Root Uptake Rates Research Articles

Root Nitrogen Uptake in Wastewater-Irrigated Pepper Fields

Given the shortage of fresh water in semi-arid and arid zones, wastewater is becoming an increasingly important source of irrigation. This study analyzes the principles of root water/nitrogen uptake dynamics in wastewater-irrigated pepper fields. Comparison of pepper root uptake rates of nitrogen show that the root uptake rate of NO3-N is an order of magnitude higher than that of NH4-N. When the original NO3-N content in the soil is high, additional wastewater irrigation will not increase the root uptake of NO3-N. The pepper root water uptake rates gradually increase with increasing light intensity and air temperature

Inorganic nitrogen acquisition by the tropical seagrass Halophila stipulacea

AbstractThe tropical seagrass Halophila stipulacea is dominant in most regions of the Indo‐Pacific and the Red Sea and was introduced into the Mediterranean Sea after the opening of the Suez canal. The species is considered invasive in the Mediterranean Sea and has been progressively colonizing new areas westward. Growth and photosynthetic responses of H. stipulacea have been described but no information is yet available on the nitrogen nutrition of the species. Here we simultaneously investigated the uptake kinetics of ammonium and nitrate and the internal translocation of incorporated nitrogen in H. stipulacea using 15N‐labelled substrates across a range of Ni levels (5, 25, 50 and 100 μm). The ammonium uptake rates exceeded the nitrate uptake rates 100‐fold, revealing a limited capacity of H. stipulacea to use nitrate as an alternative nitrogen source. The uptake rates of ammonium by leaves and roots were comparable up to 100 μm 15NH4Cl. At this concentration, the leaf uptake rate was 1.4‐fold higher (6.22 ± 0.70 μmol·g−1 DW h−1) than the root uptake rate (4.54 ± 0.28 μmol·g−1 DW h−1). The uptake of ammonium followed Michaelis–Menten kinetics, whereas nitrate uptake rates were relatively constant at all nutrient concentrations. The maximum ammonium uptake rate (Vmax) and the half‐saturation constant (Km) of leaves (9.79 μmol·g−1 DW h−1 and 57.95 μm, respectively) were slightly higher than that of roots (6.09 μmol·g−1DW h−1 and 30.85 μm, respectively), whereas the affinity coefficients (α = Vmax/Km) for ammonium of leaves (0.17) and roots (0.20) were comparable, a characteristic that is unique among seagrass species. No substantial translocation (<2.5%) of 15N incorporated as ammonium was detected between plant parts, whereas the translocation of 15N incorporated as nitrate was higher (40–100%). We conclude that the Ni acquisition strategy of H. stipulacea, characterized by a similar uptake capacity and efficiency of leaves and roots, favors the geographical expansion potential of the species into areas with variable water‐sediment N levels throughout the Mediterranean.

Dynamics of root water uptake and water use efficiency under alternate partial root-zone irrigation

The distribution of root length density of maize in ridge culture under alternate partial root-zone furrow irrigation (APFI) and conventional furrow irrigation (CFI) was investigated, and two-dimensional models of root length density and root uptake were developed to investigate the water use efficiencies under different furrow irrigation modes. Results indicated that APFI increased root depth and horizontal extended distance of maize, as well as water use efficiency (WUE). Values of the mean absolute error (MAE), the root mean square error (RMSE), and the index of agreement (di) between measured and simulated values of root length density of maize under APFI ranged from 0.26 to 0.68, 0.03 to 1.51, and 0.64 to 0.85 cm m−3, respectively; and the coefficient of determination (R2) was greater than 0.80. Values of MAE, RMSE, and di between measured and simulated values of root length density of maize under CFI ranged from 0.01 to 0.26, 0.01 to 0.60, and 0.61 to 0.96 cm m−3, respectively; and R2 was higher than 0.86. Dynamics of root uptake at different sites were simulated using the model of root water uptake. Root growth of maize under furrow irrigation was modeled using the 2D model of root length density with reasonable accuracy. Compared with CFI, water consumption rate of the maize in APFI was lower in key water requirement period, root uptake rate was higher in the end period, and WUE in APFI was higher by 5% than that in CFI.

