Root exudation (net efflux of amino acids) may increase rhizodeposition under elevated CO2

Increases in atmospheric CO₂ concentration ([CO₂]) can lead to global climate change and theoretically could enhance carbon (C) deposition in soil, but data on this complex issue are contradictory. One approach for clarifying the diverse forces influencing plant‐derived C in the rhizosphere involves defining how elevated [CO₂] alters the fundamental process of C transfer from plant roots to the soil. We examine here how a step increase in [CO₂] affects the innate influx and efflux components of root exudation in axenic plants, as one foundation for understanding how climate change may affect rhizodeposition. Increasing [CO₂] from 425 to 850 μmol mol^?1 during short‐term trials enhanced shoot and root dry weight (P<0.01) of annual rye grass (Lolium multiflorum Lam.) and medic (Medicago truncatula L.) but had no effect on growth of maize (Zea mays L.). Root amino‐acid flux in the same plants changed only in maize, which increased the efflux rate (nmol g root fresh weight^?1 h^?1) of six amino acids (arginine, alanine, proline, tyrosine, lysine and leucine) significantly (P<0.05) under elevated [CO₂]. None of the three plant species altered the steady‐state concentration of 16 amino acids released into a hydroponic solution with changing [CO₂], apparently because amino‐acid influx rates, measured at 2.5 μm , consistently exceeded efflux rates. Indeed, plants recovered amino acids at rates 94–374% higher than they were lost from roots regardless of [CO₂]. These results indicate that, in theory, any effect of [CO₂] doubling on amino‐acid efflux can be offset by innately higher rates of influx. In practice, however, higher rates of amino‐acid cycling (i.e., efflux+influx) for each root segment (in C₄ maize) or from more root tissue (in the two C₃ species) should increase root exudation by plants exposed to elevated [CO₂] as additional amino acids would be adsorbed to soil particles or be taken up by soil microorganisms.     

Amino acids as a nitrogen source for tomato seedlings: The use of dual-labeled (C, N) glycine to test for direct uptake by tomato seedlings

Direct uptake of organic nitrogen (ON) compounds, rather than inorganic N, by plant roots has been hypothesized to constitute a significant pathway for plant nutrition. The aim of this study was to test whether tomatoes (Solanum lycopersicum cv. Huying932) can take up ON directly from the soil by using ¹⁵NH₄Cl, K¹⁵NO₃, 1, 2-¹³C₂¹⁵N-glycine labeling techniques. The ¹³C and ¹⁵N in the plants increased significantly indicating that a portion of the glycine-N was taken up in the form of intact amino acids by the tomatoes within 48 h after injection into the soil. Regression analysis of excess ¹³C against excess ¹⁵N showed that approximately 21% of the supplied glycine-N was taken up intact by the tomatoes. Atom% excesses of ¹⁵N and ¹³C in the roots were higher than in any shoots. Results also indicated rapid turnover of amino acids (e.g., glycine) by soil microorganisms, and the poor competitive ability of tomatoes in absorbing amino acids from the soil solution. This implies that tomatoes can take up ON in an intact form from the soil despite the rapid turnover of organic N usually found under such conditions. Given the influence of climatic change and N pollution, further studies investigating the functional ecological implications of ON in horticultural ecosystems are warranted.     

Amino acid uptake in deciduous and coniferous taiga ecosystems

We measured in situ uptake of amino acids and ammonium across deciduous and coniferous taiga forest ecosystems in interior Alaska to examine the idea that late successional (coniferous) forests rely more heavily on dissolved organic nitrogen (DON), than do early successional (deciduous) ecosystems. We traced ¹⁵N-NH₄⁺ and ¹³C-¹⁵N-amino acids from the soil solution into plant roots and soil pools over a 24 h period in stands of early successional willow and late successional black spruce. Late successional soils have much higher concentrations of amino acid in soil solution and a greater ratio of DON to dissolved inorganic N (DIN) (ammonium plus nitrate) than do early successional soils. Moreover, late successional coniferous forests exhibit higher rates of soil proteolytic activity, but lower rates of inorganic N turnover. Differences in ammonium and amino acid uptake by early successional willow stands were insignificant. By contrast, the in situ uptake of amino acid by late successional black spruce forests were approximately 4-fold greater than ammonium uptake. The relative difference in uptake of ammonium and amino acids in these forests was approximately proportional to the relative difference of these N forms in the soil solution. Thus, we suggest that differences in uptake of different N forms across succession in these boreal forests largely reflect edaphic variation in available soil N (composition), rather than any apparent physiological specialization to absorb particular forms of N. These finding are relevant to our understanding of how taiga ecosystems may respond to increases in temperature, fire frequency, N deposition, and other potential consequences of global change.     

