Cation/anion uptake ratio

Especially in dicotyledonous plant species such as tomato, chickpea, and white lupin (82,111), with a high cation/anion uptake ratio, PEPC-mediated biosynthesis of carboxylates may also be linked to excessive net uptake of cations due to inhibition of uptake and assimilation of nitrate under P-deficient conditions (Fig. 5) (17,111,115). Excess uptake of cations is balanced by enhanced net re-lea,se of protons (82,111,116), provided by increased bio.synthesis of organic acids via PEPC as a constituent of the intracellular pH-stat mechanism (117). In these plants, P deficiency-mediated proton extrusion leads to rhizosphere acidification, which can contribute to the. solubilization of acid soluble Ca phosphates in calcareous soils (Fig. 5) (34,118,119). In some species (e.g., chickpea, white lupin, oil-seed rape, buckwheat), the enhanced net release of protons is associated with increased exudation of carboxylates, whereas in tomato, carboxylate exudation was negligible despite intense proton extrusion (82,120). 

Pro 5 True, effects of phenolic acids on seedlings occur most readily under acidic conditions. However, soils have substantial buffering capacity, and thus pH changes due to root and microbial activity are unlikely to occur at the bulk-soil levels over short time intervals. Changes in pH resulting from root and microbial activity do occur within the rhizosphere and on the rhizoplane (Heckman and Strick 1996 Rao et al. 2002 Ortas and Rowell 2004). Alkalization or acidification of the rhizosphere occurs for many species because of changing cation-anion uptake ratios, particularly uptake of nitrate or ammonium, respectively. In cowpea, for example, the rhizosphere is alkalinized in the dark and acidified wifh lighf exposure of fhe shoofs even when supplied with nitrate (Marschner and Romheld 1983 Rao et al. 2002). 

In NOj -fed plants (10 mAf NO3 ), the cation-to-anion uptake ratio ranges from 1.2 to 1.6, leading to internal OH" generation additional to that associated with shoot reduction of nitrate and sulphate. On 20 mA/N03" there is a slight excess anion uptake, probably balanced by OH excretion to the medium. As to be expected, malate level in xylem of 20 mAf N03"-fed plants is only one-third of that of plants feeding on N2 or urea. 

Uptake measurements were made [16] at several oxide/solution ratios, reported as surface loading (SL) or m2 oxide surface/liter of solution, as PdCl, 2 concentration was increased and pH was held constant at the optimal value (Figure 6.10a). Each SL indeed indicated a plateau near the steric value [16], For Pt and Pd ammine cations, the maximum surface density over many oxides appears to be a close-packed layer, which retains two hydration sheaths representative results for PTA uptake over silica from a recent paper [19] are shown in Figure 6.10b. The physical limit of cationic ammine surface density thus appears to be 0.84 pmol/m2, or about 1 cationic complex/2 nm2. Cationic uptake, therefore, is inherently half of anion uptake in many cases. 

The Nernst equation predicts that because of the cell electronegativity, passive facilitated diffusion should allow a concentration ratio cell water/external medium approximating 10 to 15 at steady state. In isolated unperfused proximal tubules from rabbits, ratios exceeding 100 for TEA, have been measured [44] and one can wonder if another mechanism exists, for example a cation exchanger, as demonstrated in rabbits (mechanism 2), but which has generally not been observed in rats and dogs [44], which might be implicated in basolateral uptake. However, as reported for anions. 

Fig, 3,6), higher uptake, and/or higher ao. When uptake of the anion and the cation was studied, the formula of the salt is given in the column adsorbate . When only uptake of one of these ions was studied, the formula of the other ion is given in the column electrolyte . Table 3.10 reports also experimental conditions, namely, the solid to liquid ratio and equilibration time. The abbreviation in the column method is used for classical electrokinetic methods as opposite to the... 

The other common representation of the uptake studies is in form of percentage of uptake (pH) or distribution coefficient (pH) curves at constant initial concentration of the adsorbate and solid to liquid ratio. The typical course of the percentage of uptake (pH) curves for cations and anions is presented in Fig. 4.5-4.7. In a few publications the dissolved fraction rather than uptake is plotted as a function of pH. In such a representation anion type curves look like the cation type curves shown in Fig. 4,5 (A) and vice versa. The typical uptake curves for cations (A) and anions (B) (for low concentration of the solute) are shown in Fig. 4.5. The uptake... 

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