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• Earlier work on the submergence-tolerant species Rumex palustris revealed that leaf anatomical and morphological changes induced by submergence enhance underwater gas exchange considerably. Here, the hypothesis is tested that these plastic responses are typical properties of submergence-tolerant species. • Submergence-induced plasticity in leaf mass area (LMA) and leaf, cell wall and cuticle thickness was investigated in nine plant species differing considerably in tolerance to complete submergence. The functionality of the responses for underwater gas exchange was evaluated by recording oxygen partial pressures inside the petioles when plants were submerged. • Acclimation to submergence resulted in a decrease in all leaf parameters, including cuticle thickness, in all species irrespective of flooding tolerance. Consequently, internal oxygen partial pressures (pO2) increased significantly in all species until values were close to air saturation. Only in nonacclimated leaves in darkness did intolerant species have a significantly lower pO2 than tolerant species. • These results suggest that submergence-induced leaf plasticity, albeit a prerequisite for underwater survival, does not discriminate tolerant from intolerant species. It is hypothesized that these plastic leaf responses may be induced in all species by several signals present during submergence; for example, low LMA may be a response to low photosynthate concentrations and a thin cuticle may be a response to high relative humidity. [ABSTRACT FROM AUTHOR]

Resistance to complete submergence was tested in three Rumex species that occur in the Dutch river forelands. The species differ in both habitat and life history characteristics. The annual or biennial R.maritimus and the biennial or short lived perennial R. palustris grow on frequently flooded mud flats of low elevation, while the perennial R. thyrsiflorus can he found on dykes and river dunes that are seIdom flooded. The flooding characteristics of the habitats of the three species were determined. These data were used to design experiments to determine the survival and biomass development of the three species during submergence and and the influence of plant size and light level on these parameters. It was shown in all three species that plants submerged during daytime were much more resistant to flooding than those submerged at night. This is most probably due to the generation of oxygen or carbohydrates by underwater photosynthesis. Mature plants of the three species showed higher survival after submergence than juvenile plants, which might by caused by higher carbohydrate levels in the taproots of mature plants. In addition, the three species clearly differed in survival and biomass development during submergence. Rumex thyrsiflorus, the species least subjected to flooding, is least tolerant to complete submergence. Rumex maritimus, which can avoid the floods In having a short life cycle, is less tolerant to submergence than R. palustris which has to survive the floods as a vegetative plant. It was noted that some plants that survived the flooding period itself, still died in the following period of drained conditions, possibly due to post-anoxic injury. [ABSTRACT FROM AUTHOR]

The flooding resistance of Rumex maritimus L. and Rumex palustris Sm., two species from frequently flooded areas of river forelands, was studied in two large-scale outdoor experiments. Plants were raised in cohorts and in several stages of development subjected to four weeks of submergence in early summer or in midsummer. The role of under-water shoot clongation in raising foliage above the water line was not equally important in both species. In R. maritimus, survival following either early-or midsummer flooding was restricted mainly to plants which emerged from the water surface by clongating their main shoot. R. palustris was better able to survive submergence, especially under early-summer conditions. As a consequence of survival being restricted to plants in the stern clongation stage, flooded plants of R. maritimus all reproduced in the first growing season and seed output correlated positively with stem length immediately after flooding. This was also the case for R. palustris flooded at midsummer. However, for plants of R. palustris which were flooded in early summer and remained below the water line, flowering was delayed or even postponed to the following season. Variation in seed weight was much larger in R. maritimus than in R. palustris and more clearly related to cohort and treatment effects. Differences between both species in flooding resistance did not seem to be primarily caused by different abilities of shoot elongation, but by differences in submergence tolerance in relation to life-history characteristics. [ABSTRACT FROM AUTHOR]