Uptake of amino acids by plants from the soil: A comparative study with castor bean seedlings grown under natural and axenic soil conditions

Castor bean seedlings grown in different media (soil, quartz sand, or liquid culture) under natural or axenic conditions take up¹⁴C labelled proline when offered to the rooting medium at concentrations similar to those occuring in the soil. Most of the absorbed proline was transferred through the root into the xylem without metabolic conversion, though some conversion to glutamine and alamine occurred.It is concluded that roots successfully compete with microorganisms for free amino acids in the soil for the following reasons: (a) The initial rate of appearance of radioactivity in the xylem sap was the same in plants grown in natural or in axenic soil, and (b) the specific activity of proline in the xylem sap was approximately the same in plants grown in natural conditions and in axenic soil (even somewhat higher under natural condition).The role of soil microorganisms became evident however in long-term experiments (e.g. 5h), because the soil solution was much more rapidly depleted of labelled amino acids in natural soil than in axenic soil. Therefore after 20 hours roots grown in sterilized soil or quartz sand always contained more¹⁴C label than those grown in natural soil.It is suggested that viable roots use free amino acids from the soil and that the main flux of carbon to the rhizosphere might be in the form of organic acids.     

Phosphorus efficiency of wheat and sugar beet seedlings grown in soils with mainly calcium,or iron and aluminium phosphate

Phosphorus is often limiting crop growth in soils low in P supplying capacity. The objective of this study was to investigate whether there are differences in P efficiency between sugar beet and wheat and to search for the plant properties responsible for different P efficiencies encountered and furthermore to see whether the kind of P binding in soil affects the P efficiency of crops. For this a pot experiment with an Oxisol with P mainly bound to Fe and Al (Fe/Al-P) and a Luvisol with P mainly bound to Ca (Ca-P) was run with increasing P fertilizer levels from 0 to 400 mg kg^–1 in a climate chamber. Shoot dry weights of wheat and sugar beet increased strongly with P application in both soils. Both crops, despite their large differences in plant properties, had the same P efficiency in both soils. Therefore none of the species was especially able to use either Fe/Al-P or Ca-P. Wheat relied on a somewhat lower internal requirement, a large root system (high root/shoot ratio) and a low shoot growth rate with a low influx while sugar beet with a small root system and a large shoot growth rate relied on a 5 to 10 times higher influx. A mechanistic mathematical model for calculation of uptake and transport of nutrients in the rhizosphere was used to assess the influence of morphological and physiological root properties on P influx. A comparison of calculated and measured P influx showed that prediction by the model is reasonably accurate for Luvisol. For Oxisol, the predicted P influx was much less than the observed one, even when P influx by root hairs was considered. A sensitivity analysis showed that physiological uptake parameters like I _max, K _m, and C_{L min} had no major influence on predicted influx. The greatest influence on influx had the P soil solution concentration C _{L i}. It is assumed that both species had used mechanisms to increase P availability in the rhizosphere similar to an increase of C _{L i}. Such mechanisms could be the exudation of organic acids, which are known as a sorption competitor to phosphate bound to Fe/Al-oxides or humic-Fe-(Al) complexes or to build soluble complexes with Fe and P. The close agreement between calculated and measured P influx in the Luvisol even at P deficiency indicates that root exudates were not able to mobilize Ca-bound P, whereas Fe/Al-P could be mobilized easily.     

