Assessment of a rapid method,using soil cores,for estimating the amount and distribution of crop roots in the field

Summary A study was made of the relationship between the number of roots (N_r) observed on unit area of the freshly exposed, horizontal faces of soil cores, and the amounts of roots (per unit volume) present in the same cores. Soil cores, 7 cm diameter, were extracted to depths of 1 m from cereal crops in 1976 at three field sites located on clay soils. Sampling was either at the start of stem elongation, or at anthesis. Estimates of root length per unit soil volume (L) were derived from N_r by assuming random orientation of roots in the soil.Values of L were found to be highly correlated with the measured lengths of both the main roots (root axes) and the total roots (axes and laterals) washed from the soil at a given growth stage, for each of the soils. On average, L was 3.3 times the length of root axes washed from the soil, and was 0.42 times the length of total roots, but there was appreciable variation between different growth stages and field sites. Possible factors giving rise to differences between L and the measured lengths of roots are discussed. Estimates of root length from observation of soil cores may nonetheless provide a suitable basis for rapidly comparing therelative distribution of roots down the soil profile under field conditions.     

Development of wheat roots under zinc deficiency

Summary Wheat seedlings were cultured in nutrient solutions with and without zinc supply. Shoot: root ratio was reduced in plants without zinc and the greater biomass of –Zn roots was a result of enhanced 1st order lateral root numbers and length. The implications of morphological changes under zinc stress are discussed in relation to knowledge of ion absorption pathways across roots.     

Growth of wheat and subterranean clover on soil artificially compacted at various depths

Summary Wheat crops with stunted chlorotic patches are widespread in northern Victoria, Australia, and are often associated with dense, compacted layers of soil. Poor growth of subterranean clover, with symptoms of cupped and reddened leaflets, is also a problem in these cropping regions during the pasture phase of the rotation. Artificially compacted soils were created to test the hypothesis that these symptoms of poor growth were caused by soil compaction. Soil compacted from 0–20 cm with a bulk density similar to that measured in problem fields reproduced these symptoms in wheat and subterranean clover. Surface compaction alone also reproduced the symptoms in clover.     

Soil carbon dioxide efflux determined from large undisturbed soil cores collected in different soil management systems

The aim of this study was to evaluate a measuring technique for determining soil CO₂ efflux from large soil samples having undisturbed structure under controlled laboratory conditions. Further objectives were to use the developed measuring method for comparing soil CO₂ efflux from samples, collected in three different soil management systems at various soil water content values. The experimental technique was tested and optimised for timing of sampling by taking air samples after 1, 3 and 6 hours of incubation. Based on the results, the incubation time was set to three hours. The CO₂ efflux measured for different soil management systems was the highest in the no-till and the lowest in the ploughing treatment, which was in accordance with measurements on accessible organic carbon for microbes. An increase in CO₂ efflux with increasing soil water content was found in the studied soil water content range. Our results indicate that soil respiration rates, measured directly after tillage operations, can highly differ from those measured long after.     

Plant uptake and leaching of copper during EDTA-enhanced phytoremediation of repacked and undisturbed soil

EDTA-enhanced phytoremediation of copper contaminated soil was evaluated. Up to 740 g g^–1 of Na₂H₂ EDTA in solution was added to repacked soil columns, and intact cores of a sandy loam of volcanic origin, that was growing Agrostis tenuis. The soil contained up to 400 g g^–1 of copper due to a history of fungicide spraying. EDTA application increased the herbage copper concentration of the grass growing in repacked soil from 30 to 300 g g^–1, but the same application to an intact core only brought about an increase from 10 to 60 g g^–1. More copper accumulated in the herbage when the EDTA was applied in numerous small doses than in just one or two larger amounts. Calculation of the concentration of copper in the water taken up by the grass revealed this to be two orders of magnitude lower than that in the soil solution. As a result of the EDTA applications, about 100 times more copper was leached than was taken up by the herbage. This means that a strategy for managing leaching losses needs to be part of any plan for EDTA-enhanced phytoremediation.     