Allocation of new growth between shoot, root and mycorrhiza in relation to carbon, nitrogen and phosphate supply: Teleonomy with maximum growth rate

Treating resource allocation within plants, and between plants and associated organisms, is essential for plant, crop and ecosystem modelling. However, it is still an unresolved issue. It is also important to consider quantitatively when it is efficient and to what extent a plant can invest profitably in a mycorrhizal association. A teleonomic model is used to address these issues. A six state-variable model giving exponential growth is constructed. This represents carbon (C), nitrogen (N) and phosphorus (P) substrates with structure in shoot, root and mycorrhiza. The shoot is responsible for uptake of substrate C, the root for substrates N and P, and the mycorrhiza also for substrates N and P. A teleonomic goal, maximizing proportional growth rate, is solved analytically for the allocation fractions. Expressions allocating new dry matter to shoot, root and mycorrhiza are derived which maximize growth rate. These demonstrate several key intuitive phenomena concerning resource sharing between plant components and associated mycorrhizae. For instance, if root uptake rate for phosphorus is equal to that achievable by mycorrhiza and without detriment to root uptake rate for nitrogen, then this gives a faster growing mycorrhizal-free plant. However, if root phosphorus uptake is below that achievable by mycorrhiza, then a mycorrhizal association may be a preferred strategy. The approach offers a methodology for introducing resource sharing between species into ecosystem models. Applying teleonomy may provide a valuable short-term means of modelling allocation, avoiding the circularity of empirical models, and circumventing the complexities and uncertainties inherent in mechanistic approaches. However it is subjective and brings certain irreducible difficulties with it.

A Push–Pull Test To Measure Root Uptake of Volatile Chemicals from Wetland Soils

This paper introduces a novel modification of the single-well "push-pull" test that uses nonvolatile and multiple volatile tracers to investigate the transport and root uptake kinetics of volatile chemicals in saturated soils. This technique provides an estimate of potential volatilization fluxes without relying on enclosure-based measurements. The new push-pull methodology was validated with mesocosm experiments, and bench-scale hydroponic measurements were performed to develop an empirical relationship for scaling root uptake rates between chemicals. A new modeling approach to interpret data using sulfur hexafluoride and helium as dual volatile tracers was developed and shown to decrease errors relative to existing analytical techniques that utilize bromide as a conservative tracer. Root uptake of the volatile tracers was diffusion-limited, and uptake rate constants (kv) in vegetated experimental mesocosms ranged from 0.021 ± 9.0 × 10(-4) h(-1) for CFC-12 to 2.41 ± 0.98 h(-1) for helium. Hydroponic and mesocosm experiments demonstrate that the molecular diameter is a robust empirical predictor of kv.

Arsenic-induced morphogenic response in roots of arsenic hyperaccumulator fern Pteris vittata

On the assumption that arsenic induces stress morphogenetic responses involved in As tolerance and hyperaccumulation in the Pteris vittata fern, we analyzed the root system of young sporophytes grown in 250, 334, and 500μM As for five days and for 14 days. Anatomical and histological analyses were performed in plants grown for five days to evaluate the number, position, length and differentiation pattern of root hairs. AgNOR staining, employed to study nucleolus behavior in root apices, showed that arsenic influences nucleolar activity (evaluated by nucleolus size, number and absorbance) in the root meristem. In plants treated with 250 and 334μM As an acropetal shift of root hair development and an increase in hair length and density were observed, linked to an ectopic pattern of differentiation. The opposite trend was recorded in plants treated with 500μM As. It is worth noting the presence of living border-like cells, not yet observed in ferns, and their increase following As treatments. Analysis and vitality of border-like cells were surveyed after 14 days of treatments. In conclusion As treatments elicited a stress-induced morphogenic response which, by modifying the differentiation pattern, number and length of root hairs, modulating nucleolar activity and interacting with the rhizosphere by inducing border-like cell production, may adjust the rate of root uptake and its metabolic activity.

