Stelar Tissues Research Articles

The effect of aluminium on uptake and distribution of magnesium and calcium in roots of mycorrhizal Norway spruce seedlings

Spruce seedlings (Picea abies (L.) Karst.) colonized with Lactarius rufus (Scop.) Fr. or Lactarius theiogalus Fr. were grown in an axenic silica sand culture system with frequently renewed nutrient solution. After successful mycorrhizal colonization the seedlings were exposed to 800 μM Al(NO3)3 (pH 3.9) for 13 or 17 weeks. Concentrations of Al, Mg, and Ca in the tissues of the mycorrhizal root tips were determined by X‐ray microanalysis. After 13 or 17 weeks of exposure to Al, high Al concentrations were found in cell walls of all mycorrhizal tissues except the stele tissues. Compared to the controls Mg levels in most of the mycorrhizal structures were reduced by Al treatment. Calcium levels in cortex cell walls of root tips colonized with L. rufus were reduced by exposure to Al. However, in cell walls of the stele Ca levels were significantly increased. No differences in Al or Mg levels were detected in structures of mycorrhizal and non‐mycorrhizal root tips from the same individual seedlings. These results suggest that (1) the endodermis is the primary barrier to radial Al transport and (2) the presence of a hyphal sheath did not prevent Al from reaching the root cortex and from displacing Mg and Ca.

The effects of drought and subsequent rehydration on the structure and vitality of Zea mays seedling roots

Drought was induced in Turface and vermiculite-grown seedlings of Zea mays L. by withholding water. Despite a 34-day drought period, the roots of stressed plants increased in dry weight by continued elongation of existing roots and initiation of new roots. A differential response to desiccation, with respect to structure and cell vitality, was observed along the root axes. In apical regions (terminal 20 mm) of seminal roots, all cell layers typically remained alive. In midlength regions (half way between the apex and base), the epidermis died in both control and treatment plants. The cortex of control roots displayed some distortion, but most of the cells were alive; the cortex of stressed roots displayed severe collapse and death of the majority of these cells. In basal regions (20 mm distal to the kernel), cells of the outer cortex were sclerified and less susceptible to collapse. Cells of stelar tissues retained their normal shapes and vitality. Upon rehydration, the root systems of stressed plants resumed growth by elongation of existing seminal, adventitious, and lateral roots and, in some cases, also by emergence of previously dormant lateral root primordia. The root systems' capacity for recovery diminished with increasing stress. Key words: drought, roots, Zea mays L., exodermis, hypodermis.

A root maceration method for studying fungal invasion in the root rot complex of red clover.

A root maceration method for quantitative assessment of invasion of roots of red clover (Trifolium pratense) by soil-borne fungi was evaluated using naturally infected glasshouse-grown and field-grown plants. Washed, surface-sterilised roots were macerated in a blender in sterile distilled water and quantitatively plated on potato-dextrose agar containing 10/μg/mL oxytetracycline. Colonies of fungi, including several known pathogens of red clover, were identified on plates and counted. The method detected a similar range of fungi to that found by plating 1–2 mm long root segments, but yielded more colonies and showed less variation. Maximum recovery of fungal colonies was obtained by macerating 2 g tissue for 60 sec; 120 sec was required for stele tissue from thick tap roots.

Effect of colonization by a sterile red fungus on viability of seed and growth and anatomy of wheat roots

Invasion patterns of the sterile red fungus (SRF) into tissues of seminal roots of wheat were examined 4, 8 and 10 d after inoculation. At 4 d, 2·7% of the cortical cells were free of the fungus (category A), 16·2% had 1–50% (category B) and 81·1%, 50–100% (category C) of the cell lumen filled by hyphae. The percentage of cortical cells in category C increased at 8 and 20 d, with a concurrent decrease in categories B and A. Most cells within the stele were free of the fungus, especially at 4 and 8 d. Even when they were found within stelar tissues they were only at low densities. The thickness of the walls of endodermis was not affected by the presence of the SRF. The SRF colonized wheat seeds without affecting their viability. The SRF was recovered at a greater proportion from seeds incubated with the fungus for 12 h than for 8 or 10 h, although some reduction in the viability was noted with seeds incubated for 12 h. Wheat seeds exposed to the SRF for 8, 10 and 12 h germinated more rapidly than untreated seeds, the seeds incubated for 10 h showing the best effect. Forty eight hours after planting there was no difference evident in the germination percentage among seeds inoculated for 8 h, 10 h and the control treatment. The SRF or its exudate increased the shoot and root weights and root length compared to the control. The SRF also increased the diameter of wheat roots which was not affected by the exudate. The root length, however, was greater in the exudate treatment than in the treatment receiving the fungus.

