, Volume 53, Issue 5, pp 933-948
Date: 10 Jan 2006

Patterns of Morphological Variation in a Sample of Cacao (Theobroma Cacao L.) Germplasm from the International Cocoa Genebank, Trinidad

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Abstract

The International Cocoa Genebank, Trinidad (ICG,T) contains 2300 accessions representing a broad range of diversity and geographic origins. This study was undertaken to examine the phenotypic relationships in a diverse sample of 600 cacao accessions from the Genebank. The major objectives were to test the applicability of the accepted nomenclature, viz., the definition of classes, as well as to elucidate the relationships between wild and cultivated germplasm and among accession groups. This will facilitate efficient utilisation, management and improvement of cacao genetic resources through the identification of potentially heterotic groups. In addition, information on morphological variation may be considered along with allelic richness or gene frequencies in selecting core collections, and when formulating strategies for future collections in the wild. Considerable phenotypic variation (high coefficients of variation and Shannon Weaver Diversity Index values) in the germplasm studied was found, and the main groups of cacao were clearly separated. Wild cacao was differentiated from cultivated types by all of the quantitative descriptors except pod length. Recognised genetic groups (Trinitario and Forastero) were discriminated by several quantitative traits including sepal length, cotyledon weight, length and width, and pod index. Principal Component Analysis differentiated among 14 accession groups according to geographic origin and genetic grouping. Cotyledon weight, length and number, pedicel column colour, mature pod ridge colour, sepal length, pod basal constriction and surface texture, and ovule number accounted for most of the variation recorded. The observed improvement in the cultivated germplasm, in terms of agronomic traits such as seed weight and pod index, attests to the success of past selection activities. This apparent improvement (particularly among Refractario germplasm) appears to be accompanied by increased style length, a finding that warrants further study. The reduced diversity in sepal length, ligue width, ovule number, and cotyledon length in the cultivated compared to the wild germplasm is consistent with the narrowing of the genetic base in the former. These results have significance for cacao breeding programmes, conservation and future collecting strategies.