Nutrient Uptake and Fiber Yield of Abaca (Musa textilis var. Laylay) as Affected by Shade, Irrigation and Fertilizer Application

Abaca, being a shallow rooted plant and a gross feeder, is able to exploit a limited zone soil. Hence, a careful evaluation of its nutrient uptake is needed, particularly under reduced light condition, irrigation, and NPK fertilization. These field trials were performed to investigate the effect of different shade conditions, irrigation, and fertilizer application on NPK plant uptake and fiber yield of abaca. Light infiltration was reduced by 30%, 40%, and 50% of full sunlight using polypropylene shade nets. Irrigation was applied at a rate of 5 liters plant” application” day”. Placement application of N, P2O5 , K2O using complete fertilizer was done at 14 grams plant-1 quarter-1 for the first six months and was increased to 40 grams plant-1 quarter-1 for the next six months after planting. Results showed that shade (p ≤ 0.01) and irrigation- fertilization (p ≤ 0.05) significantly influenced NPK plant uptake, root and leaf uptake rates of abaca from seedling stage until flagleaf stage. The amount of NPK absorbed was proportional to the amount of growth made as influenced by shade and irrigation- fertilization at different stages of plant growth. The abaca grown in 0% shade was negatively affected by high radiation causing photoinhibition and photooxidative damage of the crop at seedling and early vegetative stages that significantly affected NPK uptake rates and fiber yield. The combination of irrigation and fertilization could further enhance fiber yield to as much as 41% but this was not enough to offset the effects of shade on the growth performance and NPK plant uptake of abaca which significantly (p < 0.01) increased fiber yield to as much as 165%.

Variability of nitrogen uptake and assimilation among N-efficient and N-inefficient wheat (Triticum aestivum L.) genotypes

Ten genotypes of wheat (Triticum aestivum L.) were grown for 20 days in complete nutrient solution with 1 mM (nitrogen-insufficient), 4 mM (nitrogen-moderate) and 10 mM (nitrogen-sufficient) nitrogen (N) levels, and nitrogen efficiency (NE) was analyzed. Of these 10 genotypes, VL829 was identified as the most N-efficient, while HUW234 the most N-inefficient. To find out the physiological basis of this difference, we investigated the possible role of influx across the root cell plasma membrane in conferring NE by measuring the short-term uptake in selected 10 genotypes. Uptake experiments revealed the presence of two separate nitrate transporter systems mediating high- and low-affinity nitrate uptake. Interestingly, the nitrate uptake by the roots of VL829 is mediated by both high- and low-affinity nitrate transporter systems, while that of HUW234 by only low affinity nitrate transporter system. The study suggests that root uptake rate may play an important role in conferring N- efficiency of wheat genotypes under N-limiting condition. Also the activities and expression levels of nitrate assimilatory enzymes in N-efficient and N-inefficient wheat genotypes showed that nitrate reductase (NR) and glutamine synthetase (GS) play important roles in N assimilation under low-nitrogen conditions.

Inorganic nitrogen uptake kinetics and whole-plant nitrogen budget in the seagrass Zostera noltii

The uptake rates of ammonium and nitrate through the leaves and roots, the leaf–root interactions in the nitrogen uptake and the internal translocation of incorporated nitrogen were simultaneously investigated in the seagrass Zostera noltii. Leaf and root uptake rates, which were measured using two-compartment polyethylene chambers that physically separated the leaves from below-ground plant parts, were quantified based on tissue incorporation of 15N-labeled ammonium and nitrate. The maximum leaf uptake rates (V max) of ammonium were 100 times higher than those of nitrate. Both V max and affinity for ammonium were one order of magnitude higher in the leaves (28.3–31.9 μmol g −1 DW h −1 and 0.93–0.99, respectively) than in the roots (2.3–3.0 μmol g −1 DW h −1 and 0.06–0.08, respectively). The uptake of ammonium and nitrate by one plant part did not affect the uptake of the other plant part, and no translocation of inorganic nitrogen was detected between plant parts. The 15N enrichment detected in the rhizomes suggests either a direct uptake of inorganic nitrogen or its transference from the roots. The estimated total inorganic nitrogen uptake of Z. noltii (645 μmol m −2 h −1) in the peak production season (spring) under typical nutrient concentrations, using the rates obtained during the surge uptake phase, exceeded by 3-fold the species estimated nitrogen requirement for growth (236 μmol N m −2 h −1). However, using the stabilized values of the uptake rates obtained after several hours of incubation, the estimated whole-plant nitrogen budget (215 μmol m −2 h −1) was slightly lower than the total nitrogen requirement for growth. We conclude that the growth of Z. noltii in Ria Formosa lagoon is not limited, or is only slightly limited, by nitrogen.

