Influence of phosphate status on phosphate uptake kinetics of maize (Zea mays) and soybean (Glycine max)

To obtain plants of different P status, maize and soybean seedlings were grown for several weeks in flowing nutrient solution culture with P concentrations ranging from 0.03–100 µmol P L^-1 kept constant within treatments. P uptake kinetics of the roots were then determined with intact plants in short-term experiments by monitoring P depletion of a 3.5 L volume of nutrient solution in contact with the roots. Results show maximum influx, I_max, 5-fold higher in plants which had been raised in solution of low compared with high P concentration. Because P concentrations in the plants were increased with increase in external P concentration, I_max was negatively related to % P in shoots. Michaelis constants, K_m, were also increased with increased pretreatment P concentration, only slightly with soybean, but by a factor of 3 with maize. The minimum P concentration, C_min, where net influx equals zero, was found between 0.06 and 0.3 µmol L^-1 with a tendency to increase with pretreatment P concentration. Filtration of solutions at the end of the depletion experiment showed that part of the external P was associated with solid particles.It was concluded that plants markedly adapt P uptake kinetics to their P status, essentially by the increase of I_max, when internal P concentration decreases. Changes of K_m and C_min were of minor importance.     

Kinetics of mercury uptake by oilseed rape and white lupin: influence of Mn and Cu

Mercury influx in oilseed rape and white lupin was studied using short time influx experiments. The effect of Cu and Mn in Hg influx was also tested. Plants were grown for 2 weeks and then roots were incubated with increasing Hg concentrations (0–50 μM HgCl₂), both at 20 °C and ice-cold temperature. An active, saturable component in Hg uptake was found in oilseed rape and white lupin, with K _m and V _max values in the range of low affinity transporters for essential micronutrients. A reduction in Hg uptake was observed in the presence of Mn for oilseed rape, suggesting that Hg influx is mediated by a Mn transporter. No effects of Cu on Hg influx were observed for any of the two plant species, suggesting a different transport system for Hg and Cu in roots of oilseed rape and white lupin.     

Potassium uptake efficiency and dynamics in the rhizosphere of maize (Zea mays L.), wheat (Triticum aestivum L.), and sugar beet (Beta vulgaris L.) evaluated with a mechanistic model

Plant species differ in nutrient uptake efficiency. With a pot experiment, we evaluated potassium (K) uptake efficiency of maize (Zea mays L.), wheat (Triticum aestivum L.), and sugar beet (Beta vulgaris L.) grown on a low-K soil. Sugar beet and wheat maintained higher shoot K concentrations, indicating higher K uptake efficiency. Wheat acquired more K because of a greater root length to shoot dry weight ratio. Sugar beet accumulated more shoot K as a result of a 3- to 4-fold higher K influx as compared to wheat and maize, respectively. Nutrient uptake model NST 3.0 closely predicted K influx when 250 mg K kg^?1 were added to the soil, but under-predicted K influx under low K supply. Sensitivity analysis showed that increasing soil solution K concentration (C_Li) by a factor of 1.6–3.5 or buffer power (b) 10- to 50-fold resulted in 100% prediction of K influx. When both maximum influx (I_max) and b were increased by a factor of 2.5 in maize and wheat and 25 in sugar beet, the model could predict measured K influx 100%. In general, the parameter changes affected mostly calculated K influx of root hairs, demonstrating their possible important role in plant K efficiency.     

Phosphorus efficiency of plants

Föhse et al. (1988) have shown that P influx per unit root length in seven plant species growing in a low-P soil varied from 0.6×10^-14 to 4.8×10^-14 mol cm^-1s^-1. The objective of this work was to investigate the reasons for these differences. No correlation was found between P influx and root radius, root hairs, cation-anion balance and Ca uptake. However, when root hairs were included in mathematical model calculations, the differences of P influx could be accounted for. These calculations have shown that in soils low in available P, contribution to P uptake by root hairs was up to 90% of total uptake. The large contribution of root hairs to P uptake was partly due to their surface area, which was similar to that of the root cylinder. However, the main reason for the high P uptake efficiency of root hairs was their small radius (approx. 5×10^-4 cm) and their perpendicular growth into the soil from the root axis. Because of the small radius compared to root axes, P concentration at root hair surfaces decreased at a slower pace and therefore P influx remained higher. Under these conditions higher I_max (maximum influx) or smaller K_m values (Michaelis constant) increased P influx. The main reasons for differences found in P influx among species were the size of I_max and the number and length of root hairs. In a soil low in available P, plant species having more root hairs were able to satisfy a higher proportion of their P demand required for maximum growth.     

