δC as a tool for iron and phosphorus deficiency prediction in crops.

Many studies proposed the use of stable carbon isotope ratio (δC) as a predictor of abiotic stresses in plants, considering only drought and nitrogen deficiency without further investigating the impact of other nutrient deficiencies, that is, phosphorus (P) and/or iron (Fe) deficiencies. To fill this knowledge gap, we assessed the δC of barley ( L.), cucumber ( L.), maize ( L.), and tomato ( L.) plants suffering from P, Fe, and combined P/Fe deficiencies during a two-week period using an isotope-ratio mass spectrometer. Simultaneously, plant physiological status was monitored with an infra-red gas analyzer. Results show clear contrasting time-, treatment-, species-, and tissue-specific variations. Furthermore, physiological parameters showed limited correlation with δC shifts, highlighting that the plants' δC, does not depend solely on photosynthetic carbon isotope fractionation/discrimination (Δ). Hence, the use of δC as a predictor is highly discouraged due to its inability to detect and discern different nutrient stresses, especially when combined stresses are present.