Variability in Juice Quality of 'Valencia∑ Sweet Orange and Sample Size Estimation for Juice Quality Experiments

Sources of variation in juice quality of 'Valencia∑ sweet orange (Citrus sinensis (L.) Osb.) were quantified and their relative contributions to variability in juice quality were determined, from which sample sizes were estimated. Commercial orchards of 'Valencia∑ sweet orange trees on Carrizo citrange (C. sinensis x Poncirus trifoliata (L.) Raf.) rootstock were selected at four geographic locations representing the major citrus-producing regions in Florida. Within- and between-tree variation in soluble solids concentration (SSC) and titratable acidity (TA) were estimated in two experiments over two or three seasons, respectively. Variance components for all treatment effects were estimated to partition total variation into all possible component sources of variation. Seasonal variation in SSC and TA was relatively small, but larger for TA than SSC. Variation in SSC among blocks within a location was intermediate to low, and was less than variation among locations. In contrast, tree-to-tree variation in SSC and TA was large, in spite of sampling from trees of similar vigor and crop load, and variation in SSC and TA among fruit was relatively large. Based on results of this study, samples consisting of 35 fruit are required to detect differences (Pi0.05) of 0.3% SSC and 0.06% TA, whereas 20-fruit samples can be used to detect differences of 0.4% SSC and 0.08% TA. Seven replications are required to detect differences of 0.5% SSC and 0.1% TA, with small gains in precision when tree numbers exceed 10. Juice soluble solids concentration (SSC) and titratable acid- ity (TA) of citrus are inherently variable. This variability can be large, and is a result of various factors that affect juice quality. Some factors affect individual fruit (Denny, 1922), whereas most effects on variation in juice quality are at the whole-tree level (Appleman and Richards, 1939; Jones et al., 1957; Reitz and Sites, 1948; Sites and Reitz, 1949), resulting in variability in juice quality among orchards within a region, among trees, within a tree, and among fruit. In addition, seasonal variation in juice quality, due to prevailing weather conditions during fruit development, is also a potentially important contributor to total variability in juice quality. A knowledge of the factors affecting juice quality, and the magnitude of their effects, allows those factors to be manipulated to maximize juice quality and produce fruit of consistent qual- ity. Knowing the amount of variability in juice quality is also a prerequisite for juice quality research, and plays a pivotal role in experimental design. The concept of estimating and partitioning variance into its component sources of variation (Hanson, 1955) has extensive application to horticultural work in the field of