Effects of crop residues,soil type and temperature on emergence and early growth of wheat

Summary Two controlled environment experiments were conducted to examine the germination and early growth of wheat (Triticum aestivum L. cv. Songlen) growing under crop residues of rape, sorghum, field pea and wheat. Additional treamments also included were soil type (Lithic Vertic Ustochrept and Plinthustalf) and temperature (8°C and 24°C to simulate winter and autumn sowing conditions). At low temperature, wheat and sorghum residues produced the most adverse effects on germination with all residues reducing emergence at high temperatures. Shoot lengths were also reduced by most residues at high temperatures whilst root lengths and shoot and root dry weights were unaffected by residue treatments. These results suggest major phytotoxic effects of residues during early growth (up to 14 days after sowing) with, in general, few interactions with soil type or temperature.     

A study of the tensile force required to pull wheat roots from soil

Experiments were carried out to determine the tensile properties of mature wheat roots and the force necessary to pull roots from undisturbed soils at a range of moisture contents using an Instron materials testing rig. Roots decreased in cross sectional area from 1.5 to 0.1 mm²and in tensile strength from 7.0 to 2.3 Newtons (N) along the first 12 cm of their length. Breaking strain was constant along the root but the breaking stress increased. Increased seeding rate decreased root diameter and tensile strength but plant growth regulators and fertiliser nitrogen level did not affect the tensile properties. Roots were pulled from mesh tubes of soil (25 mm ± 75 mm) into which they had grown. The peak loads for sandy clay loam and sandy loam soils were 4.8 and 3.9 N respectively and increasing the soil moisture from 17% to 26% reduced the peak load from 5.2 N to 3.5 N. With the drier soil the inner stele pulled free leaving the outer periderm in the soil in a higher proportion of the roots indicating a stronger root/soil bond than in the wetter soil. The load us displacement relationship when pulling roots from soil resembled that for a fibre reinforced composite material. The presence of branch roots resulted in an uneven trace in which there were a number of sub-peaks as branch roots gave way. It is suggested that soil wetting could contribute to lodging of wheat by reducing the resistance of roots to slippage and breaking.     

Nitric oxide, polyamines and Cd-induced phytotoxicity in wheat roots

To further explore the biochemical basis of Cd toxicity in developing wheat seedlings, we studied the possible role of nitric oxide (NO) and polyamines as signaling molecules involved in metal-induced root growth inhibition. When used at 0.1 mM, sodium nitroprusside, a NO-releasing compound, inhibited root growth to a similar extent as Cd and enhanced the polyamine contents as Cd also did. Putrescine and spermidine treatments caused significant decreases in root growth with spermine giving the greatest level of inhibition (77% reduction). The simultaneous addition of Cd and inhibitors of putrescine biosynthesis (DFMA and DFMO) prevented increases in putrescine levels but did not restore normal root growth. NO content, as evidenced by the fluorescent probe DAF-FM diacetate, was found to be significantly increased in the roots of both Cd and polyamine treated plants, especially in those exposed to spermine. The effect was specific for NO since the NO scavenger cPTIO almost suppressed the fluorescent signal. Concerning the oxidative status of the root system, only Cd and spermine enhanced lipid peroxidation in roots. At the same time, all treatments led to a significant increase in levels of the non-enzymatic antioxidant defense glutathione. Our results strongly suggest that Cd and spermine treatments induce NO formation in wheat roots which, in turn, is involved in root growth inhibition.     

氮素形态对不同专用型小麦根系及氮素利用率影响的研究

采用盆栽方法研究了3种氮素形态对不同专用型小麦根系及氮素利用率的影响．结果表明,拔节期以后,强筋型小麦豫麦34在酰胺态氮处理下,根系生物量、根系活力、氮素利用率、氮收获指数和籽粒蛋白质含量最高,铵态氮处理次之,硝态氮处理最低．中筋型小麦豫麦49的各测定指标以铵态氮处理最高,其它指标在酰胺态氮和铵态氮间的趋势不同;弱筋型小麦豫麦50在酰胺态氮处理下各项指标最高,而铵态氮处理下蛋白质含量最低,符合品种优质和专用,氮收获指数与籽粒蛋白质含量之间呈现极显著正相关。     

Growth rates,biomass and distribution of selected woody plant roots in Burkea africana-Ochna pulchra savanna

