Exudation rate

Diffusion-mediated release of root exudates is likely to be affected by root zone temperature due to temperature-dependent changes in the speed of diffusion processes and modifications of membrane permeability (259,260). This might explain the stimulation of root exudation in tomato and clover at high temperatures, reported by Rovira (261), and also the increase in exudation of. sugars and amino acids in maize, cucumber, and strawberry exposed to low-temperature treatments (5-10°C), which was mainly attributed to a disturbance in membrane permeability (259,262). A decrease of exudation rates at low temperatures may be predicted for exudation processes that depend on metabolic energy. This assumption is supported by the continuous decrease of phytosiderophore release in Fe-deficient barley by decreasing the temperature from 30 to 5°C (67). 

Figure S The iimula(ed distribution of (u) soluble carbon and (b) microbial biomass in the rhizosphere of maize over a period of 10 days. The Jf axis represents distance (cm) from die rout surface and the Y axis represents time (h). A unifonn exudation rate (S3) is compared to the situation where the. siime amount of exudate is released in the first 24 h (S4). Note that the Z axes have different scales although both represent pg C cm (From Ref. 48.)...

Information on active exudation of DMSP is limited. Laroche et al. (1999), using a modelling study, indicated that DMSP exudation in the dinoflagellate Prorocentrum minimum is independent of growth phase at the level of 1 % of its DMSP quota per day. In contrast, they showed that Phaeocystis sp. exudation rates, measured as cumulative DMS concentration in... 

It is well estabhshed that phytoplankton cells hmited in N or P may continue the photosynthetic production of cellular and extracellular compounds not having the hmiting nutrient as a constituent, often leading to high PER values. On the other hand, the specific exudation rate may not be higher than under nutrient replete conditions. High medium N/P ratio has been shown to increase the production of extracellular polysaccharides. 

Mechanishc uptake models have considered the effect of polycarboxylic acids on P uptake (Hoffiand, 1992 Kirk, 1999 Kirk et al, 1999 Geelhoed et al., 1999 Gerke et al., 2000). Exudation rates are typically applied as a boundary condition at the root-soil interface. Two methods to treat the problem of simultaneous transport and interaction between root exudates and solutes have been employed (1) coupled diffusion, and (2) separation of transport and reaction (equilibration). 

Note is the concentration of exudates in solution, the exudation rate, the diffusion coefficient of... 

The values for the exudation rate F, interaction coefficient (A), buffer power of exudate in soil b and the decomposition rate constant for the exudate k were adopted from Kirk (1999). The value of the forward rate constant was estimated from Scheckel and Sparks (2001), who evaluated kinetic adsorption data of Ni to different minerals where ranged from 2.5 x 10 to 9.78 X 10 s For the simulation, an average value of 5.00 x 10 was used. This value also coincides with the values that Kirk and Staunton (1989) suggested for the kinetic adsorption of Q to soil, where the values ranged from lO" to 10 2 s f This same value was assumed for the rate constant for the two-stage sorption model, a2- The fraction of type 1 sites (F ) was assumed to be 0.3. Table 7 summarizes all input parameter values. 

Varying the values of the interaction coefficient (A) and exudation rate resulted in similar concentration profiles (Fig. 6). The higher the values of or... 

A, the higher were the concentrations of dissolved Ni. Therefore, the uptake rate was higher (data not shown). Due to this, a contrary effect was observed for lahile Ni the higher or X, the lower was the concentration of lahile Ni at the root surface. The effect on labile Ni extends only up to 1 mm from the root. Generally, increased root exudation rates caused larger discrepancies between simulated and experimentally obtained results. The model used here belongs to the class of models that use lumped parameters, meaning that several processes are described by one combined parameter. The parameter X expresses the total effect of exudates on dissolved Ni. Increased Ni concentration could be due to several processes, but the contributions of each of them are not explicitly described. Furthermore, the solution species of Ni as well as other important ions are not explained. 

A requisite of high exudation rate of viscous liquid Cl, to which contact inhibitors belong, is the formation of a jellygel-like structure in the polymer matrix with a developed system of pores. With this aim, polymers are impregnated with PI that also function as transporting agents for contact Cl [3,104,106,107]. 

Fig. 5. TEA exudation rate for 50% TEA plasticized films from voltammetric analysis versus water immersion time and HCHO/e-NHj molar ratios reported on the front side.

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