MODELING GROWTH AND ION CONCENTRATION OF LILIUM IN RESPONSE TO NITROGEN:POTASSIUM:CALCIUM MIXTURE SOLUTIONS

Nutrient solution composition plays an important role in root uptake rate due to interactions among nutrients and internal regulation. Studies to determine the optimum nutrient solution concentration are focused on individual ions, ignoring the adaptation mechanisms triggered by plants when growing in a varying external nutrient concentration. The objective of the present study was to determine the response in growth and tissue ion concentration of lilium cv. ‘Navona’ to nutrient mixtures of varying proportions of nitrogen (N), potassium (K+), and calcium (Ca2+) in solution using mixture experiments methodology in order to determine the optimum concentration. Bulbs of lilium were transplanted in plastic crates and drip-irrigated with the treatment solutions, which consisted of a mixture of N, K+, and Ca2+ whose total concentration was 340 mg L−1 and minimum concentrations of each ion was 34 mg L−1. Chlorophyll concentration (SPAD), shoot fresh weight (FW), leaf FW, and leaf area were measured 60 days after transplanting and ion analysis was performed on shoot tissues from selected treatments. Lilium exhibited a moderate demand for N and K+ (136–170 mg L−1 N and 116–136 mg L−1 K+) and a very low demand for Ca2+ (34–88 mg · L−1). This low demand may be due to the remobilization of the nutrients stored in the bulbs. Integrating the predictions of the models estimated to produce >90% of maximum growth, the optimum nutrient solution should contain Ca2+ at a concentration between 34 and 126 mg · L−1, K+ between 119 and 211 mg · L−1, and N between 92 mg · L−1 and 211 mg · L−1. Increasing external N concentration affected internal N concentration but not internal K+ or Ca2+ concentrations, despite that the increase in external N was associated with a decrease in external K+ and Ca2+. Similar trends were observed for external K+ and Ca2+ concentration. In conclusion, lilium was able to maintain a relatively constant K+ and Ca2+ concentration regardless of the lower concentration in the nutrient solution when N was increased (similar response was observed for K+ and Ca2+) and it has a low Ca2+ demand and moderate N and K+ supply.

Soil moisture depletion under simulated drought in the Amazon: impacts on deep root uptake

*Deep root water uptake in tropical Amazonian forests has been a major discovery during the last 15 yr. However, the effects of extended droughts, which may increase with climate change, on deep soil moisture utilization remain uncertain. *The current study utilized a 1999-2005 record of volumetric water content (VWC) under a throughfall exclusion experiment to calibrate a one-dimensional model of the hydrologic system to estimate VWC, and to quantify the rate of root uptake through 11.5 m of soil. *Simulations with root uptake compensation had a relative root mean square error (RRMSE) of 11% at 0-40 cm and < 5% at 350-1150 cm. The simulated contribution of deep root uptake under the control was c. 20% of water demand from 250 to 550 cm and c. 10% from 550 to 1150 cm. Furthermore, in years 2 (2001) and 3 (2002) of throughfall exclusion, deep root uptake increased as soil moisture was available but then declined to near zero in deep layers in 2003 and 2004. *Deep root uptake was limited despite high VWC (i.e. > 0.30 cm(3) cm(-3)). This limitation may partly be attributable to high residual water contents (theta(r)) in these high-clay (70-90%) soils or due to high soil-to-root resistance. The ability of deep roots and soils to contribute increasing amounts of water with extended drought will be limited.

