Chlorophyll Meter Calibrations for Chlorophyll Content Using Measured and Simulated Leaf Transmittances

Konica-Minolta SPAD-502 leaf chlorophyll meters provide a relative value of leaf chlorophyll content, and from the literature, there are considerable variations among the calibration equations between total chlorophyll contents (mu g chlorophyll a + b cm(-2)) and SPAD-502 values. Our objective was to determine the leaf properties that contributed to the variations in calibration. We determined the internal calibration coefficient of five SPAD-502 meters so that leaf transmittances in the red (650 nm) and near-infrared (940 nm) could be used to calculate SPAD-502 values. A leaf optics model, PROSPECT, was used to simulate transmittances and the chlorophyll-SPAD-502 relationship for different leaf optical properties. Spectral and leaf data from maize (Zea mays L.) showed that PROSPECT predicted leaf transmittances within 2%. Maize leaf data used in the PROSPECT model predicted the relationship between chlorophyll content and the SPAD-502 value, although a polynomial regression was a better fit to the data. There was a physical interaction between chlorophyll content and optical leaf structure affecting leaf transmittances, which is not in the equation for calculating SPAD-502 values. Changing the PROSPECT leaf structure parameter resulted in different chlorophyll-SPAD-502 meter relationships, which were similar to the measured range of variation from calibration equations found in the literature. If the red and near-infrared transmittances are saved for each chlorophyll meter reading, then leaf radiative transfer models such as PROSPECT may be inverted to determine the actual leaf chlorophyll content.