5.5.1 Epiphytic plants and lichens and surface-living microorganisms
A large number of autotrophic algae, lichens and higher plants such as bromeliads and orchids hitch rides on the external surfaces of living plants, especially in moist subtropical and tropical climates. They take nothing from the host except a safe anchorage and so are classed as externally attached autotrophs (epiphytes). These obvious large epiphytes are accompanied by a much more widespread and varied microflora of bacteria, yeasts and filamentous fungi. Few of these can be classified as true epiphytes; most are hetero-trophs, deriving a living from their hosts. The rhizosphere community (in the sheath of soil directly surrounding, and influenced by, the root, including the root surface - the rhizoplane) is mainly derived from the soil (Wood, 1989). It can have up to one hundred times the density of microorganisms as the surrounding soil (see Section 7.2.1 for further discussion). By contrast, the phyllosphere community (the leaf surface - the phylloplane - and surrounding space) derives mostly from species found on or in seeds and buds (Dickinson and Preece, 1976). Leaves often carry 106 to 107 bacteria per cm2 (Lindow and Brandl, 2003). Some are epiphytes but most are heterotrophs that feed on either plant exudates and animal products (including the sugary excreta -honeydew - of aphids) or on dead tissues, such as the sloughed cells produced in even the earliest stages of root growth.
Roots secrete or leak out a number of compounds. Sugars, amino acids, enzymes, growth factors, organic acids and cyst-nematode hatching factors have all been identified in root exudates. Smith (1976) estimated that root exudates from hardwood trees at the Hubbard Brook Experimental Forest (see Box 8.1) amounted to 4 kg of carbon per hectare, mostly as organic acids. These compounds are mainly produced from the elongating region a few centimetres from the tip, and also from lateral roots, root hairs and senescing or damaged tissues. Such exudates are believed to stimulate the germination of fungal propagules, young roots being especially susceptible to colonization by saprotrophs and pathogens. Particular bacterial species also build up on young root surfaces, living in the numerous crevices. Some invade and disrupt epidermal and cortical cells, causing the sloughing of organic debris. Although some rhizosphere microorganisms compete with roots for essential nutrients, others, including the bacteria Rhizobium (associated with legumes) and Frankia (found on alders Alnus spp.) benefit higher plants by fixing nitrogen. An extra dimension is added by certain bacteria recognized by Garbaye (1994) that act to promote mycorrhizal development - the mycorrhization-helper bacteria (MHB). A similar symbiosis occurs in the case of nitrogen-fixing actinomycetes growing on tropical leaves.
The activities of microbes that penetrate the surface defences of leaves, using enzymes including pectinases, and then exploit living tissues, are in stark contrast. Besides the pathogenic bacteria and fungi, there are many weak parasites, some of which cause no visible disease symptoms; others invade only tissues that are already damaged or ageing. As tissues age they release more exudates, develop a rougher surface and lower their defences against attack, so species diversity amongst the microorganisms increases with senescence. The succession of organisms recorded during the lives of leaves probably reflects the changing availability of different nutrients. The hyphomycete Aureobasidium pullulans, which has been noted on the buds and leaves of many coniferous and broadleaved species, grows on and in sycamore leaves for the first 2 months after the buds open. It then survives as resting chlamydospores.
Another fungus in the same group, Epicoccum nigrum, is amongst those colonizing mature sycamore leaves, active until leaf fall; it does not invade internal tissues until senescence. Many of these epiphytes probably hasten senescence, particularly when they have the potential to be pathogens. The fact that decomposition starts in the seedling stage emphasizes the difficulty of separating the herbivore and decomposer subsystems.
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