in leaf-level chlorophyll concentration [Chl] [17; 18]; and (c) the architecture of the photosynthetic apparatus and the relative contributions of photosystem II (PSII) and photosystem I (PSI) to the ChlF emission [11; 19; 20; 21]. At ambient temperature it is, however, challenging to ascribe specific emission bands to populations of PSI and PSII as there is a large degree of spectral overlap between the peaks [19; 22; 23]. Interestingly, the ChlF emission of the two photosystems can be separated by measuring ChlF spectra at 77 Kelvin (77 K) [15; 24; 25], where the red ChlF wavelengths correspond to PSII and the far-red bands to PSI. Typically, protocols for spectral measurements are based on setups that use a light source (artificial or the Sun) coupled with a short-pass filter that removes radiation above a given threshold (e.g., 650 nm). The setups then include a chamber or some other means to hold a sample at a given geometry with respect to the light source and spectrometer fiber. Irrespective of the selected methodology, it is a particular challenge to measure small and narrow leaves of complicated geometries, like those of conifers, in comparison to the more easily measured, relatively wider and flatter broadleaves. Nonetheless, conifers represent a significant fraction of terrestrial ecosystems--in 2010, 39% of the world's total growing stock [26]. It is, therefore, important to face the technical challenge of measuring needles with varying geometrical shapes and thicknesses. Measuring a single needle inevitably leads to a diminution of the signal-to-noise ratio (SNR) [27].