Plants of purslane (Portulaca uleracea L.), either inoculated with cucumber mosaic virus CMV) or not, were grown to seeding at a range of densities up to 628 plants m-2 , both in monoculture and together. CMV infection reduced vegetative growth and flower production in monoculture and in mixture. However, in mixture, unequal interference amplified the effect of infection on growth. At high densities, intraphenotypic interference between healthy plants grown in monoculture was very important, whereas it did not affect infected plants. When grown in mixture, infected plants had a weaker competitive ability than did healthy plants. Healthy plants showed a plasticity which allowed them to compensate for she reduced growth of infected plants when in mixture. It is concluded that partial infection of a natural population causes no overall decrease in production, but might induce a qualitative change owing to the reduced participation of infected plants in providing seed for the next generation.
Keywords: Partulaca oleracea L. (purslane); competitive ability; cucumber mosaic virus; planting density; weed.