Genomic fingerprinting of Frankia strains by PCR-based techniques. Assessment of a primer based on the sequence of 16S rRNA gene of Escherichia coli

Submergence stimulates elongation of the leaves of Rumex palustris and under laboratory conditions the maximum final leaf length (of plants up to 7 weeks old) was obtained within a 9 day period. This elongation response, mainly determined by petiole elongation, depends on the availability of storage compounds and developmental stage of a leaf. A starch accumulating tap root and mature leaves and petioles were found to supply elongating leaves with substrates for polysaccharide synthesis in expanding cell walls. Changes in the composition of cell wall polysaccharides of elongated petioles suggest a substantial cell wall metabolism during cell extension. Reduced starch levels or removal of mature leaves caused a substantial limitation of submerged leaf growth. From the 5th leaf onward enough reserves were available to perform submerged leaf growth from early developmental stages. Very young petioles had a limited capacity to elongate. In slightly older petioles submergence resulted in the longest final leaf lengths and these values gradually decreased when submergence was started at more mature developmental stages. Submerged leaf growth is mainly a matter of petiole elongation in which cell elongation has a concurrent synthesis of xylem elements in the vascular tissue. Mature petioles still elongated (when submerged) by cell and tissue elongation only: the annular tracheary elements stretched enabling up to 70% petiole elongation.     

Growth characteristics ofNelumbo nucifera Gaertn. in response to water depth and flooding

Three experiments on the effects of water depth and flooding onNelumbo nucifera Gaertn. were made in the artificial environment of concrete ponds. First, plants were harvested in autumn after growing under seven different water levels ranging from 0.2–3 m The number of floating leaves, the total number of leaves and the leaf area index of emergent leaves were greatest in the tanks at 0.5 m depth. The petiole dry weight per unit length of emergent leaves and the ratio of aboveground to belowground biomass rose with increasing water depth up to 2 m. In contrast, that of floating leaves was constant at about 10 mg dry weight cm⁻¹. The proportion of biomass in tubers fell from 20% at 0.2 m to 6% at 2 m. Second, petiole elongation responses to the amplitude of flooding were investigated in early summer. The maximum rate of petiole elongation was 25 cm per day at 2.4 m water depth. This was the maximum depth at whichN. nucifera could grow. No petioles could elongate from 3 m to 5 m depth. Finally, the effects of timing of flooding on growth were investigated. At the end of growing season, the belowground biomass of plants in the flooding treatment in late summer was smallest among the flooding treatment plants (P<0.05), and was most severe when flooding occurred in this season. Based on the results of these experiments, the growth characteristics ofN. nucifera in relation to petiole elongation, biomass allocation, and flooding tolerance were discussed.     

Differential responses of the floating-leaved aquatic plant Nymphoides peltata to gradual versus rapid increases in water levels

We compared the growth responses of the floating-leaved species Nymphoides peltata to gradual and rapid rising water levels under two nutrient concentrations (1 g and 12 g of slow released fertilizer (N-P-K: 16-8-12) per container filled with 8 kg washed sand), and predicted the population expansion after these floods. The results showed that the capacity for petiole elongation was dependent on leaf age, and only leaves that were no more than five days old had the capability to reach the water surface when the water level increased rapidly from 50 cm to 300 cm. Plants subjected to a gradual rising water level tracked the increase in water depth whose petioles elongated at 3.96 ± 1.70 cm per day and 4.80 ± 0.16 cm per day under low and high nutrient concentrations respectively throughout the experiment period. When water levels were rapidly raised, leaf petioles elongated rapidly at 25.48 ± 1.51 cm per day and 26.64 ± 2.24 cm per day under low and high nutrient concentrations respectively during the first ten days. Plants under a constant water level maintained highest mean leaf recruitment (mean 3.0 ± 0.33 leaves and 24.4 ± 5.87 leaves every ten days under low and high nutrient concentrations, respectively). Therefore, more young leaves existed in the canopy ensuring that when the water level increases, young leaves can rapidly emerge after submergence. Gradual water level rise did not significantly affect biomass and ramet production (4.75 ± 1.41 g and 5.50 ± 1.22 ramets in low nutrient; 48.49 ± 21.45 g and 35.67 ± 11.78 ramets in high nutrient), but rapid water level rise negatively affected ramet production in both nutrient concentrations (3.00 ± 1.26 ramets and 11.25 ± 4.19 ramets in low and high nutrients, respectively). The results indicated that continual leaf recruitment and rapid petiole elongation were both important ways in which N. peltata adapted to increasing water levels. Extreme flooding may be a disturbance factor that affects plant growth and the population expansion of N. peltata, while small gradual water level rise should not harm this species.     

