Influence of L-tryptophan and auxins applied to the rhizosphere on the vegetative growth of Zea mays L.

Glasshouse experiments were conducted to evaluate the influence of L-TRP in comparison with indole-3-acetamide (IAM), tryptophol (TOL) and indole-3-acetic acid (IAA) on the growth of Zea mays L. var. Early Sunglow. L-TRP (25 to 2.5×10^–5 mg kg^–1 soil), IAM (22 to 2.2×10^–5 mg kg^–1 soil), TOL (20 to 2.0×10^–5 mg kg^–1 soil), and IAA (22 to 2.2×10^–5 mg kg^–1 soil) were applied as a soil drench to established uniform seedlings. All treatments were applied in a completely randomized design with 10 replicates. IAM had no significant effect on the plant growth parameters. Shoot height, uppermost leaf collar base distance, internodal distance, and shoot dry and fresh weights were significantly improved upon the addition of TOL (2.0×10^–2 mg kg^–1 soil), however, the highest concentration (20 mg kg^–1 soil) caused a 14.6% reduction in leaf width. L-TRP (2.5×10^–3 mg kg^-1 soil) also had a significant influence on shoot height, uppermost leaf collar base distance, internodal distance and fresh weight of shoot compared with the control. The highest concentration of L-TRP (25=mg kg^–1 soil) had a negative effect on leaf width and dry weight of the shoot. The most pronounced response on the corn growth parameters was observed with the application of IAA at lower concentrations (2.2×10^–5 to 2.2×10^–2 mg kg^–1 soil) specifically improving root growth. The highest concentration (22 mg kg^–1 soil) of IAA had a significant negative effect on plant height, leaf width, stem diameter, shoot fresh and dry weight. These findings indicate that L-TRP applied at the appropriate concentrations can have positive effects on corn growth comparable to pure auxins (TOL and IAA).     

L-Tryptophan-dependent biosynthesis of indole-3-acetic acid (IAA) improves plant growth and colonization of maize by <Emphasis Type="BoldItalic">Burkholderia phytofirmans</Emphasis> PsJN

Burkholderia phytofirmans PsJN is a well-known plant growth-promoting bacterium that establishes rhizospheric and endophytic colonization in different plants. PsJN inoculation promotes growth of different horticultural crops. L-Tryptophan (L-TRP) application may further improve its effectiveness, due to substrate (L-TRP)-dependent inoculum (PsJN)-derived auxins in the rhizosphere. In the present study, the substrate (L-TRP)-dependent response of PsJN inoculation to maize growth and auxin biosynthesis was evaluated under pot conditions. In vitro auxin biosynthesis by PsJN was determined in the absence and presence of L-TRP, a physiological precursor of auxins. Surface-disinfected seeds were treated with peat-based inoculum and L-TRP solutions (10^?4 and 10^?5 M). Results revealed that L-TRP application and PsJN inoculation, when applied separately, significantly increased the growth parameters of maize compared to untreated control. However, PsJN inoculation supplemented with L-TRP (10^?5 M) gave the most promising results and significantly increased plant height, photosynthesis, chlorophyll content, root biomass and shoot biomass up to 18, 16, 45, 62 and 55 %, respectively, compared to the uninoculated control. Similarly, higher values of N, P and IAA content were observed with precursor (L-TRP)–inoculum (PsJN) interaction. The inoculant strain efficiently colonized maize seedlings and was recovered from the rhizosphere, root and shoot of plants. The results imply that substrate (L-TRP)-derived IAA biosynthesis in the rhizosphere by PsJN inoculation could be a useful approach for improving the growth, photosynthesis and nutrient content of maize plants.     