Effect of inoculation ofAzospirillum lipoferum on nitrogen fixation in rhizosphere soil,their association with root,yield and nitrogen uptake by mustard (Brassica juncea)

Summary A microplot field experiment was conducted in the presence or absence of P and N application to evaluate the influence of the seed inoculation of mustard (cv. Baruna T₅₉) withAzospirillum lipoferum on N₂-fixation in rhizosphere, association of the bacteria with the roots and grain yield and N uptake. Inoculation significantly increased the N content in rhizosphere soil particularly at early stage (40 days) of plant growth, which was accompanied by the increased association of the bacteria (A. lipoferum) in rhizosphere soil, root surface washing and surface-sterilized macerated root. A significant increase in grain yield and N uptake was also observed due to inoculation. Application of P particularly at the 20 kg. ha^–1 level further enhanced the beneficial effect ofAzospirillum lipoferum inoculation, while N addition markedly reduced such an effect.     

Uptake of amino acids by the aquatic resurrection plant Chamaegigas intrepidus and its implication for N nutrition

Chamaegigas intrepidus is a poikilohydric aquatic plant that lives in rock pools on granitic outcrops in Central Namibia. The pools are filled intermittently during the summer rains, and the plants may pass through up 20 rehydration/dehydration cycles during a single wet season. Rehydrated plants also have to cope with substantial diurnal fluctuations in the pH and extreme nutrient deficiency. Ammonium concentrations are normally around 30 μM. Additional nitrogen sources are amino acids. Total free amino acids are up to 15 μM with glycine and serine as the predominant amino acids. Experiments on uptake of radiolabelled amino acids into roots of C. intrepidus showed high␣affinity (K _M= 16 μM) and low-affinity (K _M= 159 μM) uptake systems. The K _M of the high-affinity system is well in accordance with the free amino acid concentration found in the water of the pools. We conclude that amino acids, predominantly glycine and serine, can be utilised by C. intrepidus in its natural habitat. Since glycine uptake showed a strong reduction at pH 10, nitrogen uptake from glycine or serine should occur mainly in the morning when the pH of the pool water is slightly acid. Further experiments with ¹⁵N-labelled ammonium in combination with non-labelled glycine demonstrated high ¹⁵N values in plant tissues. Under experimental conditions C. intrepidus preferred ammonium as a nitrogen source. The implication of amino acids for nitrogen nutrition of C. intrepidus may depend on the relation of inorganic and organic nitrogen available in the pool water and the preferential utilisation of one or the other nitrogen source may change during the day corresponding with pH changes in the water. Received: 28 January 1998 / Accepted: 24 July 1998     

The effect of inoculation with Azospirillum brasilense on growth and nitrogen utilization by wheat plants

Azospirillium brasilense is a rhizosphere bacteria that has been reported to improve yield when inoculated on wheat plants. However, the mechanisms through which this effect is induced is still unclear. In the present work, we have studied the effects of inoculating a highly efficient A. brasilense strain on wheat plant grown in 5 kg pots with soil in a greenhouse, under three N regimes (0, 3 or 16 mM NO₃ ^–, 50 ml/pot once or twice-a -week), and in disinfected or non-disinfected soil. At the booting stage, the inoculated roots in both soils showed a similar colonization by Azospirillum sp. that was not affected by N addition. The plants grown in the disinfected soil showed a higher biomass, N content and N concentration than those in the non-disinfected soil, and in both soils the inoculation stimulated plant growth, N accumulation, and N and NO₃ ^– concentration in the tissues.At maturity, the inoculated plants showed a higher biomass, grain yield and N content than the uninoculated ones in both soils, and a higher grain protein concentration than the uninoculated. It is concluded that in the present experiments, A. brasilenseincreased plant growth by stimulating nitrogen uptake by the roots.     

