Geometric morphometrics of leaves of Anacardium microcarpum Ducke and A. occidentale L. (Anacardiaceae) from the coastal region of Piauí, Brazil

Anacardium microcarpum (cajuí) differs from A. occidentale (cashew) by its smaller drupe and hypocarp and more restricted range in cerrados and coastal plateaux of Brazil’s Pará, Maranhão, Piauí, Ceará and Rio Grande do Norte states. Taxonomists treat A. microcarpum as conspecific with A. occidentale, but many agronomists treat it as distinct. This study used geometric morphometrics to investigate leaf shape differences between the two taxa in ten populations from coastal Piauí state, Brazil with samples of 300–380 leaves. Configurations of two landmarks and 14 semilandmarks were digitized along the leaf outline from standardized images and subjected to multivariate analysis after Procrustes alignment. Principal component analysis produced four significant principal component shape variables accounting for 79.5 % total variance. These were visualized using thin-plate spline analysis. Discriminant analysis showed a significant difference between the taxon means (P = 0.003). Jackknife cross-validation correctly classified only 61.3 % of A. occidentale individuals and 56.7 % of A. microcarpum. Non-Parametric MANOVA of the ten populations showed significant population differences (P = 0.0001). The most isolated population (Cocal da Estação)—the only one from the interior of the state—differed most. A Mantel test found no significant correlation between morphological dissimilarity and geographical distance. Although leaf shape differences both between taxa and populations were statistically significant, overlap made leaf shape difference alone ineffective for separating A. microcarpum from A. occidentale.