Sensitivity to ethylene: the key factor in submergence-induced shoot elongation of Rumex

Rosettes of flooding-resistant Rumex 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 ethylene that accumulates in submerged tissues. Flooding-intolerant Rumex 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. palustris plant result from differences in internal ethylene concentration or in sensitivity to the gas. Concentrations of free and conjugated ACC 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 concentration 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 ethylene sensitivity rather than content explains the differences in submergence-induced shoot elongation between the two Rumex species and between leaves of R. palustris.     

Ethylene and growth control in the fringed waterlily (Nymphoides peltata): Stimulation of cell division and interaction with buoyant tension in petioles

The effect of ethylene on petiole growth of the Fringed Waterlily (Nymphoides peltata (S.G. Gmelin) O. Kuntze) changes during leaf ontogeny. During early development (before expansion of laminae), ethylene causes an increase in both cell number and cell size; later in development, promotion of rapid cell expansion is the dominant effect. The early effects may contribute to the accommodation of new leaves to water columns of different depth. The later effects on cell expansion only are shown to contribute to the rapid accommodation of floating leaves when changes in water level submerge the laminae. This kind of accommodation results from an interaction between accumulated ethylene, which increases wall extensibility, and the tension in petioles due to natural buoyancy which, it is suggested, supplements the driving force for cell expansion. Cell age (position) within a petiole and age of the whole petiole influence the growth response to ethylene alone and the amount of extra growth produced by applying tension when ethylene is present. In young petioles, apical cells are highly sensitive to ethylene and tension causes little further growth; older cells in both immature and mature petioles show little response to ethylene unless the petiole is under tension. Young (but not mature) petioles respond slowly to applied tension even in the absence of ethylene. It is concluded that as cells age the driving force for expansion limits increasingly their capacity to respond to the wall-loosening effects of ethylene. Dual sensitivity to ethylene and buoyant tension facilitates rapid accommodation responses but sensitivity of young petioles to tension alone may exclude Nymphoides from habitats where current velocity is appreciable.     

Intraspecific variation in the magnitude and pattern of flooding-induced shoot elongation in Rumex palustris

Background and Aims

Intraspecific variation in flooding tolerance is the basic pre-condition for adaptive flooding tolerance to evolve, and flooding-induced shoot elongation is an important trait that enables plants to survive shallow, prolonged flooding. Here an investigation was conducted to determine to what extent variation in flooding-induced leaf elongation exists among and within populations of the wetland species Rumex palustris, and whether the magnitude of elongation can be linked to habitat characteristics.

Methods

Offspring of eight genotypes collected in each of 12 populations from different sites (ranging from river mudflats with dynamic flooding regimes to areas with stagnant water) were submerged, and petioles, laminas and roots were harvested separately to measure traits related to elongation and plant growth.

Key Results

We found strong elongation of petioles upon submergence, and both among- and within-population variation in this trait, not only in final length, but also in the timing of the elongation response. However, the variation in elongation responses could not be linked to habitat type.

Conclusions

Spatio-temporal variation in the duration and depth of flooding in combination with a presumably weak selection against flooding-induced elongation may have contributed to the maintenance of large genetic variation in flooding-related traits among and within populations.     

