Results/Conclusions: Three distinct components explain more than 95% of the variance in leaf fluorescence spectra under both steady-state and changing illumination conditions. A single spectral shape of fluorescence explains ~85% of the variance across a wide range of species. The magnitude of this shape responds to absorbed light and photosynthetic up/down regulation; meanwhile, chlorophyll concentration and non-photochemical quenching (NPQ) control ~7% and ~2% of the remaining spectral variance, respectively. The spectral shape of fluorescence is remarkably stable where most current satellite retrievals occur (‘far-red’, >740nm), and dynamic downregulation of photosynthesis reduces fluorescence magnitude similarly across the 670-850nm range. We conduct an exploratory analysis of hourly red and far-red canopy SIF in soybean, which shows a subtle change in red:far-red fluorescence coincident with photosynthetic downregulation; but is overshadowed by longer-term changes in canopy chlorophyll and structure. Based on our leaf and canopy analysis, caution should be taken when attributing large changes in the spectral shape of remotely sensed SIF to plant stress, particularly if data acquisition is temporally sparse. Ultimately, changes in SIF magnitude at wavelengths greater than 740nm alone may prove sufficient for tracking photosynthetic dynamics.