Woody plants in an African Burkea africana-Ochna pulchra savanna on deep sandy soil were found to have characteristically bimorphic root systems. The shallow lateral root component was often well developed and roots extended up to seven times the extent of the plant canopy in several species. Exponential tapering of lateral roots was found in Terminalia sericea. The wide-ranging roots, together with the high degree of multispecies root system interpenetration, result in the so-called, open grassy areas in the savanna mosaic often containing a competitively significant woody plant component. Root systems of Ochna pulchra were found to be relatively specialized and included: negatively geotropic, superficial roots; sinker roots to bedrock; high suckering response to damage in roots; belowground lignotuber-type organs; and sustained subterranean interconnections between some aboveground stems. These features are likely to contribute substantially to the resilience of this plant species to various climatic and veld management stress factors. Root/shoot mass ratios averaged unity but depended on plant size and aboveground growth form in Ochna pulchra. The dependence of these ratios on sizes of plant also applied to plant clones. Initiation of root tip growth occurred in early summer in one year and late spring in another. Main root tip growth occurred in late summer and early autumn, well after completion of most growth of leafy shoots in spring. It is suggested that some active uptake of water and nutrients by non-extending roots allows this form of phased growth in the plant. In an analysis of the seasonal growth of individual root tip systems, it was clear that transitory states of rest occur in fine root development but that these are far more frequent in the branching (and hence proliferation) of roots than in the continuing development of any root axis.Nomenclature follows the present system of the Botanical Research Institute, Pretoria, and the Flora of Southern Africa.I thank M.D. Panagos, P.S. Carr and J. Steyn for assistance at various stages of this work.     

Biosynthesis of lectin in roots of germinating and adult cereal plants

Root tips of wheat, rye, barley and rice seedlings contain lectins which are identical to the respective embryo lectins with respect to their molecular weight, sugar-specificity and serological properties. Using in vivo labelling techniques, it could be demonstrated that lectin is synthesized de novo in these tissues. The presence of lectin mRNA in seedlings was confirmed by in-vitro synthesis of lectin in root-tip extracts. Lectin synthesis occurs both in primary and first adventitious roots and is confined to the apical part (2mm) of the root. As seedling development proceeds, lectin synthesis in root tips gradually decreases. Adventitious roots of adult (five to six months old) wheat, rye and barley, but not rice, plants also contain lectins which are indistinguisable from the embryo lectins by the above-mentioned criteria. These lectins are synthesized in vivo in isolated root tips (5 mm) with labelled cysteine and in vitro in cell-free extracts prepared from root tips. Synthesis of lectin in roots of adult plants is also confined to the apical (2 mm) tip of the roots. At the molecular level, root lectin synthesis is very similar to that in embryos. All root lectins are synthesized as 23 000-M_r precursors which are post-translationally converted into the mature 18 000-M_r polypeptides. The observation that seedling roots and adventitious roots of six-month-old plants actively synthesize lectins strongly indicates that lectin genes are expressed in these tissues. In addition, since the root lectins are indistinguishable from the embryo lectins, we postulate that the same lectin genes are expressed.Abbreviations ABA abscisic acid - SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis - WGA wheat-germ agglutinin     

Effects of applying stem-shortening plant growth regulators to leaves on root elongation by seedlings of wheat, oat and barley: mediation by ethylene

Several plant growth regulators (PGRs) commonly used in practicalfarming to restrict shoot height and control lodging were examined for theirimpact on root growth in naturally short or tall cultivars of barley (cvs.Kymppi and Saana), oat (cvs. Veli and Pal), and wheat (cvs. Mahti and Tjalve).The possible involvement of ethylene in the responses was also examined. Shootswere sprayed at the two-leaf stage with the gibberellin biosynthesis inhibitorsCycocel (chlormequat chloride) (CCC) or Moddus (Trinexapac-ethyl) (TE), or withthe ethylene-releasing agent Cerone (ethephon) (ETH) at 0, 0.1, 1, 10 or 50times the recommended agricultural rate (RR). Root elongation and ethyleneproduction by roots or shoots were unaffected by CCC at all application ratesorby TE at ×0.1 or ×1.0 RR. At ×10 and ×50 RR, TE wasinhibitory to root extension but did not increase ethylene biosynthesis bytheseroots or the shoots. ETH at ×0.1 or ×1.0 RR did not affect rootextension or ethylene production in roots or shoots. At all higher rates ofapplication ETH stimulated ethylene production strongly in shoots and roots ofall three species, while root elongation was retarded severely in barley,moderately in oat and only slightly in wheat. These differences in elongationresponse are attributed to differences in sensitivity to ethylene released byethephon. Accordingly, root elongation in wheat was only slightly affected whenethylene gas was supplied at concentrations up to 100 ppm for 3d. In contrast, root elongation in barley was strongly inhibitedbyethylene, with oat demonstrating an intermediate responsiveness.     

