Morphologic and isozyme variation in barnyardgrass (Echinochloa) weed species

Mature seed samples from 35 populations of Echinochloa were collected in fi elds of the Eastern Iberian Peninsula and evaluated for nine morphologic tr aits. Four kernels per population were separately assayed for four isozyme systems. Genetic variability components were studied by correspondence anal ysis and chi-square distance using the isozyme pattern as variables. Geneti c uniformity was found within populations as would be expected from their a utogamous mating system. Intraspecific isozyme variability covered a wide s pectrum, from none in E. colonum to very high in E. oryzoides. All the popu lations of E. colonum clustered together based on morphometric and isozymat ic characters, and also those of E. oryzicola, but were distinct from the p opulations of all other species. Populations of E. crus-galli, E. hispidula , and E. oryzoides clustered within species based on morphology, but did no t cluster based on isozymatic characters, being mixed in the isozymatic den drogram. Our results support the hypothesis that E. hispidula and E. oryzoi des are infraspecific taxa of E. crus-galli. The large between-population g enetic variability may explain the differential response to herbicides with in a given species and indicate that weed specialists should study the diff erences in response to a wide spectrum of Echinochloa populations to severa l herbicides instead of testing many plants from a small number of populati ons.