Calibrations Between Chlorophyll Meter Values and Chlorophyll Contents Vary As the Result of Differences in Leaf Structure.

In order to relate leaf chlorophyll meter values with total leaf chlorophyll contents (μg cm^-2), calibration equations are established with measured data on leaves. Many studies have documented differences in calibration equations using different species and using different growing conditions for the same species. Chlorophyll meters measure leaf transmittances at red (650 nm) and near-infrared (NIR, 940 nm) wavelengths; red light is strongly absorbed by chlorophyll, whereas there is minimal absorption of NIR light. Thin leaves generally transmit more light compared to thick leaves that generally reflect more light. We hypothesize differences among calibration equations result primarily from variations in internal leaf structure, which may be quantified by differences in the ratio of reflected to transmitted NIR light. SPAD-502 chlorophyll meter reading, chlorophyll content, and leaf spectral reflectances and transmittances were measured for different plant species, including maize and sunflower.  Transmittances of NIR and red light were used to predict SPAD-502 values, and predicted values followed the one-to-one line with measured values. When leaves were grouped by the ratio of NIR reflectance to transmittance, regardless of species, we obtained consistent calibration equations with high R² and low root-mean-square error, supporting the hypothesis.  Leaf thickness and internal leaf structure are in part determined by species and crop variety, but also determined in part by growing conditions such as light levels during leaf development. Therefore, it is not recommended to use calibration equations developed by others. Instead, chlorophyll meters should report the measured red and NIR transmittances, and not just a single value.