Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 23, 1998
Publication Date: N/A
Interpretive Summary: When a crop of wheat matures, the color changes from dark green to a yellow-brown color. While the process of maturation of a grain crop is apparent by visual observation, quantifying the process is often difficult. During the ripening process, the crop changes rapidly, which means that finding differences in the field requires frequent measurement of the crop. .Visual observations can be made frequently but tend to be subjective and dependent on the person making the observations. A method was developed to measure the greenness of a wheat crop using a color digital camera costing less than $1000 and off-the-shelf-software. While there were differences among the methods, overall the new method compared favorably with established methods using hand-held radiometers, which cost several times as much as the camera system, and with measurement of the chlorophyll content of the leaves. With calibration, digital cameras have the potential to measure nutrient status of plants, pest infestations, etc. A the cost of digital imaging drops and the quality of the cameras improves, these devices may gain widespread acceptance for many field applications in agriculture. It is likely in the near future that as more data are developed, digital camera systems will be used commonly to monitor the status of crops. This work should be of benefit to other researchers, action agencies, consultants, and producers.
Technical Abstract: Documenting crop senescence rates is often difficult because of the need for frequent sampling and the subjective nature of human visual observations. Several types of greenness measurements were made as part of an experiment to determine the effects of elevated CO2 and limited soil nitrogen on spring wheat at the University of Arizona's Maricopa Agricultural Center (MAC),near Phoenix, Arizona. A Free-Air CO2 Enrichmen (FACE) apparatus was used to expose the crop to 200 umol/mol above ambient CO2 levels. Each CO2 treatment was divided into high and low levels of nitrogen application. High-N treatments received 350 kg ha-1 of N, and low-N treatments received 15 kg/ha of N. "Greenness" measurements were made during senescence of the crop using a color digital camera, a hand-held radiometer, and a SPAD chlorophyll meter. The average of the ratio of green to red (G/R) digital numbers was computed for a cropped image from a digital camera representing 1 m2 for each treatment and sample date. The normalized difference vegetation index (NDVI) was calculated from the red and near-infrared canopy reflectances measured with a hand-held radiometer. A SPAD reading was obtained from flag leaves. The relationships between G/R with NDVI and SPAD were linear over most of the range of G/R. However, NDVI was more sensitive at low values than G/R. G/R was more sensitive above G/R values of 1.2 because the upper limits of SPAD measurements were constrained by the amount of chlorophyll in the leaf, while G/R responded to both chlorophyll concentration in the leaves as well as the number of leaves present. Color digital imaging appears useful for quantifying the senescence of crop canopies. The cost of color digital cameras is expected to decrease and the quality and convenience of use to improve.