lives extracellularly in special intercellular air spaces of the fern fronds. Azolla grows equally well on fresh-water surfaces and on mud and is successfully used for mulching in tropical rice culture.
10.2.3.2.2 N2-fixing Endosymbioses: Root and Stem Nodules
In the neotropics and palaeotropics there are 40-45 species of Gunnera which form a fascinating symbiosis with cyanobacteria of the genus Nostoc. Various Gunnera species range from small creeping stoloniferous herbs up to tall plants of a height reaching 6 m. They inhabit nutrient poor super-humid areas and bogs, leached soils with high rainfall and are also pioneers on bare lands. There is a preference of high altitudes in the genus, which also can inhabit sites with mesic climate. Gunnera is the only genus of angiosperms that forms an endosymbiosis with cyanobacteria. Nostoc is incorporated via peculiar stem glands secreting a mucilage. These glands are induced by nitrogen deprivation of the plants but independent of the cyanobacte-rial symbiont. Inside the host tissue internal nodules are formed which are invested with vascular tissue. This is required for the supply of the nodules with carbohydrates. Light does not penetrate the host organs towards the cyanobacterial sym-bionts which are photosynthetically inactive. Venation of the internal nodules is also necessary for the export of combined nitrogen. The N2-fixing cyanobacterial sym-bionts provide ammonia to the host which has been shown to be sufficient for fulfilling the entire N-demand even of the larger plants of Gunnera species (Bergman et al. 1992; Osborne et al. 1992; Johansson and Bergman 1994; Stock and Silvester 1994; Silvester et al. 1996; Rai et al 2000; Parsons and Sunley 2001; Chiu et al. 2005).
Of a much more general importance are the external root and stem nodules of higher plants with N2-fixing bacterial endosymbionts. Nodule eliciting bacterial en-dosymbionts are rhizobia of the genera Azorhizobium, Bradyrhizobium, Rhizobium and Sinorhizobium. Nodules (Fig. 10.20) are of a structurally high complexity providing the anatomical basis for various physiological requirements, especially for oxygen compartmentation and venation. Oxygen compartmentation is required to solve the oxygen dilemma of nodules, where on the one hand the oxygen sensitive nitrogenase needs a hypoxic environment and on the other hand the high energy demand of N2-reduction as well as other essential metabolic activities in the nodules require high respiratory activity which needs oxygen. In addition to appropriate anatomical differentiations keeping these functions spatially separated leghe-moglobin, which binds oxygen and diffuses between the different compartments of the nodules, effectively lowers and increases oxygen levels at the sites of nitrogenase and respiration, respectively (Werner 1992; Pimenta et al. 1998). Venation is essential for import of assimilates needed as substrate for respiratory energy supply as well as acceptors of reduced nitrogen (ammonia) and for export of organic nitrogen compounds to the nitrogen sinks of the plants. Here, an interesting detail of ecological biochemistry is the nature of the organic compounds functioning in nitrogen export from the nodules. These are often amides or nitrogen-rich ureide molecules. Amides are more water soluble than ureides and therefore their transport requires less water. Thus, among the Sahelian Acaciae, amide transporting plants may colonize more arid regions and ureide transporting plants inhabit areas of greater water availability (Campa et al. 2000).
The symbiotic formation of root nodules with N2-fixing bacteria is best documented in the Leguminosae. With N-supply limiting so much the productivity of sa-
vannas, one might expect that plants capable of fixing atmospheric dinitrogen would be particularly frequent. An important leguminous savanna tree in South America is Bowdichia and in Africa various species of Acacia (Figs. 1.8A and 9.9) play an equivalent role. Open woodlands tend to contain more nodulated trees than adjacent forests. There is a progressive increase in the proportion of nodulated trees along a gradient from humid to arid areas, which is negatively correlated with the N-content of the soils (Hogberg 1986b) and N2-fixation carried out by N2-fixing trees is more important in woodland than in rainforest (Hogberg and Alexander 1995). Such a negative correlation is expected. N2-fixation requires much input of energy, of carbon skeletons for binding of reduced N, and also special morphological differentiation (nodules), and in view of these costs N2-fixation should not give a competitive advantage when sufficient N is available in the soil. Alternatively, the symbiotic association may be more susceptible to drought stress although nodulated Leguminosae such as Prosopis and Acacia are phreatophytes (see Fig. 10.7).
In Africa Fabaceae/Leguminosae trees are important and often dominant elements of savannas (Fig. 9.9). In the Etosha National Park in Namibia there are often sharp separations between Acacia nebrownii and Colophospermum mopane dominated savannas, which are determined edaphically and where the former appears to be somewhat more salinity-tolerant (Fig. 10.21; Berry and Loutit 2000). It has been noted by Ethiopian scientists that in agro-forestry systems it should be sufficient
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