# Ground Based Hyperspectral Imaging to Characterize Canopy-Level Photosynthetic Activities
## 1 Introduction
Ground-based hyperspectral imaging would be a viable solution to address those issues because it provides a higher scanning throughput (usually line by line) and a better spatial resolution (sub meter or higher). Many researchers have explored the use of ground-based hyperspectral imaging for extracting vegetative indices that can be correlated with plant aboveground biomass [34], leaf area index [35], and various plant pigments (e.g., chlorophyll) [36]. Although these studies showed some success, they required either sophisticated model calibration and validation [36] or artificial illumination [34]. A few studies examined the use of ground hyperspectral imaging to measure SIF, showing high measurement accuracies and the potential of spatial variation analysis [37; 38]. However, these studies were restricted to the scope of instrumentation validation and did not utilize retrieved SIF for characterizing whole-canopy photosynthesis. It should be noted that passive hyperspectral sensing systems can also measure canopy reflectance indices to monitor photosynthetic activities. For instance, the photochemical reflectance index (PRI) was introduced by [PERSON] et al. [39], and has been studied extensively [40; 41; 42]. It is calculated as \((R_{531}-R_{570})/(R_{531}+R_{570})\) and has been correlated with photosynthetic efficiencies previously. However, some studies reported that the PRI is somewhat limited, in that measurements are highly sensitive to viewing angle and are strongly influenced by soil background at leaf area index (LAI) values less than 3 [12; 41]. In particular, a previous study found no correlation between PRI and light use efficiency (LUE) [43]. Thus, it would be of great interest to focus on the measurement of SIF rather than other reflectance indices for photosynthetic activity evaluations, especially at the canopy level. A particularly notable limitation to using SIF to estimate canopy-level photosynthetic efficiency is the method for estimation of maximal fluorescence (\(F^{\prime}{}_{m}\)) at the canopy level, which has not been reported previously [44]. If \(F^{\prime}{}_{m}\) could be determined for the canopy using SIF, real-time estimates of crop-level photosynthetic efficiencies could be obtained using passive sensing approaches. Direct measurement of \(F^{\prime}{}_{m}\) faces a significant challenge: the maximum intensity of solar illumination on the ground is far less than a \"saturating\" flash intensity used by PAM fluorometers, so maximal fluorescence at the canopy level cannot be directly measured. In fact, it has been demonstrated that leaves of plants previously acclimated to high light environments often do not close all reaction centers despite exposure to such a \"saturating\" flash intensity, preventing even direct measurement of \(F^{\prime}{}_{m}\) by using PAM fluorometry. A multi-phase flash approach was developed, however, to estimate \(F^{\prime}{}_{m}\) and electron transport rate (ETR) without the \"saturating\" light [45]. Using this method, a leaf sample is successively exposed to flash intensities in either ascending or descending order, and the intensity of chlorophyll fluorescence is quantified at each step. Fluorescence intensity (\(F\)) is then plotted against the reciprocal of PAR. A function is fit to the data to obtain the y intercept and an estimate of apparent \(F^{\prime}{}_{m}\) at an infinite light intensity when all reaction centers would hypothetically be closed. Since it is often necessary to expose single leaf samples to different PAR levels in succession to obtain an estimate of \(F^{\prime}{}_{m}\), it might also be possible to estimate \(F^{\prime}{}_{m}\) for the canopy by taking advantage of natural, diurnal variations in PAR and SIF. The overall goal of this study was to develop a ground-based hyperspectral imaging approach to characterize photosynthesis at the canopy level.