Uptake by roots

A problem with the solution of initial-value differential equations is that they always have to be solved iteratively from the defined initial conditions. Each time a parameter value is changed, the solution has to be recalculated from scratch. When simulations involve uptake by root systems with different root orders and hence many different root radii, the calculations become prohibitive. An alternative approach is to try to solve the equations analytically, allowing the calculation of uptake at any time directly. This has proved difficult becau.se of the nonlinearity in the boundary condition, where the uptake depends on the solute concentration at the root-soil interface. Another approach is to seek relevant model simplifications that allow approximate analytical solutions to be obtained. 

J. S. Gcelhoed. L. J. M. Sipko, and G. R. Findernegg, Modelling zero sink nutrient uptake by roots with root hairs from. soil compari.son of two models. Soil Sci. 162 544 (1997). 

R. F. Grant and J. A. Robert.son, Phosphorus uptake by root systems mathematical modelling in ecosys. Plant Soil 188 219 (1997). 

The model shows that the spread of urea and NH4+ into the soil is typically only a centimetre or two in a week (Fignre 8.9). The recovery of broadcast fertilizer N in the crop mnst therefore depend entirely on the superficial root system in the soil-floodwater interface. The good recovery of broadcast fertilizer N obtained if the fertilizer is added when the crop demand is maximal (Peng and Cassman, 1998) therefore indicate rapid uptake by roots in the soil-floodwater interface. 

On the other hand, at a pH value of 5.0 there were no large differences in the strength of bonds between humic acids and metals such as Ca, Mg, Mn, Co, Ni, and Zn, whereas bonds with Pb, Cu, and Fe were stronger than with other metals (Schnitzer and Kahn, 1972) this behavior indicates that at different pH values, metal humic substance complexes of different stability are formed in the soil. This aspect is of particular relevance in an environment such as the rhizosphere, where dynamic pH gradients are present mainly due to the availability of nutrients and to their selective uptake by roots. With regard to plant availability, great importance lies in the molecular dimension and solubility of humic substances (Briimmer and Herms,... 

Dacey, J.W.H., and Howes, B.L. (1984) Water uptake by roots control water table movement and sediment oxidation in a short Spartina marsh. Science 224, 487-489. 

The phenolic acids of interest here [caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), p-coumaric acid (p-hydroxycinnamic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), sinapic acid (3,5-dimethoxy-4-hydroxyxinnamic acid), p-hydroxybenzoic acid, syringic acid (4-hydroxy-3,5-methoxybenzoic acid), and vanillic acid (4-hydroxy-3-methoxybenzoic acid)] (Fig. 3.1) all have been identified as potential allelopathic agents.8,32,34 The primary allelopathic effects of these phenolic acids on plant processes are phytotoxic (i.e., inhibitory) they reduce hydraulic conductivity and net nutrient uptake by roots.1 Reduced rates of photosynthesis and carbon allocation to roots, increased abscisic acid levels, and reduced rates of transpiration and leaf expansion appear to be secondary effects. Most of these effects, however, are readily reversible once phenolic acids have been depleted from the rhizosphere and rhizoplane.4,6 Finally, soil solution concentrations of... 

Mechanisms and kinetics of uptake by roots (or fungal mycelia) or leaves (especially in aquatic plants)... 

A more detailed model is shown in this Fig. 5b. The throughfall Fa of Fig. 5a is replaced by the partial fluxes stem flow (Fu) and canopy drip (Fu). There are also included additional fluxes as Utter fall F e, dry deposition to soil Fse and internal fluxes of the system as uptake by roots and flow to the crown Fee, leaching of leaves Fse and flow from roots to soil F9e. 

