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
Lexie McClymont, Ian Goodwin, Desmond Whitfield, Mark O’Connell, and Susanna Turpin
Vegetative growth, orchard productivity, fruit quality and marketable yield were evaluated for rootstock (D6, BP1 and Quince A), tree density (741–4444 trees/ha), and training system (Open Tatura trellis, two-dimensional vertical and three-dimensional traditional) effects on young trees of the blush pear cultivar ‘ANP-0131’. ‘ANP-0131’ is a vigorous scion and vegetative growth, precocity, and yield were influenced by the selected rootstocks. Tree density and training system treatments exerted a substantial effect on canopy radiation interception while increasing tree density improved yield. Increasing tree density from 2222 (high density) to 4444 (ultra-high density) trees/ha did not improve cumulative yield. Crop load affected fruit size, such that “marketable” yield (yield of fruit weighing between 150 and 260 g) was greatest for trees on D6 rootstock and trained to Open Tatura trellis at high and ultra-high densities.