The flooding tolerance of Carex species was studied in relation to their field distribution and their capacity to form root aerenchyma under controlled conditions. In an alpine meadow, six Carex species were selected which were distributed in a clear zonation correlating with water content of the soil. Carex sempervirens and C. ferruginea were only found on nonflooded soil, the latter species preferring moister conditions. Carex davalliana and C. nigra were both associated with water-saturated soil, whereas C. limosa and C. rostrata preferred partially submerged conditions. Carex davalliana and C. limosa were bound to flooded soils with a relatively high redox potential and horizontally flowing groundwater. Carex rostrata and C. nigra grew in stagnant soil-flooded conditions with low soil redox potentials. The amount of aerenchyma in the roots of all species increased when grown in oxygen-deficient stagnant agar. This increase in root porosity, combined with increased root diameter, presumably improved internal aeration of the roots. Although all species survived experimental soil flooding, partial submergence was lethal to C. sempervirens and, surprisingly, also to the wetland species C. davalliana. Carex ferruginea showed a reduced growth rate during partial submergence. The three other species, all wetland plants, reached highest biomass production under soil-flooded and partially submerged conditions, with slower growth on free-draining soil. It is concluded that aerenchyma is not constitutive in the Carex species under study, and is best developed in Carex species from wetlands. Species with less aerenchyma perform poorly when soil-flooded, but conditions of partial submergence could even affect species with a considerable amount of root aerenchyma.

Symptoms which are associated with die-back in Phragmites: growth inhibition, root and bud death, premature shoot senescence, blocked aeration and vascular systems, especially in rhizomes and roots, and abnormal surface and internal cell-wall lignification and suberization of roots were induced by each of three of the lower volatile organic acids, propionic, butyric and caproic. These acids were applied in nutrient media in concentrations similar to those previously associated with die-back sites and/or in sediments containing rotting rhizomes and roots of the plant. At concentrations of 1.4 and 0.56 mM, respectively, butyric and caproic acids were each found to be highly toxic at pH 4.5, but relatively innocuous at pH 6. Propionic acid, applied at a much higher concentration of 10.4 mM, was highly toxic at both pH 4.5 and 6. The results support previous findings that the undissociated forms of the organic acids are the more toxic. Rhizomes and roots, rotting in water or waterlogged sand, released cocktails of acids and produced pH in the range 4.8–5.4. Phragmites seedlings planted in these media died within 12 h. Overall, the results support the theory that die-back in Phragmites can be induced and/or perpetuated by organic acids released from the decaying underground parts of the plant or other sources of organic matter.

Natural flooding is one of the major factors affecting vegetation dynamics in many regions of the world. The Flooding Pampa Grasslands (Argentina) are frequently exposed to flooding events of diverse intensity and duration, some of which Leontodon taraxacoides, an exotic dicot. frequent in these grasslands, seems to survive. Its responses to four different water depths (0, 1, 7 and 13 cm) were studied. The results indicate that plants in conditions of total submergence (depth of 13 cm) did not survive. In less severe flood conditions, increases in the leaf insertion angle resulted in the maintenance of a large proportion of the total leaf area above the water. Differences in leaf length and a decrease in the width and the proportion of lobes per leaf were also found under partial submergence conditions (depth of 7 cm). Root and leaf aerenchyma, present in unflooded plants, showed a significant increase in flood conditions. In spite of the anatomical and morphological responses, total biomass and leaf area were severely affected by water depth. Control plants allocated more biomass to reproductive organs, while partly submerged plants allocated more to leaves and less to reproductive organs. Mature L. taraxacoides plants presented a wide range of plastic adjustment as a survival strategy in soil anaerobiosis, and appear to be able to survive short spring floods in a vegetative state; in contrast, they might not tolerate total submergence conditions imposed by more intense and long-lasting floods.