Assimilation of exogenous 2′-14C-indole-3-acetic acid and 3′-14C-tryptophan exposed to the roots of three wheat varieties

This study was conducted to determine if plants can assimilate indole-3-acetic acid (IAA) from rooting media and if exogenous L-tryptophan (L-TRP) can be assimilated and converted by plants into auxins. The addition of 2-¹⁴C-IAA (3.7 kBq plant^-1) to wheat (Triticum aestivum L.) seedlings of three varieties grown in nutrient solution resulted in the uptake (avg.=7.6%) of labelled IAA. Most of the label IAA was recovered in the shoot (avg.=7.2%) with little accumulation in the root (avg.=0.43%). A portion of the assimilated IAA-label in the plant was identified by co-chromatography and UV spectral confirmation as IAA-glycine and IAA-aspartic acid conjugates. Little of the assimilated IAA label was found as free IAA in the wheat plants. These same assimilation patterns were observed when 2-¹⁴C-IAA was added to wheat plants grown in sterile and nonsterile soil. In contrast, the wheat varieties assimilated considerably less (avg.=1.3%) of the added microbial IAA precursor, 3-¹⁴C-L-TRP (3.7 kBq plant^-1) and thus much lower amounts of IAA conjugates were detected. Glasshouse soil experiments revealed that 2 out of 3 wheat varieties had increased growth rates and increased yields when L-TRP (10^-5 and 10^-7 M) was added to the root zone. It is surmised that this positive response is a result of microbial auxin production within the rhizosphere upon the addition of the precursor, L-TRP. The amino acid composition of the root exudates plays a critical role in microbial production of auxins in the rhizosphere. This study showed that wheat roots can assimilate IAA from their rooting media, which will supplement the endogenous IAA levels in the shoot tissue and may positively influence plant growth and subsequent yield.     

Enhanced suppression of plant growth through production of L-tryptophan-derived compounds by deleterious rhizobacteria

Plant-growth-suppressive activity of deleterious rhizobacteria (DRB) may be due to production of metabolites absorbed through roots. Auxins produced in high concentrations in the rhizosphere by DRB contribute to reduced root growth. Selected DRB able to produce excessive amounts of auxin compounds for suppression of weed seedling growth may be effective for biological control of weeds. The objectives to this study were to assess the ability of DRB originating from weed seedlings to synthesize auxins from L-tryptophan (L-TRP), determine effects of DRB with or without L-TRP on seedling root growth, and characterize auxins produced from L-TRP using high performance liquid chromatography (HPLC). Auxins expressed as indole-3-acetic acid (IAA)-equivalents were produced by 22.8% of the DRB tested based on a colorimetric method. Under laboratory conditions, a DRB isolate classified as Enterobacter taylorae with high auxin-producing potential (72 mg L^–1 IAA-equivalents) inhibited root growth of field bindweed (Convolvulus arvensis L.) by 90.5% when combined with 10^–5 M L-TRP compared with non-treated control. Auxin derivatives produced by E. taylorae from L-TRP in broth culture after 24 h incubation identified by HPLC included IAA (102 g L^–1), indole-3-aldehyde (IALD; 0.4 g L^–1), and indole-3-lactic acid (ILA; 7.6 g L^–1). Results suggest that providing L-TRP with selected auxin-producing DRB to increase phytotoxic activity against emerging weed seedlings may be a practical biological control strategy.     

Response of Raphanus sativus to the auxin precursor,L-tryptophan applied to soil

Soil microorganisms are capable of producing auxins in the presence of the physiological precursor, L-tryptophan (L-TRP). This study was designed to assess the influence of L-TRP on radish (Raphanus sativus) yield when applied to soil. The amount of L-TRP added to soil to give optimum radish growth in glasshouse studies was 3.0 mg kg^-1 soil which enhanced the root yield by 1.31-fold over the control. The root/shoot ratio was increased by 1.10-fold upon this amendment. One L-TRP application was sufficient to promote growth. The best time to apply L-TRP was at the onset of seedling emergence. The application of L-TRP promoted radish yield comparable to those plants treated with indole-3-acetic acid, indole-3-acetamide and indole-3-lactic acid. Foliar application of L-TRP had no effect on the root and shoot dry weight. A field study was conducted in which L-TRP applications at a rate of 20.4 and 204 mg m^-2 significantly enhanced the radish yield in fertilized plots receiving fertilization. The shoot dry weight was increased by 1.29-fold and the root dry weight by 1.15-fold over the control in response to 20.4 mg L-TRP m^-2. These findings indicate that L-TRP, applied at the appropriate times and concentrations, can increase radish yield. The effect of L-TRP on radish growth could be attributed to i) substrate-dependent auxin production in soil by the indigenous microflora, ii) uptake directly by plant roots followed by metabolism within their tissues, and/or iii) a change in the balance of rhizosphere microflora affecting plant growth.     

