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).     

Response ofZea mays andLycopersicon esculentum to the ethylene precursors,L-methionine and L-ethionine applied to soil

Glasshouse experiments were conducted to evaluate the influence of L-methionine (L-MET) and L-ethionine (L-ETH) added to soil on the growth of corn (Zea mays L.) and tomato (Lycopersicon esculentum), respectively. The application of L-MET and L-ETH stimulated C₂H₄ production in soil by 299- and 313-fold, respectively, over an unamended control. An L-MET treatment of 1.85 mg kg⁻¹ soil was the most effective in increasing shoot height, shoot fresh weight, internodal distance, and stem diameter in two corn cultivars, Kandy Korn and Miracle, while shoot and root dry weights, leaf width, uppermost leaf collar base distance and resistance to stem breaking were increased in the case of Kandy Korn only. A significant epinastic response was observed in the second and third leaves of tomato plants when soil was treated with L-ETH. An L-ETH treatment of 0.2 mg kg⁻¹ soil resulted in the maximum fresh fruit yield, while 0.02 and 2.0 mg kg⁻¹ gave the most fruit and greater average weight of fresh fruit, respectively. Concentrations ranging from 0.002 to 2.0 mg L-ETH kg⁻¹ soil initiated early fruit formation. Early fruit ripening was observed with an application rate of 20 mg L-ETH kg⁻¹ soil. The mechanism of action of these chemicals could either be attributed to i) substrate-dependent C₂H₄ production in soil by the indigenous microflora, ii) uptake directly by plant roots followed by metabolism within the tissues, and/or iii) a change in the balance of rhizosphere microflora affecting plant growth.     

Influence of adenine,isopentyl alcohol and Azotobacter chroococcum on the vegetative growth of Zea mays

Glasshouse experiments were conducted to evaluate the influence of pretested cytokinin precursors, adenine (ADE) and isopentyl alcohol (IA), and a cytokinin producing bacterium, Azotobacter chroococcum added to soil, on the vegetative growth of maize (Zea mays L.). The combination of 2.0 mg ADE kg^-1 soil, 13 mg IA kg^-1 soil, plus an inoculum of A. chroococcum was the most effective in enhancing the vegetative growth of maize compared with the application of ADE plus IA, ADE plus A. chroococcum, or ADE, IA or A. chroococcum alone. The dry weight of root and shoot tissues was increased up to 5.57- and 5.01-fold, respectively, in comparison to the controls; however, the root/shoot ratios were similar. The increases in shoot height, internodal distance, stem and leaf width over the controls under the optimum treatment were: 2.07-, 2.81-, 1.46-, and 2.11-fold, respectively. The improvement in plant yield was primarily attributed to A. chroococcum production of cytokinins in the rhizosphere.     

土施L-蛋氨酸、L-苯基丙氨酸、L-色氨酸对玉米生长和养分吸收的影响

通过盆栽试验,研究了3种植物生长调节剂前体物质对玉米生长发育和养分吸收的影响,并确定了其适宜用量.结果表明,土施L蛋氨酸(L-MET)、L-苯基丙氨酸(L-PHE)和L-色氨酸(L-TRP)能不同程度地增加玉米株高、地上部和地下部干重,提高玉米根系活力、体内硝酸还原酶和过氧化氢酶的活性,促进玉米对氮、磷、钾、锌养分的吸收.在所有供试浓度中,以土施L-MET0.0185～0.185mg·kg^-1、L-PHE0.2mg·kg^-1和L-TRP0.03～0.3mg·kg^-1效果最佳,而且L-PHE和L-TRP对玉米生长的促进作用和提高养分的吸收能力均优于L-MET.     

Influence of adenine,isopentyl alcohol and Azotobacter chroococcum on the growth of Raphanus sativus

The effect of adenine (ADE), isopentyl alcohol (IA) and a cytokinin-producing bacterium, Azotobacter chroococcum, on the morphological plant characteristics of Raphanus sativus (radish) was studied in sand under axenic-inoculated conditions and in soil under glasshouse and field conditions. The application of the combination of 0.2 mg kg^–1 ADE, 13 mg kg^–1 IA plus the inoculum enhanced the dry weight of root and shoot tissues, leaf area and chlorophyll a content, to a much greater degree than when in the presence of the cytokinin precursors (ADE or IA) or the bacterium alone. Enhanced plant growth observed under axenic conditions upon the addition of ADE and IA indicated that the plant has the ability to assimilate and utilize ADE and IA for growth and metabolism. While the addition of the inoculum without precursors was also stimulatory, greater enhancement of plant growth was observed following the application of ADE, IA and A. chroococcum together being attributed primarily to the increase in microbial production of cytokinins within the rhizosphere.     

