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

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Abstract
Effects of flooding of the roots on ethylene levels and water relations of flood-tolerant and flood-susceptible eucalypts were studied to investigate mechanisms of adaptation to waterlogging. The order of most flood tolerant to least tolerant was E. camaldulensis Dehnh., E. globulus Labill and E. obliqua L''Herit. 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. Exogenous ethylene gas (500 .mu.l l-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. E. globulus was intermediate regarding 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-suceptible species and leaf water potential did not decrease in any of the 3 spp. in response to flooding. Apparently, the high degree of flood-tolerance in E. camaldulensis may be due to increased ethylene production which results in 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 O2 to the roots. The other species lacked one or other of these adaptive mechanisms and exhibited a lesser degree of flood tolerance.