Reflectance measurements at leaf and canopy scales were made in a red-blush pear (Pyrus communis) orchard for two growing seasons. Canopy reflectance measurements were obtained using a multispectral camera flown on board an unmanned aerial vehicle (UAV), and leaf reflectance measurements were undertaken in a laboratory using a portable spectrometer. These measurements were used to compute reflectance indices as surrogates for direct leaf nitrogen (N) concentration measurements. The indices were evaluated against laboratory analysis of leaf N concentration. Regression results for leaf %N on canopy-level measurements with the multispectral camera resulted in the highest R 2 value [R 2 = 0.67; root mean square error (RMSE) = 0.24%N] with a new index, Modified Canopy Chlorophyll Content Index (M3CI)_710 nm. Regression results for leaf %N on leaf-level measurements in-laboratory resulted in the highest R 2 value (R 2 = 0.65) with two other indices, Normalized Difference Vegetation Index (NDVI) and Normalized Difference Red-edge Index (NDRE)_720 nm. The corresponding RMSE values were 0.26%N. The results indicate that reflectance indices measured at the leaf level, with a controlled light source and calibration, could be used to estimate leaf %N. An analysis of uncertainty indicated that if leaf %N is estimated from leaf-level reflectance values, 10 or more leaves (from the same tree) should be averaged. The results support the use of a UAV-based assessment for canopy %N using the M3CI_710 nm, which could provide spatial information of leaf N concentration across an orchard.
Eileen M. Perry, Ian Goodwin, and David Cornwall
Suphasuk Pradubsuk, Joan R. Davenport, Robert G. Stevens, and Eileen M. Perry
Collection and estimation of root material are likely some of the greatest challenges of whole-plant sampling. As with other perennial crops, season of sample collection is also a challenge in grape whole-plant sampling. Our interest is in collecting grape whole-plant samples from an established (>25-year-old) vineyard to study plant nutrient partitioning. Before launching into routine sampling, two techniques were compared for very fine, fine, and coarse root distribution. For very fine and fine root sampling, soil cores were collected in a radial pattern around the vine trunk at eight sample points, each either 20, 60, 120 cm from the trunk or 50, 100, and 150 cm from the trunk. Roots were washed from the soil material, separated into fractions and weighed. For evaluation of techniques for sampling fine and coarse roots, roots were either excavated by tracing them from the trunk in about a 1-m3 soil volume or by extracting about the same soil volume using a backhoe and shaking the soil free of the roots. Overall, the more narrow soil core sampling gave a greater total root mass and both the tracing and backhoe methods gave similar results. In addition, pruning weight measurement is also frequently measured in grape research. We compared using the NDVI (Normalized Difference Vegetation Index) device, the “Greenseeker”™, with pruning mass to determine if this device could be used as a non-destructive measurement for grape pruning weight.