In vitro binding of auxin to particulate fractions from the shoots of dark-grown wheat seedlings

A binding site for auxins was found in the 50,000g pellet from a homogenate of shoots from dark-grown wheat seedlings. The optimum conditions for the binding of native auxin, IAA, were within the range of physiological conditions of growth (pH 5.2, temperature 20° C). The binding site displayed a high affinity to IAA (affinity constant about 10⁷ M ^–1, i.e. dissociation constant about 10^–8 M) and low capacity, 60 p mol per 1 g of fresh weight. The binding capacity of 3.5-days-old shoots is represented by about 56% and 44% of that of leaves and coleoptiles, respectively. The more rapidly growing leaves also contained more endogenous free IAA (64%) than the coleoptiles from the same seedlings (36%). The binding site was very specific, distinguishing well between strong auxins and structurally related substances which exhibit very weak auxin activity. These physiological properties of this binding site indicate that it may have a certain role in the regulation of physiological processes, such as elongation growth and cell division.     

Auxin carriers in Cucurbita vesicles

Carrier-mediated uptake of indole-3-acetic acid (IAA) by microsomal vesicles from Cucurbita pepo L. hypocotyls was strongly inhibited by 2,4-dichlorophenoxyacetic acid (2,4-D; i ₅₀= 0.3 M) but only weakly by 1-naphthylacetic acid (NAA). The fully ionised auxin indol-3-yl methanesulphonic acid also inhibited (i ₅₀=3 M). The same affinity ranking of these auxins for the uptake carrier, an electroimpelled auxin anion-H⁺ symport, is demonstrable in hypocotyl segments. The specificity of the auxin-anion eflux carrier was tested by the ability of different nonradioactive auxins to compete with [³H]IAA and reduce the stimulation of net radioactive uptake by N-1-naphthylphthalamic acid (NPA), a noncompetitive inhibitor of this carrier. By this criterion, NAA and IAA had comparable affinities, with 2,4-D interaction more weakly. Stimulation of [³H]IAA uptake by NAA, as a result of competition for the efflux carrier, could also be demonstrated when a suitable concentration of 2,4-D was used selectively to inhibit the uptake carrier. However, when [³H]NAA was used, no stimulation of its association with vesicles by NPA, 2,3,5-triiodobenzoic acid, or nonradioactive NAA was found. In hypocotyl segments, [³H]NAA net uptake was much less sensitive to NPA stimulation than was [¹⁴C]IAA uptake. The apparent contradictions concerning NAA could be explained by carrier-mediated auxin efflux making a smaller relative contribution to the overall transport of NAA than of IAA. The relationship between carrier specificity as manifested in vitro and the specificity of polar auxin transport is discussed.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - ION3 mixture of 4 M carbonylcyanide m-chlorophenylhydrazone, nigericin and valinomycin - IMS indol-3-yl methanesulphonic acid - NAA 1-naphthylacetic aci - NPA N-1-naphthylphthalamic acid     

The specificity of auxin transport in intact pea seedlings (Pisum sativum L.)