In ecosystems, Pu is present mainly in the form of sparingly soluble Pu(IV) dioxide or hydroxide and is therefore rather immobile. It stays mainly in the upper layers of the soil and its uptake by roots is very small (soil-plant transfer coefficients <0.001 d/kg). However, plants may be contaminated with Pu by deposition from the air. Resorption factors in the gastrointestinal tract of animals are also very small (/r < 10 " ). On the other hand, up to 5% of inhaled Pu is found in blood and up to 15% in the lymph glands. About 80% of resorbed Pu is deposited in bones, the rest in kidneys and liver. Biological half-lives reported in the literature vary between 500 and 1000 d for the lymph glands and between 1 and lOO y for the skeleton. 

Plant roots take up cesium readily because of its similarity with the essential nutrient, potassium. Plants differ in their accumulation of cesium (Broadley and Willey, 1997 Lasat et al., 1998 Broadley et al., 1999a,b). This may be due to differences in rooting pattern, root uptake, or translocation within the plant, but the cause is not clearly identified. Cesium is mobile within plants and has similar trends in the translocation within plants to K (Broadley and Willey, 1997 Zhu and Smolders, 2000). Rather little is known about differences between species in their capacity to discriminate between Cs and K. There appears to be a threshold below which Cs uptake by roots increases with decreasing K concentration. The value of this threshold has been reported to be 1 mmol L , 250 Itmol L , or 20 Itmol (Shaw and Bell, 1989, 1991 Shaw et al., 1992 Buysse et al., 1996 Smolders et al., 1996 Zhu, 2001). This inhibition may be due in part to increased efflux of absorbed Cs (Zhu et al., 1999). Model prediction of Cs uptake often overpredict uptake and are improved if account is take of soil-solution potassium concentration below 1 mM (Smolders et al., 1997 Absolom et al., 1999 Roca-Jove and Vallejo-Calzada, 2000), Cs uptake is found to be enhanced by potassium starvation (Jones et al., 1998 Broadley et al., 1999b Zhu et al., 2000), but the effect is short-lived (Willey and Martin, 1997 Staunton et al., 2003). 

Biological acid production (bicarbonate leaching) Excess base uptake by roots =1.1 Acid rainfall (rainwater pH = 4.3-4.4) = 0.67 KCl-exchangeable acidity = 100-275 Total titratable acidity = 500-800... 

The main feature of the collective data of F1g. 3 1s that higher burdens of actinide elements occur under field conditions. The higher CR s determined under field conditions represent both uptake by roots and contamination of foliar surfaces. The array of values given in Fig. 3 make one point certain a constant coefficient of uptake should not be used for all actinide elements nor should a single coefficient be applied to diverse environments. Parameters for simulation models should be a function of the respective actinide, should depend on the environmental chemistry of the actinide, and should be related to specific conditions prevailing in the environment being evaluated. 

The limited information on the plant uptake of other actinide elements (U, Np, Am, Cm) indicates that higher CR values can be expected relative to those observed for Pu. Values for Am based on uptake by roots and from deposition on foliage approach or exceed the 10-1 value used in the LMFBR assessment thus, the value used in dose assessments is probably realistic but not conservative. Price (32) reported CR values of 10-1 to 10-2 for 237Np assimilated by the root pathway. Based on these data and on the low K. for Np (Table II), it appears that this element exhibits a higher mobility than the other actinides. A potential CR > 10-1 due to uptake from soil and from direct contamination of foliage is hypothesized for Np. Curium-244 uptake by the root pathway yielded CR values of 10 3 to 10-1, according to pot culture experiments (32, 52). 

It appears evident therefore that the main function of most nutrient elements is as a constituent of the enzymes required to build up the organic matter within plants. Many such reactions run side by side in cells and tissues, and this is made possible by the presence of biomembranes that allow the build-up and decomposition of compounds, without mixing the components. Biomembranes subdivide cells into reaction spaces (e.g., nucleus, plastids, mitochondria, ribosomes, vacuoles, cytosol), and they permit well-ordered substance exchange between the compartments. Such processes are also responsible for ion uptake by root cells from the soil solution. Despite certain differences, all biomembranes have a similar chemical structure, the basic components being double lamellae of P-containing lipids (Figure 2.2) such as phosphatidylserine and glycolipids (Table 2.4). Proteins are movably incorporated into these double lamellae (see... 