Uptake and resource allocation of ammonium and nitrate in temperate seagrasses Posidonia and Amphibolis

Ecologically relevant estimates of seasonal variability in nitrogen uptake and allocation in two species of temperate seagrasses were obtained using in situ isotope-labelling approach. Significantly higher uptake rates of ammonium by leaves, roots and epiphytes of Amphibolis than Posidonia were observed. Overall, root uptake rates were lower than other components. Effect of season was not significant for leaves, roots or epiphytes of the two species. However, plankton uptake varied seasonally with higher rates in winter (0.98mgNg−1DWh−1). In contrast, nitrate uptake rates for various components were significantly affected by seasons. Uptake rates by plankton were highest ranging from 0.003mgNg−1DWh−1 (summer, Amphibolis) to 0.69mgNg−1DWh−1 (winter, Posidonia). Uptake of nitrate by roots was negligible. Biotic uptake rates for nitrate were an order of magnitude slower than ammonium, demonstrating an affinity for ammonium over nitrate as a preferred inorganic nitrogen source. Adelaide coastal waters have lost over 5000ha of seagrasses, much of this attributed to nutrient inputs from wastewater, industrial and stormwater. Managing these inputs into future requires better understanding of the fate of nutrients, particularly biological uptake. This study attempts to quantify uptake rates of nitrogen by seagrasses.

Morphological and physiological variation among seagrass (Zostera marina) genotypes

Intraspecific variation in habitat-forming species can have important ecological consequences at the population, community, and ecosystem level. However, the contribution of genetic variation among individuals to these effects is seldom documented. We quantified morphological and physiological variation among genotypes of a marine foundation species, the seagrass Zostera marina. We grew replicate shoots of eight genetically distinct Zostera individuals collected from Bodega Bay, California, in a common garden environment and then quantified shoot production and morphology, nutrient uptake, and key photosynthetic parameters. We found that genotypes differed in shoot production, biomass, and both root and shoot nutrient uptake rates, even when corrected for genotype-specific biomass differences. In addition, the rank order of uptake ability differed for ammonium and nitrate, indicating that genotypes may exhibit resource partitioning of different forms of nutrients. Our results suggest that both niche complementarity among genotypes and the sampling/selection effect could contribute to previously observed positive effects of seagrass clonal diversity on resource utilization and biomass production. Further, they highlight that genotypic variation in key traits of habitat-forming species could have measurable effects on community structure and function.

Root uptake of cationic amino acids by Arabidopsis depends on functional expression of amino acid permease 5

* Specific transporters mediate uptake of amino acids by plant roots. Earlier studies have indicated that the lysine histidine transporter 1 and amino acid permease 1 participate in this process, but although plant roots have been shown to absorb cationic amino acids with high affinity, neither of these transporters seems to mediate transport of L-arginine (L-Arg) or L-lysine (L-Lys). * Here, a collection of T-DNA knockout mutants were screened for alterations in Arabidopsis root uptake rates of L-Arg and it was found that only the AAP5 mutant displayed clear phenotypic divergence on high concentrations of L-Arg. A second screen using low concentrations of (15)N-labelled L-Arg in the growth media also identified AAP5 as being involved in L-Arg acquisition. * Momentaneous root uptake of basic amino acids was strongly affected in AAP5 mutant lines, but their uptake of other types of amino acids was only marginally affected. Comparisons of the root uptake characteristics of AAP5 and LHT1 mutants corroborated the hypothesis that the two transporters have distinct affinity spectra in planta. * Root uptake of all tested amino acids, except L-aspartic acid (L-Asp), was significantly affected in double AAP5*LHT1 mutants, suggesting that these two transporters account for a major proportion of roots' uptake of amino acids at low concentrations.

Enhanced root-to-shoot translocation of cadmium in the hyperaccumulating ecotype of Sedum alfredii.