Influx,efflux and net uptake of nitrate in Quercus suber seedlings

Nitrate influx, efflux and net nitrate uptake were measured for the slow-growing Quercus suber L. (cork-oak) to estimate the N-uptake efficiency of its seedlings when grown with free access to nitrate. We hypothesise that nitrate influx, an energetically costly process, is not very efficiently controlled so as to avoid losses through efflux, because Q. suber has relatively high respiratory costs for ion uptake. Q. suber seedlings were grown in a growth room in hydroponics with 1 mM NO₃ ^-. Seedlings were labelled with ¹⁵NO₃ ^- in nutrient solution for 5 min to measure influx and for 2 h for net uptake. Efflux was calculated as the difference between influx and net uptake. Measurements were made in the morning, afternoon and night. The site of nitrate reduction was estimated from the ratio of NO₃ ^- to amino acids in the xylem sap; the observed ratio indicated that nitrate reduction occurred predominantly in the roots. Nitrate influx was always much higher than net acquisition and both tended to be lower at night. High efflux occurred both during the day and at night, although the proportion of ¹⁵NO₃ ^- taken up that was loss through efflux was proportionally higher during the night. Efflux was a significant fraction of influx. We concluded that the acquisition system is energetically inefficient under the conditions tested. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chickpea and white lupin rhizosphere carboxylates vary with soil properties and enhance phosphorus uptake

Chickpea and white lupin roots are able to exude large amounts of carboxylates, but the resulting concentrations in the rhizosphere vary widely. We grew chickpea in pots in eleven different Western Australian soils, all with low phosphorus concentrations. While final plant mass varied more than two-fold and phosphorus content almost five-fold, there were only minor changes in root morphological traits that potentially enhance phosphorus uptake (e.g., the proportion of plant mass allocated to roots, or the length of roots per unit root mass). In contrast, the concentration of carboxylates (mainly malonate, citrate and malate, extracted using a 0.2 mM CaCl₂ solution) varied ten-fold (averaging 2.3 mol g^–1 dry rhizosphere soil, approximately equivalent to a soil solution concentration of 23 mM). Plant phosphorus uptake was positively correlated with the concentration of carboxylates in the rhizosphere, and it was consistently higher in soils with a smaller capacity to sorb phosphorus. Phosphorus content was not correlated with bicarbonate-extractable phosphorus or any other single soil trait. These results suggest that exuded carboxylates increased the availability of phosphorus to the plant, however, the factors that affected root exudation rates are not known. When grown in the same six soils, three commonly used Western Australian chickpea cultivars had very similar rhizosphere carboxylate concentrations (extracted using a 0.2 mM CaCl₂ solution), suggesting that there is little genetic variation for this trait in chickpea. Variation in the concentration of carboxylates in the rhizosphere of white lupin did not parallel that of chickpea across the six soils. However, in both species the proportion of citrate decreased and that of malate increased at lower soil pH. We conclude that patterns of variation in root exudates need to be understood to optimise the use of this trait in enhancing crop phosphorus uptake.     

Interactions in the uptake of amino acids, ammonium and nitrate ions in the Arctic salt-marsh grass, Puccinellia phryganodes

The uptake of amino acids and inorganic nitrogen by roots of Puccinellia phryganodes was examined to assess the potential contribution of soluble organic nitrogen to plant nitrogen uptake in Arctic coastal marshes, where free amino acids constitute a substantial fraction of the soil‐soluble N pool. Short‐term excised root uptake experiments were performed using tillers grown hydroponically under controlled conditions in the field. The percentage reductions in ammonium uptake at moderate salinity (150 mm NaCl) compared with uptake at low salinity (50 mm NaCl) were double those of glycine, but glycine uptake was more adversely affected than ammonium uptake by low temperatures. Glycine uptake was higher at pH 5·7 than at pH 7·0 or 8·2. The glycine uptake was up‐regulated in response to glycine, whereas ammonium uptake was up‐regulated in response to ammonium starvation. Nitrate uptake was strongly down‐regulated when tillers were grown on either ammonium or glycine. In contrast to N‐starved roots, which absorbed ammonium ions more rapidly than glycine, the roots grown on glycine, ammonium and nitrate and not N‐starved prior to uptake absorbed glycine as rapidly as ammonium and nitrate ions combined. Overall, the results indicate that amino acids are probably an important source of nitrogen for P. phryganodes in Arctic coastal marshes.     

