Measuring wheat senescence with a digital camera

Documenting crop senescence rates is often difficult because of the need fo r frequent sampling during periods of rapid change and the subjective natur e of human visual observations. The purpose of this study was to determine the feasibility of using images produced by a digital camera to measure the senescence rate of wheat and to compare the results with changes in greenn ess determined by two established methods. Measurements were made as part o f an experiment to determine the effects of elevated CO2 and limited soil n itrogen on spring wheat (Triticum aestivum L.) at the University of Arizona 's Maricopa Agricultural Center, near Phoenix, AZ. "Greenness" measurements were made during senescence of the crop with a color digital camera, a han d-held radiometer, and a SPAD chlorophyll meter. The green to red (GIR) for each pixel in an image was calculated and the average GIR computed for cro pped images from a digital camera representing 1 m(2) for each treatment an d sample date. The normalized difference vegetation index (NDVI) was calcul ated from the red and near-infrared canopy reflectances measured with a han d held radiometer. A SPAD reading was obtained from randomly selected flag leaves. All three methods of measuring plant greenness showed similar tempo ral trends. The relationships between GIR with NDVI and SPAD were linear ov er most of the range of GIR. However, NDVI was more sensitive at low values than GIR. GIR was more sensitive above G/R values of 1.2 than SPAD because the upper limits of SPAD measurements were constrained by the amount of ch lorophyll in the leaf, while GIR responded to both chlorophyll concentratio n in the leaves as well as the number of leaves present. Color digital imag ing appears useful for quantifying the senescence of crop canopies. The cos t of color digital cameras is expected to decrease and the quality and conv enience of use to improve.