Physiological and agronomical responses of common bean subjected to tryptophol

With the increasing global demand for food, fuel and fibre, the use of plant growth regulators in agriculture has become an agricultural practice aimed to improve physiological and productive responses. Our work aimed to evaluate the effect of tryptophol (Tol), a precursor of auxin, on common bean (Phaseolus vulgaris L.). The experiment was conducted in pots under greenhouse conditions, where we evaluated the Tol effect on bean crop under two different application forms: T_Soil – soil application of Tol (4.10^?4 mg L^?1) and T_Leaf – leaf tryptophol application (4.10^?4 mg L^?1), plus a reference treatment (0 mg L^?1 of Tol). We analysed the variables: shoot fresh and dry matter; root dry matter, area and volume; leaf macro and micronutrients; CO₂ net assimilation rate (A); stomatal conductance (g_S); internal CO₂ concentration (C_I); foliar transpiration (E); photosynthetic pigment content and some crop production attributes. The application of Tol through the foliar pathway proved to be more advantageous because it improved the shoot fresh and dry matter, increased the root volume and area, favoured less foliar transpiration and improved the length of pods, while the application of Tol in soil induced higher nitrogen accumulation in leaves. Our observations allow the characterization of Tol as a bioactive metabolite, suggesting an important potential for use in agricultural systems.     

Effect of Arbuscular Mycorrhizal Fungi On Yield and Phytoremediation Performance of Pot Marigold (Calendula officinalis L.) Under Heavy Metals Stress

In order to study the effect of mycorrhizal fungi (inoculated and non-inoculated) and heavy metals stress [0, Pb (150 and 300 mg/kg) and Cd (40 and 80 mg/kg)] on pot marigold (Calendula officinalis L.), a factorial experiment was conducted based on a randomized complete block design with 4 replications in Research Greenhouse of Department of Horticultural Sciences, University of Tehran, Iran, during 2012–2013. Plant height, herbal and flower fresh and dry weight, root fresh and dry weight and root volume, colonization percentage, total petal extract, total petal flavonoids, root and shoot P and K uptakes, and Pb and Cd accumulations in root and shoot were measured. Results indicated that with increasing soil Pb and Cd concentration, growth and yield of pot marigold was reduced significantly; Cd had greater negative impacts than Pb. However, mycorrhizal fungi alleviated these impacts by improving plant growth and yield. Pot marigold concentrated high amounts of Pb and especially Cd in its roots and shoots; mycorrhizal plants had a greater accumulation of these metals, so that those under 80 mg/kg Cd soil^?1 accumulated 833.3 and 1585.8 mg Cd in their shoots and roots, respectively. In conclusion, mycorrhizal fungi can improve not only growth and yield of pot marigold in heavy metal stressed condition, but also phytoremediation performance by increasing heavy metals accumulation in the plant organs.     

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.     

The effect of soil compaction on growth and P uptake by Trifolium subterraneum: interactions with mycorrhizal colonisation

The effects of vesicular-arbuscular mycorrhizal (VAM) colonisation on phosphorus (P) uptake and growth of clover (Trifolium subterraneum L.) in response to soil compaction were studied in three pot experiments. P uptake and growth of the plants decreased as the bulk density of the soil increased from 1.0 to 1.6 Mg m^-3. The strongest effects of soil compaction on P uptake and plant growth were observed at the highest P application (60 mg kg^-1 soil). The main observation of this study was that at low P application (15 mg kg^-1 soil), P uptake and shoot dry weight of the plants colonised by Glomus intraradices were greater than those of non-mycorrhizal plants at similar levels of compaction of the soil. However, the mycorrhizal growth response decreased proportionately as soil compaction was increased. Decreased total P uptake and shoot dry weight of mycorrhizal clover in compacted soil were attributed to the reduction in the root length. Soil compaction had no significant effect on the percentage of root length colonised. However, total root length colonised was lower (6.6 m pot^-1) in highly compacted soil than in slightly compacted soil (27.8 m pot^-1). The oxygen content of the soil atmosphere measured shortly before the plants were harvested varied from 0.18 m³m^-3 in slightly compacted soil (1.0 Mg m^-3) to 0.10 m³m^-3 in highly compacted soil (1.6 Mg m^-3).     

