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Time of flooding id)

Fig. 8.11 Survival percentages (n — 6) of Rumex species after complete submergence in light (open circles) or dark (closed circles) after allowing 14 days for recovery and regrowth. Juvenile plants: 35-40 days old; mature plants: 85—110 days old. (Reproduced with permission from Nabben, 2001.)

respiration, and to remove waste products, e.g. CO2 removed in the opposite direction. Analyses of the internal atmosphere have shown that the oxygen concentrations are high and in most cases match the needs for respiration. In the common club-rush (Schoenoplectus lacustris) the oxygen content in the submerged stalks and rhizomes is maintained at 13—15% (Braendle, 1991).

Underwater studies of the submergence-tolerant marsh dock (Rumex palustris) have demonstrated a remarkable ability to carry out photosynthesis under water (Mommer et al., 2004; Mommer et al., 2005) and maintain relatively high internal oxygen pressures under water, sufficient even to cause a release of oxygen via the roots into the sediment. This acclimation to submergence was found to involve both an increase in specific leaf area and a significant reduction of diffusion resistance for gas exchange between leaves and the water column with the development of thinner cuticles (Fig. 8.12).

8.5.1 Disadvantages of flooding tolerance

As with all adaptations to specific habitats it is necessary to consider the disadvantages that can result in relation to specialization for one particular type of environment. The fact that wetland plants grow in wetland areas is so obvious that the question is seldom asked as to why they cannot also live in areas where flooding is unlikely. The question has rarely been investigated as to whether adaptation to drought is fundamentally incompatible with flooding tolerance. Most flood-tolerant species can be cultivated successfully in pots without flooding and the presence of other species, which suggests that under natural surroundings flood tolerance renders plants in some way uncompetitive in drier habitats. One possible disadvantage of the presence of aerenchyma in dry land species is that it reduces the capacity of the roots for nutrient uptake (Koncalova, 1990). Roots adapted to flooding with aerenchyma are usually thick, with little branching, and with a consequently reduced surface/ volume ratio. An added adverse effect of this strategy is the diminution of the total surface areas for nutrient uptake. The upper regions of the roots are frequently suberized against premature oxygen leakage, which further hinders their absorptive powers.

Research on root growth in a selection of species with varying tolerances of flooding that are common on Dutch flood plains has shown that in general the more flood-tolerant species are less selective than the flood-sensitive ones in placing their roots in soil patches enriched with nutrients (Jansen et al., 2005). Metabol-ically, long-term flooding tolerance which enables plants to over winter in flooded habitats is achieved only by incurring extra costs in terms of carbohydrate utilization. Despite the down-regulation of metabolism to conserve carbohydrate reserves against the dangers of accelerated glycolytic activity, the need for high levels of these reserves to resume growth from submersion or waterlogged soils in spring is a cost that is imposed by anaerobic habitats. The provision of adequate antioxi-dants to counter post-anoxic injury in spring is also dependent on adequate carbohydrate reserves. The use of carbohydrate reserves to meet these costs will thus place adapted species at a competitive disadvantage

Fig. 8.12 Comparison of photosynthetic efficiency in terrestrial and aquatic leaves of Rumex palustris as measured with carbon dioxide response curves (a) under water from oxygen production and (b) in air from net carbon dioxide assimilation. Note that each leaf type achieves maximum effectiveness in its own particular environment. (Reproduced with permission from Mommer, 2005.)

COj concentration in water (nM) C02 concentration in air (Pa)

Fig. 8.12 Comparison of photosynthetic efficiency in terrestrial and aquatic leaves of Rumex palustris as measured with carbon dioxide response curves (a) under water from oxygen production and (b) in air from net carbon dioxide assimilation. Note that each leaf type achieves maximum effectiveness in its own particular environment. (Reproduced with permission from Mommer, 2005.)

when growing in unflooded habitats (Crawford, 2003). There are therefore a number of attributes, both morphological and physiological, associated with flooding tolerance that will disadvantage adapted species if they attempt to grow in natural communities that are flooded only rarely.

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