Root uptake

Roots modify their environment quite extensively in many ways. The most important of these are pH change, exudation and microbiological activity in the rhizosphere. Root exudates contain compounds such as hydroxycarboxylic acids and amino acids and these are capable of complexing trace metals. Bowling (1976), Farago (1986) and Streit and Stumm (1993) have discussed the theories of mineral uptake by plant roots the first suggests that there are four links in the uptake chain movement of ions or complexes in the soil to the roots uptake into the root transport across the root to the vascular system and movement to the shoot. 

The absorption of elements by roots can be both active (metabolic) and passive (nonmetabolic). It was known from early experiments that a number of metal ions are able to diffuse passively in and out of the root up to the Casparian band although uptake in some cases, such as K+ and Na+, by epidermal and cortex cells seems to be by active transport. There is evidence that ions cross the root by two general pathways  

cell wall pathway ions cross the cortex by means of the cells walls  

symplasm pathway ions cross the cortex by cytoplasmic drift through the continuum of cytoplasm that extends from cell to cell through the plasmodesmata, and is known as the symplasm. 

Transport into the xylem can be passive or active and is a problem still to be resolved (Streit and Stumm, 1993). 

---

Numerical techniques are iterative and require considerable computer processing power. With modern desktop computers, this is usually not an issue and solutions of root uptake over days or weeks typically take a few seconds to generate. However, for some strongly nonlinear problems, such as the development of rhizosphere microbial populations (Sect. Ill), where the increase in microbial biomass may be exponential over time, processing time may become important with solutions requiring >60 min to calculate on a modern PC. 

Another commonly used approximation for which there is an analytical solution is to assume that the root acts as a zero sink for uptake. Here the solute concentration at the root surface is taken to be zero and uptake is therefore completely controlled by the diffusive flux to the root (21,40,41). The implicit assumption is that root uptake is very rapid in comparison to resupply by transport and hence the root very rapidly depletes the solute concentration at the root surface to zero and maintains it there. The validity of this assumption depends on the value of X and it is inapplicable unless X is greater than or about 10 (38). For such large X, there is a nondimensional critical time (/,.) after which it is reasonable to assume a zero sink (38,42). Approximate values of /, are... 

S. Itoh and S. A. Barber. A numerical solution of whole plant uptake for soil-root uptake including root hairs. Plant Soil 70 403 (1983). 

Because of the limited root uptake and slow rate of systemic translocation, the diphenyl ether herbicide residues detected in the aerial plant portion are low. 

Briggs, G.G., Bromilow, R.H. and Evans, A.A., Relationships between lipophilicity and root uptake and translocation of non-ionized chemicals by barley, Pestic. Sci., 13, 495-504, 1982. 

Most trace metals may be precipitated with phosphate into insoluble metal phosphates (Table 7.5). Most metal phosphates have low solubility. High localization of phosphates reduces the bioavailability of Zn in arid soils. The banded application of P near the seeds depresses Zn uptake by com (Adriano and Murphy, 1970 Grant and Bailey, 1993), causing Zn deficiency. However, both N and P fertilizers increase Cd concentration in plants. Cadmium and Zn are antagonistic in root uptake and distribution within plants. 

In soils of agroecosystems, above ground biomass (foliar) uptake and metal cycling by mineralization and total root uptake can be lumped into a net removal term due to harvest (indicated as growth uptake, Mgu) when the critical load is calculated for the root zone, e.g., for upper 20-30 cm. In this situation we can calculate root uptake as a function of the growth uptake, whereas the net effect of litterfall and foliar uptake is assumed to be negligible. 

Root uptake Derivation of a relationship with deposition and land use... 

Input data for the most detailed soil model include parameters describing atmospheric deposition, precipitation, evapotranspiration, litterfall, foliar uptake, root uptake, weathering, adsorption and complexation of Pb, Cd, Cu, Zn, Ni, Cr and Hg. The input data mentioned above vary as a function of location (receptor area) and receptor (the combination of land and soil type) as shown in Table 6. 

There have been several investigations of plutonium uptake by plants and several authors have reviewed the various articles (124-126). Based on studies with plant-soil systems direct root uptake into the plant appears to be low. Concentration factors for (pCi/g dry acceptor)/(pCi/g donor) are of the order 10-5 to 10 4 (127). However, variations in the chemical form of plutonium and the presence of chelating agents in the soil can result in the concentration of more plutonium in the plant. Over five years Romney et aL (33) observed that the plutonium concentration in ladino clover increased from 3.1 dpm/g for the first year to 22.6 dpm/g in the fifth year. It should be noted that clovers release citric acid into the soil (113). 

Shaw, G., 1993, Blockade by fertilisers of caesium and strontium uptake into crops effects on the root uptake process. 5ci. Total Environ. 137 119-133. 

Skaggs, T.H., Shouse, P.J., Poss, J.A. Irrigating Forage Crops with Saline Waters 2. Modeling Root Uptake and Drainage. Vadose Zone J. 5(3), 824—837 (2006b)... 

To calculate the inflow, Cta must be found from the concentration in the soil bulk taking into account rates of transport through the soil. Kirk and Solivas (1997) have done this for N uptake by rice growing in flooded soil and used the resulting model to assess the relative importance of root uptake properties and transport through the soil. Their results are summarized in the following. 

Note that the above conclusions refer to uptake of soil N by the main body of the rice root system in the anoxic soil beneath the soil-floodwater interface. Uptake of fertilizer N broadcast into ricefield floodwater and absorbed by roots in the floodwater or soil near the floodwater is not likely to be limited by root uptake properties or transport (Kirk and Solivas, 1997). 

McFarlane C, Wickliff C. 1985. Excised barley root uptake of several carbon-14-labeled organic compounds. Environ Monit Assess 5 385-392. 

Polybrominated Biphenyls. PBBs do not readily translocate from soil to plants via root uptake (Jacobs et al. 1976, 1978). Therefore, PBBs may not bioconcentrate in plants. However, plant uptake data are limited, audit will be helpful to develop additional plant uptake data. Brominated biphenyls with bromine substitution 6 or less will bioconcentrate in aquatic organisms (Gobas et al. 1989 Norris et al. 1973 Opperhuizen et al. 1985 Zitko 1979 Zitko and Hutzinger 1976). PBBs are preferentially stored in the adipose tissue of animals (Kimbrough 1987). Although PBBs have been detected in fish-eating birds... 

Teixeira, VS, Franco EP. 1986. Root uptake of exogenous radium-226 by three edible vegetables grown in farm soils from the vicinity of the first Brazilian uranium mine and mill [Abstract], Chem Environ Proc Int Conf 837-842. (CA 110 7068b)... 

This group of herbicides can be characterized as having relatively high water solubility marginal selectivity towards annual field crops relatively rapid action through both leaf uptake and root uptake and broad spectrum of activity after pre- and post-emergence treatments. 

Triazine herbicides have been important tools in the development and implementation of conservation tillage systems. Postemergence and residual activity, low vapor pressure, weak adsorption to surface crop residue, root uptake, broad weed control spectrum, and crop safety make these herbicides compatible with the high crop residue conditions of conservation tillage. 

The first term on the right-hand side of equation (2.14), termed the rate factor, represents the direct contamination of herbage by fallout during the growing season. The second term, lag rate factor, is the contribution from the previous year s fallout. This includes the contribution of uptake from the surface soil and matt and also the effects of carry-over of silage and other feeding stuffs from one year to the next. The third term, soil factor, represents the contribution of root uptake, allowing for radioactive decay and reduced availability as nuclides move down the soil profile and become fixed to clay minerals. 

---