Aeration roots

Figure 2 Microbial degradation of citrate in aerated root washings of P-deficient white lupin 5.5 h after removal of the root systems from the trap solution.

Examples of such technology include the ability to develop grazing grass and crops with an aerated root system that will resist drought, floods, and also potentially neutralize toxic mineral compotuids by oxidizing them. 

Water-soluble root exudates are most frequently collected by immersion of root systems into aerated trap solutions for a defined time period (Fig. 1 A). The technique is easy to perform and permits kinetic studies by repeated measurements over time using the same plants. While it is possible to get a first impression about qualitative exudation patterns and even quantitative changes in response to different preculture conditions, the technique also includes several restrictions that should be taken into account for the interpretation of experimental data. 

Under aerated conditions at neutral to alkaline pH, inorganic iron is extremely insoluble (8), such that plants and microorganisms rely absolutely on iron uptake from organic matter complexes or iron that has been solubilized by siderophores and organic compounds contained in root exudate. Low-molecular-weight root exudates that dissolve iron include organic acids that are secreted by plant roots as a specific response to iron deficiency (9) or that are released constitutively at... 

Other important soil properties of the ET landfill cover may be controlled by adequate design and good construction practices. The properties that govern root and plant growth and are important to design of ET landfill covers include soil density, aeration, pH, and nutrients. For a complete list of soil properties refer to Table 25.2. 

The structure of the rice root is therefore apparently dominated by the need for internal gas transport. On the face of it, this structure may conflict with the needs for efficient nutrient absorption (Kirk and Bouldin, 1991). The development of gas-impermeable layers in the root wall seems likely to impair the ability of those parts of the root to absorb nutrients, and the disintegration of the cortex might impair transport from the apoplasm to the main solute transport vessels in the stele, though these points are uncertain (Drew and Saker, 1986 Kronzucker et al, 1998a). It seems likely that the short fine lateral roots are responsible for the bulk of the nutrient absorption by the root system and compensate for any impairment of nutrient absorption by the primary roots as a result of adaptations for internal aeration. 

Although O2 leakage compromises the root s internal aeration, some leakage is desirable for a number of purposes. These include oxidation of toxic products of anaerobic metabolism in submerged soil such as ferrous iron (van Raalte, 1944 Bouldin, 1966 van Mensvoort et al., 1985) nitrification of ammonium to nitrate, there being benefits in mixed nitrate-ammonium nutrition (Kronzucker et al., 1999, 2000) and mobilization of sparingly soluble nutrients such as P (Saleque and Kirk, 1995) and Zn (Kirk and Bajita, 1995) as a result of acidification due to iron oxidation and cation-anion intake imbalance. 

MODEL OF ROOT AERATION VERSUS NUTRIENT ABSORPTION... 

Kirk (2003) has developed a simple model to compare root requirements for aeration with those for efficient nutrient acquisition in rice. The main features of the rice root system are summarized in Figure 6.4. The model considers roots in the anoxic soil beneath the fioodwater—soil interface, receiving their oxygen solely from the aerial parts of the plant. 

Figure 6.12 Effect of hypoxia on influx of NH4+ into rice roots. Seedlings were cultivated in nutrient solutions containing 100 p.M NH, aerated for 3 weeks then at 15 % O2 for indicated times (Kronzucker et al, 1998a). Reproduced by permission of the American Society of Plant Biologists...

Armstrong W, Beckett PM. 1987. Internal aeration and the development of stelar anoxia in submerged roots. A multishelled mathematical model combining axial diffusion of oxygen in the cortex with radial losses to the stele, the wall layers and the rhizosphere. New Phytologist 105 221-245. 

Armstrong W, Beckett PM, Justin SHFW, Lythe S. 1991a. Modelling, and other aspects of root aeration by diffusion. In Jackson MB, Davies DD, Lambers H, eds. Plant Life... 

Kirk GJD. 2003. Rice root properties for internal aeration and efficient nutrient acquisition in submerged soil. New Phytologist 159 185-194. 

Salicylic Acid Absorption. The apical 5 cm of the primary and two seminal roots from each of three plants were out into 1-cm segments to form an experimental unit (ca. 0.08 g). Incubation solution cc ijjtalned 0.5 mM KCl, 0.25 mM CaSOjj, 0.5 mM salicylic acid, 10 nCl/mL [ C]-sallcyllc acid, with 25 mM Tris and 25 mM Mes buffers mixed to obtain pH 6.5. Because the salicylic acid was dissolved in absolute ethanol, the final concentration of ethanol in the incubation solution was 1 (v/v). Root segments were transferred to test tubes containing 10 mL continuously aerated incubation solution. After the predetermined absorption time, segments were collected from the incubation solution by rapid filtration on Whatman No. 2 filter paper. 

Active zones of Fe oxide reduction in soils can be easily recognized as bleached areas showing the grey colour of the matrix minerals after removal of the staining Fe oxides. Such zones can only form where a microbially metabolizable biomass is available, for example in the lower top soil or along roots. In poorly aerated soils with large structural units (e.g. prisms), root mats often develop only at the surface of these units and bleach their surfaces, whereas the interior is still coloured... 

---