Flooding resistance and shoot elongation in relation to developmental stage and environmental conditions in Rumex maritimus L. and Rumex palustris Sm.

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 elongation 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 elongating 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 stem elongation 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 Hooding 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.