Location: Hydrology and Remote Sensing Laboratory
Title: Assessment of chlorophyll meter calibrations for chlorophyll content using leaf spectral transmittances Authors
Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 15, 2014
Publication Date: April 23, 2014
Citation: Hunt, E.R., Daughtry, C.S. 2014. Assessment of chlorophyll meter calibrations for chlorophyll content using leaf spectral transmittances. Agronomy Journal. 106:931-939. Interpretive Summary: Hand-held chlorophyll meters provide rapid estimates of leaf nutrient status, and meter values are compared to measured leaf chlorophyll contents in order to develop statistical calibration equations. With a calibration equation, subsequent meter values may be used to determine chlorophyll contents quantitatively, which is much more useful than simply having a relative measure of nutrient status. However, calibration equations vary significantly from one crop to another, and often from one crop variety to another or with the same variety grown under different environmental conditions. We found there was a physical interaction between leaf chlorophyll content and optical leaf structure, which was not included in the calculation of the meter value. Optical leaf structure is only weakly correlated to leaf thickness, leaf dry weight per area and leaf fresh weight per area, so including more data will not help improve calibration equations for chlorophyll meters. Instead of a meter value, leaf transmittances measured by the chlorophyll meter should be used as inputs into computer simulation models, with outputs of leaf chlorophyll contents independent of variations in leaf optical structure. By upgrading the computer software inside chlorophyll meters, elaborate calibration of chlorophyll meters will not be necessary. Furthermore, manufacturers could add additional sensors to chlorophyll meters, which will increase the accuracy of estimated chlorophyll content for nutrient management.
Technical Abstract: Konica-Minolta SPAD-502 leaf chlorophyll meters provide a relative value of plant nitrogen status useful for agricultural nutrient management. From previous studies, there is not a single accurate calibration equation between leaf chlorophyll content (µg chl. a+b cm-2) and SPAD-502 value. 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. The leaf radiative transfer model, PROSPECT, was used to simulate transmittances and the chlorophyll/SPAD-502 relationship for different leaf optical properties based on leaf structure. Spectral and leaf data from maize (Zea mays L.) showed that the PROSPECT model predicted transmittances within 2%. There was a strong interaction between chlorophyll content and leaf structure that was not included in the equation used to calculate SPAD-502 values. It is possible that this interaction created variation among the calibration regressions published in the literature. Leaf dry matter content and water content had no effect on the predicted SPAD-502 values; however leaf structure was correlated with these leaf variables, so it is possible to increase the accuracy of SPAD-502 calibrations for chlorophyll content.