The Australian pear industry produced 105,243 t of fruit in 2015, at a value of $125 million (ABS, 2016a, 2016b); however, this production represents a steady decline since a peak in 2004. Pear breeding programs throughout the world have released new fresh market cultivars that are of premium quality and aim to increase world consumption of pears. Most of the selections are either red-blushed (e.g., ‘Celina’, ‘Gem’, and ‘ANP-0131’) or full-red skinned cultivars (e.g., PremP009) that are more popular with consumers (Human, 2013). Australian growers, predominantly in the Goulburn Valley of north-central Victoria, are switching to these new cultivars. Moving away from high yields of traditional green-skinned cultivars to these new red-blush cultivars brings new challenges, in particular, the need to manage crops to promote good color to meet market expectations.
Nitrogen is an essential element for plant growth. N deficiency in pear orchards results in low vigor and reduced yield, whereas N excess causes high vigor and poor fruit quality (Brunetto et al., 2015; Sugar et al., 1992). Timing of the N application is also important. For example, applications of urea (5% by weight) at full bloom was found to enhance fruit size (Curetti et al., 2013; Sánchez et al., 2008). Red and pink skin color in new blush pear cultivars is a critical quality factor for markets. Increasing N supply to Gala apple delayed skin red color development by decreasing both anthocyanin synthesis and chlorophyll degradation (Wang and Cheng, 2011). Although the effect of excess N on pear skin color has not been established, N will most likely affect pear skin color.
As part of pear N management, growers need a way to assess their orchard’s N status. Destructive sampling of leaves for N analysis is one approach. However, laboratory analysis of leaves is too costly to provide a whole-of-orchard assessment of the N status and does not provide immediate results. Being able to accurately, easily, and cheaply measure leaf N across an entire orchard would be ideal. Whereas nondestructive methods to estimate %N for pear leaves have been presented (Jie et al., 2014), methods to estimate %N at canopy scales for pear orchards have yet to be developed. Previous research (Fitzgerald et al., 2010; Perry et al., 2012) shows that using remote sensing to estimate N in wheat canopies is effective, and calculation of the Canopy Chlorophyll Content Index (CCCI) from canopy spectral reflectance of wheat showed good correlation with N uptake (kg N/ha). In this research, we extend the previous research on wheat to red-blush pears, acquiring and analyzing datasets of remote sensing measurements and corresponding leaf %N from laboratory analysis to determine the feasibility of remote sensing to characterize N status. This research also differs from previous research in the use of high spatial resolution imagery from UAV platforms to measure canopy reflectance to estimate leaf N concentration.
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