Sedum alfredii (Crasulaceae) is the only known Cd-hyperaccumulating species that are not in the Brassica family; the mechanism of Cd hyperaccumulation in this plant is, however, little understood. Here, a combination of radioactive techniques, metabolic inhibitors, and fluorescence imaging was used to contrast Cd uptake and translocation between a hyperaccumulating ecotype (HE) and a non-hyperaccumulating ecotype (NHE) of S. alfredii. The Km of 109Cd influx into roots was similar in both ecotypes, while the Vmax was 2-fold higher in the HE. Significant inhibition of Cd uptake by low temperature or metabolic inhibitors was observed in the HE, whereas the effect was less pronounced in the NHE. 109Cd influx into roots was also significantly decreased by high Ca in both ecotypes. The rate of root-to-shoot translocation of 109Cd in the HE was >10 times higher when compared with the NHE, and shoots of the HE accumulated dramatically higher 109Cd concentrations those of the NHE. The addition of the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP) resulted in a significant reduction in Cd contents in the shoots of the HE, and in the roots of the NHE. Cd was distributed preferentially to the root cylinder of the HE but not the NHE, and there was a 3–5 times higher Cd concentration in xylem sap of the HE in contrast to the NHE. These results illustrate that a greatly enhanced rate of root-to-shoot translocation, possibly as a result of enhanced xylem loading, rather than differences in the rate of root uptake, was the pivotal process expressed in the Cd hyperaccumulator HE S. alfredii.

Do oaks have different strategies for uptake of N, K and P depending on soil depth?

The uptake of nutrients from deep soil layers has been shown to be important for the long-term nutrient sustainability of forest soils. When modelling nutrient uptake in forest ecosystems, the nutrient uptake capacity of trees is usually defined by the root distribution. However, this leads to the assumption that roots at different soil depths have the same capacity to take up nutrients. To investigate if roots located at different soil depths differ in their nutrient uptake capacity, here defined as the nutrient uptake rate under standardized conditions, a bioassay was performed on excised roots (<1 mm) of eight oak trees (Quercus robur L.). The results showed that the root uptake rate of 86Rb+ (used as an analogue for K+) declined with increasing soil depth, and the same trend was found for \( {\text{NH}}^{ + }_{4} \). The root uptake rate of \( {\text{H}}_{2} {\text{PO}}^{ - }_{4} \), on the other hand, did not decrease with soil depth. These different physiological responses in relation to soil depth indicate differences in the oak roots, and suggest that fine roots in shallow soil layers may be specialized in taking up nutrients such as K+ and \( {\text{NH}}^{ + }_{4} \) which have a high availability in these layers, while oak roots in deep soil layers are specialized in taking up other resources, such as P, which may have a high availability in deep soil layers. Regardless of the cause of the difference in uptake trends for the various nutrients, these differences have consequences for the modelling of the soil nutrient pool beneath oak trees and raise the question of whether roots can be treated uniformly, as has previously been done in forest ecosystem models.

Responses in nutrient uptake in rice roots to infestation of brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae)

This study investigates the uptake of nitrogen (N), phosphorus (P), and potassium (K) by rice roots as a response to infestation by the rice brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae) at two developmental stages (i.e. beginning-heading and grain-filling) of two rice varieties, Zhendao 2 (japonics rice) and Xieyou 63 (hybrid indica rice), respectively. The results show that the impact of N. lugens infestation at the beginning-heading stage on N, P, and K uptake was not only more serious, but also that the recovery rate of root uptake after the removal of N. lugens was slower, as compared to the grain-filling stage. The percent reduction of N uptake following N. lugens infestations at the beginning-heading stage of Xieyou 63 was significantly greater in comparison to Zhendao 2 at the same stage. There was no significant difference in N uptake between two developmental stages, but P and K uptake by roots following N. lugens infestation were significantly different between the two developmental stages. The maximum reduction of K uptake was shown at the beginning-heading stage in both rice varieties. The recovery rate of N, P, and K uptake by roots after the removal of N. lugens from rice plants varied with the rice variety, developmental stage of rice and infestation duration. In Xieyou 63, the recovery rate of K uptake by roots of the plants infested at the beginning-heading stage was slower than that at the grain-filling stage. Plants of Zhendao 2 infested at the two developmental stages had showed slower recovery rate of K uptake compared with N and P uptakes. These findings partially explain the virulence of N. lugens feeding on rice plants, and thus may have important implications for integrated management of N. lugens.

