There was a P-flow from maize to soybean, while soybean could provide maize with N (Bethlenfalvay et al. 1991).
Although tropical wetlands and floodplains are not only characteristic of some savanna types (Fig. 9.10) but also typical elements of tropical forests (Sect. 3.2.3) it may be the best place here to mention N2-fixing nodule symbioses in these habitats, some of which are heavily leached, and where under flooded conditions mineralization of organic matter is slow so that they are nutrient poor and biological N2-fixation is of great importance. Many nodulated Leguminosae also grow on floating mats of vegetation in the flooded areas. Flooding and especially cycles of fluctuating water levels highly amplify the dilemma of the oxygen metabolism of nodules. Oxygen levels are low in inundated and flooded soils (see also in mangroves, Sect. 7.3.1). This may appear beneficial for the oxygen sensitive nitrogenase but on the other hand it hampers the required respiratory activity. Hence, adaptations are important which include nodule formation on more superficial adventitious roots and even on stems to overcome the oxygen constraints by the submerged main root system, structural facilitation of oxygen diffusion pathways from stems via particularly developed lenticels (see Sects. 3.2.3 and 7.3.1) and aerenchymas to the root and the root nodules, and special structural features of the root nodules themselves (Loureiro et al. 1998). However, these adaptations facilitating oxygen supply of nodules under flooded conditions prove disastrous when nodules are exposed to the air in flooding and emergence cycles and emergent nodules deteriorate due to rapidly increasing oxygen levels (Loureiro et al. 1998; James et al. 2001). Stem nodules primarily may have evolved in response to flooding. Stem nodule forming species are in the genera of Aeschimone, Discolobium, Sesbania and Vigna (Loureiro et al. 1998). Nitrogen accumulation by stem-nodulated Leguminosae can be very high, ranging from 41 to 532kgNha-1 in 1.5-2 months (Loureiro et al. 1998). These plants can also be used successfully in green-manuring to improve tropical agriculture. Sesbania rostrata fixing up to 200kgNha-1 season-1 contributes 50-150kgNha-1 year-1 to the soil nitrogen. In this way rice production can be increased two- to threefold (Clarkson et al. 1986).
Carnivory has already been mentioned in relation to lianas and epiphytes, as a potential strategy for nutrient acquisition (Sect. 6.6.3). In the temperate climate, carnivorous plants are particularly frequent in moist and acidic sites and especially in peat bogs, which are very poor in nutrients. Similarly, in the tropics, carnivorous plants of the genus Drosera are frequently found in great numbers in the wet and often peaty soils of upland herbaceous vegetation types with savanna-like meadows at 1,000-2,800m a.s.l. (Fig. 10.22; see Huber 1988 for site description). The carnivorous genus Heliamphora (Sarraceniaceae) is endemic to the Tepuis, the characteristic table mountains of the Guayana highlands in tropical South America.
Drosera attracts its prey by the numerous brilliant droplets of mucilage ("sun dew") secreted via special glands on the surface of colourful, often reddish, tentacles
(Fig. 10.23). The sticky mucilage usually prevents the escape of small insects once they have touched it. The tentacles move in response to mechanical and chemical stimuli caused by the captured animals making thigmotropic and chemotropic as well as thigmo- and chemo-nastic movements. The prey is thus enveloped and then digested by proteases secreted from the tentacle glands. Mineral elements like N, S, head stalk neck head stalk neck
Fig. 10.23 Scheme of a tentacle of Drosera after Gilchrist and Juniper (1974; from Lüttge 1983). The centre of the tentacle is served by tracheids. It is separated from the peripheral gland epithelium by an endodermis, whose radial walls are suberized so that apoplastic transport is blocked and transport between the periphery and the interior must use a symplastic route
P, Mg2+, K+ from the prey then stimulate growth and productivity of the Drosera plants (Lüttge 1983).
Heliamphora is a genus of pitcher plants with several species (H. nutans, H. heterodoxa, H. minor, H. ionasii, H. tatei). The pitchers are formed of single leaves. They are morphogenetically derived from peltate leaves, so that the interior of the pitcher wall corresponds to the upper leaf surface and the exterior to the lower leaf surface. In the middle of the pitchers there is a small opening, which allows water to flow out and thus prevents over-filling in the high rainfall habitats of Heliamphora. Animals are attracted by coloration and nectar secretion at the pitcher orifice. Escape is hindered by hairs and trichomes directed downwards to the bottom. Most of the criteria of true carnivory are fulfilled by all Heliamphora species, such as:
• attraction of prey through special visual and chemical signals,
• trapping and killing of prey,
• presence of wax scales and other structures preventing escape of prey,
• absorption of nutrients.
Most of the Heliamphora species, however, lack one important trait of true carnivory, i.e. digestive glands and enzyme secretion. In these cases digestion of prey is mediated by bacterial commensals (Schmucker and Linnemann 1959). There is one noticeable exception though, which is H. tatei. In this species there is enzymatic activity in closed pitchers just as they maturate and open. Since microbes have no access to the closed pitchers, this proves to be genuine enzyme secretion by the pitcher tissue. Capture of small animals is very effective in Heliamphora species in their natural habitat. The carnivorous traits are lost, however, in low light conditions, which indicates that nutrient supply is limiting only under conditions of higher growth rates, and in terms of cost-benefit optimization the sophisticated carnivorous traits are not affordable under limited light (Jaffe et al. 1992). The occurrence of enzyme secretion in only one of the species of Heliamphora also suggests evolutionary trends in carnivory within the genus, with enzyme secretion being the most advanced trait in carnivory.
The expression of true carnivory is more dubious in the tanks formed by the leaf rosettes of bromeliads. Jolivet and Vasconcellos-Neto (1993) note that in general, in contrast to dicotyledonous carnivorous plants, among the monocotyledons there is only "protocarnivory" (see Sect. 6.6.3). Examples include Catop-sis berteroniana, Brocchinia reducta and Brocchinia hechtioides among bromeliads or Paepalanthus bromelioides (Eriocaulaceae) of upland plateaus in northern Brazil. In moist upland savannas of Venezuela the terrestrial bromeliad Brocchinia reducta shows such extensive developments in some areas, that one may speak of a "Brocchinia-savanna". It catches many animals and has a waxy inner surface to prevent escape (Fig. 10.24). There is breakdown of the bodies of small animals and absorption of solutes via the bromeliad scales. The outer walls of the scale cells have an unusual structure. They have a labyrinthine-like appearance and particularly large pores (6.6 nm) allowing the passage of rather large molecules, which possibly is followed by cellular uptake via endocytosis-vesicles (Owen and Thomson 1991). The species has been considered as a true carnivorous plant (Givnish et al. 1984), although glands and enzyme secretion are totally absent.
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