Molecular and Physiological Mechanisms of Flooding Avoidance and Tolerance in Rice

Submergence is one of the major constraints in rice production. The main factor limiting rice survival during submergence is oxygen deprivation. To cope with flooding conditions, rice has developed two survival strategies: either rapid elongation of the submerged tissues to keep up with the rising water level or no elongation to save carbohydrate resources for maintenance of energy production under submerged and concomitant hypoxic conditions. The survival strategies used by rice have been studied quite extensively and the role of several phytohormones in the elongation response has been established. The mechanisms of submergence tolerance include metabolic changes, for instance, the shift to an ethanolic fermentation pathway, reduced elongation growth to save carbohydrates and energy for maintenance processes, and protective antioxidant systems. Current molecular technology can provide tools for the understanding of mechanisms developed by rice to survive submergence. In addition, cloning of genes related to submergence tolerance might open new ways to genetic improvement of this crop.     

De-submergence-induced ethylene production in Rumex palustris: regulation and ecophysiological significance

Rumex palustris responds to total submergence by increasing the elongation rate of young petioles. This favours survival by shortening the duration of submergence. Underwater elongation is stimulated by ethylene entrapped within the plant by surrounding water. However, abnormally fast extension rates were found to be maintained even when leaf tips emerged above the floodwater. This fast post-submergence growth was linked to a promotion of ethylene production that is presumed to compensate for losses brought about by ventilation. Three sources of ACC contributed to post-submergence ethylene production in R. palustris: (i) ACC that had accumulated in the roots during submergence and was transported in xylem sap to the shoot when stomata re-opened and transpiration resumed, (ii) ACC that had accumulated in the shoot during the preceding period of submergence and (iii) ACC produced de novo in the shoot following de-submergence. This new production of ethylene was associated with increased expression of an ACC synthase gene (RP-ACS1) and an ACC oxidase gene (RP-ACO1), increased ACC synthase activity and a doubling of ACC oxidase activity, measured in vitro. Out of seven species of Rumex examined, a de-submergence upsurge in ethylene production was seen only in shoots of those that had the ability to elongate fast when submerged.     

Submergence research using Rumex palustris as a model; looking back and going forward

Flooding is a phenomenon that destroys many crops worldwide. During evolution several plant species evolved specialized mechanisms to survive short- or long-term waterlogging and even complete submergence. One of the plant species that evolved such a mechanism is Rumex palustris. When flooded, this plant species is capable to elongate its petioles to reach the surface of the water. Thereby it restores normal gas exchange which leads to a better survival rate. Enhanced levels of ethylene, due to physical entrapment, is the key signal for the plant that its environment has changed from air to water. Subsequently, a signal transduction cascade involving at least four (classical) plant hormones, ethylene, auxin, abscisic acid, and gibberellic acid, is activated. This results in hyponastic growth of the leaves accompanied by a strongly enhanced elongation rate of the petioles enabling them to reach the surface. Other factors, among them cell wall loosening enzymes have been shown to play a role as well.     

Differences in mechanical and structural properties of surface and aerial petioles of the aquatic plant Nymphaea odorata subsp. tuberosa (Nymphaeaceae)

Lily pads (Nymphaea odorata) exhibit heterophylly where a single plant may have leaves that are submerged, floating, or above (aerial) the surface of the water. Lily pads are placed in a unique situation because each leaf form is exposed to a distinctly different set of mechanical demands. While surface petioles may be loaded in tension under conditions of wind or waves, aerial petioles are loaded in compression because they must support the weight of the lamina. Using standard techniques, we compared the mechanical and morphological properties of both surface and aerial leaf petioles. Structural stiffness (EI) and the second moment of area (I) were higher in aerial petioles, although we detected no differences in other mechanical values (elastic modulus [E], extension ratio, and breaking strength). Morphologically, aerial petioles had a thicker rind, with increased collenchyma tissue and sclereid cell frequency. Aerial petioles also had a larger cross-sectional area and were more elliptical. Thus, subtle changes in the distribution of materials, rather than differences in their makeup, differentiate petiole forms. We suggest that the growth of aerial petioles may be an adaptive response to shading, allowing aerial leaves to rise above a crowded water surface.     

