Effect of Bentazon, a Hill Reaction Inhibitor, on Symbiotic Nitrogen-fixing Capability and Apparent Photosynthesis

Symbiotic associations of bean plants (Phaseolus vulgaris L. cv. Blue Lake) and Rhizobium phaseoli strain 127K17 were treated with the Hill reaction inhibitor bentazon (3-isopropyl-1 H-2,1,3-benzothiadiazin-4-(3H)-one-2,2-dioxide). Plants receiving foliar and root treatments of 1.8 kilograms per hectare bentazon were assayed at 6 hour intervals for N(2)-fixing capacity by measuring C(2)H(2)-dependent C(2)H(4) production and H(2) evolution and for CO(2) exchange rates. In foliar treated plants greatest measured inhibition of CO(2) exchange rates and N(2)-fixing capacity occurred 6 and 12 hours after treatment, respectively. In root-treated plants maximum inhibition of both processes was delayed by 6 hours, and was less severe than in foliar treated plants. Nitrogen-fixing capacity and CO(2) exchange rate recovered to control levels in all plants. Application of higher rates of bentazon resulted in greater inhibition of CO(2) exchange rate and N(2)-fixing capacity. Inhibition of the two processes was positively correlated (r = 0.985). The results indicate that inhibition of N(2)-fixing capacity was not caused by bentazon directly, but indirectly through limiting the availability of photosynthate to support root nodule activity.