Generic Calibration of Chlorophyll Meter Values for Leaf Chlorophyll Content Using Spectral Reflectances and Transmittances.

The Minolta SPAD-502 chlorophyll meter measures leaf transmittances at 650 and 940 nm to provide a relative value of total chlorophyll a and b contents based on Beer's Law.  Many studies have calibrated chlorophyll meter values with measured chlorophyll contents, and generally found that different equations are required for different plant species or cultivars, and differences in leaf thickness within the same species or cultivar.  We examined the potential for creating a generic calibration using leaf reflectances and transmittances measured with a portable spectrometer and simulated with the PROSPECT leaf radiative transfer model.  Subtraction of leaf reflectances from the incident radiation made the calibration much more linear between 0 and 65 μg/cm² of chlorophyll.  Chlorophyll meters have reduced sensitivity at high chlorophyll concentrations due to the well-known limitations of Beer's Law.  Increased leaf thickness increased chlorophyll-meter values at a given chlorophyll content, so changes of the calibration slope may largely correct for changes in leaf thickness.  However, the change in slope is difficult to predict from leaf thickness alone, because light scattering within a leaf depends on both the amount and arrangement of leaf tissues.