Bare Roots Research Articles

First Report of Ilyonectria robusta Causing Rusty Root of Asian Ginseng in China.

Asian ginseng (Panax ginseng) is an economically important perennial herb, mainly cultivated in Jilin Province, China. In September 2013, Asian ginseng plants in Jilin showed rusty root symptoms. Typical symptoms included rusty superficial lesions of irregular shapes and margins. Ten symptomatic roots were collected from each of five fields for investigation. To isolate the pathogen, root epidermal tissues with typical lesions were excised, surface-sterilized, and placed on potato dextrose agar (PDA) amended with 50 μg/ml tetracycline. After incubation at 20 ± 1°C in the dark for a week, 18 single-spore isolates out of 50 samples were obtained and identified as Ilyonectria robusta (A.A. Hildebr.) A. Cabral & Crous based on morphological characters and DNA sequence analysis (1). After incubating 7 days on PDA in the dark at 20°C, colonies were cottony to felty in texture and orange white to brownish grey in color with average diameters of 60 ± 3 mm. Isolates were cultured on synthetic nutrient-poor agar for conidial measurements. Macroconidia formed on simple conidiophores predominately, with mostly one and occasionally up to three septa, and were cylindrical with both ends broadly rounded. Macroconidia varied in size depending on the number of cells as follows: one-septate, 7.0 ± 0.6 × 27.7 ± 2.7 μm; two-septate, 7.3 ± 0.7 × 33.3 ± 2.1 μm; three-septate, 7.4 ± 0.6 × 33.4 ± 2.2 μm. Microconidia that formed on complex conidiophores were ellipsoid to ovoid and ranged in size from aseptate 3.7 ± 0.5 × 8.7 ± 1.1 μm to one-septate 5.0 ± 0.6 × 13.1 ± 1.6 μm. Brown chlamydospores were abundantly produced on PDA, globose to subglobose in shape, and in size of 10.9 ± 1.3 × 11.8 ± 1.5 μm (n ≥ 30 observations per structure for each measurement). The isolates were further classified by amplifying and sequencing the ITS1-5.8S rRNA-ITS2 region and histone H3 gene with primer pairs ITS5 and ITS4 (4), and H3-1a and H3-1b (3), respectively. Sequences of the two loci (GenBank Accession Nos. KM015300 and KM015299) showed 100% identity among the three examined isolates and the published I. robusta isolates (JF735268 and JF735517). To confirm the pathogenicity, bare roots of 3-year-old Asian ginseng were inoculated with mycelial plugs of three isolates of I. robusta selected randomly. Four roots were inoculated as replicates for each isolate with pathogen-free agar plugs as a control. One week post-inoculation in the dark at 20 ± 1°C, all the inoculated ginseng roots showed light-brown to dark-brown lesions. I. robusta was recovered from symptomatic roots and confirmed by analyzing the DNA sequence of the histone H3 gene. The inoculation experiment was repeated, and both trials showed the same results. The ginseng tissue under the control agar plugs remained symptomless, and no fungi were isolated. To our knowledge, this is the first report of I. robusta causing rusty root of P. ginseng in China (1,2,5).

First Report of Rhexocercosporidium panacis Causing Rusty Root of Panax ginseng in Northeastern China.