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.     

Increased root length and branching in cotton by soil application of the plant growth regulator PGR-IV

Field and growth chamber studies were used to determine the effect of in-furrow application of PGR-IV on root and shoot development, and yield of cotton. In the field study, an in-furrow application of PGR-IV @ 73 mL ha^–1 at planting increased yield by 18% compared to the untreated control, and by 11% compared to 2-foliar applications of 292 mL/ha^–1 each at pinhead square stage of flower development and at first flower appearance. Growth chamber studies revealed that the in-furrow applications of PGR-IV @ 1.131L/plant dramatically increased root length (+47%), root dry weight (+29%), number of lateral roots per plant (+75%), and nutrient uptake one week after planting. These differences were still apparent five weeks later at pinhead square but to a lesser degree. The yield enhancement from the foliar applications was associated with increases in leaf growth, nutrient uptake, and boll number, whereas the yield enhancement from the soil application was associated with enhanced root growth and nutrient uptake. The positive effect of PGR-IV on root growth and accelerated early-season growth could have very substantial benefits in cotton production.     

Growth response of coffee tree shoots and roots to subsurface liming

In the greenhouse growth of two coffee-tree varieties, Catuaí (sensitive) and Icatu (tolerant) to aluminum, was evaluated in surface-fertilized and limed soil following subsurface treatment with seven lime levels (0.0; 0.49; 1.7; 2.9; 4.1; 6.6 and 9.3 t/ha). Plants were grown for 6.5 months in soils in PVC columns, subdivided into two horizons. In the lower 12 – 34 cm depth horizon, soil Al saturation varied between 93 and 0%. For both varieties evaluated, shoot dry weight and leaf area remained unchanged following limestone application. This fact shows that surface layer correction permitted normal shoot growth. High Al saturation resulted in decrease of root dry weight percent, root length percent and root surface percent in the 12–34 cm horizon, which were compensated by higher percentages of these properties in the upper 0–12 cm horizon. The ratio between root surface – root dry matter (cm²/g) of Catuaí variety was increased by limestone application to the lower soil horizons, indicating that roots turn longer and thinner, when Al soil saturation decreased. This also shows a great sensitivity to Al of the Catuaí variety. In contrast, in the Icatu variety, all root characteristics remained stable at all levels of Al tested.     

Tryptophan-dependent biosynthesis of auxins in soil

The presence of auxins in soil may have an ecological impact affecting plant growth and development. A rapid and simple colorimetric method was used to assess California soils for their potential to produce auxins upon the addition of L-tryptophan (L-TRP). The auxin content measured by colorimetry was expressed as indole-3-acetic acid (IAA)-equivalents. A substrate (L-TRP) concentration of 5.3 g kg^-1, glucose concentration of 6.7 g kg^-1, no nitrogen, pH 7.0, 40°C, shaking (aeration) and 48 h incubation time were selected as standardized conditions to assay for auxin biosynthesis in soil. IAA was confirmed as a major microbial metabolite derived from L-TRP in soil by use of high performance liquid chromatography (HPLC). Under standardized conditions, L-TRP-derived auxins in 19 soils varied greatly ranging from 18.2 to 303.2 mg IAA equivalents (auxins) kg^-1 soil. This study suggests that the phenotypic character of the soil microbiota has more of an influence on auxin production than the soil physicochemical properties (e.g., pH, organic C content, CEC, etc.).     

