The hormonal regulation of adventitious root formation induced by flooding of the root system was investigated in the wetland species Rumex palusfris Sm. Adventitious root development at the base of the shoot is an important adaptation to flooded conditions and takes place soon afler the onset of flooding. Decreases in either endogenous auxin or ethylene concentrations induced by application of inhibitors of either auxin transport or ethylene biosynthesis reduced the number of adventitious roots formed by flooded plants, suggesting an involvement of these hormones in the rooting process. The rooting response during flooding was preceded by increased endogenous ethylene concentrations in the root system. The endogenous auxin concentration did not change during flooding-induced rooting, but a continuous basipetal transport of auxin from the shoot to the rooting zone appeared to be essential in maintaining stable auxin concentrations. These results suggest that the higher ethylene concentration in soil-flooded plants increases the sensitivity of the root-forming tissues to endogenous indoleacetic acid, thus initiating the formation of adventitious roots.Flooding causes many changes in the hormone physiology of plants. For instance, transport of auxin from shoots to roots may be inhibited by soil flooding, resulting in accumulation of auxin at the base of the shoot (Phillips, 1964;Wample and Reid, 1979). Other hormones such as ethylene in hypoxic roots are produced in larger amounts during flooding (Voesenek et al., 1990; Brailsford et al., 1993). In wetland plants, the change in hormonal status of the flooded plant is followed by a number of responses that alleviate the negative effects of flooding on plant growth. The mechanisms that underlie these adaptations to flooding have been explained in terms of changes in hormone concentrations or sensitivity to a hormone (see reviews by Reid and Bradford, 1984;Jackson, 1990;Voesenek et al., 1992; Blom et al., 1994). However, for one major adaptation to flooding, i.e. adventitious root formation, persuasive evidence for a hormone-mediated regulation is still lacking.

Submergence-induced ethylene synthesis and entrapment were studied in two contrasting Rumex species, one flood-resistant (Rumex palustris) and the other flood-sensitive (Rumex acefosa). The application of a photoacoustic method to determine internal ethylene concentrations in submerged plants is discussed. A comparison with an older technique (vacuum extraction) is described. For the first time ethylene production before, during, and after submergence and the endogenous concentration during submergence were continuously measured on a single intact plant without physical perturbation. Both Rumex species were characterized by enhanced ethylene concentrations in the shoot after 24 h of submergence. This was not related to enhanced synthesis but to continued production and physical entrapment. In R. palustris, high endogenous ethylene levels correlated with enhanced petiole and lamina elongation. No dramatic change in leaf growth rate was observed in submerged R. acetosa shoots. After desubmergence both species showed an increase in ethylene production, the response being more pronounced in R. palustris. This increase was linked to the enhanced postsubmergence growth rate of leaves of R. palustris. Due to the very rapid escape of ethylene out of desubmerged plants to the atmosphere (90% disappeared within 1 min), substantia1 underestimation of internal ethylene concentrations can be expected using more conventional vacuum extraction techniques.Enhanced stem or petiole elongation in response to submergence enables aquatic, semi-aquatic, and terrestrial plants to avoid the constraints of oxygen deficiency, toxins, and ion deficiency imposed by the flooded environment. The gaseous growth regulator ethylene plays a crucial role in the stimulation of shoot extension under water (Ku et al., 1970;Musgrave et al.,

Abstract. The role of gibberellin (GA) and ethylene in submergence-induced petiole elongation was studied in two species of the genus R u m e x. Analysis of endogenous GAs in the flooding-tolerant R u m e x pa/ustris Sm. and the intolerant R u m e x acetosa L. by gas chromatographvmass spectrometry showed for both species the presence of GA], G A 4, G A 9, G A 19, G A 20 and G A 53. Gas chromatography-mass spectrometry analysis of R. palusiris petiole tissue of submerged plants showed an increase in levels of 13-OH GAs, especially G A h compared with drained plants. This effect could be mimicked by application of 5 f.tL L_l ethylene. In R. acetosa, no differences between levels of GAs in drained or submerged plants were found. In R. p a lu stris, both submergence and ethylene treatment sensitized petioles to exogenous gibberellic acid (GA3). In R. acetosa the effect was opposite, i.e. submergence and ethylene de-sensitized petioles to G A 3. Our results demonstrate the dual effect of ethylene in the submer gence response related to flooding tolerance, i.e. in the flooding-tolerant R. palustris ethylene causes an increased concentration of and sensitivity to GA with respect to petiole elongation while in the intolerant R. acetosa ethylene reduces growth independent of GAs.

Siliqua development was studied in the wild type line Landsberg erecta and the GA‐sensitive mutant ga‐1 of Arabidopsis thaliana (L.) Heynh. Reciprocal crosses between wild type and ga‐l mutant, and self pollinations of either parent have shown that siliqua growth is determined by endogenous GAs originating from maternal tissues and embryo. The ga‐1 mutant either self pollinated or cross‐pollinated with wild type pollen showed reduced siliqua growth, which to a large extent could be overcome by exogenously applied GAs. The siliquae of the ga‐1 mutant possessed very reduced ent‐kaurene synthesizing capacity and no detectable endogenous GA‐activity indicating an early block in the GA‐biosynthetic pathway. Seed weight is not affected by GA‐deficiency during development.

Adventitious rooting in Rumex plants, in which the root systems were in hypoxic conditions, differed considerably between two species. R. palustris. a species from frequently flooded river forelands, developed a large number of adventitious roots during hypoxia, whereas adventitious root formation was poor in R. thyrsiflorus, a species from seldom Hooded dykes and river dunes. Adventitious rooting could also be evoked in aerated plants of both species by application of auxin ( l-naphthaleneacetic acid or indoleacetic acid) to the leaves. The response to auxin was dose-dependent, but even high auxin doses could not stimulate R. thyrsijlorus to produce as many ad ventitious roots as R. palustris. Consequently, the difference between the species in the amount of adventitious root formation was probably genetically determined, and not a result of a different response to auxin. A prerequisite for hypoxia-induced adventitious root formation is the basipetal trans port of auxin within the shoot, as specific inhibition of this transport by N-i-naphthylphthalamic acid severely decreased the number of roots in hypoxia-treated plants. It is suggested that hypoxia of the root system causes stagnation of auxin transport in the root system. This can lead to an accumulation of auxin at the base of the shoot rosette, w resulting in adventitious root formation.