The structure of the littoral: effects of waterlily density and perch predation on sediment and plant‐associated macroinvertebrate communities

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The role of oxygen in submergence-induced petiole elongation in Rumex palustris: in situ measurements of oxygen in petioles of intact plants using micro-electrodes

In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c . 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c . 8–12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5–4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1- aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.     

Germination and early growth of Nymphaea odorata at different water depths

We experimentally determined the effects of water depth on seed germination and seedling growth and morphology, and we documented the transition from submerged to emergent plants in the white water lily, Nymphaea odorata. Seeds of N. odorata were germinated at 30, 60, and 90 cm water depth in outdoor mesocosms and percent germination and morphology measured after a month. The presence of self-seeded seedlings in pots at the same 3 water levels was also recorded over two years. To examine juvenile growth, seeds planted in soil were placed at the same mesocosm depths; germination and growth were monitored for three months, when the plants were harvested for morphological and biomass measurements. N. odorata germinated equally well in 30, 60 and 90 cm water; seedlings grew as submerged aquatics. After one month, seedlings in 90 cm water had less biomass than those in 30 cm (1.1 vs. 3.3 mg and 1.0 vs. 1.8 mg for different seed sources, respectively) and allocated relatively more biomass to shoots (97.5 vs. 67.8% and 73.1 vs. 58.0%, respectively). Seedlings in 60 cm water were intermediate. After 3 months of submerged growth, plant biomass remained less in 90 vs. 60 and 30 cm water (22.5 vs. 36.4 and 33.3 mg, respectively). Plants in 90 and 60 cm water had greater biomass allocation to shoots than plants in 30 cm water (85.7 and 72.6% vs. 64.4%, respectively) and produced larger laminae on longer petioles (lamina length = 33.3 vs. 25.2 mm in 90 vs. 30 cm; petiole length = 99.0 vs. 36.0 mm, respectively). After about 3 months, submerged plants produced floating leaves that had 39% shorter laminae but 267% to 1988% longer petioles than submerged leaves on the same plant. Lamina length to width allometric relations of submerged leaves were >1 at all water levels, distinguishing them from the equal allometry of adult floating leaves. The switch from production of submerged to emergent leaves resembles submergence-escape growth in other aquatics, but because the seedlings have been submerged throughout their life, submergence itself cannot be the stimulus to produce emergent leaves in these totally immersed plants. Our data show that N. odorata plants can establish from seeds in up to 90 cm water and that seedlings grow as submerged aquatics until they switch abruptly to production of floating leaves.     

Relationship of dehydration rate to drought avoidance,dehydration tolerance and dehydration avoidance of cabbage leaves,and to their acclimation during drought-induced water stress*

Abstract. Drought avoidance due to cuticular control increases with leaf number to a maximum in the intermediate leaves, decreasing to a minimum in the upper leaves. Dehydrated intermediate leaves do not rehydrate detectably when floated on water for several days. Excision of their petioles when submerged, permits full rehydration, presumably via the xylem. Stressing the plant by withholding water for 1–3 weeks fails to increase this already high resistance to water movement through the leaf surface. It does, however, markedly decrease the loss of water from the fully rehydrated (100% RWC) leaves during the first hour of dehydration, presumably due to a more rapid stomatal closure than in the non-stressed leaves. Dehydration tolerance increases with leaf number, without an intermediate maximum. The intermediate and upper leaves were markedly more tolerant of dehydration after drought-induced stress than when non-stressed. Dehydration tolerance in some cases, was inversely proportional to dehydration rate. It was possible, however, to equalize the rates of dehydration of drought-stressed and non-drought-stressed leaves without affecting the greater tolerance of the drought-stressed leaves. Dehydration avoidance by osmotic adjustment was markedly developed in the slowly dehydrated attached leaves of drought-stressed plants, but not in the rapidly dehydrated excised leaves. This is evidence of drought acclimation. It must, therefore, be concluded that the slow dehydration of the drought-stressed plants also leads to the increase in dehydration tolerance by permitting drought-induced acclimation. The overall drought resistance of cabbage leaves depends on the three components: drought avoidance, dehydration avoidance and dehydration tolerance. The latter two increase during acclimation but the cuticular control of drought avoidance does not.     