Summary When eight ¹⁴C-labelled auxin and non-auxin compounds were applied to the apical buds of intact dwarf pea seedlings (Pisum sativum L.), only [1-¹⁴C]indoleacetic acid ([¹⁴C]IAA) and -[1-¹⁴C] naphthaleneacetic acid ([¹⁴C]NAA) underwent appreciable basipetal transport during the first 24 h; over a longer period (72 h) considerable basipetal transport of the auxin [1-¹⁴C]2,4-dichlorophenoxyacetic acid ([¹⁴C]2,4-D) also occurred, but at a very much lower velocity (ca. 1.4–2.2 mm·h^-1). The movement of 2,4-D possessed many of the characteristics of a typical auxin transport. During uptake and transport IAA and NAA were extensively metabolised to the corresponding aspartates, and to ethanol-insoluble/NaOH-soluble compounds; little metabolism of 2,4-D was observed. None of the non-auxin compounds applied (sorbose, sucrose, leucine, adenine and kinetin) underwent appreciable basipetal transport from the apical bud. All but sorbose were extensively metabolised by the apical tissues. Little metabolism of sorbose itself was detected.The results suggest that the long-distance basipetal auxin transport system from the apical bud of intact plants is specific for auxins; the specificity may result from the affinity of auxins for specific transport sites.     

Regulative Systems in the Developing Citrus Fruit I. The Hormonal Balance in Orange Fruit Tissues

The regulative systems of auxins, gibberellin-like substances and their inhibitors in citrus fruit were studied. Masking caused by a high content of inhibitors was eliminated by using a refined method of solvent partition. Considerable amounts of auxins and gibberellin-like substances were detected in all stages of fruit development. The auxin system of the citrus fruit is highly complex and consists of various elements which undergo dynamic changes throughout the growth period. The identity of the auxins was studied using IAA-2-¹⁴C, and 88% of the radioactivity specifically migrated into the etheric pH 6.0 fraction. Although the prominent zones of promotion do not coincide with IAA, it can be concluded that the auxin promoters are probably not the “citrus auxin”. Abscisic acid-like inhibitors were found to accumulate in the external layers of the fruit, increasing in content as time advances.     

Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring

The growth and chemical composition of most plants are influenced by elevated CO₂, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short‐term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO₂ spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO₂ for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of ‘available’ P. Two soils were present ? a gley soil and an organic soil – but only the gley soil is examined here. Average atmospheric CO₂ concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0–5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg^?1, respectively. Under standardized moisture conditions at 25 °C, average rates of CO₂‐C production (7–14 days) were 5.4 mg kg^?1 h^?1 and of net mineral‐N production (14 ?42 days) 0.40 mg kg^?1 h^?1. These properties were all correlated positively and significantly (P < 0.10) with atmospheric CO₂ concentrations, but not with soil moisture (except for CO₂‐C production) or with clay content; they were, however, correlated negatively and mainly significantly with soil pH. In spite of uncertainties associated with the uncontrolled environment of naturally occurring springs, we conclude that storage of C and N can increase under prolonged exposure to elevated CO₂, and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.     

K+ channels of stomatal guard cells: bimodal control of the K+ inward-rectifier evoked by auxin

