Abstract
Abstract: The objective of this work was to evaluate five nonparametric selection indices for the selection of hybrid citrus rootstocks grafted with 'Valência' sweet orange, using horticultural traits relevant for the juice processing industry. Forty-six rootstocks were evaluated in a randomized complete block design, with three replicates and five trees in the plot, in the period from 2009-2015, in a rainfed cultivation. The means of the variables plant height, accumulated fruit yield, fruit yield efficiency, total soluble solids concentration, juice yield, and drought-tolerance were used to calculate the following indices: multiplicative index (IEi), sum of classification (IMMi), genotype-ideotype distance (DiI), and ranking indices (IRKi, based on simple means; and IRKii, based on linear normalization). The indices were efficient to classify the hybrids in relation to general performance. Spearman’s correlation showed a high similarity between most nonparametric indices, notably between IRKi and IRKii. The ranking indices, mainly IRKii, provide a more coherent classification of the hybrids, which allows of the selection of more productive and drought-tolerant rootstocks to produce high-quality fruit for processing.
Highlights
Despite its socioeconomic importance, Brazilian citrus production is vulnerable to several abiotic and biotic stresses because of the low variability of the available genetic material, mainly of rootstock cultivars (Bastos et al, 2014)
All assessed variables showed significant differences, which allowed of the ranking of the hybrid citrus rootstocks, confirming the variability within the evaluated genotypes
The coefficient of variation (CV) of the variables used to calculate the indices were accumulated fruit production (AP) (16.13%), EF (20.37%), SS (3.37%), juice yield (JC) (4.13%), DT (10.37%), and tree height (TH) (6.33%) (Table 1). This wide genetic variability reflects the diverse parental background of the evaluated genotypes, but it is commonly reported within populations of hybrid citrus rootstocks (Raga et al, 2012; Schinor et al, 2013)
Summary
Brazilian citrus production is vulnerable to several abiotic and biotic stresses because of the low variability of the available genetic material, mainly of rootstock cultivars (Bastos et al, 2014). The citrus rootstock influences more than 20 characteristics of the scion variety, from drought-tolerance to pest resistance, tree size, and fruit traits (Castle, 2010) This fact poses a challenge for the appropriate selection of superior genotypes in breeding programs that, in turn, have been introducing hundreds of new hybrid rootstocks to promote varietal diversification (Soares Filho, 2012). Due to the high genetic segregation of citrus (Navarro et al, 2002), obtaining hybrids that combine good performance in multiple traits is seldom an easy task In this sense, nonparametric indices are auxiliary tools that involve the simultaneous combination of several attributes of interest, to allow a more efficient selection of promising genotypes (Vilarinho et al, 2003)
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