Classification of plant defences

Plant defences can be classified in several ways. Defences can be classified as physical or chemical. The former alter the mechanical properties of the foliage, making it more difficult to attack. The latter involve molecules that deter or poison the attacker. The distinction between the two is not always clear. For instance, the irritant latexes and resins produced by some plants may act both physically in gumming up the herbivore and chemically in containing a toxic substance. Stinging and glandular hairs are physical defences, but may deliver noxious chemicals.

It is the physical defences of mature leaves, notably their high toughness, that makes immature leaves the most attractive to herbivores. The low fibre concentration and relatively high protein and water contents make young, expanding leaves easier to digest and more rewarding food.

Chemical defences consist of a wide range of molecules produced by secondary metabolism in plant cells. It has been estimated that there are over 100 000 plant secondary metabolites (Waterman 1996), of which we know the probable function of a tiny proportion only. Some may be involved with defence, but we cannot assume that they all are. Apparently defensive chemicals may play additional roles. For instance, high concentrations of tannins in leaves may interfere with the decomposition process. The tannins bind the protein in the decaying leaves and reduce nitrogen availability to the micro-organisms responsible for decomposition (Kuiters 1990; Bruijnzeel et al. 1993; Northup et al. 1995). This results in a build-up of litter and a reduced supply rate of nitrogen to the plants. There is a general inverse correlation between soil nitrogen availability and foliar tannin concentrations, so there tends to be a positive feedback increasing the litter and soil concentration of polyphenolic compounds. It has been speculated that this is important in determining distribution patterns of forest formations and individual species (Bruijnzeel et al. 1993; Northup et al. 1995). The production of tannin-rich litter may give tree species an advantage if, by reducing nutrient availability and increasing soil acidity and toxic phenol concentrations, they compromise the growth potential of competitors. Alternatively, on highly infertile soils the tannins may help prevent loss of nitrogen from the system, improve the cation exchange capacity and provide a thick litter layer more hospitable to tree roots than the mineral soil (Northup et al. 1995). They may also bind potentially toxic free aluminium ions, though these are unlikely to be present in white sand soils.

Chemical defences may be concentrated at the sites poorly protected physically. For instance, the mesophyll tissues of leaves may store relatively high concentrations of secondary compounds as they are low in toughness (Choong 1996). Increasing wall thickness of cells in the mesophyll would reduce their effectiveness at photosynthesis, so chemical defence is better for these tissues.

Another classification of defences contrasts constitutive defences, which are always present, with inducible ones that are produced in response to attack. Most research on inducible defences has been conducted on species from outside the tropics. However, Agrawal & Rutter (1998) compiled published and unpublished data that provided support for ant-based defences sometimes being inducible. These are largely reports of increases in volume or sugar concentrations in secretions from extra-floral nectaries in response to real or simulated herbivory. Pubescence in Endospermum labios from New Guinea may be induced by attack from stem-boring insects (Letourneau & Barbosa 1999). The extra hairs on the plant's surface may interfere with oviposition by more borers. Other inducible defences, such as the production of proteinase inhibitors, seem likely to occur in at least some tropical tree species.

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