Growth responses of Rumex species in relation to submergence and ethylene

Abstract. Submergence stimulates growth of the petioles of Rumex palustris and Rumex crispus under field, greenhouse and laboratory conditions. Growth of Rumex acetosa petioles was hardly influenced by submergence. These growth responses under flooded conditions can be partially mimicked by exposing non-submerged Rumex plants to ethylene-air mixtures. Submergence of intact plants in a solution of AgNO₃ inhibited the elongation of all petioles of R. palustris and the youngest petiole of R. crispus and stimulated growth of the youngest petiole of R. acetosa , The ethylene-air mixture experiments, the effect of AgNO₃ and observed increase of the endogenous ethylene concentration during submergence suggest that ethylene plays a regulatory role in the growth responses of these Rumex species under submerged conditions. The three Rumex species showed a gradient in elongation responses to submergence, which correlates with the field distribution of the three species in a flooding gradient.     

Contrasting interactions between ethylene and abscisic acid in Rumex species differing in submergence tolerance

Complete submergence of flooding-tolerant Rumex palustris plants strongly stimulates petiole elongation. This escape response is initiated by the accumulation of ethylene inside the submerged tissue. In contrast, petioles of flooding-intolerant Rumex acetosa do not increase their elongation rate under water even though ethylene also accumulates when they are submerged. Abscisic acid (ABA) was found to be a negative regulator of enhanced petiole growth in both species. In R. palustris, accumulated ethylene stimulated elongation by inhibiting biosynthesis of ABA via a reduction of RpNCED expression and enhancing degradation of ABA to phaseic acid. Externally applied ABA inhibited petiole elongation and prevented the upregulation of gibberellin A(1) normally found in submerged R. palustris. In R. acetosa submergence did not stimulate petiole elongation nor did it depress levels of ABA. However, if ABA concentrations in R. acetosa were first artificially reduced, submergence (but not ethylene) was then able to enhance petiole elongation strongly. This result suggests that in Rumex a decrease in ABA is a prerequisite for ethylene and other stimuli to promote elongation.     

Effects of submergence on the growth of Phragmites australis seedlings

Colonisation by reed seedlings, Phragmites australis (Cav.) Trin. ex Steud. is rare and usually occurs after drawdown and when shallow water prevails. P. australis seeds have high rates of germination but successful colonisation is dependent upon subsequent water depths. We investigated the capacity of young reed plants to resist a 4 weeks submergence stress within a 5 months period, and their subsequent recovery. A pond experiment examined the interactions between submergence depth and the age of the seedlings at submergence. Four submergence treatments were used. In two partial submergence treatments, 50 and 80% of the initial leaf area was submerged. In two total submergence treatments, plants were either submerged at 125% of their initial height with possible subsequent development of emerged leaves, or the water was deepened as they grew to maintain total submergence for 4 weeks. The ages at submergence were 40, 60 and 80 days. Plants were harvested at 5 months. Shoot elongation, biomass allocations to aerial biomass, roots and rhizomes, and photosynthetic activity of aerial leaves were measured. Redox potential was measured for a subsample.Mortality (18.7%) occurred only in the permanent submergence treatment for 40-day-old seedlings. In all treatments, submerged leaves senesced, except the terminal (youngest) leaves of permanently submerged plants. Submergence differentially affected shoot length and biomass, depending upon the intensity of the treatment and the seedling age. The major differences were found between the two partial and two total submergence treatments. Partial submergence (50 and 80%) significantly enhanced biomass accumulation and growth, whereas total submergence largely decreased biomass production and growth in length, with less effect on shoot numbers. The 80-day-old seedlings tolerated submergence better but growth was poorest in medium-aged plants (60-day-old). Increased elongation of the growing internodes of up to 140% was caused by submergence, and photosynthetic activity was enhanced by 85% in emergent leaves of plants initially submerged but allowed to produce emerged leaves during the treatment period.Young P. australis plants require shallow water levels without long lasting submergence to grow and survive. Tolerance to submergence increases with age. These processes contribute to define the conditions for colonisation via seeds in P. australis.     

