INVITRO SELECTION OF IMAZETHAPYR-TOLERANT TOMATO (LYCOPERSICON-ESCULENTUM MILL)

Effectiveness of cyclical in vitro selection for imazethapyr tolerance in tomato was studied. Cotyledons and leaf explants from shoots regenerated on media containing imazethapyr were used to initiate three successive cycles of selection. Increases in tissue viability, callus initiation, and callus proliferation were observed following three cycles of selection on 10(-7) M imazethapyr. Dry matter accumulation of unselected tomato seedlings was reduced by 50% when imazethapyr was applied postemergence at a rate of 28 g ae ha-1. Progeny of eight tomato lines selected after one. cycle of selection had greater tolerance to imazethapyr than control plants (60 to 70% dry matter accumulation versus 50%, respectively). Variability was also generated for imazethapyr tolerance among progeny of the same tomato line.