Responses to Total Submergence in Tolerant and Intolerant Riverside Species

Abstract

Submergence during the growing season often occurs in river areas throughout the world. In order not to succumb, plants have developed mechanisms to overcome periods of flooding that may differ greatly in duration, depth and frequency. To study these mechanisms, a group of related species, mainly from the genus Rumex, has been investigated as a model. Rumex occurs in regularly flooded to non-flooded sites in river areas. Adaptive behaviour ranges from changes in life-history strategies to responses at the level of hormones. Submergence may occur at several stages of the life cycle and these should all be included in studies concerning flooding resistance. In this life-history approach, adaptive responses at stages from seed bank to reproduction are described and experimentally proved. Rumex species from frequently flooded areas have persistent seed banks and are characterised by germination flushes after flooding in the spring. Species from higher elevations have transient seed banks and the seeds germinate in the autumn. A trade-off between survival of submergence and early reproduction was found in species occurring in the most frequently flooded sites. Submergence of the wetland species causes leaf re-orientation. This change, from prostrate to vertical, is accompanied by a stimulation of the extension growth of both petioles and internodes. Petiole elongation is due to increased cell expansion and is functionally related to the restoration of contact between shoots and air. This re-establishes oxygen-dependent metabolic processes. Both petiole and stem elongation are correlated with increased internal concentrations of ethylene. Measurements with a laser-driven photoacoustic detection technique reveal variations in endogenous ethylene levels among the various Rumex species. The differences in elongation reactions between the species are the result of differences in the internal concentration of ethylene and, predominantly, in sensitivity to this hormone. In wetland species, protrusion of a substantial part of the shoot above the water results in the formation of new aerenchymatous roots. Their high porosities enhance internal aeration, resulting in restored aerobic respiratory activities. Underwater photosynthesis is another adaptive response leading to the maintenance of basic physiological and biochemical processes during submergence. In some tolerant species the older plants survive the adverse underwater conditions through the slow consumption of carbohydrates stored in their tap roots. Thus, a broad spectrum of adaptive responses at the level of avoidance or tolerance strategies is involved in the survival of resistant riverside plants during and after total submergence.