Ethylene Promotes Elongation Growth and Auxin Promotes Radial Growth in Ranunculus sceleratus Petioles

Submergence induces elongation in the petioles of Ranunculus sceleratus L., after a rise in endogenous ethylene levels in the tissue. Petioles of isolated leaves also elongate 100% in 24 hours when treated with ethylene gas, without a change in the radius. Application of silver thiosulfate, aminoethoxyvinylglycine (AVG), abscisic acid (ABA), or methyl jasmonate inhibits this elongation response. Gibberellic acid treatment promotes ethylene-induced elongation, without an effect on the radius. Indoelastic acid (IAA) induces radial growth in the petioles, irrespective of the presence or absence of added ethylene. High concentrations of IAA will also induce elongation growth, but this is largely due to auxin-induced ethylene synthesis; treatment with silver thiosulfate, AVG, ABA, or methyl jasmonate inhibit this auxin-promoted elongation growth. However, the radial growth induced by IAA is not affected by gibberellic acid, and not specifically inhibited by ABA, methyl jasmonate, silver thiosulfate, or AVG. These results support the idea that petiole cell elongation during “accommodation growth” can be separated from radial expansion. The radial expansion may well be regulated by IAA. However, effects of high levels of IAA are probably anomalous, since they do not mimic normal developmental patterns.     

The Response of Petioles of Glechoma hederacea to a Dynamic Light Climate

Abstract: In herbaceous vegetation patterns of light distribution may change over time. Prostrate plants growing in such a dynamic light environment may benefit from petioles that respond plastically to changing light conditions. In an experiment, the response of petioles of Glechoma hederacea to changing light conditions was analyzed. Treatments included continuous shade, continuous high light, a shift from shade to high light and from high light to shade when the plants had formed 10 ramets. In all four treatments, even petioles that had apparently ceased growing, were still able to elongate slightly but the extent of elongation decreased with the age of the petiole. In the oldest petioles relative extension rates were higher in shade than in high light. In plants that were exposed to full daylight in the second half of the experiment, even newly formed petioles were longer than those in plants that grew in full daylight continuously though they had elongated over a shorter period. In plants that were shaded in the second half of the experiment, only the youngest 4 to 5 petioles reached lengths similar to that in continuous shade. This mechanism may enable plants to keep young (productive) leaves in the upper layers of the canopy while other less productive leaves remain at lower levels of the vegetation.     

The contrasting role of auxin in submergence-induced petiole elongation in two species from frequently flooded wetlands

The involvement of auxin in the submergence-induced petiole elongation has been investigated in Rumex palustris and Ranunculus sceleratus. Both wetland species are capable of enhanced petiole elongation upon submergence or treatment with exogenous ethylene (5μl l⁻¹). Treatment of intact Rumex palustris plants with 1-naphthalene acetic acid (NAA) at 10⁻⁴ M enhanced petiole elongation, while treatment with N -1-naphthylphthalamic acid (NPA) had no effect on petiole elongation. The elongation response after NAA or NPA treatment was comparable for plants in both submerged and drained conditions. Pre-ageing of detached petioles of Rumex palustris for 3 h in light or in dark conditions had no effect on the submergence-induced elongation. In comparison to intact plants, detached petioles of Rumex palustris , with or without lamina, did not show significant differences in responsiveness to IAA between drained or submerged conditions. This was in contrast to Ranunculus sceleratus where submergence caused a clear increase in responsiveness towards IAA. Removal of the lamina, the putative source of auxin, or treatment with NPA did not hinder the submergence-induced elongation of detached Rumex palustris petioles, but severely inhibited elongation of detached Ranunculus sceleratus petioles. This inhibition could be restored by application of NAA, suggesting the specific involvement of auxin in the submergence response of Ranunculus sceleratus. It is concluded that, in contrast to Ranunculus sceleratus , auxin is probably not involved in the submergence-induced petiole elongation of Rumex palustris.