In northeastern China, Asian ginseng (Panax ginseng) roots exhibited reddish brown lesions of various sizes, irregular shapes, and diffuse margins, typical of rusty root disease. The lesions remain superficial, smooth, and limited to the epidermal and peridermal tissues. In September 2013, 10 symptomatic roots were collected from each of three fields in Jilin and Heilongjiang provinces. One piece of symptomatic skin tissue from each root was excised, surface-disinfested in 1% NaClO for 3 min, rinsed three times with sterile water, and then placed on tetracycline-amended (50 μg/ml) potato dextrose agar. After incubation at 22 ± 1°C in the dark for a week, small olivaceous black colonies developed from the symptomatic tissue from five of the 30 samples. No spores were observed. A single hyphal tip of each colony was transferred to a fresh V8 agar plate to purify the culture. Two-week-old colonies on V8 agar were olivaceous gray, and 42 to 46 mm in diameter with an outer white margin (3 to 5 mm wide). Conidia produced in V8 broth after 3 weeks with a 12-h photoperiod were straight and hyaline, cylindrical or subcylindrical with no or one septum. Mature conidia were 12.8 to 21.8 × 2.2 to 4.5 μm (mean 18.2 × 3.0 μm, n = 100 conidia for each of three isolates). Three isolates selected randomly were further identified by analyzing the partial sequences of the ITS region of rDNA with primers ITS4 and ITS5 (5), and partial sequences of β-tubulin with the primers tub2F and tub2R (1). Sequences of the three isolates (GenBank Accession Nos. KJ149287, KJ149288, and KJ149290 to 93) showed 99% to 100% homology with previously identified and deposited Rhexocercosporidium panacis isolates (DQ2499992 and DQ457119) for both loci (3). Therefore, the three isolates were identified as R. panacis and deposited in China General Microbiological Culture Collection Center (CGMCC3.17259 to 61). Pathogenicity of R. panacis in Asian ginseng was investigated using these three isolates as described previously with slight modifications (4). Bare roots of 3-year-old Asian ginseng were surface-disinfested as described above, and inoculated with mycelial plugs (4 mm diameter) cut from the margin of actively growing colonies of the isolates on V8 agar. Three mycelial plugs were placed on each root at 3-cm intervals and four roots (replicates) were inoculated for each isolate. Four additional roots were inoculated with non-colonized agar plugs as control. The treated roots were placed on moist filter paper in an enamel tray. The plates were sealed with plastic wrap to prevent desiccation and incubated in the dark at 18 ± 1°C. Four weeks post inoculation, all the inoculated ginseng roots showed red-brown lesions, which turned to dark red or black over time. R. panacis was recovered from symptomatic roots for all isolates and confirmed by ITS sequence analysis. The mock-inoculated control roots remained symptomless and no R. panacis was isolated. The inoculation experiment was repeated and showed the same results. R. panacis was reported in 2006 to infect roots of Panax quinquefolius (2,3,4). To our knowledge, this is the first report of R. panacis causing rusty root of P. ginseng.

Strawberry yield submitted to different root pruning intensities of transplants

This study aimed to evaluate the growth of plants and the precocity of strawberry production under different root pruning intensities at planting time. Bare roots plants with 12 millimeters crown diameter produced in nurseries from the Patagonia region, Argentina were used. The planting was carried out on May 12th 2010 into experimental plots with non-fumigated soil. The treatments consisted of three cultivars (Camarosa, Florida Festival and Camino Real) and three pruning intensities (1/3, 2/3 and no pruning) on the total root length of the plants. The experimental design used was a randomized block design in a 3x3 factorial arrangement with three replications and 12 plants per plot and density of 11.1 plants m-2. Mature fruits were harvested from July 15th to December 14th 2010 and the production of fresh fruit was determined. There was no significative interaction between cultivars and pruning intensity. 'Camarosa' and 'Florida Festival' plants showed precocity and had the most abundant and heavier fruits during the precocity period. The different root pruning intensities did not affect the assessed variables. It was concluded that, in order to facilitate strawberry planting of the cultivars Camarosa, Florida Festival and Camino Real root pruning is possible, with no damages on plant growth and development, precocity and early fruit production.

Initial morphological learning in preverbal infants

How do children learn the internal structure of inflected words? We hypothesized that bound functional morphemes begin to be encoded at the preverbal stage, driven by their frequent occurrence with highly variable roots, and that infants in turn use these morphemes to interpret other words with the same inflections. Using a preferential looking procedure, we showed that French-learning 11-month-olds encoded the frequent French functor /e/, and perceived bare roots and their inflected variants as related forms. In another experiment an added training phase presented an artificial suffix co-occurring with many pseudo-roots. Infants learned the new suffix and used it to interpret novel affixed words that never occurred during the training. These findings demonstrate that initial learning of sub-lexical functors and morphological alternations is frequency-based, without relying on word meaning.

