Long-term flooding stimulates stilit-root production in Avicennia marina

Plants of Avicennia marina (Forssk.) Vierh. Var. Australasica (Walp.) Moldenke ex N. C. Duke that had been continuosly flooded for about two months produced numerous downward-growing roots well above the soil surface, as they sometimes do after pollution events in the field. These downward-growing roots (stilt roots) were not produced by the non-flooded plants. The stilt roots generally resembled inverted normal pneumatophores, and had many lenticels on their surface. We suggest that a stimulus for stilt root production in this species may be an increase in ethene (ethylene) resulting from restricted ventilation. We suspect that threre is a requirement for some availability of oxygen to support the growth of new ventilating root structures, and that this criterion is satisfied near the air-water interface. The number of aerenchymatous organs including all types was the same in both flooded and non-flooded plants, and we speculate that this number may be pre-determined in some way in plants of this size.