Cadmium Dynamics in the Rhizosphere and Cd Uptake of Different Plant Species Evaluated by A Mechanistic Model

Maize, sunflower, flax, and spinach differed in the accumulation of Cd when grown on a Cd contaminated soil. This was mainly due to the different Cd net influx, I_n , that varied among species by a factor of up to 30. The objective of this study was to find possible reasons for the different Cd I_n by using a mechanistic model. After 14 days of Cd uptake the model calculated only a small Cd depletion at the root surface, e.g. from 0.22 μmol L^?1down to 0.19 μmol L^?1for maize and from 0.48 μmol L^?1down to 0.35 μmol L^?1for spinach. Even so the model always overestimated the Cd I_n , for spinach by a factor of 1.5 and for maize by a factor of 10. Only simulating a decrease of C_Li or the root absorbing power, α, by 40% to 90% gave an agreement of calculated and measured I_n . This may be interpreted as that about 40% in the case of spinach and 90% in the case of maize of the Cd in soil solution were not accessible for plant uptake. The high sensitivity to α also shows that not the Cd transport to the root but α was limiting the step for Cd uptake.     

Nutrient uptake rates by the alien alga Undaria pinnatifida (Phaeophyta) (Nuevo Gulf, Patagonia, Argentina) when exposed to diluted sewage effluent

In the early nineties, Undaria pinnatifida has been accidentally introduced to Nuevo Gulf (Patagonia, Argentina) where the environmental conditions would have favored its expansion. The effect of the secondary treated sewage discharge from Puerto Madryn city into Nueva Bay (located in the western extreme of Nuevo Gulf) is one of the probable factors to be taken into account. Laboratory cultures of this macroalgae were conducted in seawater enriched with the effluent. The nutrients (ammonium, nitrate and phosphate) uptake kinetics was studied at constant temperature and radiation (16?°C and 50 μE m^?2 s^?1 respectively). Uptake kinetics of both inorganic forms of nitrogen were described by the Michaelis–Menten model during the surge phase (ammonium: V _max ^sur: 218.1 μmol h^?1 g^?1, K _s ^sur: 476.5 μM and nitrate V _max ^sur: 10.7 μmol h^?1 g^?1, K _s ^sur: 6.1 μM) and during the assimilation phase (ammonium: V _max ^ass: 135.6 μmol h^?1 g^?1, K _s ^ass: 407.2 μM and nitrate V _max ^ass: 1.9 μmol h^?1 g^?1, K _s ^ass: 2.2 μM), with ammonium rates always higher than those of nitrate. Even though a net phosphate disappearance was observed in all treatments, uptake kinetics of this ion could not be properly estimated by the employed methodology.     

KINETICS OF NUTRIENT UPTAKE AND GROWTH IN PHYTOPLANKTON1

Microscopic algae ran grow rapidly in natural waters that are extremely low in essential macro and micro nutrients. Yet, their nutrient uptake systems exhibit only mediocre nutrient affinities, the saturation constants being often 10–1000 times the (estimated) ambient concentrations. The large difference which exists between the saturation constants for growth (K_μ) and short term uptake (K_ρ) are due to the acclimation capabilities of the organisms. Over the acclimation range, K_μ to K_ρ, the algae can maintain maximum growth rate by modulating both their internal nutrient quotas (Q) and their maximum short term nutrient uptake rates (ρ_max) in response to variations in external nutrient concentrations. The commonly assumed hyperbolic relationships for steady growth and uptake (viz “chemostat theory”) are coherent with a hyperbolic expression for short term uptake including a variable maximum (ρ_max). The ratio of the saturation constants for growth and uptake is then directly related to the extreme in quotas and maximum uptake rates: K_μ/K_ρ= Q_min/Q_max·ρ^lo_max/ρ^hi_max. This result is applicable even when the exact hyperbolic laws are not. Published data on Fe, Mn, P and N limitation in algae are generally in accord with the theory and demonstrate a wider acclimation range for trace than for major nutrients.     

