Morphological plasticity of wheat and barley roots in response to spatial variation in soil strength

Root systems of individual crop plants may encounter large variations in mechanical impedance to root penetration. Split-root experiments were conducted to compare the effects of spatial variation in soil strength on the morphological plasticity of wheat and barley roots, and its relationship to shoot growth. Plants of spring barley (Hordeum vulgare cv Prisma) and spring wheat (Triticum aestivum cv Alexandria) were grown for 12 days with their seminal roots divided between two halves of a cylinder packed with sandy loam soil. Three treatment combinations were imposed: loose soil where both halves of the cylinder were packed to 1.1 g cm^–3 (penetrometer resistance 0.3 MPa), dense soil where both halves were packed to 1.4 g cm^–3 (penetrometer resistance 1 MPa), and a split-root treatment where one half was packed to 1.1 and the other to 1.4 g cm^–3. In barley, uniform high soil strength restricted the extension of main seminal root axes more than laterals. In the split-root treatment, the length of laterals and the dry weight of main axes and laterals were increased in the loose soil half and reduced in the dense soil half compared with their respective loose and dense-soil controls. No such compensatory adjustments between main axis and laterals and between individual seminal roots were found in wheat. Variation in soil strength had no effect on the density of lateral roots (number per unit main axis length) in either barley or wheat. The nature and extent of wheat root plasticity in response to variation in soil strength was very different from that in response to changes in N-supply in previous experiments. In spite of the compensatory adjustments in growth between individual seminal roots of barley, the growth of barley shoots, as in wheat, was reduced when part of the root system was in compacted soil.     

Death of the cereal root cortex: its relevance to biological control of take-all

Cell death in the root cortex of cereals was assessed by an inability to detect nuclei, using acridine orangelfluorescence microscopy after fixation and mild acid hydrolysis. Seminal roots were scanned at x 100 magnification and their cortices were considered dead when nuclei were absent from all cell layers except the innermost one, adjacent to the endodermis; this cell layer remains alive long after the rest of the cortex has died. Cortical death of wheat and barley roots occurred in the absence of major pathogens. Cell death started behind the root hair zone of the main root axis, initially in the outermost cell layer of the cortex and then progressively inwards towards the endodermis; however, the cortex remained alive for a distance of c. 800 μm around emerging root laterals. The rate of cortical death was more rapid in wheat than in barley, both under field conditions and in the glasshouse at 20 °C. Thus, field-grown spring wheat (Sicca) showed 50% death of the root cortex in the top 6 cm of first seminal roots after 35 days (growth stage 1–2), whereas spring barley (Julia) showed 50% death of the root cortex after 67 days (growth stage 8). In the glasshouse, the top 9 cm of first seminal roots on 16-day plants showed 55% cortical death in wheat (Cappelle-Desprez) but only 2.5% cortical death in barley (Igri). The same rates of death were found in all subsequent seminal roots. The wheat root cortex died at the same rate in sterile and unsterile conditions, and at the same rate in the presence/absence of Phialophora radicicola Cain var. graminicola Deacon or Aureobasidium bolleyi (Sprague) von Arx. Hence, although P. radicicola and other soil microorganisms may benefit from root cortex death they do not exert biological control of take-all by enhancing or retarding the rate of this process. To study the effects of cortical death on take-all, Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker was point-inoculated at the tips and on older (5 and 15 day) regions of wheat seminal roots. After 17 days at 20 °C the fungus had grown to the same extent as runner-hyphae in all cases, but the severity of disease decreased with increasing age of the root cortex prior to inoculation; thus, G. graminis caused most extensive vascular discoloration and most intense vascular blockage in roots inoculated at their tips. Similar experiments on wheat and barley roots inoculated separately with P. radicicola and G. graminis suggest that at least three factors associated with cortical death influence infection by these fungi: (1) initially, cell death may enhance infection because nutrients are made available to the parasites and host resistance within the cortex is reduced; (2) weak parasites and soil saprophytes may colonise dead and dying cortices in competition with G. graminis and P. radicicola and thereby reduce infection by these fungi; (3) changes in the endodermis and adjacent cell layers may be associated with cortical death and may retard invasion of the stele. Future work will seek to establish the relative importance of these factors and extend this study to other cereal host-fungus combinations.     