Chemistry of Cr in some Swedish soils

The effect of Si(OH)₄ on Cr toxicity and elemental concentrations in ryegrass were investigated in a growth chamber using an acid and a neutral mineral soil. Each soil was treated with 50 mg Cr, as CrO₃, kg⁻¹ soil dry weight, singly, or in combination with 25 mg Si as Si(OH)₄. Plants growing in unamended soils were used as controls. Chromium toxicity, expressed as decrease in shoot or root dry weight, was increased by the Si. This increase was accompanied by a higher Cr uptake particularly on the acid soil. The shoot and root dry weights were significantly correlated (P=1%) with the concentration of Cr, where r=−0.80 and −0.65, respectively. Uptake of Al, Cu, Fe, P and Zn did not show any consistent relationship to the magnitude of Cr toxicity.     

Growth response to subsurface soil acidity of wheat genotypes differing in aluminium tolerance

Subsurface soil acidity coupled with high levels of toxic Al is a major limiting factor in wheat production in many areas of the world. This study examined the effect of subsurface soil acidity on the growth and yield of two near-isogenic wheat genotypes differing in Al tolerance at a single genetic locus in reconstructed soil columns. In one experiment, plants were grown in columns with limed topsoil and limed or acidic subsurface soils, and received water only to the subsurface soil at a late part of the growth period. While shoot dry weight, ear number and grain yield of Al-tolerant genotype (ET8) were not affected by subsurface soil acidity, liming subsurface soil increased shoot weight and grain yield of Al-sensitive genotype (ES8) by 60% and ear number by 32%. Similarly, root length density of ET8 was the same in the limed and acidic subsurface soils, while the root length density of ES8 in the acidic subsurface soil was only half of that in the limed subsurface soil. In another experiment, plants were grown with limed topsoil and acidic subsurface soil under two watering regimes. Both genotypes supplied with water throughout the soil column produced almost twice the dry weight of those receiving water only in the subsurface soil. The tolerant genotype ET8 had shoot biomass and grain yield one-third higher than ES8 when supplied with water throughout the whole column, and had yield 11% higher when receiving water in the subsurface soil only. The tolerant genotype ET8 produced more than five times the root length in the acidic subsurface soil compared to ES8. Irrespective of watering regime, the amount of water added to maintain field capacity of the soil was up to 2-fold higher under ET8 than under ES8. The results suggest that the genotypic variation in growth and yield of wheat grown with subsurface soil acidity results from the difference in root proliferation in the subsurface soil and hence in utilizing nutrient and water reserves in the subsurface soil layer.     

Differential Tolerance of Cool- and W arm-Season Grasses to TNT -Contaminated Soil

Plants can be used for effective and economical remediation of soil provided they are tolerant or resistant to the contaminants. Greenhouse experiments were conducted to determine the tolerance of the cool-season grasses: smooth bromegrass (Bromus inermus Leyss.) and tall fescue (Festuca arundinaceae Schreb), and the warm-season grasses: big bluestem (Andropogon gerardii Vitman) and switchgrass (Panicum virgatum L.) to TNT (2,4,6-trinitrotoluene) in soil. TNT-contaminated soil was mixed with uncontaminated soil to obtain water-extractable TNT concentrations ranging from 71 to 435 mg kg^-1, corresponding to acetonitrile-extractable concentrations of 278 to 3115 mg kg^-1. Germination, shoot and root dry weight, and root area were measured in response to TNT concentrations in the soil mixtures. Germination and height of the warm-season grass species were more sensitive than the cool-season grass species to increasing TNT concentrations in soil. Significant reductions in shoot and root growth were observed in cool-season grasses at lower TNT concentrations in soil compared with warm-season grasses in the soil mixtures. Results indicated that the warm-season grasses can be established in soil containing less than 86 mg of water-extractable TNT kg^-1, based on 80% of measured growth in uncontaminated control soil.     

