FLOODING RESISTANCE OF RUMEX SPECIES STRONGLY DEPENDS ON THEIR RESPONSE TO ETHYLENE - RAPID SHOOT ELONGATION OR FOLIAR SENESCENCE

Rumex palustris is a flooding-resistant amphibious species from freque ntly flooded riversides, whereas Rumex acetosella is flooding-sensitiv e and grows on dry sandy soils. Upon complete submergence, both specie s accumulate ethylene to similar levels. After more than four days, ho wever, the ethylene concentration in R. acetosella plants strongly ris es to an extremely high level, whereas it remains much lower in R. pal ustris 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 durin g the first 8 h of submergence. A comparison of the elongation rates o f R. palustris, R. acetosella and silver-treated R. palustris plants d emonstrated that R. palustris plants responded to ethylene within 1 h of submergence. In R. acetosella, clear symptoms of senescence and dec ay were observed within two weeks of submergence. In R. palustris plan ts, only the oldest leaf was senescent. To investigate the role of eth ylene in the senescence process, the effects of silver ions on submerg ed plants, and the effects of prolonged exposure to an extremely high ethylene level on drained plants were studied in both Rumex species. T he 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 cau sed 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.