Effects of saline root environment (NaCl) on nitrate and potassium uptake kinetics for rose plants: a Michaelis–Menten modelling approach

Greenhouse-grown cut flower roses are often irrigated with moderately saline irrigation water. The salt/ballast ions are either present initially in poor quality raw water or reclaimed municipal water, or accumulated in greenhouse irrigation water that is captured and reused. Such ions can inhibit root absorption of essential nutrients. The objective of this work was to quantify the influence of NaCl concentration on the uptake of nitrate and potassium by roses and develop a predictive model of uptake inhibition based on NaCl, NO₃ ^?, and K⁺ concentration. One year-old rose plants (Rosa spp. ‘Kardinal’ on ‘Natal Briar’ rootstock) were moved into growth chambers where nitrogen and potassium depletion were monitored during 6 days. Eight different initial NaCl treatments varying from zero to 65 mol m^?3 were used and within these there were two initial NO₃ ^? and K⁺ concentrations: high concentration (HC, 7.0 mol m^?3 and 2.6 mol m^?3 NO₃ ^? and K⁺ respectively) or low concentration (LC, 3.5 mol m^?3 and 1.3 mol m^?3 NO₃ ^? and K⁺ respectively). Plant NO₃ ^? uptake was negatively affected by NaCl concentration. NO₃ ^? maximum influx (I_max) declined from 5.1 µmol to 2.5 µmol per gram of plant dry weight per hour as NaCl concentration increased from zero to 65 mol m^?3. A modified Michaelis–Menten (M–M) equation taking into account inhibition by NaCl provided the best fit for NO₃ ^? uptake in response to varying NaCl concentration. K⁺ uptake was unaffected by NaCl concentration. A M–M equation that did not include inhibition was suitable for describing K⁺ uptake at varying NaCl concentration. The resulting empirical models could assist with decision making, such as: adjustment of NO₃ ^? fertilization based on NaCl concentration, necessity of water desalinization, or determination of the desired leaching fraction.     

Changes in the kinetics of phosphate and potassium absorption in nutrient-deficient barley roots measured by a solution-depletion technique

From measurements of the rates of depletion of labelled ions from solution in the low concentration range, we described the phosphate and potassium uptake characteristics of the roots of intact barley plants in terms of the kinetic parameters, K _m and I _max (the maximum rate of uptake). In relatively young (13 d) and older (42 d) plants, cessation of phosphate supply for 4 d or more caused a marked increase in I _max (up to four times), without concomitant change in K _m, which remained between 5 and 7 M. By contrast, 1 d of potassium starvation with 14-d plants caused a decline in the K _m (i.e. an increased apparent affinity for potassium) from 53 M to 11 M, without alteration to I _max. After longer periods of potassium starvation, I _max increased (about two times) while the K _m remained at the same low value. Growth of shoots and roots were unaffected by these treatments, so that concentrations of ions in the tissues declined after 1 d or more of nutrient starvation, but we could not identify a characteristic endogenous concentration for either nutrient at which changes in kinetic parameters were invariably induced. The possible mechanisms regulating carriermediated transport, and the importance of changes induced in kinetic parameters in ion uptake from solution and soil are discussed.Symbol I _max the maximum rate of absorption at saturating concentrations     

Manganese dynamics in the rhizosphere and Mn uptake by different crops evaluated by a mechanistic model

