The involvement of ethylene, (C₂H₄) in the formation and persistence of
legume root nodules, and its interaction with carbon dioxide, (CO₂) have been
studied in three different legume species, namely, pea (Pisum sativum L.), bean
(Phaseolus vulgaris L.) and lentil (Lens culinaris Medik.). 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, and 1 ppm of C₂H₄. In the closed -vessel system C₂H₄ and /or CO₂
produced by the roots were allowed to accumulate around them.Exogenous C₂H₄ at 1 ppm significantly inhibited root -nodule formation in pea
and bean. Nodule fresh weight (individual and total), nitrogenase activity and
leghaemoglobin content per plant were significantly reduced. Total N
accumulation in the shoots and shoot dry weight per plant were also
significantly lower than in the other treatments. However, root dry weight and
primary root length were not significantly affected in either species. Significant
positive correlations were observed between leghaemoglobin content and
nodule fresh weight per plant, nitrogenase activity and leghaemoglobin, total N
accumulation in the shoots and nitrogenase activity, and root dry weight and
total nodule fresh weight. Shoot N concentration, on the other hand, was
negatively correlated with shoot dry weight and total nodule fresh weight.No comparable effect of 1 ppm C₂H₄ on the lentil cultivar, which is known to
be moderately waterlogging tolerant, was observed.Similar results were observed in the closed- vessel experiments, where C₂H₄
(produced by the plant roots) reached concentrations of 0.6 -0.8 ppm. Carbon
dioxide (0.6 -1 %) in the root atmosphere of pea and bean plants caused an
increase in individual and total nodule fresh weight and the nitrogenase activity
of the nodules. However, increased CO₂ in the root atmosphere did not
significantly increase total N accumulation and shoot dry weight in pea. Again,
no significant effect was observed with lentil. No significant interactions
between C₂H₄ and CO₂ were observed in either experimental system.Comparisons of a range of bean cultivars in sealed vessels showed large
differences in the amounts of C₂H₄ produced by the root systems. Abnormal
shoot growth and defoliation occurred when the endogenous C₂H₄ was allowed
to accumulate, suggesting that it was at least partly responsible for the effects.The relationship between sensitivity to C₂H₄ of different species and cultivars,
and tolerance to waterlogging is discussed
