Assessment of Canopy Chlorophyll Content Retrieval in Maize and Soybean: Implications of Hysteresis on the Development of Generic Algorithms

Yi Peng,Timothy Arkebauer,Anthony Nguy-Robertson,Anatoly Gitelson

doi:10.3390/rs9030226

Abstract

Canopy chlorophyll content (Chl) closely relates to plant photosynthetic capacity, nitrogen status and productivity. The goal of this study is to develop remote sensing techniques for accurate estimation of canopy Chl during the entire growing season without re-parameterization of algorithms for two contrasting crop species, maize and soybean. These two crops represent different biochemical mechanisms of photosynthesis, leaf structure and canopy architecture. The relationships between canopy Chl and reflectance, collected at close range and resampled to bands of the Multi Spectral Instrument (MSI) aboard Sentinel-2, were analyzed in samples taken across the entirety of the growing seasons in three irrigated and rainfed sites located in eastern Nebraska between 2001 and 2005. Crop phenology was a factor strongly influencing the reflectance of both maize and soybean. Substantial hysteresis of the reflectance vs. canopy Chl relationship existed between the vegetative and reproductive stages. The effect of the hysteresis on vegetation indices (VI), applied for canopy Chl estimation, depended on the bands used and their formulation. The hysteresis greatly affected the accuracy of canopy Chl estimation by widely-used VIs with near infrared (NIR) and red reflectance (e.g., normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and simple ratio (SR)). VIs that use red edge and NIR bands (e.g., red edge chlorophyll index (CIred edge), red edge NDVI and the MERIS terrestrial chlorophyll index (MTCI)) were minimally affected by crop phenology (i.e., they exhibited little hysteresis) and were able to accurately estimate canopy Chl in two crops without algorithm reparameterization and, thus, were found to be the best candidates for generic algorithms to estimate crop Chl using the surface reflectance products of MSI Sentinel-2.

Highlights

Since chlorophyll is the main driver of light absorption and conversion of the absorbed light to stored chemical energy, it is an essential factor governing plant photosynthetic potential (e.g., [1,2])
The hysteresis greatly affected the accuracy of canopy Chl estimation by widely-used vegetation indices (VI) with near infrared (NIR) and red reflectance (e.g., normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and simple ratio (SR))
In the photosynthetically-active radiation (PAR) Multi Spectral Instrument (MSI) spectral bands, maize reflectance decreased with an increase in canopy Chl for both the vegetative and reproductive stages

Summary

Introduction

Since chlorophyll is the main driver of light absorption and conversion of the absorbed light to stored chemical energy, it is an essential ( not unique) factor governing plant photosynthetic potential (e.g., [1,2]). Foliar chlorophyll content (Chl) provides useful information on leaf photosynthetic capacity as defined by the maximum rate of carboxylation (Vmax , e.g., [3,4,5]). The leaf RuBisCO content is highly correlated with the foliar nitrogen (N) content because of the large proportion of N in photosynthetic machinery [6]. In addition to RuBisCO, much of the Remote Sens. 2017, 9, 226 incorporated foliar N is in chlorophyll, and strong correlations exist between foliar Chl and N content [7,8,9,10]. It was found that the relationship between Chl and N content at the canopy level is much stronger than at the leaf level (e.g., [10])

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