The influence of the auxins indole-3-acetic acid (IAA) and 1-napthylene acetic acid (NAA) on K⁺ channels and their control was examined in stomatal guard cells of Vicia faba L. Intact guard cells were impaled with multibarrelled microelectrodes to record membrane potentials and to monitor K⁺ channel currents under voltage clamp during exposures to 0.1–100 µM IAA and NAA. Following impalements, challenge with either IAA or NAA in the presence of 10 mM KCl resulted in the concerted modulation of at least four different currents with distinct kinetic characteristics and concentration dependencies. Equivalent concentrations of benzoic acid were wholly without effect. Most striking, current carried by inward-rectifying K⁺ channels (I_K,in) exhibited a bimodal response to both IAA and NAA which was reversed on washing the auxins from the bathing medium. The steady-state current was augmented 1.3- to 2-fold at concentrations between 0.1 and 10 µM and antagonized at concentrations near 30 µM and above. Auxin agonism of I_K,in was time- and voltage-independent. By contrast, I_K,in inactivation at the higher auxin concentrations was marked by a voltage-dependence and slowing of the kinetics for current activation. Inactivation of I_K,in by the auxins was relieved when cytoplasmic pH (pH_i) was clamped near 7.0 in the presence of 30 mM Na⁺-butyrate. In addition to the control of I_K,in, current carried by a second class of (outward-rectifying) K⁺ channels rose in a monotonic and largely voltage-independent manner with auxin concentrations about 10 µM and above, and IAA and NAA also activated an inward-going current with a voltage dependence characteristic of guard cell anion channels. Further changes in background current were consistent with a limited activation of the H⁺-ATPase. Over the concentration range examined, the auxins evoked membrane hyperpolarizations and depolarizations of up to ±12–19 mV, depending on the free-running membrane potential prevailing before auxin additions. Prolonging exposures to 100 µM auxin beyond 3–5 min frequently elicited rapid transitions to voltages near E_K as well as regenerative action potentials. However, in every case the voltage response was a predictable consequence of auxin action on the K⁺ channels and, at 100 µM auxin, on the anion current. These results demonstrate a control of K⁺ channel activity by auxin, consistent with the roles of these channels in mediating K⁺ flux for stomatal movements; the data associate a bimodal characteristic with the activity of I_K,in, implicating pH_i as a putative intermediate in its control, and offer strong evidence for a multiplicity of signal cascades evoked by auxin; finally, they highlight a coordinate modulation of transport activities by auxin, thereby drawing a close analogy to the pattern of stimulus-response coupling in abscisic acid.     

Effect of soil cadmium application and pH on growth and cadmium accumulation in roots, leaves and fruit of strawberry plants (Fragaria × ananassa Duch.)

Three strawberry (Fragaria × ananassa Duch.) cultivars Rainier, Totem and Selva were grown under greenhouse conditions in a Parkhill sandy loam soil with a background DTPA-extractable Cd concentration of 0.18 mg kg^-1 and a pH of 5.1. Experimental treatments included combinations of 4 Cd applications (0, 15, 30 and 60 mg Cd kg^-1 soil) applied as CdSO₄ and 2 soil pH values 5.1 and 6.8. Both the application of Cd and pH of the soil significantly affected plant growth, yield and Cd accumulation in plant tissue anf fruit. Although roots accumulated the highest concentrations of Cd of all plant parts investigated, increased soil Cd application reduced leaf weight more than root weight. In general, yield of strawberries was decreased by an increase in amount of soil-applied Cd, however the yield response varied among cultivars. At 60 mg Cd kg^-1 soil, yield of Rainier cultivar was reduced to 17.6% of control plants. Over 90% of total Cd taken up by plants grown in Cd-treated soil accumulated in roots, regardless of the Cd level in the soil. Root Cd concentrations ranged from 2.6 mg kg^-1 (control plants) to 505.7 mg kg^-1 (Totem plants grown in soil at highest Cd and a soil pH 5.1) and were directly related to soil Cd concentrations. Cd translocation from roots to leaves and fruit was very limited, resulting in a maximum Cd concentration in root leaf tissue of 10.2 mg kg^-1. Accumulation of Cd in fruit was found to correlate well with leaf Cd, although even at the highest amount of applied Cd, fruit Cd concentration did not exceed 700 g kg^-1 of fresh weight.Contribution no. 951     

Assessing mineralization rates of petroleum hydrocarbons in soils in relation to environmental factors and experimental scale