Intra-specific variation in growth and P accumulation of Leucaena leucocephala and Gliricidia sepium as influenced by soil phosphate status

Twenty-three provenances of Gliricidia sepium and eleven isolines of Leucaena leucocephala were examined at a low and at high phosphate levels (20 and 80 mg P kg^-1 soil) for growth, phosphate (P) uptake and use efficiency. Large differences in growth at the low P level, and in growth response to the higher P rate occurred among L. leucocephala isolines and G. sepium provenances. Shoot dry weight at low P varied from 1.30 to 3.01 g plant^-1 for L. leucocephala and from 1.44 to 3.07 g plant^-1 for G. sepium.Leucaena isolines had only half the root weight of G. sepium provenances yet produced approximately 90% of the shoot weight of the corresponding G. sepium treatments, i.e. more than a 2-fold difference in root/shoot ratios. Total P in shoots of G. sepium was some 85% greater than of the respective L. leucocephala isolines in corresponding treatments. Physiological phosphate use efficiency (g shoot/mg P in shoots) (PPUE) was not a simple reciprocal relation, being markedly lower at higher shoot % P and content. However, for the same shoot P both species produced the same shoot weight. Nevertheless, there were apparent genotypic differences within species in the root development, shoot P and PPUE.In another study, the numbers of rhizobia in the rhizosphere of L. leucocephala, nodulation, N₂ fixation at five different levels of P were determined. The numbers of rhizobia in the rhizosphere of inoculated L. leucocephala during the first two weeks were lower when P was added but later became similar to those without added P. Nodules formed earlier than inoculated plants fertilized with P and in greater numbers (4- to 5-fold) and dry weights than in those without P. However, the percentage of N₂ derived from fixation did not change with increasing levels of P application. These results suggest that the observed P effect did not operate via stimulated growth of rhizobia in the rhizosphere, nor through increased N₂ fixation rate. The major effect appeared to be due to effects via plant growth.     

Heavy Metal Phytotoxicity to Radish (Raphanus sativus L.) in a Digested Sludge-amended Gangetic Alluvium

A limiting factor in land application of sewage sludge is the resultant heavy metal accumulation in soils followed by biomagnification in the food chain, posing a potential hazard to animal and human health. In view of this fact, pot experiments were conducted to evaluate the effect of digested sludge application to soil on phytotoxicity of heavy metals such as Cd, Cr, Ni, and Pb to radish (Raphanus sativus L.) plants. Increasing sludge levels resulted in increased levels of DTPA-extractable heavy metals in the soil. Cadmium was the dominant metal extracted by DTPA followed by Ni, Pb, and Cr. The extractability of metals by DTPA tended to decrease from the first to the second crop. Dry matter yield of radish increased significantly as a function of increasing sludge treatments. Soil application of sludge raised the concentration of one or more heavy metals in plants. Shoots contained higher concentrations of Cd, Cr, and Ni than the roots of radish plants. Shoot concentrations of Cd, Cr, Ni, and Pb were within the tolerance levels of this crop at all rates of sludge application. Shoot as well as root concentration of Cd was above 0.5 mg kg^?1, considered toxic for human and animal consumption. The levels of DTPA-extractable Cd and Ni were less correlated while those of Cr and Pb were more correlated with their respective shoot and root contents. The results emphasize that accumulation of potentially toxic heavy metals in soil and their build-up in vegetable crops should not be ignored when sludge is applied as an amendment to land.     

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.     

Effect of silicate on the growth and arsenate uptake by rice (Oryza sativa L.) seedlings in solution culture

A solution culture experiment was conducted to investigate the effect of silicate on the yield and arsenate uptake by rice. Rice seedlings (Oryza sativaL. cv. Weiyou 77) were cultured in modified Hoagland nutrient solution containing three arsenate levels (0, 0.5 and 1.0 mg L ^–1 As) and four silicate levels (0, 14, 28 and 56 mg L ^–1 Si). Addition of Si significantly increased shoot dry weight (P=0.001) but had little effect on root dry weight (P=0.43). Addition of As had no significant effect on shoot dry weight (P=0.43) but significantly increased root dry weight (P=0.01). Silicon concentrations in shoots and roots increased proportionally to increasing amounts of externally supplied Si (P < 0.001). The presence of As in the nutrient solution had little effect on shoot Si concentration (P=0.16) but significantly decreased root Si concentration (P=0.005). Increasing external Si concentration significantly decreased shoot and root As concentrations and total As uptake by rice seedlings (P <0.001). In addition, Si significantly decreased shoot P concentration and shoot P uptake (P <0.001). The data clearly demonstrate a beneficial effect of Si on the growth of rice seedlings. Addition of Si to the growth medium also inhibited the uptake of arsenate and phosphate by the rice seedlings.