Variation in Populations of Tribulus terrestris (Zygophyllaceae) .1. Burr Morphology

Abstract

Variation in bun morphology was investigated as part of a study to identify the origins of the widespread weed and potential biological control target Tribulus terrestris L. s.1. (Zygophyllaceae). Measurements were made of four size variables, four spine angles and the number of seeds in each bun from 31 Australian and overseas collection sites. Cluster analysis using all variables (valid for 26 collection sites) identified four groups of burrs, one of which contained a single site from Israel that differed due to the angle of the spines. All 31 sites were included by removing the measurement of the basal spine, which is sometimes absent from burrs. Four clusters were found on reanalysis. The minimum number of variables that produced the four clusters were length and width of burrs and abaxial spine length. Most southern Australian sites were grouped with southern African, Indian and Israeli sites. Three Western Australian sites formed one group, as did Northern Territory and Queensland sites. A fourth group included one site each from Australia, Iran, Israel and USA. The relationship between base length and width of the bun distinguished the probably native northern Australian collections, which have either more squat or more elongate burrs, from the probably introduced populations of T. terrestris in southern Australia. However, analysis of morphological variation only approximately detected groupings within the southern Australian and overseas collections that were subsequently identified by isozyme and cytogenetic studies.