Phenylalanine Ammonia-lyase Activity in Barley After Infection with Bipolaris sorokiniana or Treatment with its Purified Xylanase

Phenylalanine ammonia-lyase (PAL) activity was determined from leaves and roots of two barley (Hordeum vulgare L.) cultivars after infection with a necrotrophic pathogen, Bipolaris sorokiniana (Sacc.) Shoem., and treatment with its purified xylanase. PAL activity increased in leaves of both cultivars 16 h after fungal inoculation but two phases, with activity peaks at 24–32 h and 40 h, were recorded only for the more resistant cultivar, Agneta. Attempts to use a PAL inhibitor, χ-amin, ooxyacetic acid, to increase susceptibility to B. sorokiniana in barley leaves were unsuccessful. Treatments of leaves with purified xylanase resulted in more rapid (4–12 h after injection), although reduced, induction of PAL compared with fungal injection. The higher the concentration of xylanase applied the earlier the activity peaks were detected. Fungal inoculation only slightly increased PAL activity in barley roots while xylanase treatment had no effect. The basal level of PAL was however much higher in roots than in leaves. In wheat, Triticum aestivum L. resistant to B. sorokiniana, the time-course of PAL induction after fungal infection and xylanase treatment resembled that for cv. Agneta, while in oats, Avena sativa L. (non-host) PAL activity did not change after the treatments. The results suggest that the second phase of PAL induction, associated only with responses of barley cv. Agneta and wheat, is linked with their resistance to B. sorokiniana infection. The possible role of xylanase as an elicitor of PAL is discussed.     

Comparison of rates of natural senescence of the root cortex of wheat (with and without mildew infection), barley,oats and rye

Summary In comparative tests in a glasshouse, the cortex of oat and rye roots senesced more slowly than the cortex of wheat and barley roots. Of the cereals tested, wheat showed the most rapid rate of root cortical senescence, and the rate was unaffected by inoculation of leaves withErysiphe graminis. The results are discussed in relation to infection by root pathogens.     

A comparative study of Phialophora radicicola, an avirulent fungal root parasite of grasses and cereals

The biology and infection-behaviour of a typical isolate of Phialophora radicicola Cain have been compared with those of a representative isolate of Ophiobolus graminis (Sacc.) Sacc. Both species can utilize a nitrate source of nitrogen and both require thiamine and biotin for growth on inorganic nitro-gen; P. radicicola, but not O. graminis, was able to synthesize biotin when grown on asparagine as a nitrogen source. The pH range for good growth of P. radicicola in nutrient solution was narrower than that for O. graminis, and its growth rate on agar was only one-third. P. radicicola was the more active decomposer of cellulose, and its cellulolysis adequacy index was I.66 as com-pared with a value of 0.33 for 0. graminis. In agreement with prediction from Garrett's (I966) hypothesis on the cellulolysis adequacy index, saprophytic survival of P. radicicola in wheat straw was shortened by additional soil nitrogen, which prolongs survival of O. graminis.P. radicicola was found to spread ectotrophically over the roots of wheat, oats and barley by runner hyphae indistinguishable from those of O. graminis, but cortical infection caused no necrosis and no discernible check to growth of the infected cereals, nor any significant decrease in grain yield of inoculated wheat grown to maturity. Pre-existing infection of wheat roots by P. radicicola retarded spread of infection by O. graminis; inoculation of several grass species with P. radicicola reduced the extent of infection by O. graminis of wheat following the grasses.     

