Hormone Sensitivity and Plant Adaptations to Flooding

Plant hormones play a key role as mediators between environmental signals and adaptive plant responses. Auxin, ethylene and gibberellins are involved in the initiation of adaptive plant responses such as the development of adventitious roots and stimulated shoot elongation upon flooded conditions. These adaptive plastic responses in plants are frequently linked to changes in the concentrations of the hormones involved, but only rarely to shifts in sensitivity. Examples from ecophysiological research performed with species from the genus Rumex demonstrate the importance of the hormone sensitivity concept in plant adaptations to flooding: (a) Rumex species can be grouped into three response categories according to the ethylene sensitivity of the youngest petioles: positive, negative and indifferent; (b) Sub-ambient oxygen concentrations sensitize petioles of wetland Rumex species to ethylene; (c) Enhanced ethylene levels sensitize petioles of wetland Rumex species to gibberellin; (d) Auxin is the primary plant hormone responsible for the initiation of adventitious roots in wetland Rumex species. However, a factor related to waterlogging, possibly ethylene, is required to sensitize the root-shoot junction to endogenous auxin.