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 (μg 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.