Tree age and canopy position affect rind quality, fruit quality and rind nutrient content of 'Kinnow' mandarin (Citrus nobilis Lour x Citrus deliciosa Tenora)

Rind quality is a critical factor affecting the external appearance and marketability of citrus fruit, especially for fresh consumption. Among many factors affecting citrus rind quality, tree age is the most important, but remained unexplored. This study aimed to explore the effects of tree age on rind and fruit quality of 'Kinnow' mandarin, the most important mandarin cultivar in Pakistan. For this purpose 'Kinnow' mandarin trees of different age (3, 6, 18 and 35 years), growing in a commercial orchard in the main citrus growing district, Sargodha, (latitude 32 degrees 03'N and longitude 72 degrees 40'E), were used. Fruit were harvested at commercial maturity from internal and external canopy positions and assessed for various physicochemical characteristics as well as rind macro- and micro-nutrients. Rind thickness (2.5 mm), percentage of rind mass (24.8%) and rag mass (24.1%), ascorbic acid (M) (32 mg/100 mL), pH (3.07), non reducing sugars (5.85%), rind manganese (Mn) (0.125 ppm) and iron (Fe) (1.27 ppm) contents were higher in young 3-year-old trees, whereas total sugars (7.6%), reducing sugars (2.2%), acidity (0.9%) and total soluble solids (TSS) (10.4 degrees Brix) were higher in 18-year-old trees. A correlation matrix was developed for nutrient content and rind quality parameters. Rind thickness correlated negatively with rind P (r=0.75), Ca (r=0.79), Cu (r=0.92), Mn (r=0.74), Fe (r=0.85) and Zn (r=0.80) contents in 3-year-old trees, and with K (r=0.73) content in 6-year-old trees, whereas 18- and 35-year-old trees exhibited no correlation between rind nutrient contents and rind thickness. For internal fruit quality. K correlated positively with rag mass (%) and TSS in 3-year-old trees, and was negatively correlated with rind mass (%) and TSS in 6-year-old trees and acidity in 35-year-old trees. This study highlights the role of potential nutrients which can help to improve rind and fruit quality in young 'Kinnow' mandarin orchards in the future. (C) 2011 Elsevier B.V. All rights reserved.