RESUMO
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.
RESUMO
The flooding resistance of four Ranunculus species was studied under controlled conditions and related to the tactics used by these species to survive in their natural habitat in river floodplains. R. bulbosus, a species from seldom-flooded river levées, was relatively intolerant of both waterlogging and complete submergence, due to a constitutively low level of aerenchyma in the root system. This lack of gas spaces resulted in high mortality rates during flooding treatments and an inability to use photosynthetically derived oxygen for root respiration during complete submergence. The pioneer R. sceleratus, predominantly abundant in low lying mudflats, was very resistant to waterlogging and shallow floods. Due to its constitutively high root porosity and its ability to greatly increase the elongation rate of petioles under water this species can ameliorate flooding stress. However, when leaf blades of R. sceleratus were unable to reach the water surface, this species died as quickly as the flooding-intolerant R. bulbosus. This indicates that fast elongation of petioles under water competes for energy and respirable reserves with maintenance processes. R. repens, a species from lower, frequently inundated floodplains, was very tolerant of prolonged waterlogging and submergence. Its high resistance to complete submergence under continuous darkness indicates that this species tolerates hypoxic and/or anoxic tissue conditions via metabolic adjustments. Lysigenous aerenchyma was also induced in the primary root system and in newly developed laterals, and it was able to use oxygen generated by underwater photosynthesis, for root respiration. R. acris, a species from less frequently flooded areas, was as resistant to waterlogging and submergence in the light as R. repens. However, it has a lower resistance than R. repens to complete submergence in the dark. A submergence pre-treatment increased the maximum net underwater photosynthetic rate in R. bulbosus, whereas a significant decrease of light compensation points was observed in R. repens when it had previously been submerged. This study shows that Ranunculus species exhibit various strategies to cope with different flooding conditions. R. repens responds to flooding by its tolerance mechanism and R. sceleratus by avoidance. R. acris ameliorates submergence and R. bulbosus was not able to adapt high water tables.