The influence of soil-applied fungicides and previous cropping on the development of violet root rot of sugar beet

The proportion of sugar-beet roots infected by Helicobasidium purpureum increased most rapidly in September and October. Violet root rot was not controlled by fungicides applied at drilling or in July. Heavily infected roots yielded 31% less sugar than healthy or lightly infected roots. Sugar beet following infected carrots lifted or ploughed in during July had no more violet root rot than when following barley or fallow, but the beet crop became heavily infected when it followed carrots left in the ground until December, whether they were then lifted or ploughed in. Eight varieties of sno-ar beet did nnt differ in siiscenrihilitv to violet root rot.     

Effect of Ophiobolus graminis infection on the assimilation and distribution of 14C in wheat

The uptake of ¹⁴C and movement of ¹⁴C-labelled assimilates in wheat plants inoculated with Ophiobolus graminis was examined following exposure of the second youngest leaf to ¹⁴CO₂. Autoradiographs of plants with infected seminal roots showed that assimilates were not translocated past the sites of root infection but accumulated in the undamaged portions of infected root systems, in particular the developing crown roots. There was no evidence that assimilates accumulated in the vicinity of O. graminis lesions. The net assimilation of ¹⁴CO₂ by wheat plants over a 5 h feeding period was not significantly affected by O. graminis infection. However, infection reduced the amount of ¹⁴C lost through respiration. Infection delayed the transfer of labelled assimilates from the fed leaf to the remainder of the plant but increased the proportion translocated to the roots. The latter effect was not apparent when infected plants were continuously irrigated during, and for 20 h following, the feeding period.     

Effect of Helminthosporic Root Rot on the Lipid Content in Wheat Seedlings

The concentration of dry matter and the content of esterified fatty acids in total lipids of roots and etiolated shoots of 3- to 10-day-old seedlings of wheat (Triticum aestivum L.) infected with Bipolaris sorokiniana (Sacc.) Schoemaker, the agent of helminthosporic root rot, were determined in the course of germination. At the onset of germination, fungal infection caused a considerable increase in the dry matter concentration in both roots and shoots due to the enhanced mobilization of seed reserves. However, after the 7th day of germination, dry matter concentration fell below the level of noninfected control seedlings as a result of infection. The content of total lipids rose immediately after infection and always exceeded the control index up to the end of germination, in spite of a continuous decrease in this index in both control and infected seedling. It is concluded that an increase in the content of cellular lipids is a characteristic response of both shoots and roots to the root rot infection of wheat seedlings.     

Features of Resistance to Take-all Disease in Cereal Species Evaluated by Laboratory Assays

Development of take-all disease, caused by Gaeumannomyces graminis var. tritici, on wheat, barley and rye, respectively, susceptible, moderately susceptible and resistant under field conditions, was evaluated by laboratory assays. The root system, which grew either in vermiculite in plastic tubes or polyethylene sleeves, or in a sand mix in plastic pots, was inculated, and lesion frequency and extension, both considered as resistance factors, were monitored. In assays in tubes and pots, both enabling multiple infection points, the infection scores of wheat, barley and rye on a scale of 0–5 were in decreasing order. In assays in sleeves, with a single point inoculation, lesion length on barley and rye was similar and less than thaton, wheat, but percentage of infected roots was markedly lower on rye than on barley. For large-scale screening programs we suggest to employ both the Tube Assay, which is reliable and easy to perform and the Slanted Sleeve Assay, which is more sensitive allowing detection of even a small degree of resistance. Selected accessions could be subsequently evaluated by the Pot Assay, under more natural conditions, and the resistance to take-all must eventually be verified under field conditions.     

