| Summary: | The involvement of ethylene, C<SUB>2</SUB>H<SUB>4</SUB> (exogenous and endogenous) in the formation and persistence of legume root nodules, and its interaction with carbon dioxide, CO<SUB>2</SUB> have been studied in three different legume species, namely, pea (<I>Pisum sativum </I>L.), bean (<I>Phaseolus vulgaris</I> L.) and lentil (<I>Lens culinaris Medik.</I>). This work was undertaken because of the possible connection between tolerance to ethylene in the root zone and tolerance to waterlogging. Two different techniques for root aeration were employed: a 'constant flow-through' system and a 'closed-vessel' (recirculatory) system. In the constant flow-through system, legume roots were exposed to an air stream containing 0, 0.11, 0.33, or 1 ppm of C<SUB>2</SUB>H<SUB>4</SUB>. In the closed-vessel system endogenous C<SUB>2</SUB>H<SUB>4</SUB> and/or CO<SUB>2</SUB> was allowed to accumulate around the roots. Exogenous C<SUB>2</SUB>H<SUB>4</SUB> at 1 ppm significantly inhibited root nodule formation and the nitrogenase activity of existing nodules in pea and bean, but had no comparable effect on the lentil cultivar which is known to be moderately waterlogging tolerant. However, nodule biomass production in lentil was inhibited by 1 ppm of C<SUB>2</SUB>H<SUB>4</SUB>. The inhibition of root formation in pea appears to have occurred during the infection process. In bean it appeared that inhibition occurred later during nodule development. Nodule production and nitrogenase activity of pea and bean were inhibited by 0.4-0.7 ppm of endogenous C<SUB>2</SUB>H<SUB>4</SUB>. Carbon dioxide (0.6-1%) in the root atmosphere stimulated nodule biomass production and nitrogenase activity. The presence of CO<SUB>2</SUB>, however, could not counteract the inhibitory effects of C<SUB>2</SUB>H<SUB>4</SUB>. The effect of endogenous C<SUB>2</SUB>H<SUB>4</SUB> appears to have been mediated through the induction of early nodule senescence. In contrast to these inhibitory effects of endogenous C<SUB>2</SUB>H<SUB>4</SUB> in pea and bean, a significant stimulation of nodule production and nitrogenase activity by C<SUB>2</SUB>H<SUB>4</SUB> was observed in lentil, in the initial stages of growth.
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