Morphologic and isozyme variation in barnyardgrass (Echinochloa) weed species

Mature seed samples from 35 populations of Echinochloa were collected in fields of the Eastern Iberian Peninsula and evaluated for nine morphologic traits. Four kernels per population were separately assayed for four isozyme systems. Genetic variability components were studied by correspondence analysis and chi-square distance using the isozyme pattern as variables. Genetic uniformity was found within populations as would be expected from their autogamous mating system. Intraspecific isozyme variability covered a wide spectrum, from none in E. colonum to very high in E. oryzoides. All the populations of E. colonum clustered together based on morphometric and isozymatic characters, and also those of E. oryzicola, but were distinct from the populations of all other species. Populations of E. crus-galli, E. hispidula, and E. oryzoides clustered within species based on morphology, but did not cluster based on isozymatic characters, being mixed in the isozymatic dendrogram. Our results support the hypothesis that E. hispidula and E. oryzoides are infraspecific taxa of E. crus-galli. The large between-population genetic variability may explain the differential response to herbicides within a given species and indicate that weed specialists should study the differences in response to a wide spectrum of Echinochloa populations to several herbicides instead of testing many plants from a small number of populations.