Non-monotonic relationship of sun-induced fluorescence to photosynthesis

Due to its close link to the photosynthetic process, sun-induced fluorescence (SIF) is one of the most promising signals to assess spatio-temporal variation in photosynthesis. Yet the positive linear relationship between SIF and photosynthesis, often reported from satellite and proximal remote sensing, contradicts findings from leaf-level studies, particularly under stress conditions. In two separate experiments, we grew Mays (Zea mays L.) under increasing phosphorus limitation and potato (Solanum tuberosum L.) under increasing drought stress to assess whether SIF can detect the phosphorus and drought induced reduction in photosynthesis. We demonstrate that the relationship between photosynthesis and APAR (absorbed photochemical active radiation) normalized SIF (FY) is non-monotonic under increasing environmental stress conditions, rendering the prediction of photosynthesis by FY alone unfeasible. The use of FY in combination with a pigment corrected photochemical reflectance index (PRI) as an indicator of the stress stage, allows the estimation of photosynthesis. However, this approach is strongly affected by uncertainties in PRI and we therefore propose the pigment-corrected ratio of the two SIF peaks (cF_ratio) as a precise and robust estimator of photosynthesis (R² = 0.90, rRMSE = 10%). Due to its independence on the absorbed photosynthetic active radiation, the cF_ratio is a promising novel estimator of spatio-temporal variation in photosynthesis.