Understanding of the mechanisms of Mn supply from the soil and uptake by the plants can be improved by using simulation models that are based on basic principles. For this, a pot culture experiment was conducted with a sandy clay loam soil to measure Mn uptake by summer wheat (Triticum aestivum L. cv. Planet), maize (Zea mays L. cv. Pirat) and sugar beet (Beta vulgaris L. cv. Orbis) and to simulate Mn dynamics in the rhizosphere by means of a mechanistic model. Seeds of three crops were sown in pots containing 2.9 kg soil in a controlled growth chamber. Root and shoot weight, Mn content of plants, root length and root radius were determined 8 (13 days in case of sugar beet) and 20 days after germination. Soil and plant parameters were determined to run nutrient uptake model calculations. Manganese content of the shoot varied from 25 mg kg^-1 for sugar beet to 34 mg kg^-1 for maize. Sugar beet had the lowest root length/shoot weight ratio but the highest relative shoot growth rate, resulting in the highest shoot demand on the root. This is reflected by the Mn influx which was 0.9 × 10^-7, 1.7 × 10^-7 and 2.5 × 10^-7 nmol cm^-1 s^-1 for wheat, maize and sugar beet, respectively. Nutrient uptake model calculations predicted similar influx values. Initial Mn concentration of 0.2 μM in the soil solution decreased to only 0.16 μM for wheat, 0.13 μM for maize and 0.11 μM for sugar beet at the root surface. This shows that manganese transport to the root was not a limiting step. This was confirmed by the fact that an assumed 20 times increase in maximum influx (I_max) increased the calculated Mn influx by 3.7 times. Sensitivity analysis demonstrated that for controlling Mn uptake the initial soil solution concentration (C _Li), the root radius (r₀), I_max and the Michaelis constant (K _m) were the most sensitive factors in the listed order. This revised version was published online in August 2006 with corrections to the Cover Date.     

In Vitro Functional Characterisation of Cytochrome P450 (CYP) 2C19 Allelic Variants <Emphasis Type="Italic">CYP2C19*23</Emphasis> and <Emphasis Type="Italic">CYP2C19*24</Emphasis>

Cytochrome P450 (CYP) 2C19 is essential for the metabolism of clinically used drugs including omeprazole, proguanil, and S-mephenytoin. This hepatic enzyme exhibits genetic polymorphism with inter-individual variability in catalytic activity. This study aimed to characterise the functional consequences of CYP2C19*23 (271 G>C, 991 A>G) and CYP2C19*24 (991 A>G, 1004 G>A) in vitro. Mutations in CYP2C19 cDNA were introduced by site-directed mutagenesis, and the CYP2C19 wild type (WT) as well as variants proteins were subsequently expressed using Escherichia coli cells. Catalytic activities of CYP2C19 WT and those of variants were determined by high performance liquid chromatography-based essay employing S-mephenytoin and omeprazole as probe substrates. Results showed that the level of S-mephenytoin 4′-hydroxylation activity of CYP2C19*23 (V _max 111.5 ± 16.0 pmol/min/mg, K _m 158.3 ± 88.0 μM) protein relative to CYP2C19 WT (V _max 101.6 + 12.4 pmol/min/mg, K _m 123.0 ± 19.2 μM) protein had no significant difference. In contrast, the K _m of CYP2C19*24 (270.1 ± 57.2 μM) increased significantly as compared to CYP2C19 WT (123.0 ± 19.2 μM) and V _max of CYP2C19*24 (23.6 ± 2.6 pmol/min/mg) protein was significantly lower than that of the WT protein (101.6 ± 12.4 pmol/min/mg). In vitro intrinsic clearance (CL_int = V _max/K _m) for CYP2C19*23 protein was 85.4 % of that of CYP2C19 WT protein. The corresponding CL_int value for CYP2C19*24 protein reduced to 11.0 % of that of WT protein. These findings suggested that catalytic activity of CYP2C19 was not affected by the corresponding amino acid substitutions in CYP2C19*23 protein; and the reverse was true for CYP2C19*24 protein. When omeprazole was employed as the substrate, K _m of CYP2C19*23 (1911 ± 244.73 μM) was at least 100 times higher than that of CYP2C19 WT (18.37 ± 1.64 μM) and V _max of CYP2C19*23 (3.87 ± 0.74 pmol/min/mg) dropped to 13.4 % of the CYP2C19 WT (28.84 ± 0.61 pmol/min/mg) level. Derived from V _max/K _m, the CL_int value of CYP2C19 WT was 785 folds of CYP2C19*23. K _m and V _max values could not be determined for CYP2C19*24 due to its low catalytic activity towards omeprazole 5′-hydroxylation. Therefore, both CYP2C19*23 and CYP2C19*24 showed marked reduced activities of metabolising omeprazole to 5-hydroxyomeprazole. Hence, carriers of CYP2C19*23 and CYP2C19*24 allele are potentially poor metabolisers of CYP2C19-mediated substrates.     

