Physiological basis for tolerance of four Zea mays hybrids to RPA 201772

Greenhouse and laboratory experiments were conducted to determine the physi ological basis for differential tolerance of four Zea mays L. hybrids to RP A 201772. Differences in Zea mays tolerance were quantified by determining the herbicide rate required to injure and reduce Z. mays height 50% (GR(50) ) GR(50) values indicated that the Z. mays hybrids 'Pioneer 3751' and 'Pion eer 3737' were less tolerant to RPA 201772 than the hybrids 'Pioneer 3394' and 'Pioneer 3963.' Experiments using C-14-RPA 201772 were conducted to det ermine if hybrid sensitivity was due to differential uptake, translocation, or metabolism of the herbicide. Differences in hybrid tolerance were prima rily due to differential herbicide metabolism rates. The time required for 50% inactivation (T-1/2) of RPA 201772 was 42 and 52 h for the more toleran t hybrids and 66 and 93 h for the more sensitive hybrids. Increased uptake of RPA 201772 was also a contributing factor to the sensitivity of one of t he hybrids.