Fluorescence ratio and photochemical reflectance index as a proxy for photosynthetic quantum efficiency of photosystem II along a phosphorus gradient

Sun-induced chlorophyll fluorescence (SIF) is one of the most promising remote-sensing signals to assess spatiotemporal variation in photosynthesis. Yet, it has been shown that the positive linear relationship of SIF and photosynthesis, often reported from satellite and proximal remote sensing, is mainly driven by the amount of absorbed photosynthetic active radiation (APAR). By normalizing SIF by APAR these structural first-order effects can accounted for and SIF is then reflecting physiological regulation of photosynthetic efficiency. However, because of the confounding contribution of non-photochemical energy dissipation, the relationship between SIF and photosynthetic efficiency is non-linear, and therefore additional measurements have to be included to constrain the predictions of photosynthetic efficiency and photosynthetic electron transport. We grew Zea mays at different phosphorus (P) levels to assess if P-induced reduction in quantum efficiency of PSII (phi PSII), can be estimated by the fluorescence efficiency parameters, APAR normalized fluorescence (Fyield) and the ratio of the two emitted fluorescence peaks (F up arrow ratio), at leaf level. Results were compared to the photochemical reflectance index (PRI), a well-established index related to the activity of the xanthophyll cycle, a protection mechanism which activates under light-stress conditions. We demonstrate that the relationship between phi PSII and Fyield is non-monotonic across a P limitation gradient, rendering the prediction of phi PSII by Fyield alone unfeasible. We show, however, that the pigment corrected PRI (cPRI) and F up arrow ratio (cF up arrow ratio) share a strong linear relationship with phi PSII, thereby enabling the estimation of phi PSII. We demonstrate that a compensation for reabsorption effects improved the estimation of phi PSII by Fratio at foliar level. This may allow improved predictions of photosynthetic light use efficiency parameters without the need of measuring green APAR.

Automated summary

Sun-induced chlorophyll fluorescence (SIF) is a promising remote-sensing signal for assessing variation in photosynthesis, but its relationship with photosynthetic efficiency is non-linear. By normalizing SIF by absorbed photosynthetic active radiation (APAR), the physiological regulation of photosynthetic efficiency can be better reflected. The impact of phosphorus on photosynthetic efficiency can be estimated using fluorescence efficiency parameters, but predicting photosynthetic efficiency solely based on these parameters is challenging.