Selection for persistence in red clover: Influence of root disease and stem nematode

Abstract The cause of death of red clover plants (Trifolium pratense L.), and attributes associated with persistence of survivors, were investigated in two field plot experiments on land previously used by plant breeders for evaluating persistence of different red clover genotypes. In the first experiment, using ‘Grasslands Hamua’ red clover, most plants developed foliage symptoms of stem nematode (Ditylenchus dipsaci (Kühn) Filipjev) infestation and/or developed severe cortical rot or stele necrosis in their tap roots, during the first year. Cylindrocladium scoparium Morgan, Trichocladium basicola (Berk. & Br.) Carmichael, Verticillium dahliae Kleb., Fusarium solani (Mart.) Sacc., and F. oxysporum Schlecht. were isolated from diseased roots. An internal breakdown of the upper tap root occurred following necrosis of pith tissue. The proportion of dead plants increased rapidly during the second year, and only 12.5% of plants had survived 2 years after sowing. Most plants which persisted for 2 years had developed vigorous, deeply penetrating adventitious roots. In the second experiment, plants from a mixed sowing of the Swiss cultivars ‘Renova’, ‘Mont Calme’, ‘Leise’, and ‘Changins’ were more persistent than plants of the New Zealand cultivars ‘Grasslands Hamua’, ‘Grasslands Turoa’, and ‘Grasslands Pawera’. The Swiss cultivars developed significantly less root rot and stele necrosis, and yielded fewer fungal colonies from stele tissue. In a nationwide survey of fungi in red clover tap roots from 24 sites, V. dahliae was found in two seed crops in Blenheim, and T. basicola and F. solani, in pasture at several sites in the northern North Island where root rot was present Stele tissue of apparently healthy roots from pasture often yielded many fungal isolates, particularly Phoma medicaginis Mal. & Roum., other Phoma spp., Gliocladium roseum Bain., and Acremonium spp.

A procedure for isolating Pyrenochaeta terrestris from onion roots

SummarySurface sterilised stelar tissues from onion roots exhibiting typical pink root symptoms were plated on water agar and incubated under cool white fluorescent light (CWFL) for 12 h day‐1 at 20°C. After 4–5 days, the plates were examined with the 10† objective of a compound microscope and tissues producing conidia of Fusarium spp. were discarded. Sub‐cultures of isolates suspected to be Pyrenochaeta terrestris, grown from Fusarium‐negative tissues, were transferred to chloroamphenicol‐amended (500 ppm) corn‐meal (CCMA) agar plates and incubated at 24°C under CWFL for a further 10–14 days. Cultures with setose pycnidia were identified as P. terrestris. Plates were again scanned for conidia of Fusarium spp., and cultures negative for Fusarium spp., but not producing pycnidia, were suspected to be P. terrestris. These were compared with known isolates on CCMA. On this medium growth of P. terrestris was slow, producing appressed, pinkish colonies which were circular with a smooth margin. All isolates of P. terrestris isolated by the procedure caused pink root of onion when tested, whereas none of the Fusarium spp. isolated produced the disease. However, a mixed inoculums of P. terrestris and Fusariurn solani produced typical pink root symptoms.

Histochemical localization of desoxyhemigossypol, a phytoalexin in Verticillium dahliae-infected cotton stems.