Boron response in wheat is genotype-dependent and related to boron uptake, translocation, allocation, plant phenological development and growth rate.

Wheat genotypes often differ significantly in their response to low and high boron (B) supply, although the underlying mechanisms for such differences are poorly understood. The stable isotopes 10B and 11B were used to investigate the contribution of root retention, uptake rates, translocation and allocation of B within wheat (Triticum aestivum L.) genotypes known to differ in B response. At high B supply, the tolerant GREEK had reduced B concentrations in main shoot leaves associated with lower uptake rates and increased allocation of B to tillers. The equally tolerant BT-SCHOMBURGK exhibited high uptake rates, but accumulation was low because of rapid development, lower concentrations of soluble B in the cell sap and lower B translocation to the shoot. In WlMMC, high uptake rates, slow development, high translocation and allocation to main shoots resulted in high B accumulation and poor tolerance. Retention in roots was not substantial in any of the genotypes. The results suggest that B tolerance is multi-faceted and genotype specific. Mechanisms contributing to B tolerance include reduced uptake rates and differential translocation and allocation within plants. Additionally, plant growth rate and leaf morphology can influence B response by affecting tissue concentrations and allowing completion of plant maturation before B accumulation impairs growth. These mechanisms are expressed to different extents depending on the genotype.

Solute transport and extraction by a single root in unsaturated soils: model development and experiment

A contaminant transport model was developed to simulate the fate and transport of organic compounds such as TNT (2,4,6-trinitrotoluene), using the single-root system. Onions were planted for this system with 50-ml plastic tubes. Mass in the soil, soil solution, root and leaf was monitored using 14C-TNT. Model parameters were acquired from the experiments in the single-root system and were used to simulate total TNT concentration in soil, providing the average concentrations in the rhizosphere and bulk soil as well as root and leaf compartments. Because the existing RCF (root concentration factor) and TSCF (transpiration stream concentration factor) equations based on log K ow (octanol–water partition coefficient) were not correlated to TNT uptake, a new term, root uptake rate ( R ur), and a new T scf equation, based on the experimental data, were introduced in the proposed model. The results from both modeling and experimental studies showed higher concentrations of TNT in the rhizosphere than in the bulk soil, because mass transported from the surrounding soil into the rhizosphere was higher than that by root uptake.

Control of salt transport from roots to shoots of wheat in saline soil.

Wheat genotypes with 5-fold difference in shoot Na+ concentrations were studied over a salinity range of 1-150 mm NaCl and CaCl2 of 0.5-10 mm to assess their performance in saline and sodic soils. All genotypes had a maximum shoot Na+ concentration at 50 mm external NaCl when the supplemental Ca2+ provided an activity of 1 mm or more. Shoot Na+ concentrations either stayed constant from 50 to 150 mm external NaCl, or decreased in some genotypes at the higher salinity. Calculated rates of root uptake, and root : shoot transport, were at a maximum at 50 mm NaCl in all genotypes, and decreased at higher NaCl in some genotypes, indicating feedback regulation. K+ showed a pattern inverse to that of Na+. Cl- uptake and transport rates increased linearly with increasing salinity, and differed little between genotypes. Increasing external Ca2+ concentration reduced the accumulation of Na+ in the shoot, the effects being greater in the low Na+ genotypes, and greater as the salinity increased, indicating that the plateau in shoot Na+ concentration relied on the maintenance of a minimal Ca2+ activity of 1 mm. Increasing external Ca2+concentration did not reduce the root Na+ concentration, however, suggesting that Ca2+ influenced the loading of Na+ in the xylem.