Phosphorus retention of forested and emergent marsh depressional wetlands in differing land uses in Florida,USA

The translocation of phosphorus (P) from terrestrial landscapes to aquatic bodies is of concern due to the impact of elevated P on aquatic system functioning and integrity. Due to their common location in depressions within landscapes, wetlands, including so-called geographically isolated wetlands (GIWs), receive and process entrained P. The ability of depressional wetlands, or GIWs, to sequester P may vary by wetland type or by land use modality. In this study we quantified three measures of P sorption capacities for two common GIW types (i.e., emergent marsh and forested wetlands) in two different land use modalities (i.e., agricultural and least impacted land uses) across 55 sites in Florida, USA. The equilibrium P concentration (EPC₀) averaged 6.42 ± 5.18 mg P L^?1 (standard deviation reported throughout); and ranged from 0.01–27.18 mg P L^?1; there were no differences between GIW type or land use modality, nor interaction effects. Significant differences in phosphorus buffering capacity (PBC) were found between GIW types and land use, but no interaction effects. Forested GIWs [average 306.64 ± 229.63 (mg P kg^?1) (µg P L^?1)^?1], and GIWs in agricultural settings [average 269.95 ± 236.87 (mg P kg^?1) (µg P L^?1)^?1] had the highest PBC values. The maximum sorption capacity (S_max) was found to only differ by type, with forested wetlands (1274.5 ± 1315.7 mg P kg^?1) having over three times the capacity of emergent GIWs (417.5 ± 534.6 mg P kg^?1). Classification trees suggested GIW soil parameters of bulk density, organic content, and concentrations of total P, H₂O-extractable P, and HCl-extractable P were important to classifying GIW P-sorption metrics. We conclude that GIWs have high potential to retain P, but that the entrained P may be remobilized to the wetland water column depending on storm and groundwater input P concentrations. The relative hydrologic dis-connectivity of GIWs from other aquatic systems may provide sufficient retention time to retain elevated P within these systems, thereby providing an ecosystem service to downstream waters.     

Characterization of phosphate absorption in maize root cortex segments

Uptake of phosphate ions by 1 mm segments of isolated maize root cortex layers was studied. Cortex segments (from roots of 8 days old maize plants) absorb phosphate ions from 1 mM KH₂PO₄ in 0.2 mM CaSCO₄ at the average rate of 34.3 ±3.2 μg P_i g^?1 (fr. m.) h^?1,i.e. 0.35± 0.02 μmol P_i g^?1 (fr. m.) h^?1. Phosphate uptake considerably increases after a certain period of “augmentation”,i.e. washing in aerated 0.2 mM CaSO₄. This increase is completely blocked by the presence of 10 μg ml^?1 cycloheximide. The relation of uptake rate to phosphate concentration in the medium was shown to have 3 phases in the concentration range of 0.02 - 40 mM. Transition points were found between 0.8–1 mM and 10–20 mM. Following K_m and V_max values were found: K_m[mM] : 0.37 - 3.82 - 27.67 V_max[μg P_i g^?1 (fr. m.) h^?1] : 3.33 - 39.40 - 66.67 We have found no sharp pH optimum for phosphate uptake. It proceeds at almost constant rate till pH 6.0 and then the uptake rate drops with increasing pH. At low phosphate concentrations (1 mM) the lowest uptake rate was found at 5 and 13 °C, while the uptake is higher at 5 °C than at 13 °C at phosphate concentrations higher than 1 mM. At these concentrations uptake rate at 35 °C is lower than at 25 °C. Phosphate uptake considerably decreased in anaerobic conditions. DNP and iodoacetate (0.1 mM) completely blocked phosphate uptake from 1 mM KH₂PO₄, while uptake from 5 and 10 mM KH₂PO₄ was left unaffected by these substances. The inhibitors of active - SH groups NEM and PCMB inhibited phosphate uptake: 10^?3 M NEM by 81.6%, 10⁴ M NEM by 42% and 10^?4 M PCMB by 42%.     

