Ethylene Sensitivity and Response Sensor Expression in Petioles of Rumex Species at Low O2 and High CO2 Concentrations.

Rumex palustris, a flooding-tolerant plant, elongates its petioles in response to complete submergence. This response can be partly mimicked by enhanced ethylene levels and low O2 concentrations. High levels of CO2 do not markedly affect petiole elongation in R. palustris. Experiments with ethylene synthesis and action inhibitors demonstrate that treatment with low O2 concentrations enhances petiole extension by shifting sensitivity to ethylene without changing the rate of ethylene production. The expression level of the R. palustris gene coding for the putative ethylene receptor (RP-ERS1) is up-regulated by 3% O2 and increases after 20 min of exposure to a low concentration of O2, thus preceding the first significant increase in elongation observable after 40 to 50 min. In the flooding-sensitive species Rumex acetosa, submergence results in a different response pattern: petiole growth of the submerged plants is the same as for control plants. Exposure of R. acetosa to enhanced ethylene levels strongly inhibits petiole growth. This inhibitory effect of ethylene on R. acetosa can be reduced by both low levels of O2 and/or high concentrations of CO2.

An Ethylene-Mediated Increase in Sensitivity to Auxin Induces Adventitious Root Formation in Flooded Rumex palustris Sm.

The hormonal regulation of adventitious root formation induced by flooding of the root system was investigated in the wetland species Rumex palustris Sm. Adventitious root development at the base of the shoot is an important adaptation to flooded conditions and takes place soon after 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.

Ethylene Biosynthesis and Accumulation under Drained and Submerged Conditions (A Comparative Study of Two Rumex Species).

A model is presented of the regulation of ethylene biosynthesis in relation to submergence and flooding resistance. It is based on time-course measurements of ethylene production, ethylene accumulation, and concentrations of free and conjugated 1-aminocyclo-propane-1-carboxylic acid (ACC) in submerged and drained flooding-resistant Rumex palustris Sm. and flooding-sensitive Rumex acetosella L. 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 in submerged plants. Although submergence inhibited ethylene production, physical entrapment increased endogenous ethylene concentrations in both flooding-resistant R. palustris and flooding-sensitive R. acetosella plants. However, R. palustris plants controlled their internal ethylene levels in the long term by a negative regulation of ACC synthase induced by ethylene. In flooding-sensitive R. acetosella plants, absence of negative regulation increased internal ethylene levels to more than 20 [mu]L L-1 after 6 d of submergence. This may accelerate the process of senescence and contribute to their low level of flooding resistance.

Potamogeton pectinatus Is Constitutively Incapable of Synthesizing Ethylene and Lacks 1-Aminocyclopropane-1-Carboxylic Acid Oxidase.

A highly sensitive laser-driven photoacoustic detector responsive to [less than or equal to]2.1 nmol m-3 ethylene (50 parts per trillion [v/v]) was used for ethylene analysis. Dark-grown plants of Potamogeton pectinatus L. growing from small tubers made no ethylene. Exposure of shoots to white light, wounding, submergence in water followed by desubmergence, partial oxygen shortage, indole acetic acid, or carbon dioxide failed to induce ethylene production, although clear effects were observed in Pisum sativum L. Some ethylene was released after applying high concentrations of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC; 10 mol m-3) to P. pectinatus, but the amount was trivial compared with that released by P. sativum. More endogenous ACC was found in P. pectinatus than in P. sativum. Considerable ACC oxidase activity was present in tissue extracts of P. sativum. However, no ACC oxidase activity was found in P. pectinatus, indicating that this is where ethylene production is arrested.

Investigation of Local Ethylene Emission from Intact Cherry Tomatoes by Means of Photothermal Deflection and Photoacoustic Detection.

The function of the coronet region of the cherry tomato (Lycopersicon esculentum Cherry) as the main emission channel for ethylene was investigated. Ethylene was measured employing two laser-based detection systems, the photothermal deflection instrument and the photoacoustic detection setup. It is possible to detect minimum ethylene concentrations of 1 nL L-1 locally and rapidly with the first instrument and concentrations of 6 pL L-1 in a continuous flow system with the second setup. The continuous flow system makes it possible to change the air composition and to monitor its influence on the ethylene production of the tomato. The response times of the two instruments are 30 s and 4 min, respectively. The local character of the measurements makes it possible to determine the emission sites of the gaseous plant hormone ethylene and their relative importance. Transient anoxic conditions stop production of ethylene; return to aerobic conditions shows the evolution of the accumulated ethylene precursor 1-aminocyclopropane-1-carboxylic acid and its renewed production on the measured ethylene emission, with a time resolution of several minutes. Temporarily sealing the main emission channel yields results comparable to anoxia.

Submergence-Induced Ethylene Synthesis, Entrapment, and Growth in Two Plant Species with Contrasting Flooding Resistances.

Submergence-induced ethylene synthesis and entrapment were studied in two contrasting Rumex species, one flood-resistant (Rumex palustris) and the other flood-sensitive (Rumex acetosa). 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), substantial underestimation of internal ethylene concentrations can be expected using more conventional vacuum extraction techniques.