Morphological seed differentiation between cultivars of xoconostles and tunas (Opuntia spp.)

The classification and description of cactus pear varieties, tunas (sweet cactus pear) and its wild relatives xoconostles (acidic cactus pear), has been based on morphological traits of the fruits and cladodes. The absence of pulp and the presence of an edible thick pericarp are the most significant morphological differences between xoconostles and tunas. This work was focused on the differentiation between tunas and xoconostles by analyzing the external and internal seeds characteristics. Twenty three tunas and six xoconostles accessions were compared using sixteen variables. The ANOVA was performed and significant variables were selected by correlation and principal component analysis (PCA), and subsequently a cluster analysis was performed too. All variables were found to be significant in order to discriminate between these types of plants: the perisperm area and the whole seed area were 50 and 40% bigger in tunas than in xoconostles, respectively. While the perimeter area, the major and minor axis length, the Feret diameter, the embryo area, and 100 seeds weight, all were 20% smaller in xoconostles than in tunas. Two principal components (PC) explained 88.96% of the variability; the PC1 (Seed area, major axis length, Feret diameter, embryo area and 100 seeds weight) explained 60.61%, and the PC2 (minor axis length and elongation) explained 28.36% of the variability. The PCA plots and the cluster analysis clearly separated the xoconostles accessions from the tunas ones, but both analysis did not match up with the actual taxonomy classification. The present study demonstrated that the internal and external seeds variables are an alternative to the cactus pear characterization and it showed a tendency to increase the seeds weight in relation to the domestication level.