The participation of the cell wall hydrolytic enzymes in the initial colonization of Azospirillum brasilense on wheat roots

The effect of cellulase and pectinase on bacterial colonization of wheat was studied by three different experiments. In the first experiment, the root colonization of 3 wheat cultivars (Ghods, Roshan and Omid) by two A. brasilense strains (Sp7 and Dol) was compared using pre-treated roots with cellulase and pectinase, and non-treated with these enzymes (control). Although the root colonization varied greatly among strain-plant combinations in controls, the pre-treatment of roots with polysaccharide degrading enzymes significantly increased the bacterial count in roots, regardless of the strain-plant combination. This might be an indication that cell wall may act as an important factor in plant-Azospirillum interaction. In the second experiment, the root cellulase activity of the same wheat cultivars treated with and without the two Azospirillum brasilense, strains (Sp7 and Dol) was compared. The pre-treatment of wheat roots with Azospirillum enhanced the cellulase activity of wheat root extracts. Thus, the cellulase activity might participate in the initial colonization of wheat roots by Azospirillum. The comparison of the cellulase activity of root extracts within inoculated and non-inoculated seedlings showed that the inoculation had enhanced the cellulase activity in root extracts, but this effect was directly dependent on the strain-plant combination. Strain Sp7 stimulated the highest cellulase activity in cv. Roshan, but strain Dol induced the highest enzyme activity in cv. Ghods. In the third experiment, several growth parameters of those 3 wheat cultivars treated with and without those two bacterial strains (Sp7 and Dol) were compared. The highest magnitude of growth responses caused by Sp7 strain was in the cv Roshan, but Dol strain stimulated the highest growth in cv Ghods. Therefore, effective colonization may contribute to more growth responses.     

Heterogeneous distribution of phosphorus and potassium in soil influences wheat growth and nutrient uptake

Heterogeneous distribution of mineral nutrients in soil profiles is a norm in agricultural lands, but its influence on nutrient uptake and crop growth is poorly documented. In this study, we examined the effects of varying phosphorus (P) and potassium (K) distribution on plant growth and nutrient uptake by wheat (Triticum aestivum L.) grown in a layered or split soil culture in glasshouse conditions. In the layered pot system the upper soil was supplied with P and either kept watered or allowed to dry or left P-deficient but watered, whereas the lower soil was watered and fertilised with K. Greater reductions in shoot growth, root length and dry weight in the upper soil layer occurred in −P/wet than in +P/dry upper soil treatment. Shoot P concentration and total P content were reduced by P deficiency but not by upper soil drying. Genotypic responses showed that K-efficient cv. Nyabing grew better and took up more P and K than K-inefficient cv. Gutha in well-watered condition, but the differences decreased when the upper soil layer was dry. In the split-root system, shoot dry weight and shoot P and K contents were similar when P and K were applied together in one compartment or separated into two compartments. In comparison, root growth was stimulated and plants took up more P and K in the treatment with the two nutrients supplied together compared with the treatment in which the two nutrients were separated. Roots proliferated in the compartment applied with either P or K at the expense of root growth in the adjoining compartment with neither P nor K. Heterogeneous nutrient distribution has a direct decreasing effect on root growth in deficient patches, and nutrient redistribution within the plant is unlikely to meet the demand of roots grown in such patches.     