EFFECTS OF AMMONIUM RATIO AND NUTRIENT DELIVERY INTERVAL ON ROSES GROWING IN AEROPONICS

Roses have been grown for centuries because of their aesthetic value in landscapes, pots or vases as well as their nutritional and therapeutic significance. This has led growers to search for means of improving its production by changing the mode of cultivation and the fertilization regime. The development of a new aeroponic system is reported here. Stentings of Rosa x hybrida 'Prestige' were placed on top of separate mist chambers and nutrient solutions were sprayed on their bare roots by means of pressurized air. Aeroponically-grown roses were treated with four NH 4 + /total N ratios (0.14, 0.03, 0.02 and 0.008) of a complete nutrient solution and two spray intervals (3/15 min and 3/30 min; on/off). Subsequent growth of plants was measured by number of flowers, length and thickness of flowering stems, length and thickness of flower buds, leaf and total dry weight and total leaf number. The middle range proportions of ammonium-N in the nutrient solutions (0.03 and 0.02) improved the growth characteristics of plants including number of flowers, leaf and total dry weight, and length and thickness of flower buds. The shorter spray interval (3/15 min) resulted in roses with improved growth: length and thickness of flowering stems, length and thickness of flower buds and total leaf number were enhanced.

Closed soilless growing system for producing strawberry bare root transplants and runner tips

The objective of this work was to test a closed soilless growing system for producing bare root transplants and runner tips of two strawberry clones, using two categories of substrates. The system used corrugated roofing panels of fiber-cement, over which a substrate layer was used as a growing bed. The nutrient solution was pumped from a reservoir toward the upper end of the roofing panels and drained back to a reservoir. Plant growth and development were determined for two advanced strawberry clones, grown in sand or in Plantmax organic substrate. Growth of the stock plants and the number and dry mass of bare root transplants were similar in the substrates, but bare roots differed in their crown diameters by substrate. For number of runner tips, no significant differences were found in total, small, and medium categories in the substrates. A mean production of about 590 runner tips per square meter and 145 bare root transplants per square meter was obtained. For both clones, a large number of bare root transplants and runner tips of adequate size were produced in the closed soilless growing system using sand or organic substrate.

Efecto del sistema y densidad de la almaciguera en el cultivo de cebolla (Allium cepa)

C. Gomez, and C. Oberpaur. 2007. Effect of density and seedbed system in onion (Allium ( cepa) production. Cien. Inv. Agr. 34(3):205-214. Production of onion (Allium cepa) seedlings for transplants is done directly in fl at soil conditions or in trays using reduced cell volumes and high plant density conditions, without consideration of the possible effect that reduced cell volume can have on crop growth. Therefore, the objective of this study was to evaluate the production of onion seedlings in two seedbed systems and their subsequent behavior in the fi eld. Seedling production and transplant establishment were determined with a long-day type of onion (cv. Grano de Oro) in the Metropolitan Region of Chile (33°-34° south latitude) from 2004 to 2005. The production of covered root transplants was performed in trays with 286 (18 cm 3 ), 432 (10 cm 3 ) and 814 (5 cm 3 ) cells each, equivalent to plant densities of 1144 (low), 1728 (medium) and 3256 (high) plants·m -2 , respectively. The production of bare root transplants was done in boxes with equal plant densities as in the tray system. Next, plants obtained from each seedbed system and plant density were fi eld transplanted. Based on the results obtained, neither plant density nor transplant production system affected the pseudo stem diameter 25 days after emergency (DAE). At 77 DAE, the pseudo stem diameters were 3.8 and 5.2 mm, the root dry weights (DW) were 4.8 and 6.2 g·plant -1 produced in high and low plant density, respectively. This suggests that the tray system had a minor effect on plant development. At 23 days after transplanting (DAT) differences in the pseudo stem diameter between the three densities were obtained, although differences between the bare and covered root systems were not statistically signifi cant. Plants obtained from high-density seedbeds showed a smaller pseudo stem diameter and lower canopy DW than plants produced in low-density seedbeds until 84 DAT. At harvest, no differences were observed in formation or among the quality categories of the onion. Therefore, it is feasible to produce onion seedlings to be transplanted with bare roots or covered roots, produced in high-density boxes or in 5 cm 3 cells, without affecting the fi nal quality of harvested onion bulbs.