Mineralization rates of non-volatile petroleum hydrocarbons (HCs) in five different oil-contaminated soils with initial HC contents ranging from 0.1 to 13 g kg^-1 are estimated as a function of environmental factors. The aim of the study is threefold, (i) to study the relevance of environmental factors that may influence the mineralization rate, (ii) to compare mineralization rates estimated in two experiments at different scales, after standardizing them to environmental reference conditions, (iii) to evaluate the CO₂ production rate as a measure for the mineralization rate of HCs. Experiments were performed at laboratory scale (30–50 cm³ soil volume) in closed-jars under constant environmental conditions and in lysimeters (0.81 m³ soil volume) under dynamic climatic and hydrological conditions. A biodegradation model, coupled to transport models for soil heat, water, and gas dynamics is employed for data interpretation. The transport models are used to simulate the environmental conditions that influence the mineralization rate in the non-steady lysimeter experiments. The results show that temperature, O₂ concentration and HC content have an effect on the mineralization rates. Water content could not be identified as a direct governing environmental factor. However, an indirect effect of water content is that it influences the effective gas diffusion coefficient in soils. The CO₂ production rate seems to be a good quantity to express the mineralization rate of HCs for HC contents>1 g kg^-1. Measured CO₂ production rates standardized to reference conditions are similar for the two different experimental scales. This demonstrates that the usage of biodegradation rates obtained in the laboratory to predict the biodegradation rates under field conditions is sound, as long as the differences in environmental conditions have been taken into account.     

Soil Nitrogen Status Modifies Rice Root Response to Nematode-Bacteria Interactions in the Rhizosphere

It has been hypothesized that faunal activity in the rhizosphere influences root growth via an auxin-dependent pathway. In this study, two methods were used to adjust nematode and bacterial populations within experimental soils. One is “exclusion”, where soil mixed with pig manure was placed in two bags with different mesh sizes (1mm and 5μm diameter), and then surrounded by an outer layer of unamended soil resulting in soil with a greater populations of bacterial-feeding nematodes (1mm) and a control treatment (5μm). The second method is “inoculation”, whereby autoclaved soil was inoculated with bacteria (E. coli and Pseudomonas) and Nematodes (Cephalobus and C. elegans). In order to detect the changes in the rice’s perception of auxin under different nutrient and auxin conditions in the presence of soil bacterial-feeding nematodes, responses of soil chemistry (NH₄⁺, NO₃^- and indole acetic acid (IAA)), rice root growth and the expression of an auxin responsive gene GH3-2 were measured. Results showed that, under low soil nutrient conditions (exclusion), low NO₃^- correlated with increased root branching and IAA correlated with increased root elongation and GH3-2 expression. However, under high soil nutrient conditions (inoculation), a high NH₄⁺ to NO₃^- ratio promoted an increase in root surface area and there was an additional influence of NH₄⁺ and NO₃^- on GH3-2 expression. Thus it was concluded that soil bacterial-feeding nematodes influenced soil nutritional status and soil IAA content, promoting root growth via an auxin dependent pathway that was offset by soil nitrogen status.     

Uptake,accumulation and metabolism of auxins in tobacco leaf protoplasts

Uptake and metabolism of exogenous naphthalene-1-acetic acid (NAA) and indole-3-acetic acid (IAA) have been studied in tobacco (Nicotiana tabacum L. cv. Xanthi) mesophyll protoplasts. Both auxins entered protoplasts by diffusion under the action of the transmembrane pH gradient without any detectable participation of an influx carrier. Molecules were accumulated by an anion-trapping mechanism and most of them were metabolized within hours, essentially as glucose-ester and amino-acid conjugates. Protoplasts were equipped with a functional auxin-efflux carrier as evidenced by the inhibitory effect of naphthylphtalamic acid on IAA efflux. Basically, similar mechanisms of NAA and IAA uptake occurred in protoplasts. However, the two auxins differed in their levels of accumulation, due to different membrane-transport characteristics, and the nature of the metabolites produced. This shows the need to estimate the accumulation and the metabolism of auxins when analyzing their effects in a given cell system. The internal auxin concentration could be modulated by changing the transmembrane pH gradient, giving an interesting perspective for discriminating between the effects of intra- and extracellular auxin on physiological processes.Abbreviations BA benzoic acid - C_i/C_e accumulation ratio of auxin - IAAasp N-[3-indolylacetyl]-dl-aspartic acid - NAA naphthalene-1-acetic acid - NAAasp N-[1-naphthylacetyl]-l-aspartic acid - NPA N-1-naphthylphthalamic acid The authors thank Dr. M. Caboche (I.N.R.A, Versailles, France) for his generous gifts of some amide derivatives of 1-NAA, Mr. P. Varennes and Dr. B. Das (I.C.S.N., C.N.R.S., Gif-sur-Yvette, France) for recording and interpreting the mass spectra of NAA glucose ester, and Prof. P. Manigault (Institut des Sciences Végétales, Gif-sur-Yvette) for microscopy measurements of protoplast dimensions. This work was supported by funds from the C.N.R.S, I.N.R.A, and E.E.C.     