Jarosik L, Zvara P, Konecny J and Obdrzalek M (1988) Dynamics of cobalt-60 uptake by roots of pea plant. SdTotal Environ 71 225-229. 

Zioni, A.B., Y. Vaadia, and S.H. Lips Nitrate uptake by roots as regulated by nitrate reduction products of the shoot Physiol. Plant 24 (1971) 288-290. 

Figure 5.9 Root concentration factor in barley roots for substituted phenoxyacetic acids as a function of pH. The lines represent predictions based on the ion-trap mechanism. O, 2,4-D ( ), 3,5-D. [Reproduced from G. G. Briggs, R. L. O. Rigitano, and R. H. Bromilow, Physico-chemical factors affecting uptake by roots and translocation of weak acids in barley Pesticid. Sci. 19, 101. Copyright 1987, Society of Chemical Industry. Reproduced with permission granted by John Wiley and Sons, Ltd on hehalf of the S.C.I.]...

The rate of transfer of solutes between soil and overlying water column and from one physical or chemical state to another is defined as flux. The dimensions of flux are M T where M is the mass of material transferred by flux, L is the distance or length, and T is the time. The processes associated with flux are advection, diffusion, and dispersion. Diffusive and advective flux between soil and overlying water and elemental uptake by rooted wetland vegetation are the major transport... 

Fig. 5. Se uptake by roots in response to SeO activities in the rooting medium (A) and at the PM surface (B). Atlas 66 wheat seedlings were cultured in media variously supplemented with CaClj and MgClj, adjusted to several pH values. The figure is redrawn from Kinraide (2003a).

Geelhoed, J.S., Mous, S.L.J., Findenegg, G.R., 1997. Modehng zero sink nutrient uptake by roots with root hairs from soil comparison of two models. Sod Sci. 162, 544—553. 

There are a number of different pathways by which organic chemicals may enter vegetation [95]. The major paAways include (1) uptake by roots and subsequent translocation from roots to shoots (i.e., liquid phase transfer) in the transpiration stream, (2) foUo uptake of volatilized organic chemicals from the surrounding air (i.e., vapor phase transfer), (3) uptake by external contamination of shoots by soil and dust, followed by retention in the cuticle or penetration through it, and (4) uptake and transport in oil cells which are found in oil containing plants Hke carrots and cress. 

Although the mechanism involved in nitrate uptake by roots is not known, indirect evidence (inhibitors of protein and RNA synthesis) indicates that nitrate induces a permease that greatly enhances the rate of nitrate uptake (Minotti et a/., 1968 Jackson ct a/., 1972, 1973 Neyra and Hageman, 1975). Data of Jackson et al. (1973) indicating that maximal rates of nitrate uptake depend on continuous protein synthesis support the hypothesis for the involvement of a specific nitrate transport protein. Because of the concurrent induction of the permease and nitrate reductase activity (Jackson et al., 1973 Neyra and Hageman, 1975), it has been postulated that the permease and... 

Growth and water status of maize have been followed by Tardleu with either a favourable (0) or a compact (C) soil structure In the ploughed layer. Differences In DMP and In stomatal conductance (gs) were observed, and related to water uptake by roots (ratio 1). This was ascribed to the spatial arrangement of roots. The objective of this article Is to present the second term of rain water use efficiency leaf water use efficiency (WUE). 

Cucumber seeds and seedlings have associated with them substantial microbial populations that are difficult to eliminate because microbes are not only found on and in the cutinized surface of the seed coat but can also be found internally within the seed (Leben 1961 Mundt and Hinkle 1976). Depletion of phenolic acids from nutrient solutions thus represent uptake by roots and microbial utilization. By replacing the nutrient solution (control) and nutrient-phenolic acid solutions (treatments) every other day, microbial populations were kept in check and phenolic acid concentrations were brought back to the original treatment concentrations. However, since phenolic acid treatments changed microbial populations on the rhizoplane (Fig. 2.9)... 

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