The terpenoid phytoalexin desoxyhemigossypol (dHG) was detected histochemically in the stem xyiem of Verticillium dahliae Kleb-infected, wilt-resistant Seabrook Sea Island cotton as a green product on V. dahliae mycelium within vessel lumens and in specialized, often solitary, paravascular parenchyma cells. The SbCl3 -HClO4 histochemical reagent yielded a green-coloured Sb-dHG product specific for dHG when used as a spray on chromatograms of extracts from Verticillium-infected stele tissue. Both dHG and related terpenoid aldehyde derivatives occurred together in parenchyma cells and on V. dahliae mycelium. The presence of dHG on Verticillium mycelium reinforces previous studies that identified dHG as the most toxic and possibly most important phytoalexin in the resistance of cotton to V. dahliae.

Development of Procambium, Xylem, and Phloem in the Shoot Apex of Selaginella

A quantitative study of vascular development in shoot apices of Selaginella rupestris was undertaken, using sample sizes large enough to justify statistical treatment of the data, and with special interest in comparing results from this microphyllous species with results from megaphyllous angiosperms. Selaginella rupestris developed microphyll primordia that initially contained no procambium. When procambium formed in the leaf trace, it was at an isolated locus, as was the later-forming protoxylem and protophloem in the trace. In the stele the first xylem always differentiated closer to the apex than did the phloem; its average distance from the apex of 302 μm in main shoots was highly significant from the 807 μm of the protophloem. The air spaces, which result from the unusual endodermis development in Selaginella, always differentiated even closer to the apex than did the xylem, averaging 101 μm above the protoxylem. A qualitative survey of 15 other species showed a similar pattern in the stelar tissues.

Influence of weight of seed on hyphal growth of the take-all pathogen, Gaeumannomyces graminis var. tritici, in wheat roots grown in sand culture

This study examined extent of hyphal growth of Gaeumannomyces graminis var. tritici (Ggt) in seminal roots of wheat seedlings grown from seed of varying weight (12-60 mg). Hyphal invasion, measured microscopically in the inner cortex, endodermis and stele of root sections and in the early formation of stelar lesions, was found to be influenced in a complex manner by weight of seed. As weight of seed was increased, invasion of the inner cortex decreased linearly while invasion of stelar tissue varied in a non linear manner. These findings show that weight of seed must be considered when comparing levels of infection in seedlings of different cultivars of wheat.

Cross Protection Between Strains of Fusarium oxysporum f. sp. vasinfectum and its Effect on Vascular Resistance Mechanisms

AbstractThe severity of symptoms induced in cotton plants by stem inoculation with Fusarium oxysporum f. sp. vasinfectum was reduced by inoculation with a non‐pathogenic strain (NPS) of the fungus 3 days previously. Symptom reduction was associated with reduced populations of the pathogen in the stele tissue of the stem. This cross‐protective response became effective within 24 h of inoculation with the NPS. Although both vascular occlusion and terpenoid aldehyde synthesis were induced by theNPS, the short time required for cross protection to become effective and the duration of the effect, indicated that vascular occlusion rather than fungitoxicity of the terpenoid aldehyde compounds was the mechanism of protection.

Tissue specificity of acid action in rapid elongation responses of lupin hypocotyls

We have tested the acid‐responsiveness of hollow cylinders, peeled segments, and cores of stelar tissue of hypocotyls of light‐grown lupin seedlings (Lupinus angustifolius L. cv. New Zealand Bitter Blue), to determine how differential responsiveness of individual tissues to acid might govern the response of the intact hypocotyl. Hollow cylinders of cortex with the outer cell layers intact were less responsive to acid than was the cortex alone, which elongated strongly at pH 5. The stelar tissues were acid‐responsive (pH 5), but only if the cell layer surrounding the stele (the starch sheath) was disrupted. We conclude that all tissues were acid‐responsive, with the cortex alone most responsive, and the outer cell layers least responsive. We suggest that acid‐induced elongation of intact segments may be controlled by the response of the outer cell layers.