Phytotoxic doses of boron in contrasting soils depend on soil water content

Boron (B) affects plant growth in soil at B doses (mg added B kg^-1 soil) that appear in the range of natural background B concentrations. A study was set up to determine B bioavailability by testing B toxicity to plant as affected by soil properties and ageing after soil dosing. Nineteen soils (pH 4.4?C7.8) and 3 synthetic soils (sand-peat mixtures) were amended with 7 doses of H₃BO₃. Barley root elongation was determined immediately after B amendment and after 1 and 5 months ageing. Soil solution B concentrations increased linearly with added B concentrations with almost no detectable adsorption. In contrast, the ratio of aqua regia soluble B/soil solution B in unamended soils (no B added) was 10?C25 times higher than in B amended soils at similar aqua regia soluble B concentrations illustrating a much lower B availability in unamended soils. Soil solution B concentrations did not decrease by ageing. The toxic B doses or soil B concentrations that decreased barley root growth by 10% (EC10 values) varied about tenfold (respectively 3?C27 mg added B kg^-1 and 5?C52 mg B kg^-1) among soils. Corresponding thresholds in soil solution varied less than fourfold (16?C59 mg B l^-1). Soil ageing for 5 months did not significantly change EC10 and EC50 values, expressed either as total soil B or as soil solution B, unless in 1 soil. Variability in EC10 and EC50 values was explained by various soil properties (soil moisture content, background B, %clay, cation exchange capacity), but covariance of these properties with the soil moisture content suggest that B dilution is the critical factor explaining B toxicity. It is concluded that effects of B amendments do not decrease by ageing and that soil solution B or B doses corrected for soil moisture content may be used as an index for B toxicity across different soils.     

A saturable site responsible for polar transport of indole-3-acetic acid in sections of maize coleoptiles

The velocity of transport and shape of a pulse of radioactive indole-3-acetic acid (IAA) applied to a section of maize (Zea mays L.) coleoptile depends strongly on the concentration of nonradioactive auxin in which the section has been incubated before, during, and after the radioactive pulse. A pulse of [³H]IAA disperses slowly in sections incubated in buffer (pH 6) alone; but when 0.5–5 M IAA is included, the pulse achieves its maximum velocity of about 2 cm h^-1. At still higher IAA concentrations in the medium, a transition occurs from a discrete, downwardly migrating pulse to a slowly advancing profile. Specificity of IAA in the latter effect is indicated by the observation that benzoic acid, which is taken up to an even greater extent than IAA, does not inhibit movement of [³H]IAA. These results fully substantiate the hypothesis that auxin transport consists of a saturable flux of auxin anions (A^-) in parallel with a nonsaturable flux of undissociated IAA (HA), with both fluxes operating down their respective concentration gradients. When the anion site saturates, the movement of [³H]IAA is nonpolar and dominated by the diffusion of HA. Saturating polar transport also results in greater cellular accumulation of auxin, indicating that the same site mediates the cellular efflux of A^-. The transport inhibitors napthylphthalamic acid and 2,3,5-triiodobenzoic acid specifically block the polar A^- component of auxin transport without affecting the nonsaturable component. The transport can be saturated at any point during its passage through the section, indicating that the carriers are distributed throughout the tissue, most likely in the plasmalemma of each cell.Abbreviations A^- auxin anion - HA undissociated auxin - IAA indole-3-acetic acid - NPA N-1-napthylphthalamic acid - TIBA 2,3,5-triiodobenzoic acid     