Impact of the exodermis on infection of roots byFusarium culmorum

Patterns of infection withFusarium culmorum (W G Smith) Saccardo were observed in seedling roots of barley (Hordeum vulgare L.), wheat (Triticum aestivum L.), maize (Zea mays L.) and asparagus (Asparagus officinalis L). Apical regions of the main roots were not infected. Since penetration into the root occurred several days after inoculation and the roots were growing during the experiment, these regions had apparently not been in existence long enough to be infected. In older regions of barley, wheat and asparagus, hyphae entered through the tips of lateral roots. In barley and wheat, which had not developed any suberin lamellae in their subepidermal layer, infection occurred randomly over the remainder of the root. In maize, the fungus penetrated the epidermis at many sites but did not breach the exodermis in which all cells possessed both Casparian bands and suberin lamellae. Maize roots, therefore, sustained only minimal infections. In asparagus, the fungus grew through the short (passage) cells but never the long cells of the exodermis. In doing so, it penetrated cells possessing Casparian bands but lacking suberin lamellae. The results support the hypothesis that suberin lamellae provide effective barriers to the growth ofF. culmorum hyphae.     

The effects of barley yellow dwarf virus, aphids and honeydew on Cladosporium infection of winter wheat and barley

Prior infection of both wheat and barley plants by BYDV predisposed their ears to infection by Cladosporium spp. and Verticillium spp. Aphids and honeydew increased the incidence of Cladosporium on wheat ears but not on barley. This difference between crops was attributed to the larger number ot aphids on the wheat. In the glasshouse, aphids and honeydew, but not honeydew alone, increased Cladosporium populations.     

Effects of long-term barley monoculture on plant-affecting soil microbiota

Effects of soil microbiota on shoot and root growth of barley were tested in a greenhouse tube-growing system. Tubes were filled with a mixture of pure sand and various percentages of soils sampled from plots in three long-term field experiments measuring effects of various crop rotations on yield. Using 3% soil in the sand-soil mixture, shoot dry weight of barley test plants was reduced by about 35% and root depth by about 40% in soils from monoculture plots as compared to soils from crop-rotation plots. Typical root symptoms on poorly growing barley plants started as distinct dark-brown zones which then rapidly spread over the whole root system until the root tips ceased to grow. As tested in one experiment, the barley monoculture soil also affected wheat and oats, but to a lesser degree than it did barley. Most of the depressing effects of monoculture soil on barley were eliminated when soil samples were treated with metalaxyl or heated to 65°C for 2 hours. A Pythium sp. frequently isolated from barley roots showing typical symptoms affected barley, wheat and oats in the same way as did barley monoculture soil.     

Effect of <Emphasis Type="Italic">Pseudomonas</Emphasis> Bacteria on Peroxidase Activity in Wheat Plants when Infected with <Emphasis Type="Italic">Bipolaris sorokiniana</Emphasis>

We investigated the effect of treating soft wheat seeds (Triticum aestivum L.) with two Pseudomonas bacteria strains, isolated from earthworm coprolites, showing a significant antifungal and growth-promoting action in preliminary screening on the activity of guaiacol-dependant peroxidase under phytopathogenic load in the presence of Bipolaris sorokiniana (Sacc.) Shoemaker as a mechanism for inducing plant resistance to the pathogen. We established a statistically significant decrease (P < 0.05) in root rot disease incidence and severity during bacterization, which is indicative both of antifungal activity of the used bacterial isolates and of their successful colonizing the rhizosphere of wheat plants. We noted a response of free and weakly bound peroxidase of wheat plants to infection with B. sorokiniana: the enzyme activity increased during pathogenesis. Bacterization also increased peroxidase activity in plant leaves and roots, the greatest differences from non-bacterized plants being observed in wheat roots in the presence of the pathogen. We detected a direct link between peroxidase activity in wheat roots and leaf tissues in the absence of the pathogen and the feedback between peroxidase activity and plant infestation by the root rot pathogen. In the presence of the phytopathogen, there is a lack of correlation between peroxidase activity in wheat roots and leaves, and there is a shift of activity towards its increase in roots, which plays an important role in the development of systemic resistance against the root rot pathogen that penetrates into plants through the roots and root collar.     

