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 spatio-temporal 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 (ΦPSII), can be estimated by the fluorescence efficiency parameters, APAR normalized fluorescence (Fyield) and the ratio of the two emitted fluorescence peaks (F↑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 ΦPSII and Fyield is non-monotonic across a P limitation gradient, rendering the prediction of ΦPSII by Fyield alone unfeasible. We show, however, that the pigment corrected PRI (cPRI) and F↑ratio (cF↑ratio) share a strong linear relationship with ΦPSII, thereby enabling the estimation of ΦPSII. We demonstrate that a compensation for reabsorption effects improved the estimation of ΦPSII by Fratio at foliar level. This may allow improved predictions of photosynthetic light use efficiency parameters without the need of measuring green APAR.