Release of bound iron and aluminium from soils by the root exudates of tea (camellia sinensis) plants

Abstract

Root exudate of young tea plants contained appreciable quantities of malic acid. Malic acid was capable of solubilising P, Fe and A1 from rock phosphate fertiliser. Tea soils incubated with added malic acid released appreciable quantities of P, Fe and Al. In tea soils where most of the P added as the rock phosphate is immobilised as the iron and aluminium phosphates, malic acid may prove to be useful in chelating these elements and so releasing the phosphates for utilisation by the tea plant. IntroductonTea plants thrive on acid soils and most of the soils in which tea is grown are predominantly kaolinitic. Field experiments have shown that tea plants did not respond to applications of rock phosphate beyond 33.6 kg P~O~/hectare per annum. Subsequent fractionation of the soil for phosphorus carried out by the method of Chang and Jackson1 indicated that most of the phosphorus applied as the rock phosphate fertiliser was fixed as the relatively insoluble iron and aluminium phosphates.Many investigators have reported that certain organic substances and particularly hydroxylated organic acids normally produced in soils by the action of microorganisms are effective agents in releasing phosphates that are rendered insoluble as the iron and aluminium phosphates. Garretsen2 showed that insoluble phosphates were solubilised in the rhizosphere either by the action of microorganisms or plant root exudates.Malic acid has been detected in tea roots.3 The object of this study was to investigate whether malic acid was capable of releasing the phosphates rendered insoluble as the iron and aluminium phosphates so that the plants can utilise the phosphates so released. Materials and methodsOne year old tea plants of the clone TRI 2025 established in polythene bags were used. The roots were cleaned and washed thoroughly with demineralised water. The plants with their root systems fully immersed were allowed to remain in 250 ml Erlenmeyer flasks containing 100 ml demineralised water for 48 h in the laboratory at room temperature (23°C). At the end of this period, the plants were removed and the water 1895

Adaptive attributes of tropical forage species to acid soils. V. Differences in phosphorus acquisition from less available inorganic and organic sources of phosphate

“…Furthermore, chelation of Al alleviates the harmful effects on root growth exerted by high concentrations of monomeric Al in acid mineral soils. This mechanism is of major importance in those Eucalyptus species (Mulette et al, 1974) and tea varieties (Jayman and Sivasubramaniam, 1975) that are well adapted to low-P-supplying acid mineral soils. Enhanced exudation of organic acids from roots is often observed under low P supply in dicotyledons in general and legumes in particular (Ae et al, 1990;Liptonetal., 1987;OhwakiandHirata, 1992;Miyasaka et al, 1991).…”

Rhizosphere pH along different root zones of Douglas-fir (Pseudotsuga menziesii), as affected by source of nitrogen

“…3. Exudates at the root apex (Horst et al, 1982;Mulkey and Evans, 1981) or root exudates related to Fe or P deficiencies (Brown, 1976;Gardner et al, 1983;Jayman and Sivasubramaniam, 1975) often are (anions of) organic acids or other organic molecules, which can affect the pH of the rhizosphere. 4.…”

Effects of aluminum on organic acid, sugar and amino acid composition of the root system of sorghum (Sorghum bicolorL. Moench)

“…However, an appreciable amount of aluminum penetrates the cell; therefore, an internal mechanism must also be involved. The possibility that organic acids chelate the aluminum ion has been proposed by JONES (13) and emphasized by other authors (3,9,12). According to this idea, organic acids and/or other chelating ligands would form soluble complexes with aluminum to eliminate its toxicity.…”