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

Abstract

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.