Phosphorus intensity determines short-term P uptake by pigeon pea (<Emphasis Type="Italic">Cajanus cajan</Emphasis> L.) grown in soils with differing P buffering capacity

Phosphorus (P) uptake by plant roots depends on P intensity (I) and P quantity (Q) in the soil. The relative importance of Q and I on P uptake is unknown for soils with large P sorption capacities because of difficulties in determining trace levels of P in the soil solution. We applied a new isotope based method to detect low P concentrations (<20 μg P l⁻¹). The Q factor was determined by assessment of the isotopically exchangeable P in the soil (E-value) and the I factor was determined by measurement of the P concentration in the pore water. A pot trial was set up using four soils with similar labile P quantities but contrasting P buffering capacities. Soils were amended with KH₂PO₄ at various rates and pigeon pea (Cajanus cajan L.) was grown for 25 days. The P intensity ranged between 0.0008 and 50 mg P l⁻¹ and the P quantity ranged between 10 and 500 mg P kg⁻¹. Shoot dry matter (DM) yield and P uptake significantly increased with increasing P application rates in all soils. Shoot DM yield and P uptake, relative to the maximal yield or P uptake, were better correlated with the P concentration in the pore water (R ² = 0.83–0.90) than with the E-value (R ²=0.40–0.53). The observed P uptakes were strongly correlated to values simulated using a mechanistic rhizosphere model (NST 3.0). A sensitivity analysis reveals that the effect of P intensity on the short-term P uptake by pigeon pea exceeded the effect of P quantity both at low and high P levels. However, DM yield and P uptake at a given P intensity consistently increased with increasing P buffering capacity (PBC). The experimental data showed that the intensity yielding 80% of the maximal P uptake was 4 times larger in the soil with the smallest PBC compared to the soil with the largest PBC. This study confirms that short-term P uptake by legumes is principally controlled by the P intensity in the soil, but is to a large extent also affected by the PBC of the soil. Section Editor: N. J. Barrow     

Inorganic nitrogen uptake kinetics of sugarcane (Saccharum spp.) varieties under in vitro conditions with varying N supply

An in vitro system was established for the characterisation of inorganic nitrogen uptake by sugarcane plantlets of variety NCo376. After multiplication and rooting, plantlets (0.27–0.3 g fresh mass) were placed on N-free medium for 4 days, and then supplied with 2–20 mM N as NO₃ ^?-N only, NH₄ ⁺-N only or NO₃ ^?-N + NH₄ ⁺-N (as 1:1). With few exceptions, on all the tested N media, the in vitro plants always had a higher V_max for NH₄ ⁺-N (28.69–66.51 μmol g^?1 h^?1) than for NO₃ ^?-N uptake (10.24–30.19 μmol g^?1 h^?1) and the K_m indicated a higher affinity for NO₃ ^?-N (0.02–7.38 mM) than for NH₄ ⁺-N (0.06–9.15 mM). When N was applied as 4 and 20 mM to varieties N12, N19 and N36, the interaction between variety, N form and concentration resulted in differences in the V_max and K_m. The high N-use efficient varieties (N12 and N19), as determined in previous pot and field trials, behaved similarly under all tested conditions and displayed a lower V_max and K_m than the low N-use efficient ones (NCo376 and N36). Based on this finding, it was suggested that the N-use efficient designation (from pot and field trials) may not be ascribed solely to N uptake. Assessment of the relative preference index (RPI) for NO₃ ^?-N and NH₄ ⁺-N uptake revealed that, at present, the RPI has no application in sugarcane due to its preferential uptake of NH₄ ⁺-N.     