Gross Phloem Anatomy in the Roots of selected C4, C3, and C4‐C3 Intermediate Poaceae Species1

Is the anatomy and physiology associated with the C4 carbon pathway exclusively a feature of the leaf (photosynthate source)? To assist in establishing a more inclusive knowledge of sourcesink relations, qualitative and quantitative aspects of mature root phloem anatomy were compared between related C4, C3, and C4‐C3 intermediate species, (Triticum aestivum L., Zea mays L., and five Panicum spp.). In all species each phloem bundle was contiguous with the pericycle and separated from any tracheary element by one or two rows of stelar parenchyma. The number of cells between phloem bundles was essentially constant and the spatial relationships among the bundles and the cortex and surrounding stelar tissues were similar in the species studied. One protophloem sieve‐tube element, two companion cells, and one metaphloem sieve‐tube element constituted the basic cellular composition of a bundle. The exception was Z. mays L., in which two to six metaphloem sieve‐tube elements and one or two phloem parenchyma cells were usually present. There were significant differences among these species, but these differences were best explained in terms of species specificity rather than by photosynthetic characteristics.

Metabolism of [14C]indole-3-acetic acid by the cortical and stelar tissues of Zea mays L. roots

Reverse-phase high-performance liquid chromatography was used to analyse (14)C-labelled metabolites of indole-3-acetic acid (IAA) formed in the cortical and stelar tissues of Zea mays roots. After a 2-h incubation in [(14)C]IAA, stelar segments had metabolised between 1-6% of the methanol-extractable radioactivity compared with 91-92% by the cortical segments. The pattern of metabolites produced by cortical segments was similar to that produced by intact segments bathed in aqueous solutions of [(14)C]IAA. In contrast, when IAA was supplied in agar blocks to stelar tissue protruding from the basal ends of segments, negligible metabolism was evident. On the basis of its retention characteristics both before and after methylation, the major metabolite of [(14)C]IAA in Zea mays root segments was tentatively identified by high-performance liquid chromatography as oxindole-3-acetic acid.

Perfusion of onion root xylem vessels: a method and some evidence of control of the pH of the xylem sap

We describe a method for perfusing the xylem in the stele of excised onion roots with solutions of known composition under a pressure gradient. Tracer studies using [(14)C] polyethylene glycol 4000 and the fluorescent dye, Tinopal CBSX, indicated that perfusing solutions passed exclusively through the xylem vessels. The conductance of the xylem was small over the apical 100 mm of the root axis but increased markedly between 100 and 200 mm. Unbuffered perfusion solutions supplied in the range pH 3.7-7.8 emerged after passage through the xylem adjusted to pH 5.2-6.0, indicating the presence of mechanisms for absorbing or releasing protons. This adjustment continued over many hours with net proton fluxes apparently determined by the disparity between the pH of the perfusion solution and the usual xylem sap pH of about 5.5. Mild acidification of the xylem sap by buffered perfusion solutions increased the release of (86)Rb (K(+)) and (35)SO4 (2-) from the stelar tissue into the xylem stream. The ion-transporting properties of onion roots seemed little changed by excision from the bulbs, or by removal of the apical zones of the root axis. The pH of sap produced by root pressure resembles that found in the outflow solutions of perfused root segments.

Napropamide Binding in Corn (Zea mays) Root Tissue

Napropamide [2-(α-naphthoxy)-N,N-diethylpropionamide]-binding in excised root segments of corn (Zea maysL. ‘NC + 59′) was confined to cell wall fractions (residue and 500gpellet) remaining after homogenization and to components of the 100 000gsupernatant. Binding increased in both the cell wall and soluble fractions with continued exposure to napropamide. Microautoradiographs revealed that the napropamide bound in the cell walls was located in epidermal, cortical, and stelar tissue. Various proteins were capable of binding napropamide in vitro; however, protease treatment did not liberate the radioactivity bound in the cell wall fragments. Carbohydrate release from the cell wall material with cellulase was not correlated with the solubilization of bound radioactivity and wall carbohydrate monomers did not appear to bind to napropamide in vitro. A portion of the radioactivity found in the soluble components (at 100 000g) was associated with a molecule of MW > 600. The continued influx of napropamide was due to binding to cell wall components and molecules within the cell.