Targeting of auxin carriers to the plasma membrane: differential effects of brefeldin A on the traffic of auxin uptake and efflux carriers

Monensin and brefeldin A (BFA), inhibitors of Golgi-mediated protein secretion, rapidly perturb the transport catalytic activity of specific plasma membrane-associated efflux carriers for indole-3-acetic acid (IAA) and inhibit polar transport of IAA. To determine if these responses result solely from perturbation of the efflux carrier or whether specific auxin uptake carrier function is also affected, the influence of BFA on the cellular transport of a range of auxins with contrasting affinities for specific auxin uptake and efflux carriers was investigated in zucchini (Cucurbita pepo L.) hypocotyl tissue. In-flight addition of BFA (3 · 10⁻⁵ mol · dm⁻³) caused a rapid (lag < 10 min) and substantial (fourfold) increase in the rate of [1-¹⁴C]IAA net uptake by zucchini hypocotyl tissue. In the presence of the specific auxin efflux carrier inhibitor N-1-naphthylphthalamic acid (NPA; 3 · 10⁻⁶ mol · dm⁻³), BFA slightly reduced the rate of [1-¹⁴C]IAA net uptake. Stimulation of [1-¹⁴C]IAA net uptake by BFA was concentration-dependent. In the absence of BFA, net uptake of [1-¹⁴C]IAA exhibited the characteristic biphasic response to increasing concentrations of competing cold IAA but in the presence of BFA, [1-¹⁴C]IAA uptake decreased smoothly with increase in concentration of competing unlabelled IAA, indicating a loss of auxin efflux carrier activity but retention of functional uptake carriers. The half-time for mediated efflux of [1-¹⁴C]IAA from preloaded zucchini tissue was substantially increased by BFA (t_1/2 = 51 min, controls; 107 min, BFA-treated). Treatment with BFA and/or NPA did not significantly affect the net uptake by, or efflux from, zucchini tissue of [1-¹⁴C]2,4-dichlorophenoxyacetic acid ([1-¹⁴C]2,4-D), a substrate for the auxin uptake carrier but not the auxin efflux carrier. Uptake of [1-¹⁴C]2,4-D declined smoothly with increasing concentrations of competing unlabelled IAA whether or not BFA was included in the uptake medium, confirming the failure of BFA to perturb auxin uptake carrier function. Transport of 1-[4-³H]naphthaleneacetic acid (1-NAA) exhibited little response to BFA or NPA, confirming that it is only a weakly transported substrate for the efflux carrier in zucchini cells. Received: 12 November 1997 / Accepted: 27 January 1998     

Auxin in scapes,flower buds,flowers, and fruits of daffodil (Narcissus pseudonarcissus L.)

Summary Diffusates from flower buds, flower fruits, and scape segments, and extracts of flower stalks of Narcissus pseudonarcissus contain an auxin active in the Avena geo-curvature test. The auxin behaved like indole-3-acetic acid (IAA) in thin-layer chromatography (TLC) with neutral and basic solvents on different adsorbents. After TLC, the auxin of the extracts showed chromogenic reactions identical with those of IAA; in gas-liquid chromatography on two different columns, the purified substance, after methylation, appeared at the retention time of IAA methyl ester. The auxin content of the extracts has been estimated to be equivalent to ca. 10 g IAA kg^–1 fresh weight. Diffusates, collected at the basal end of excised flowering apices and of scape segments at different developmental stages, showed highest auxin activity when collected from old buds and young flowers, and from the basal, rapidly elongating scape regions. The diffusible auxin obtained from scape segments was very likely produced by the segments themselves. Thus, the shoot of Narcissus appears to possess two different sites of auxin production, namely, the apical region represented by the flower bud, the flower or the fruit, and the scape.Abbreviations IAA indole-3-acetic acid - IAA-OMe indole-3-acetic-acid methyl ester - TLC thin-layer chromatography - GLC gas-liquid chromatography