Effects of liming and mycorrhizal colonization on soil phosphate depletion and phosphate uptake by maize (Zea mays L.) and soybean (Glycine max L.) grown in two tropical acid soils

The effects of liming and inoculation with the arbuscular mycorrhizal fungus, Glomus intraradices Schenck and Smith on the uptake of phosphate (P) by maize (Zea mays L.) and soybean (Glycine max [L.] Merr.) and on depletion of inorganic phosphate fractions in rhizosphere soil (Al-P, Fe-P, and Ca-P) were studied in flat plastic containers using two acid soils, an Oxisol and an Ultisol, from Indonesia. The bulk soil pH was adjusted in both soils to 4.7, 5.6, and 6.4 by liming with different amounts of CaCO₃.In both soils, liming increased shoot dry weight, total root length, and mycorrhizal colonization of roots in the two plant species. Mycorrhizal inoculation significantly increased root dry weight in some cases, but much more markedly increased shoot dry weight and P concentration in shoot and roots, and also the calculated P uptake per unit root length. In the rhizosphere soil of mycorrhizal and non-mycorrhizal plants, the depletion of Al-P, Fe-P, and Ca-P depended in some cases on the soil pH. At all pH levels, the extent of P depletion in the rhizosphere soil was greater in mycorrhizal than in non-mycorrhizal plants. Despite these quantitative differences in exploitation of soil P, mycorrhizal roots used the same inorganic P sources as non-mycorrhizal roots. These results do not suggest that mycorrhizal roots have specific properties for P solubilization. Rather, the efficient P uptake from soil solution by the roots determines the effectiveness of the use of the different soil P sources. The results indicate also that both liming and mycorrhizal colonization are important for enhancing P uptake and plant growth in tropical acid soils.     

Waterlogging and fire impacts on nitrogen availability and utilization in a subtropical wet heathland (wallum)

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate. During waterlogging soil amino acids increased, particularly γ-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P < 0.05) ¹⁵N-enriched (0.3–4.3‰) compared to species with mycorrhizal associations (ericoid-type, ecto-, va-mycorrhizal) which were strongly depleted in ¹⁵N (-6.3 to -1.8‰). Lignotubers and roots had δ¹⁵N signatures similar to that of the leaves of respective species. The exceptions were fine roots of ecto-, ecto/va-, and ericoid type mycorrhizal species which were enriched in ¹⁵N (0.1–2.4‰). The 5¹⁵N signatures of δ¹⁵N_{total soil N} and δ¹⁵N_{soil NH4+} were in the range 3.7–4.5‰, whereas δ¹⁵N_{soil NO3?} was significantly (P < 0.05) more enriched in ¹⁵N (9.2–9.8‰). It is proposed that there is discrimination against ¹⁵N during transfer of nitrogen from fungal to plant partner. Roots of selected species incorporated nitrogen sources in the order of preference: ammonium > glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.     

Regulation of nitrogen partitioning in field-grown almond trees: Effects of fruit load and foliar nitrogen applications

Two treatments were employed to influence the amount of amino nitrogen (N) transport in phloem. In walnut trees (Juglans regia L.), developing fruit significantly reduced the efflux of foliar-applied ¹⁵N-enriched urea from treated spurs over a 33-day period in comparison with similarly-treated defruited spurs. Those data suggest that local aboveground demand for N influences vascular transport of amino N. In another experiment, a 1% urea solution was applied foliarly to 5-year old `Mission' almond trees [Prunus dulcis (Mill.) D. A. Webb] to increase the concentration of amino N in the phloem. The effect of foliar N treatments on a) the transport and distribution of labelled urea N within the trees over the experimental period and b) the uptake of soil-applied labelled N were determined by replicated whole tree excavation, fractionation into various tree components and mass spectrometric analyses of the ¹⁴N/¹⁵N ratios. Concentrations and composition of amino acids in the phloem and xylem saps of control trees and trees receiving foliar-applied urea were also determined. In foliar urea-treated trees, the amino acid concentrations increased significantly in leaf and bark phloem exudate, within 24 and 96 h, respectively. Foliar-applied urea N was translocated to the roots of almond trees over the experimental period and decreased soil N uptake. The results of these experiments are consistent with the hypothesis that aboveground N demand affects the amount of amino N cycling between shoots and roots, and may be involved in the regulation of soil N uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrate fluxes in soybean seedling roots and their response to amino acids: an approach using 15N