Aflatoxin B<Subscript>1</Subscript> levels in groundnut products from local markets in Zambia

In Zambia, groundnut products (milled groundnut powder, groundnut kernels) are mostly sold in under-regulated markets. Coupled with the lack of quality enforcement in such markets, consumers may be at risk to aflatoxin exposure. However, the level of aflatoxin contamination in these products is not known. Compared to groundnut kernels, milled groundnut powder obscures visual indicators of aflatoxin contamination in groundnuts such as moldiness, discoloration, insect damage or kernel damage. A survey was therefore conducted from 2012 to 2014, to estimate and compare aflatoxin levels in these products (n = 202), purchased from markets in important groundnut growing districts and in urban areas. Samples of whole groundnut kernels (n = 163) and milled groundnut powder (n = 39) were analysed for aflatoxin B₁ (AFB₁) by competitive enzyme-linked immunosorbent assay (cELISA). Results showed substantial AFB₁ contamination levels in both types of groundnut products with maximum AFB₁ levels of 11,100 μg/kg (groundnut kernels) and 3000 μg/kg (milled groundnut powder). However, paired t test analysis showed that AFB₁ contamination levels in milled groundnut powder were not always significantly higher (P > 0.05) than those in groundnut kernels. Even for products from the same vendor, AFB₁ levels were not consistently higher in milled groundnut powder than in whole groundnut kernels. This suggests that vendors do not systematically sort out whole groundnut kernels of visually poor quality for milling. However, the overall contamination levels of groundnut products with AFB₁ were found to be alarmingly high in all years and locations. Therefore, solutions are needed to reduce aflatoxin levels in such under-regulated markets.     

Inhibition of bovine heart Na+, K+-ATPase by palmitylcarnitine and palmityl-CoA.

The activity of a partially purified bovine heart Na⁺,K⁺-ATPase is inhibited by DL- and L- palmitylcarnitine (I₅₀=44–48μM). Palmitylcarnitine with a I₅₀ of 25μM also markedly inhibits K⁺-phosphatase activity. Palmityl-CoA decreases Na⁺,K⁺-ATPase activity, but to a lesser extent (I₅₀=80μM). Both palmitic acid and hexanoic acid produce 10 to 15% inhibition of activity at concentrations of 70μM and 3–5mM, respectively. These free fatty acids protect the enzyme against inhibition by 40μM palmitylcarnitine. However, at 50μM palmitylcarnitine, the protective effect by hexanoic acid is no longer apparent. Addition of 40μM palmitylcarnitine to the Na⁺,K⁺-ATPase in the presence of varying concentrations of palmityl-CoA produces an additive inhibition of enzyme activity, suggesting two different sites on the enzyme susceptible to inhibition by the two ester forms of the fatty acid.     

Relevance of allosteric conformations and homocarnosine concentration on carnosinase activity

Activity of carnosinase (CN1), the only dipeptidase with substrate specificity for carnosine or homocarnosine, varies greatly between individuals but increases clearly and significantly with age. Surprisingly, the lower CN1 activity in children is not reflected by differences in CN1 protein concentrations. CN1 is present in different allosteric conformations in children and adults since all sera obtained from children but not from adults were positive in ELISA and addition of DTT to the latter sera increased OD450 values. There was no quantitative difference in the amount of monomeric CN1 between children and adults. Further, CN1 activity was dose dependently inhibited by homocarnosine. Addition of 80 μM homocarnosine lowered V _max for carnosine from 440 to 356 pmol/min/μg and increased K _m from 175 to 210 μM. The estimated K _i for homocarnosine was higher (240 μM). Homocarnosine inhibits carnosine degradation and high homocarnosine concentrations in cerebrospinal fluid (CSF) may explain the lower carnosine degradation in CSF compared to serum. Because CN1 is implicated in the susceptibility for diabetic nephropathy (DN), our findings may have clinical implications for the treatment of diabetic patients with a high risk to develop DN. Homocarnosine treatment can be expected to reduce CN1 activity toward carnosine, resulting in higher carnosine levels.     

