References

27 Root exudation and rhizosphere biology 415
variety of rhizosphere microbiota under natural conditions. The influence
of (±)-catechin on soil communities and the consequences for pathogen
and mutualist abundances in C. maculosa soils warrant further investigation.
27.6
(±)-Catechin, Soil Processes, and Nutrient Availability
(±)-Catechin may also influence soil nutrient availability and nutrient cycling
in the C. maculosa rhizosphere, through both direct chemical interactionswithsoilnutrientsandindirecteffectsonsoilcommunities(Callaway
and Ridenour 2004). In laboratory experiments, catechin has been shown
to be a relatively strong metal chelator, able to form stable complexes with
iron, aluminum, and copper ions (Mhatre et al. 1993; Mira et al. 2002;
Khokhar and Apenten 2003). Metal chelators in root exudates are thought
to increase availability of soil micronutrients, including iron, manganese,
copper,andzinc,byformingcomplexeswiththemetalsandincreasingtheir
solubility and mobility (Dakora and Phillips 2002). Evidence that chelators
in plant root exudates increase soil micronutrient availability is particularly
strong with regard to graminoid secretion of phytosiderophores (Treeby et
al. 1989; Cesco et al. 2002; Jones et al. 2004), but many phenolics, including
catechin, produced by dicots also have the potential to form complexes
with insoluble micronutrients and may have similar effects (Olsen et al.
1981; Dakora and Phillips 2002). In addition, metal chelators in root exudates
are thought to increase availability of soil phosphorus, a frequently
limiting macronutrient in terrestrial ecosystems. A large portion of soil
phosphorus is often unavailable to plants because it is bound in insoluble
ferric, aluminum, and calcium phosphates (Mengel and Kirkby 1987).
Metal chelators, by binding to iron and aluminum in ferric and aluminum
phosphates, release plant-available phosphates at the same time that they
increase metal solubility (Masaoka et al. 1993; Dakora and Phillips 2002).
Thus, (±)-catechin exudation may increase phosphorus and micronutrient
availability in the C. maculosa rhizosphere, although effects of (±)-catechin
chelation on nutrient availability have not yet been examined.
(±)-Catechin may also affect soil nutrient availability by altering soil
communities.Asdescribedearlier,(±)-catechinisknowntobetoxicto
some soil-borne pathogens and nematodes. The nematicidal effects of (±)catechin
could have profound effects on nutrient cycling. Laboratory experiments
and field studies have demonstrated that nematodes play a critical
role in influencing turnover of soil microbial biomass and nutrient availability
(Bardgett et al. 1999; Bongers and Ferris 1999; Yeates 2003). In
some ecosystems, nematode activity accounts for up to 40% of nutrient