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Detailed physiological information from crops is essential for various applications, including breeding, farming, and ecological assessment. Ideally, such information is obtained through noninvasive techniques, among which visible spectroscopy is the most widely used. Although reflectance measurements provide reliable information, chlorophyll fluorescence (ChlF) is considered the most promising physiological indicator due to its close association with photosystem II chemistry. ChlF can be retrieved at multiple spatial scales, from chloroplasts and individual leaves to farm plots, entire fields, and even regional scales. Notably, in remote sensing applications, ChlF is primarily retrieved passively, whereas at the leaf level, it is commonly measured actively. Therefore, it is crucial to bridge both retrieval approaches through simultaneous measurements at the canopy scale, which serves as the intermediate level. In this study, we demonstrate how a combined active and passive measurement approach can provide new insights into the physiology of two wheat varieties subjected to drought conditions. Photosynthetic efficiency and electron transport rate (ETR) were estimated using active fluorescence measurements with a light-induced fluorescence transient (LIFT) sensor. Passive measurements were conducted with a fluorescence box device (FloX), equipped with two spectrometers, enabling the calculation of reflectance-based vegetation indices and the retrieval of sun-induced fluorescence (SIF) via fluorescence line discrimination (FLD) methods. Both devices were mounted on a field cart to allow systematic and reproducible measurements throughout the growing season. This integrated approach revealed a strong relationship between SIF emission and ETR, both of which are linked to total absorbed photosynthetically active radiation (APAR). We also observed that while drought conditions significantly reduced photosynthetic efficiency over the season, SIF yield remained relatively constant. Consequently, no correlation between photosynthetic and fluorescence emission yields was detected. This behavior suggests that, in this case study, fluorescence emission does not appear to be the primary mechanism by which plants mitigate photooxidative stress. The approach presented here offers a framework for future simultaneous retrieval of fluorescence at the canopy level and lays the foundation for interpreting remote SIF measurements from both physiological and photochemical perspectives.

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