Visualizing three-dimensional root networks using computed tomography

We describe a method to reconstruct the root network from a three-dimensional image generated with computed tomography. The X-ray absorption of roots and the interface of the air–soil material is very similar and the contrast extremely low. The method consists of three steps: contrast enhancement using a non-linear diffusion filter, thresholding based on Rosin's method and extraction of the main features using a morphological connectivity algorithm. With this approach, we are also capable of reconstructing fine roots with high fidelity, which is shown with a performance experiment using artificial images with increasing noise. The method is applied to an X-ray microtomography data set of two alder roots, grown for 4 months in a natural moraine soil. To simplify the soil material, the bare roots were transferred into a Plexiglas receptacle filled with quartz sand. The reconstructed image is compared with a photograph of the real roots.

Utilization of photographs for determining impact indicators for trail management.

"Indicators" are a key component of resource management, but selecting them can be difficult and controversial. This study explores different methods for selecting impact indicators for management of trail conditions by comparing photo survey methods (PSM) with written survey methods (WSM). A total of 661 subjects participated in interviews conducted within Mudeung-Mountain Provincial Park in Kwang-ju, Korea. PSM explained variation in acceptability better than WSM. Six major photo components (in order of importance: area of large rocks on trail, area of wood steps, area of bare roots, area of bare soil, area of small stones, and area of stone steps) explained 84.7% of the variation in acceptability of trail management conditions. This study suggests that PSM can be a useful tool for selecting impact indicators.

Plant responses to abiotic stresses: heavy metal-induced oxidative stress and protection by mycorrhization

The aim of this review is to assess the mode of action and role of antioxidants as protection from heavy metal stress in roots, mycorrhizal fungi and mycorrhizae. Based on their chemical and physical properties three different molecular mechanisms of heavy metal toxicity can be distinguished: (a) production of reactive oxygen species by autoxidation and Fenton reaction; this reaction is typical for transition metals such as iron or copper, (b) blocking of essential functional groups in biomolecules, this reaction has mainly been reported for non-redox-reactive heavy metals such as cadmium and mercury, (c) displacement of essential metal ions from biomolecules; the latter reaction occurs with different kinds of heavy metals. Transition metals cause oxidative injury in plant tissue, but a literature survey did not provide evidence that this stress could be alleviated by increased levels of antioxidative systems. The reason may be that transition metals initiate hydroxyl radical production, which can not be controlled by antioxidants. Exposure of plants to non-redox reactive metals also resulted in oxidative stress as indicated by lipid peroxidation, H(2)O(2) accumulation, and an oxidative burst. Cadmium and some other metals caused a transient depletion of GSH and an inhibition of antioxidative enzymes, especially of glutathione reductase. Assessment of antioxidative capacities by metabolic modelling suggested that the reported diminution of antioxidants was sufficient to cause H(2)O(2) accumulation. The depletion of GSH is apparently a critical step in cadmium sensitivity since plants with improved capacities for GSH synthesis displayed higher Cd tolerance. Available data suggest that cadmium, when not detoxified rapidly enough, may trigger, via the disturbance of the redox control of the cell, a sequence of reactions leading to growth inhibition, stimulation of secondary metabolism, lignification, and finally cell death. This view is in contrast to the idea that cadmium results in unspecific necrosis. Plants in certain mycorrhizal associations are less sensitive to cadmium stress than non-mycorrhizal plants. Data about antioxidative systems in mycorrhizal fungi in pure culture and in symbiosis are scarce. The present results indicate that mycorrhization stimulated the phenolic defence system in the Paxillus-Pinus mycorrhizal symbiosis. Cadmium-induced changes in mycorrhizal roots were absent or smaller than those in non-mycorrhizal roots. These observations suggest that although changes in rhizospheric conditions were perceived by the root part of the symbiosis, the typical Cd-induced stress responses of phenolics were buffered. It is not known whether mycorrhization protected roots from Cd-induced injury by preventing access of cadmium to sensitive extra- or intracellular sites, or by excreted or intrinsic metal-chelators, or by other defence systems. It is possible that mycorrhizal fungi provide protection via GSH since higher concentrations of this thiol were found in pure cultures of the fungi than in bare roots. The development of stress-tolerant plant-mycorrhizal associations may be a promising new strategy for phytoremediation and soil amelioration measures.