Root dynamics in barley,lucerne and meadow fescue investigated with a mini-rhizotron technique

Root development, including depth distribution, was followed in pure barley stands (Hordeum distichum, L.) with or without nitrogen fertilization and in barley undersown with lucerne (Medicago sativa L.) or meadow fescue (Festuca pratensis, Huds.). The number of roots per 5 cm depth level down to 1 m was counted frequently during the growing season using mini-rhizotrons, i.e., transparent tubes inserted into the soil. Root biomass at different depths down to 1 m was estimated from soil cores taken one month before harvest. The results from the two methods were compared and root counts in the different treatments were compared with the above-ground growth and production. Nitrogen-fertilized barley in pure stand had the highest biomass both above and below ground. According to the mini-rhizotron observations this treatment also had a deeper and denser root system, until barley harvest, than the other treatments. After barley harvest, roots from the undersown lucerne continued to increase, whereas the number of roots in the undersown meadow fescue remained the same. The root system in barley/meadow fescue did not penetrate into the subsoil, where more than 60% of the number of roots in barley undersown with lucerne were found. In general, the mini-rhizotron results indicated a higher relative abundance of roots in the deeper layers than the root biomass estimated with the soil coring method.     

禾谷多粘菌对小麦梭条斑花叶病毒的传播及其土壤侵染潜力的测定

从大田侵染小麦梭条斑花叶病毒的小麦病根中挑取禾谷多粘菌休眠孢子堆,接种受侵染小麦品种扬麦４号,经砂培养纯化,获得５个禾谷多粘菌分离物,但都为无毒。无毒多粘菌休眠孢子堆接种表现ＷＳＳＭＶ症状的小麦,经培养可饲获病毒,并可经接咱后将病毒传播给无病小麦,供试的４个大小麦禾谷多粘菌分离物都可对大小进行交叉侵染,产生同样数量的游动孢子产量。供试５个病土和２个无病土样品,都具有强大持多粘菌侵染潜力,即使稀释放     

Quantifying relationships between rooting traits and water uptake under drought in Mediterranean barley and durum wheat

In Mediterranean regions drought is the major factor limiting spring barley and durum wheat grain yields. This study aimed to compare spring barley and durum wheat root and shoot responses to drought and quantify relationships between root traits and water uptake under terminal drought.One spring barley（Hordeum vulgare L. cv. Rum） and two durum wheat Mediterranean cultivars（Triticum turgidum L. var durum cvs Hourani and Karim） were examined in soil‐column experiments under well watered and drought conditions. Root system architecture traits, water uptake, and plant growth were measured. Barley aerial biomass and grain yields were higher than for durum wheat cultivars in well watered conditions. Drought decreased grain yield more for barley（47%） than durum wheat（30%, Hourani）. Root‐to‐shoot dry matter ratio increased for durum wheat under drought but not for barley, and root weight increased for wheat in response todrought but decreased for barley. The critical root length density（RLD） and root volume density（RVD） for 90% available water capture for wheat were similar to（cv. Hourani） or lower than（cv. Karim） for barley depending on wheat cultivar. For both species, RVD accounted for a slightly higher proportion of phenotypic variation in water uptake under drought than RLD.     

Effects of shading and powdery mildew infection on senescence of the root cortex and coleoptile of wheat and barley seedlings,and implications for root- and foot-rot fungi

Summary Nuclear and cytoplasmic staining methods were used to study natural senescence of the root cortex and coleoptile of wheat and barley seedlings grown in glasshouse conditions. Coleoptiles of barley senesced more slowly than those of wheat, paralleling the known difference in rates of root cortex senescence in these cereals. The coleoptiles and root cortices of both cereals senesced more slowly in shaded than in unshaded conditions, but infection of the shoots of barley byErysiphe graminis had little effect on root cortex senescence. The results are discussed in relation to infection by root- and foot-rot fungi. Previous reports on the effects of illumination on take-all infection (Gaeumannomyces graminis) are explained. It is suggested that natural senescence of the coleoptile might affect establishment of infection by the eyespot fungus,Pseudocercosporella herpotrichoides, either directly or through the activities of competing microorganisms.     