硅酸盐细菌NBT菌株在小麦根际定殖的初步研究

对硅酸盐细菌NBT菌株进行了耐药性标记,得到稳定的链霉素抗性标记菌NBT菌株,采用选择性培养基分离计数,通过琼脂平板和盆栽试验,研究了标记菌NBT在小麦根际的定殖动态及影响因素。结果表明,在灭菌土盆栽中,播种后9d左右NBT菌株在小麦根际的定殖水平达最高(1．4×10^8cfu·g^-1根土),播种后54d左右趋向稳定,NBT菌株细胞数量为2．4×10^3cfu·g^-1根土;未灭菌土盆栽中,播种后9d左右NBT菌株的定殖水平达最高(3．8×10^8cfu·g^-1根土),60d左右趋向稳定,菌数为3．1×10^3cfu·g^-1根土,牛物和非牛物因素对NBT菌株定殖小麦根系有影响。     

冬小麦根系各种参数垂直分布实验研究

本文根据田间实测资料研究了冬小麦根系各种参数(长度、重量、活性表面和根径)的垂直分布。结果表明,在所观测地区的土壤条件下,冬小麦根系的长度、重量、表面积和体积随深度呈指数分布,而累积根系的长度、重量和表面积随深度呈双曲线型分布。根据作者的观测资料和国内外其它观测资料分析研究表明,对于不同土壤,根区各层土壤中累计根重及根长的百分比随相对深度的变化都符合下列双曲线函数形式:这一研究结果可为根系生态研究和作物对水分吸收的模拟工作提供参考。     

Influence of temperature and soil drying on respiration of individual roots in citrus: integrating greenhouse observations into a predictive model for the field

In citrus, the majority of fine roots are distributed near the soil surface – a region where conditions are frequently dry and temperatures fluctuate considerably. To develop a better understanding of the relationship between changes in soil conditions and a plant’s below‐ground respiratory costs, the effects of temperature and soil drying on citrus root respiration were quantified in controlled greenhouse experiments. Chambers designed for measuring the respiration of individual roots were used. Under moist soil conditions, root respiration in citrus increased exponentially with changes in soil temperature (Q₁₀ = 1·8–2·0), provided that the changes in temperature were short‐term. However, when temperatures were held constant, root respiration did not increase exponentially with increasing temperatures. Instead, the roots acclimated to controlled temperatures above 23 °C, thereby reducing their metabolism in warmer soils. Under drying soil conditions, root respiration decreased gradually beginning at 6% soil water content and reached a minimum at <2% soil water content in sandy soil. A model was constructed from greenhouse data to predict diurnal patterns of fine root respiration based on temperature and soil water content. The model was then validated in the field using data obtained by CO₂ trapping on root systems of mature citrus trees. The trees were grown at a site where the soil temperature and water content were manipulated. Respiration predicted by the model was in general agreement with observed rates, which indicates the model may be used to estimate entire root system respiration for citrus.     

Displacement of soil aggregates by elongating roots and emerging shoots of crop plants

Summary Models are presented in this paper for prediction of the extent to which soil aggregates in a loose seed bed can be displaced by extending roots and shoots. For roots, the maximum applied force is considered to be limited by either elastic bending, when the angle of contact with an aggregate surface is oblique, or buckling, when the root meets an aggregate perpendicularly. For emerging shoots, only the maximum forces are known. These forces are related to the known force displacement behaviour of artificially prepared beds of graded soil aggregates.It is concluded that displacement of soil aggregates which lie between 20 and 100 mm from the surface of the aggregate bed is only likely to be important for roots with diameters of 0.5 mm or less when the diameter of the aggregates in the bed is less than 1 mm. However, for plant species which have relatively large root diameters, such as pea, significant displacement of aggregates of up to 4 mm diameter may be possible. In contrast, emerging shoots are able to displace very much larger aggregates from their paths.     

The effect of soil strength on the yield of wheat

Although it is well-known that high soil strength is a constraint to root and shoot growth, it is not clear to what extent soil strength is the main physical stress that limits crop growth and yield. This is partly because it is difficult to separate the effects of soil drying and high soil strength, which tend to occur together. The aim of this paper is to test the hypothesis that for two different soil types, yield is closely related to soil strength irrespective of difference in soil water status and soil structure. Winter (Triticum aestivum L., cv. Hereward) and spring wheat (cv. Paragon) were grown in the field on two soils, which had very different physical characteristics. One was loamy sand and the other sandy clay loam; compaction and loosening treatments were applied in a fully factorial design to both. Crop growth and yield, carbon isotope discrimination, soil strength, water status, soil structure and hydraulic properties were measured. The results showed that irrespective of differences in soil type, structure and water status, soil strength gave a good prediction of crop yield. Comparison with previous data led to the conclusion that, irrespective of whether it was due to drying or compaction (poor soil management), soil strength appeared to be an important stress that limits crop productivity.