The photosynthetic activities of bare rooted Phalaenopsis during storage

Only bare rooted Phalaenopsis orchids can be exported to the United States from Taiwan. Being bare rooted, these plants may be exposed to various environmental stresses that sometimes lead to low survival rates. We have studied the photosynthetic activities of leaves and green roots of bare rooted Phalaenopsis equestris during storage at 25°C or at 35°C, with relative humidity (RH) 70% or below 10%, and with or without illumination for up to 30 days, using chlorophyll fluorescence as the monitor. A single environmental constraint did not lead to any inhibition; a combination of stresses (dehydration and dark treatment) did induce severe suppression. The results suggest that the shipment of Phalaenopsis with bare roots to foreign countries should not be a problem as long as the simultaneous presence of two stresses is avoided before replanting.

Bare-root-dip treatment of tomato for the management of the root-knot and the reniform nematodes

The efficacies of a water-soluble fraction (WSF) of rice polish or a solution of pyridoxine hydrochloride (vitamin B 6) were tested against the two important plant parasitic nematodes, the root-knot nematode ( Meloidogyne incognita) and the reniform nematode ( Rotylenchulus reniformis). At the time of transplantation of tomato seedlings, bare roots were dipped in different concentrations of the above solutions for 30 min, then transplanted. The treatments with higher concentrations resulted in improved plant growth in the uninoculated controls. Growth retardations caused by inoculated root-knot or reniform nematodes were smaller than those of the untreated controls.

Vowel raising in Basaa: a synchronic chain shift

Like many of the zone A Bantu languages of western equatorial Africa, Basaa, which is spoken over a large area to the north-east and east of Douala, Cameroon, shows an inventory of seven surface vowels (Guthrie 1953). The Basaa forms cited in this article, all of which are found in the comprehensive Dictionnaire Basaa-Français (Lemb & de Gastines 1973), are transcribed using the vowel symbols in (1):These vowel symbols differ from those used by Guthrie only in that the hooks are eliminated from underneath his [i] and [u] and the dots are eliminated from underneath his [???] and [???]. I assume for these vowels the characteristics indicated in (1), where non-parenthesised feature specifications are a property of surface phonological representation and parenthesised feature specifications are default phonetic values. Following Hyman's (1988) analysis of Esimbi, another Cameroonian language containing these vowels in its inventory, I take [E] and [ɔ] to be low.The verb roots in (2) contain instances of each of the seven surface vowels of Basaa:Though bare verb roots may surface unsuffixed, suffixal extensions may also be added to give applied, passive, habitual, direct causative, indirect causative, simultaneous, associative, possessive, reversive, reflexive, stative and nominalised forms. Of interest to us is the vowel raising induced within the verb root when certain of these suffixal extensions are added. In (3) we see the CVC verb forms of (2) along with their corresponding applied and indirect causative forms:Other suffixal extensions that induce raising are the passive, direct causative, simultaneous, reversive and stative extensions. For example, these other suffixal extensions attach to /ten/, one of the verb roots in (3), to give [tina], [tinis], [tinha], [tinil] and [tiní], respectively.

