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.,

A model i s presented of the regulation of ethylene biosynthesis in relation to submergence and flooding resistance. It is based on time-come measurements of ethylene production, ethylene accumulation, and concentrations of free and conjugated 1 -aminocyclopropane-1 -carboxylic acid (ACC) i n submerged and drained flooding-resistant Rumex palusfris Sm. and flooding-sensitive Rumex acefosella 1. plants. From these data, in vivo reaction rates of the final steps in the ethylene biosynthetic pathway were calculated. According to our model, submergence stimulates ACC formation and inhibits conversion of ACC to ethylene in both Rumex species, and as a result, ACC accumulates. This may explain the stimulated ACC conjugation observed i n submerged plants. Although submergence inhibited ethylene production, physical entrapment increased endogenous ethylene concentrations in both flooding-resistant R. palusfris and flooding-sensitive R. acefosella plants. However, R. palusfris plants controlled their internal ethylene levels i n the long term by a negative regulation of ACC synthase induced by ethylene. In flooding-sensitive R. acefosella plants, absence of negative regulation increased internal ethylene levels to more than 20 p L 1-' after 6 d of submergence. This may accelerate the process of senescence and contribute to their low leve1 of flooding resistance.

Rumex palustris is a flooding‐resistant amphibious species from frequently flooded riversides, whereas Rumex acetosella is flooding‐sensitive and grows on dry sandy soils. Upon complete submergence, both species accumulate ethylene to similar levels. After more than four days, however, the ethylene concentration in R. acetosella plants strongly rises to an extremely high level, whereas it remains much lower in R. palustris plants. This latter species responds to ethylene with enhanced leaf elongation, whereas elongation in R. acetosella is insensitive to ethylene. Elongation rates of leaves were measured continuously during the first 8 h of submergence. A comparison of the elongation rates of R. palustris, R. acetosella and silver‐treated R. palustris plants demonstrated that R. palustris plants responded to ethylene within 1 h of submergence. In R. acetosella, clear symptoms of senescence and decay were observed within two weeks of submergence. In R. palustris plants, only the oldest leaf was senescent. To investigate the role of ethylene in the senescence process, the effects of silver ions on submerged plants, and the effects of prolonged exposure to an extremely high ethylene level on drained plants were studied in both Rumex species. The results demonstrated that although ethylene accelerated senescence of submerged R. acetosella plants, the process may have been caused by other factors. The slower senescence of R. palustris plants could not be explained by their lower ethylene concentration. Rather, it was caused by a much lower sensitivity of the senescence process to ethylene. Moreover, other factors may be less unfavourable in R. palustris than in R. acetosella plants under submerged conditions.

S U M M A R YSeveral species from the genus Rumex are found in Dutch river forelands. Species such as R. palustris Sm. from the low, frequently flooded areas are well adapted to wet conditions, Rumex species from higher and less frequently flooded sites are poorly adapted and therefore sensitive to flooding. One of the adaptations to flooding is enhanced shoot elongation upon connpiete submergence, enabling plants to restore leaf-air contact, provided that the water is not too deep. This paper demonstrates the strong variation in the absolute extent of flood-induced leaf elongation among species of the genus Rumex. The effects of flooding on shoot dr>' and fresh weight and internal gas \'olunies of an elongating and a non-elongating species, R. palustris and R. acetosetla L,, were compared. Net water uptake in response to complete submergence was observed in the shoots of both species. Based on results presented here, we conclude that in plants of R. palustris water enters the cells and is used for cell expansion leading to petiole elongation, whereas in plants of R. acetosella at least part of the water taken up fills the intercelfular gas spaces. Elongation of complete))* submerged Rumex plants does not vary with different depths of submergence, Tbis was concluded from the observation that there is little effect of either hydrostatic pressure or irradiance on leaf elongation during complete submergence. Howe\ er, when R. palustris plants were subjected to a changing water depth, i.e, alternate periods of complete submergence and waterlogging, they elongated less strongly than under permanent complete submergence. Water movements did not affect leaf elongation induced by submergence.

K eyw ords: Auxin, Ethylene, Gibberellin, R um ex, Submergence A b stra ct: Plant horm ones play a key role as mediators between environmental signals and adaptive plant responses. Auxin, ethylene and gibberellins are involved in the initiation o f adaptive plant responses such as the developm ent o f adventitious roots and stimulated shoot elongation upon flooded conditions. These adaptive plastic responses in plants are frequently linked to changes in the concentrations o f the horm ones involved, but only rarely to shifts in sensitivity. Exam ples from ecophysiological research perform ed with species from the genus Rum ex demonstrate the importance o f the hormone sensitivity concept in plant adaptations to flooding: (a) R umex species can be grouped into three response categories according to the ethylene sensitivity o f the youngest petioles: positive, negative and indifferent; (b) Sub-am bient oxygen concentrations sensitize petioles of wetland R um ex species to ethylene; (c) Enhanced ethylene levels sensitize petioles o f 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.

Rosettes of flooding-resistant Rum ex palustris plants show a submergence-induced stimulation of elongation, which is confined to the petioles of young leaves. This response increases the probability of survival. It is induced by ethy lene that accumulates in submerged tissues. Floodingintolerant Rum ex acetosella plants do not show this response. We investigated whether differences in shoot elongation between the species, between old and young leaves and between the petiole and leaf blade of a R. palu s tris plant result from differences in internal ethylene con centration or in sensitivity to the gas. Concentrations of free and con jugated A C C in petioles and leaf blades of R. palustris indicated that ethylene is synthesized throughout the submerged shoot, although production rates varied locally. Nevertheless, no differences in ethylene concentra tion were found between submerged leaves of various ages. In contrast, dose-response curves showed that only elongation of young petioles of R. palustris was sensitive to ethylene. In R. acetosella, elongation of all leaves was insensitive to ethylene. We conclude that variation in ethy lene sensitivity rather than content explains the differ ences in submergence-induced shoot elongation between the two Rum ex species and between leaves of R. palustris.

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Photoacoustic spectroscopy is a highly sensitive technique for measuring low molecular weight gases such as the plant hormone ethylene. Due to its high sensitivity (10 pl 1 -ethylene), photoacoustic spectroscopy can be combined with flow-through systems that avoid the need for enclosing excised plant parts in small volumes for head-space analysis. In this way, artifacts introduced by various accumulation techniques can be avoided and ethylene production monitored at short intervals in air or other gas mixtures as it flows out of a cuvette enclosing all or part of an intact plant. The principles of this technique are described. Three case studies demonstrate the application of photoacoustic spectroscopy in flooding research. These studies concentrate on accurate measurement of endogenous ethylene concentrations in submerged shoots and roots, root ethylene production under subambient oxygen pressures and the simultaneous measurement of ethylene production and leaf growth. In addition, the qualitative and quantitive methods previously used to measure the gaseous plant hormone ethylene are briefly reviewed. Finally, the future prospects of photoacoustic spectroscopy in flooding research are discussed. © 1997 Annals of Botany Company