Ultrastructural localization and identification ofAzospirillum brasilense Cd on and within wheat root by immuno-gold labeling

Azospirillum brasilense Cd localization in wheat roots was studied by light microscopy, by scanning, and by transmission electron microscopy.A. brasilense Cd cells were specifically identified immunocytochemically around and within root tissues.A. brasilense Cd cells found both outside and inside inoculated roots were intensively labeled with colloidal gold. In non-axenic cultures other bacterial strains or plant tissue were not labeled, thereby providing a non-interfering background. The roots of axenic grown wheat plants were colonized both externally and internally byA. brasilense Cd after inoculation, whereas non-axenic cultures were colonized by other bacterial strains as well.A. brasilense Cd cells were located on the root surface along the following zones: the root tip, the elongation, and the root-hair zone. However, bacteria were located within the cortex only in the latter two zones. In a number of observations, an electron dense material mediated the binding of bacterial cells to outer surfaces of epidermal cells, or between adjacent bacterial cells.A. brasilense Cd were found in root cortical intercellular spaces, but were not detected in either the endodermal layer or in the vascular system. This study proposes that in addition to root surface colonization,A. brasilense Cd forms intercellular associations within wheat roots.     

Observations on take-all and eyespot diseases of wheat in Yorkshire

Wheat crops were surveyed in Yorkshire from 1944 to 1946 on farms where crops were reported to be unsatisfactory. Take-all ( Ophiobolus graminis (Sacc.) Sacc.) and eyespot ( Cercosporella herpotrichoides Fron.) were both found to be present; the latter is the most serious trouble on the better wheat lands.
The variation in the incidence of these diseases on selected farms during the three seasons has been compared.
The effect of rotation has been examined; both diseases were found to be encouraged by too frequent cropping with wheat or barley. The incidence of disease in wheat crops following a 1-year seeds ley was found to be influenced by the nurse crop used to undersow the seeds, and also by the time at which the ley was ploughed up. Oats were found to be preferable to either wheat or barley as a nurse crop, and in 1946 late ploughing considerably reduced the amount of eyespot disease present in the following wheat crop.
A 1-year ley is considered to be of too short a duration to ensure the disappearance from the soil of Ophiobolus graminis and Cercosporella herpotrichoides surviving from the previous crop, and a period of 2–3 years is suggested as being desirable.     

Factors affecting formation of cluster roots in Myrica gale seedlings in water culture

The effect of nutrient deficiency, aeration, phosphorus supply, and nitrogen source on the formation of cluster (proteoid) roots was examined in Myrica gale seedlings growing in water culture. Only the omission of phosphorus resulted in the formation of significant numbers to cluster roots when plants were grown in a number of 1/4 strength Hoagland's solutions, each lacking one mineral nutrient. Aeration shortened the time required for cluster root formation and increased the percentage of plants forming cluster roots. The proportion of the root system comprised of cluster roots decreased as the phosphorus concentration in the solution increased and no cluster roots formed in solutions containing 8 mg P/L. Phosphorus supply also affected total plant biomass, proportion of biomass comprising nitrogen-fixing nodules, shoot:root ratio, phosphorus concentration in the leaves and phosphorus content of the plants. The plants showed luxury consumption of phosphorus and were able to produce large amounts of biomass utilizing only stored phosphorus.Nitrogen source also affected cluster root formation. Urea-fed plants produced cluster roots more quickly and devoted a substantially larger proportion of root growth to cluster roots than did nitrate-fed plants. The longest cluster root axes were produced in nitrate-fed plants supplied with no phosphorus and the shortest were in urea-fed plants at 4 mg P L^–1.Four methods for expressing the extent of cluster root formation were examined and it was concluded that cluster roots as a proportion of total fine root dry weight is preferable in many cases. Formation of cluster roots in response to phosphorus deficiency coupled with previously demonstrated traits allows Myrica gale to adapt to a wide range of soil conditions.