Maize productivity and nutrient dynamics in maize-fallow rotations in western Kenya

One-season fallows with legumes such as Crotalaria grahamiana Wight & Arn. and phosphorus (P) fertilization have been suggested to improve crop yields in sub-Saharan Africa. Assessing the sustainability of these measures requires a sound understanding of soil processes, especially transformations of P which is often the main limiting nutrient. We compared plant production, nitrogen (N) and P balances and selected soil properties during 5.5 years in a field experiment with three crop rotations (continuous maize, maize-crotalaria and maize-natural fallow rotation) at two levels of P fertilization (0 and 50 kg P ha^?1 yr^?1, applied as triple superphosphate) on a Kandiudalfic Eutrudox in western Kenya. The maize yield forgone during growth of the crotalaria fallow was compensated by higher post-fallow yields, but the cumulative total maize yield was not significantly different from continuous maize. In all crop rotations, P fertilization doubled total maize yields, increased N removal by maize and remained without effect on amounts of recycled biomass. Crotalaria growth decreased in the course of the experiment due to pest problems. The highest levels of soil organic and microbial C, N and P were found in the maize-crotalaria fallow rotation. The increase in organic P was not accompanied by a change in resin-extractable P, while H₂SO₄-extractable inorganic P was depleted by up to 38 kg P ha^?1 (1% of total P) in the 0–50 cm layer. Microbial P increased substantially when soil was supplied with C and N in a laboratory experiment, confirming field observations that the microbial biomass is limited by C and N rather than P availability. Maize-legume fallow rotations result in a shift towards organic and microbial nutrients and have to be complemented by balanced additions of inorganic fertilizers. Abbreviations: BNF – biological nitrogen fixation; COM – continuous maize; LR – long rainy season; MCF – maize-crotalaria fallow rotation; MNF – maize-natural fallow rotation; SR – short rainy season; TSP – triple superphosphate.     

EVALUATION OF COMPETITIVE ABILITY OF STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA (CYANOPHYCEAE) UNDER P LIMITATION1

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. showed pronounced differences in chemical composition and ability to maintain P fluxes. The cellular P:C ratio (Q_p) and the surplus P:C ratio (Q_sp) were higher in M. aeruginosa, indicating a lower yield of biomass C per unit of P. The subsistence quota (Q_p) was 1.85 μg P·mg C^?1in S. luetkemuellerii and 6.09 μg P·mg C^?1in M. aeruginosa, whereas the respective Q_p of P saturnted organisms (Q_s) were 43 and 63 μg P·mg C^?1. These stores could support four divisions in S. luetkemuellerii and three divisions in M. aeruginosa, which suggests that the former exhibited highest storage capacity (Q_s/Q₀). M. aeruginosa showed a tenfold higher activity of alkaline phosphatase than S. luetkemuellerii when P starved. The optimum N:P ratio (by weight) was 5 in S. luetkemuellerii and 7 in M. aeruginosa. The initial uptake of P_i pulses in the organisms was not inhibited by rapid (<1 h) internal feedback mechanisms and the short term uptake rote could be expressed solely as a function of ambient P_i. The maximum cellular C-based uptake rate (V_m) in P starved M. aeruginosa was up to 50 times higher than that of S. luetkemuellerii. It decreased with increasing growth rate (P status) in the former species and remained fairly constant in the latter. The corresponding cellular P-based value (U_m= V_m/Q_p) decreased with growth rate in both species and was about 10 times higher in P started M. aeruginosa than in S. luetkemuellerii. The average half saturation constant for uptake (K_m) was equal for both species (22 μg P·L^?1) and varied with the P status. S. luetkemuellerii exhibited shifts in the uptake rate of P_i that were characterized by increased affinity (U^m/K^m) at low P_i, concentrations (<4 μg P·L^?1) compared to that at higher concentrations. The species thus was well adapted to uptake at low ambient P_i, but M. aeruginosa was superior in P_i uptake under steady state and transient conditions when the growth rate was lower than 0.75 d^?1. Moreover, M. aeruginosa was favored by pulsed addition of P_i. M. aeruginosa relpased P_i at a higher rate than S. luetkemuellerii. Leakage of P_i from the cells caused C-shaped μ vs. P_i curves. Therefore, no unique K_s for growth could be estimated. The maximum growth rate (μ_m) (23° C) was 0.94 d^?1for S. luetkemuellerii and 0.81 d^?1for M. aeruginosa. The steady state concentration of P_i (P*) was lower in M. aeruginosa than in S. luetkemuellerii at medium growth rates. The concentration of P_i at which the uptake and release of P_i was equal (P_c was, however, lower in S. luetkemuellerii.