Effect of Neutral and Pathogenic Fungi on Mycorrhizal and Non-Mycorrhizal Picea Roots: Transpiration and Accumulation of the Stress Metabolite Aminocyclopropane Carboxylic Acid

Summary Ectomycorrhiza of Picea abies seedlings with Pisolithus tinctorius were synthesized rapidly in a Petri dish culture system under axenic conditions. Mycorrhizal and non-mycorrhizal seedlings of P. abies were infected with either Trichoderma harzianum, Fusarium oxysporum or Sclerotium rolfsii, respectively. The transpiration rate was measured to compare the effect of the various fungi on the mycorrhizal and non-mycorrhizal plantlets. At first, the transpiration of mycorrhizal plantlets declined after inoculation with T. harzianum and F. oxysporum, but it increased again later and almost reached the value of the control. This recovery did not occur when non-mycorrhizal plantlets were inoculated with these fungi; the transpiration rate decreased in a few days to around half of the initial value and remained at this low level for the rest of the measuring period. After infection of non-mycorrhizal roots with S. rolfsii, the rate of transpiration dropped to zero in 5 days. It decreased similarly in mycorrhizal plantlets but remained at about 20 % of the initial value for some days. In this case, both non-mycorrhizal and mycorrhizal plantlets died eventually. The stress metabolite aminocyclopropane carboxylic acid (ACC) was formed in the roots of mycorrhizal as well as non-mycorrhizal plantlets upon infection with the pathogenic fungi S. rolfsii and F. oxysporum, and also with the neutral fungus T. harzianum. Bare roots, which were brought into contact with the mutualistic fungus P. tinctorius, formed no ACC at all.

The bare: ensheathed anagen root ratio as a function of the speed of epilation

Small tufts of chest hair were epilated at different rates and the roots classified as to growth phase, presence or absence of root sheaths and whether the shafts were fractured. With respect to anagen roots, a slow epilation gives bare roots whereas an increase in the rate of epilation increases the proportion of ensheathed roots. As the rate of epilation increases, the proportion of fractured hair shafts increases. These findings are relevant to epilation used as a diagnostic procedure because it is not possible to have both a high proportion of well-formed ensheathed anagen roots without some breakages.

The water status of the roots of soil‐grown maize in relation to the maturity of their xylem

The long delayed maturation of the late metaxylem of maize (Zea mays) roots imposes a high‐resistance barrier between the immature apices and the negative water potential of the leaves. These apices (20+ cm) bear strongly adhering soil sheaths to within 0.5 to 2 cm of the distal end. It was hypothesized that the sheathed immature apices should show less response to transpiration stress than bare regions. Measurements were made of the relative water content (RWC) of the sheathed and bare zones of the axile roots, both at different ages of the plant, and early and late in the day's transpiration. Sheathed roots maintained a steady RWC of about 83% irrespective of age or transpiration. Bare roots had RWCs of about 63% in the morning, but this fell to 55% in the afternoon. The first‐order branches on the bare roots in the morning had still lower values of RWC, near 50%. Plots of RWC against water potential were indistinguishable for the three root types. It is concluded that the immature apices are indeed relatively isolated from the fluctuating tensions in the stem xylem, and that these tensions reduce the water content of bare roots and their branches to low values.

Pinus sylvestris mycorrhizae and their reaction to acidity in vitro — Aspects of bioindication

Mycorrhizae of Amanita muscaria , Cenococcum geophilum , and Pisolithus tinctorius were synthesized in sterile flask cultures. The substrate was adjusted to different acid pH-values. A. muscaria showed the optimal mycorrhization above pH 4, C. geophilum at about pH 4 and P. tinctorius below pH 3.5. In addition to a variable degree of mycorrhization, no aberrant morphological and ultra-structural growth patterns could be detected. The bare roots of Pinus sylvestris tolerate pH values even below 3.0. Abortion of apical meristems in root tips is discussed as a morphological indicator for acid substrates.