This study was undertaken to determine which of the two NO₃^? fluxes (influx or efflux) across plasma membranes of root cells is the target of those amino acids which have been shown to inhibit net NO₃^? uptake (Muller & Touraine 1992, Journal of Experimental Botany 43 , 617–623). Parallel experiments were performed to mea-sure either the time course of ¹⁵NO₃^? release from roots of soybean seedlings previously labelled with this isotope into non-labelled solution, or the time course of ¹⁵N accumulation from labelled ¹⁵NO₃^? solution in non-labelled seedlings. Focusing on the fate of ¹⁵NO₃^? in the cytoplasmic compartment, a model is developed to describe the time courses of the accumulation and release of tracer across the plasma membranes of root cells. Both time courses can be described by the sum of an exponential plus a linear term. In our material, the linear part of the accumulation time course is obscured by the NO₃^? fluxes exiting the cytoplasm, and the curve thus appears to be quasilinear over several minutes. However, we show that the use of the net tracer accumulation rate during this time period as an estimate of NO₃^? influx does not provide accurate estimates of influx and efflux. By contrast, ¹⁵NO₃^? efflux analysis permits calculation of the unidirectional fluxes across plasma membranes of root cells and the kinetic parameters of the cytoplasmic NO₃^? pool. Under our experimental conditions, efflux accounted for 30 to 50% of influx, and the cytoplasmic NO₃^? content was found to be in the 70–400nmol g^?1 fw range. Using this methodology, the effect of amino acid accumulation on unidirectional fluxes of nitrate was then examined. Pretreatments of the seedlings with an amino acid which has been shown to inhibit net NO₃^? uptake led to concomitant decreases in net accumulation rates of ¹⁵NO₃^? and of reduced ¹⁵N in roots and total ¹⁵N in cotyledons. NO₃^? influx was markedly inhibited by these treatments, while NO₃^? efflux remained essentially unaffected, or even decreased. It is concluded that the target of the regulation of NO₃^? uptake by phloemtranslocated amino acids is the influx system.     

Characteristics of amino acid uptake in barley

Plants have the ability to take up organic nitrogen (N) but this has not been thoroughly studied in agricultural plants. A critical question is whether agricultural plants can acquire amino acids in a soil ecosystem. The aim of this study was to characterize amino acid uptake capacity in barley (Hordeum vulgare L.) from a mixture of amino acids at concentrations relevant to field conditions. Amino acids in soil solution under barley were collected in microlysimeters. The recorded amino acid composition, 0–8.2 μM of l-Serine, l-Glutamic acid, Glycine, l-Arginine and l-Alanine, was then used as a template for uptake studies in hydroponically grown barley plants. Amino acid uptake during 2 h was studied at initial concentrations of 2–25 μM amino acids and recorded as amino acid disappearance from the incubation solution, analysed with HPLC. The uptake was verified in control experiments using several other techniques. Uptake of all five amino acids occurred at 2 μM and below. The concentration dependency of the uptake rate could be described by Michaelis–Menten kinetics. The affinity constant (K _m) was in the range 19.6–33.2 μM. These K _m values are comparable to reported values for soil micro-organisms.     

Depletion of Selenium in Soil Solution due to its Enhanced Sorption in the Rhizosphere of Soybean

The effect of plant roots on selenium (Se) mobility in soil was studied by a large-scale pot experiment in order to understand the environmental behavior of Se in agricultural soils under plant growth conditions. Soybean plants (Glycine max (L.) Merrill) were grown in a greenhouse for 84 d. The concentrations of Se and major elements (K, Ca, Mg, Na, and Al) in the soil solutions and in the plants were measured at different growth periods. Concentrations of Se and major cations in soil solution decreased as the soybean plants grew, while the concentrations of Al increased. It was assumed that the soybean roots released H⁺ with the uptake of cations; consequently, due to the acidification of the rhizosphere, Al³⁺ was released starting from the soil solid phase. The decreased Se concentration in the soil solution should be due to the enhancement of Se sorption onto the soil solid phase. The increase of Se sorption level in the rhizosphere was examined in a small-scale pot experiment. The soil–soil solution distribution coefficient of Se (K _d-Se) was observed as an index of Se sorption level. K _d-Se clearly increased in the rhizosphere soil after cultivation. The effects of pH and Al³⁺ in the rhizosphere on Se sorption were assessed by K _d-Se measurements at different levels of HCl and AlCl₃. In this third experiment, a decrease in pH increased K _d-Se values, but no specific effect was observed on Se sorption due to increased Al³⁺. These results show that the Se mobility in agricultural soil could be decreased by plant roots under plant growth conditions due to enhanced Se sorption in the rhizosphere.     

