Characterization of maize leaf chlorophyll content using a color palette

There is a need to estimate leaf chlorophyll (CHL) content for various applications (e.g. precision agriculture, plant health status, etc.). To provide a simple and cheap alternative to commonly used hand-held sensors, this study seeks to quantify the relationship between leaf CHL and the leaf color in maize, determined by a set of four color swatches (CS: from least to most green; CS1-4). The study area included three fields located near Mead, Nebraska, USA. Two fields are irrigated continuous maize, while the third is rainfed maize/soybean rotation. All the fields were planted with maize in 2011, the year of the study. CHL content in leaves was retrieved from leaf level reflectance spectra measured by a USB2000 Ocean Optics radiometer fitted with a leaf clip and, in addition, a SPAD (Minolta) reading. A total of 183 leaf measurements were collected with 27, 31, 54, and 71 samples for each respective CS from the lowest to highest greenness. Using a technique for estimating CHL from leaf reflectance data, we found that in CS1 an average CHL content was 129.7 mg/m2 with a standard deviation (SD) of 43.3 mg/m2. For CS2, an average CHL was 218.4 mg/m2 with a SD = 60.4 mg/m2; for CS3 it was 441.2 mg/m2 with a SD =137.4 mg/m2. For the greenest class, CS4, CHL was 811.2 mg/m2 with a SD = 154.8 mg/m2. The CHL estimated for each CS determined from SPAD was closely related those found from reflectance measurements. One application for these results is to estimate total canopy chlorophyll content in conjunction with LAI measurements. We were able to use the averages determined in this study for each CS to estimate canopy CHL content throughout the growing season. This method provides a cheap and simple way to monitor leaf CHL content in crops.