Different populations of bacteria associated with sheathed and bare regions of roots of field-grown maize

It has been previously shown that soil sheaths cling tightly to some portions of all axile roots and cover all but the growing tips of the young roots of field-grown maize. These sheaths overlie immature regions of the roots which have intact epidermal cells with root hairs, and living, thus non-conducting, late metaxylem elements. Loss of the soil sheath in the proximal region coincides with the opening of these large metaxylem vessels. Now, total, and viable counts have been recorded of bacteria associated with the root surface and adhering soil of sheathed and bare regions. These showed some common features, in that populations of similar size were associated with the two root regions in plants beginning to flower. Each population included about the same numbers of bacteria that were viable on each of three selective media (nitrogen-free, Pseudomonas F or MacConkey). However, more spore-formers capable of growth on nitrogen-free media and more fluorescent bacteria were isolated from the sheathed regions. Actinomycetes were absent from sheathed but plentiful on bare regions. The high numbers of diverse types of bacteria associated with both root surfaces can be related to the previously demonstrated similarity in amounts of organic carbon released from each region. The proliferation of actionomycetes on the bare roots and their exclusion from sheathed roots may in part be due to the lower water status of the bare region, which is related to its greater axial conducting capacity. Thus the distribution of the two types of root surface within an individual root system has important implications for the choice of root and rhizosphere sampling techniques and for root bacterization work.

Accumulation of potassium by differentiating metaxylem elements of maize roots

The previous demonstration that the large late metaxylem vessels of field‐grown maize (Zea mays L. cv. Rosella) roots do not lose their crosswalls until they are 20–30 cm from the tip, and that the presence of a soil sheath outside the root was indicative of immature vessels within, greatly strengthened the hypothesis that ion accumulation into these roots was by uptake into living xylem element vacuoles. Proposals that salt movement into the xylem was by leakage or secretion into dead vessels became much less plausible. Potassium concentration in the vacuoles of late metaxylem elements was measured by X‐ray microanalysis in unetched fracture faces of bulk, frozen‐hydrated pieces of sheathed roots, and found to be in the range 150–400 mM. Potassium concentration in open vessels of bare roots, measured both with the microprobe and by spectrophotometry of aspirated sap, was in the range of 5 to 25 mM. It is concluded that uptake of potassium (and possibly other ions) is into living xylem elements, and that its release to the transpiration stream occurs by the breakdown of their crosswalls and the addition of their vacuoles to the solution in the vessels above.

Localisation of translocated 14C in roots and root exudates of field‐grown maize

The roots of a mature, field‐grown maize plant are dimorphic: the primary root and those from the oldest nodes are bare with a heavily lignified cortex arid sloughed epidermis; those from younger nodes, except for a bare elongation zone, have an intact epidermis surrounded by a persistent soil sheath. Sheathed roots consistently have more layers of cortical cells, but the ratio of volumes of cortex to stele (ca 4) and the cross‐sectional area of phloem (ca3× 10−2 mm2) are similar in each type. Assimilated carbon (from 14C applied to a small area of one leaf) was translocated to all roots and actively metabolized in cortex and stele of both types. After 1 to 2 days the proportion of 14C exuded from a given length of mature root into its soil sheath, or into the adjacent unattached soil in the case of bare roots, was the same (5%) in both root types when compared with the ethanol‐soluble 14C in the tissues of this length. Up to 75% of the ethanol‐soluble label in the roots was in a cationic fraction (amino acids and unidentified compounds), ca 1% was in an anionic fraction and the remainder was in a neutral fraction (sugars). Approximately equal amounts of soluble 14C were found in the stele, cortex and laterals.