Enhanced cadmium accumulation in maize roots—the impact of organic acids

Low molecular weight organic acids are important components of root exudates and therefore, knowledge regarding the mechanisms of cadmium (Cd) uptake and distribution within plants under the influence of organic acids, is necessary for a better understanding of Cd behavior in the plant–soil system. In this study, acetic and malic acids increased the uptake of Cd by maize (Zea mays L. cv. TY2) roots and enhanced Cd accumulation in shoots under hydroponic conditions. Concentration-dependent net Cd influx in the presence and absence of organic acids could be resolved into linear and saturable components. The saturable component followed Michaelis–Menten kinetics, which indicated that Cd uptake across the plasma membrane was transporter-mediated. While the K _m values were similar, the V _max values in the presence of acetic and malic acids were respectively 6.0 and 3.0 times that of the control. Zinc transporters were the most probable pathways for Cd accumulation. It was hypothesized that Cd(II)–organic acid complexes associated with the root zone, could decompose and liberate Cd²⁺ for subsequent absorption by maize roots; and that in the layer of the roots or within the root free space, depletion of Cd²⁺ was buffered by the presence of Cd(II)–organic acid complexes. Plant response to elevated Cd levels involved overproduction of organic acids in maize roots as a resistance mechanism to alleviate Cd toxicity.     

Oryza sativa Lysine‐Histidine‐type Transporter 1 functions in root uptake and root‐to‐shoot allocation of amino acids in rice

In agricultural soils, amino acids can represent vital nitrogen (N) sources for crop growth and yield. However, the molecular mechanisms underlying amino acid uptake and allocation are poorly understood in crop plants. This study shows that rice (Oryza sativa L.) roots can acquire aspartate at soil concentration, and that japonica subspecies take up this acidic amino acid 1.5‐fold more efficiently than indica subspecies. Genetic association analyses with 68 representative japonica or indica germplasms identified rice Lysine‐Histidine‐type Transporter 1 (OsLHT1) as a candidate gene associated with the aspartate uptake trait. When expressed in yeast, OsLHT1 supported cell growth on a broad spectrum of amino acids, and effectively transported aspartate, asparagine and glutamate. OsLHT1 is localized throughout the rice root, including root hairs, epidermis, cortex and stele, and to the leaf vasculature. Knockout of OsLHT1 in japonica resulted in reduced root uptake of amino acids. Furthermore, in ¹⁵N‐amino acid‐fed mutants versus wild‐type, a higher percentage of ¹⁵N remained in roots instead of being allocated to the shoot. ¹⁵N‐ammonium uptake and subsequently the delivery of root‐synthesized amino acids to Oslht1 shoots were also significantly decreased, which was accompanied by reduced shoot growth. These results together provide evidence that OsLHT1 functions in both root uptake and root to shoot allocation of a broad spectrum of amino acids in rice.     

Regulation of nitrate uptake by amino acids in maize cell suspension culture and intact roots

The effect of amino acids on nitrate transport was studied in Zea mays cell suspension cultures and in Zea mays excised roots. The inclusion of aspartic acid, arginine, glutamine and glycine (15mM total amino acids) in a complete cell-culture media containing 1.0 mM NO₃ ^- strongly inhibited nitrate uptake and the induction of accelerated uptake rates. The nitrate uptake rate increased sharply once solution amino acid levels fell below detection limits. Glutamine alone inhibited induction in the cell suspension culture. Maize seedlings germinated and grown for 7 days in a 15 mM mixture of amino acids also had lower nitrate uptake rates than seedlings grown in 0.5 mM Ca(NO₃)₂ or 1 mM CaCl₂. As amino acids are the end product of nitrate assimilation, the results suggest an end-product feed-back mechanism for the regulation of nitrate uptake.