Differentiation of Prosopis juliflora (Sw.) DC. and P. pallida (H. & B. ex. Willd.) H.B.K. using foliar characters and ploidy

Two species of Prosopis, Prosopis juliflora and Prosopis pallida, native to Central America and northern South America, have been widely introduced and are now found throughout the world’s hot arid and semi-arid zones. However, these species are frequently misidentified, especially where introduced, hampering genetic improvement and effective management. Samples from trees previously identified as either P. juliflora or P. pallida were divided into two groups by multivariate analysis using selected foliar characters. One group contained known tetraploids and the other contained known diploids. It was concluded that the tetraploid and diploid groups correspond to P. juliflora and P. pallida, respectively. Triploid individuals were also identified, the first report of these in the genus Prosopis. A dichotomous key was developed to differentiate the species, using number of interfoliar glands per rachis, number of leaflet pairs per pinna and leaflet length. P. juliflora and P. pallida grown from seed collected within their respective native ranges were correctly identified using the key. Where introduced, samples from Burkina Faso, Jordan and India were confirmed as P. juliflora, but those from Brazil, Cape Verde and Senegal were identified as P. pallida, rather than P. juliflora, the species assumed to be naturalized in these countries. The correct identification of these two species will facilitate the genetic improvement and management of P. juliflora and P. pallida. There is a strong case for basing future selection and genetic improvement programs on P. pallida rather than P. juliflora. Further collection and evaluation of P. pallida is therefore required, together with conservation programs in the native range.