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 kg/hectare bentazon were assayed at 6 h intervals for N2-fixing capacity by measuring C2H2-dependent C2H4 production and H2 evolution and for CO2 exchange rates. In foliar treated plants greatest measured inhibition of CO2 exchange rates and N2-fixing capacity occurred 6 and 12 h after treatment, respectively. In root-treated plants maximum inhibition of both processes was delayed by 6 h, and was less severe than in foliar treated plants. Nitrogen-fixing capacity and CO2 exchange rate recovered to control levels in all plants. Application of higher rates of bentazon resulted in greater inhibition of CO2 exchange rate and N2-fixing capacity. Inhibition of the 2 processes was positively correlated (r = 0.985). The results indicate that inhibition of N2-fixing capacity was not caused by bentazon directly, but indirectly through limiting the availability of photosynthate to support root nodule activity.