Ethylene, Water Relations and Tolerance to Waterlogging of Three Eucalyptus Species

Abstract

Effects of flooding of the roots on ethylene levels and water relations of flood-tolerant and flood-susceptible eucalypts were studied in an attempt to shed light on mechanisms of adaptation to waterlogging.

The order of most flood tolerant to least tolerant were Eucalyptus camaldulensis Dehnh., E. globulus Labill., and E. obliqua L'Hérit. Tolerance of flooding in E. camaldulensis was associated with an initial increase in ethylene production by the roots, a subsequent increase in ethylene evolution by the lower stem, and basal stem thickening and aerenchyma formation in response to flooding. In addition, exogenous ethylene gas (500 µll^-1) increased stem thickening in E. camaldulensis seedlings compared with untreated controls.

Although treatment of the flood-susceptible E. obliqua with ethylene gas resulted in stem hypertrophy, flooding did not induce either the stem thickening response nor did it stimulate ethylene production.

Eucalyptus globulus was intermediate as regards flood tolerance and basal stem thickening; ethylene production by the roots increased but it did not show marked hypertrophy of the unflooded lower stem and was unresponsive to exogenous ethylene gas.

Water stress was not associated with flooding damage in eucalypts. Early stomatal closure as shown by high leaf stomatal resistances occurred in both the flood-tolerant and the flood-susceptible species and leaf water potential did not decrease in any of the three species in response to flooding.

The results suggest that the high degree of flood-tolerance in E. camaldulensis may be due to (1) increased ethylene production which results in (2) tissue hypertrophy and basal stem thickening. Such tissue hypertrophy would permit the plant to eliminate the build-up of the potentially toxic gas ethylene and could also enhance the transport of oxygen to the roots.

The other species lacked one or other of these adaptive mechanisms and exhibited a lesser degree of flood tolerance.