Acidification root induced

In experiments with lowland rice Oiyzci saliva L.) it was found that roots quickly exhausted available sources of P and sub.sequently exploited the acid-soluble pool with small amounts deriving from the alkaline soluble pool (18). More recalcitrant forms of P were not utilized. The zone of net P depletion was 4-6 mm wide and showed accumulation in some P pools giving rather complex concentration profiles in the rhizosphere. Several mechanisms for P solubilization could be invoked in a conceptual model to describe this behavior. However using a mathematical model with independently measured parameters (19), it was shown that it could be accounted for solely by root-induced acidification. The acidification resulted from H" produced during the oxidation of Fe by Oi released from roots into the anaerobic rhizosphere as well as from cation/anion imbalances in ion uptake (18). Rice was shown to depend on root-induced acidification for more than 80% of its P uptake. 

Besides the fluxes of protons that occur to counterbalance charge imbalances, redox-coupled pH changes can also take place in the rhizosphere, as is the case for lowland rice. Kirk and Le Van Du (1997) showed that the precipitation of iron oxide occurring as a consequence of root-induced oxidation of the rhizosphere of rice was responsible for a significant proportion of the concurrent acidification. 

The root-induced pH changes, shown on the right side of Fig. 10, were very different from those caused by addition of root exudates alone (Fig. 10, left). In addition to acidification effect of root exudates, root-induced changes in pH occur as a consequence of root respiration and the excretion or reabsorption of H or HCOj (Nye, 1981 Marschner and Romheld, 1996). As shown in Fig. 10 (right), the pH values in all sterilized and some unsterilized rhizosphere soils increased less than 0.3 units compared with the bulk soil. The two exceptions were unsterilized wheat and pea rhizosphere soils, which were a little more acidic than the bulk soil. 

The first publicised hypothesis by Ulrich is that tree damage is caused by soil acidification which induces aluminium attack on the roots of trees followed by pathogens (Section 6.3.1). Others have argued that the spatial occurrence of observed tree damage and evidence of soil chemistry do not support this hypothesis. 

The acidification of the rhizosphere has been reported in a range of studies from pot experiments to field investigations (Marschner, 1986 Smith and Pooley, 1989 Courchesne et al, 2001 Wang et al, 2001). The acidification is considered to be mainly induced by the response of roots to ionic charge imbalances in the soil solution (Nye, 1986 Haynes, 1990). This imbalance is caused by the preferential uptake of cations or anions, as selected by plant roots. The acidification of the soil solution therefore results from a release of H+ by roots in response to an ionic charge imbalance caused by the preferential uptake of cations such as NH. Other factors, such as the exudation of organic substances by roots and COj enhancement by microorganism respiration, are also known to contribute to the acidification of the rhizosphere. 

Gonzalez-Reyes et al (1992) have reported that APR induces a quick and permanent plasmalemma hyperpolarization and stimulates the proton efflux in onion roots. Ascorbate and DHA also induce hyperpolarization and acidification of media although their effects are transient. More recently, Gonzalez-Reyes et al (1994a) reported that ascorbate can also stimulate root elongation when culture conditions allow ascorbate to be oxidized in an optimal way to produce APR. On the contrary, inhibition of ascorbate oxidation leads to an inhibition of ascorbate-mediated acceleration of root elongation. 

---