ELICITATION OF ACCUMULATION OF TERPENOID ALDEHYDE PHYTOALEXINS IN VERTICILLIUM DAHLIAE‐NFECTED COTTON

SummaryStem stele tissues of Verticillium wilt‐resistant (Seabrook Sea Island 12B2) cotton plants were examined for location of the pathogen and localization of terpenoid aldehyde accumulation 5 days after inoculation with Verticiliium dahliae. Mycelia of V. dahliae were confined to the vessel lumina. Terpenoid aldehyde phytoalexins were detected histochemically in fresh sections as red Sb‐chelates after application of the terpenoid aldehyde‐specific SbCl3 reagent. Most terpenoid aldehydes were detected as diffuse deposits in and around colonized xylem vessels; however, some were localized in solitary xylem parenchyma cells appressed to vessel walls or in solitary xylem and pith parenchyma cells removed from any V. dahliae‐colonized vessels. Thus, physical contact with the pathogen is not required for elicitation of terpenoid aldehyde accumulation.

Preliminary studies on Microdochium bolleyi with special reference to colonization of barley

Microdochium bolleyi was regularly recovered from the roots of healthy field-grown barley, isolation frequencies being greatest for older plants. The fungus was also present on oats and pasture grasses. Dark-pigmented chlamydospores were noticeable on seed surfaces and in root cortical and epidermal cells but not in stelar tissues. Lesions were observed on the coleoptile which could be reproduced on artificially infected plants. M. bolleyi was weakly pathogenic. The different vegetative structures of M. bolleyi are discussed in relation to host penetration, colonisation and survival of the fungus.

ROOT CONTRACTION IN FREESIA (IRIDACEAE)

The contractile roots of the horticultural variety Freesia hybrida Bailey (Iridaceae) were determined to contract via a growth/collapse mechanism. Contraction is initiated by a radial growth of middle and outer cortical parenchyma cells which is morphologically evident by an expanded diameter of the root. No concomitant decrease in length of the actively growing cells was observed. Shortening of the root is caused by axial tension produced by the radial growth of cells contiguous with nonexpanded cells distally. Centripetal collapse of expanded cells, coupled with passive shortening of inner cortical parenchyma and stelar tissues, releases axial tension slowly, returning the shortened root to equilibrium. Inner cortical parenchyma cells shorten in an accordion‐like manner facilitated by partial dissolution of middle lamellar material.

Membrane ATPase Activity of Barley Roots and Potassium Uptake by Isolated Stele and Cortex

AbstractUptake of potassium ions by isolated stelar tissues of barley from 0.5 and 10 mM K+ was respectively 13 and 3.6% of that of the cortical tissues. 0.1 mM H2PO4, LO mM ATP and 10 mM Ca(NO3)2 did not increase the potassium uptake of either stele or cortex during 5 h of uptake period.A time‐course incubation for histological demonstration of the ATPase activity of the plasmalemma and tonoplast of the matured sections of the roots demonstrated a greater activity for the cortical than the stelar tissue. In the stelar parenchyma cells, the plasma lemma showed a higher activity than the tonoplast. These results, which support the “leakiness hypothesis” of the stele, are discussed in relation to the proposed mechanisms of radial ion transport in roots.

THE TOLERANCE OF TREE ROOTS TO WATERLOGGING

SUMMARYSeedling root systems, or individual roots of cuttings, were submerged in reduced indigo‐carmine solution so that only the seedling shoot, or basal part of the cutting root, was exposed to air or oxygen. Oxygen diffusing out of the roots produced a localized blue colouration of the previously clear reduced dye; this process was employed to study internal oxygen movement.Using plants grown in freely drained soil it was evident that seedlings of the flood‐tolerant species, Lodgepole pine, had a greater capacity for oxygen transport than the flood‐intolerant Sitka spruce. Oxygen diffusion was also observed in roots detached from Lodgepole pine cuttings and was greatly enhanced in plants which had grown in waterlogged conditions; in the latter plants oxygen diffusion was detected from the root tips after a short exposure of the basal region of the root to oxygen, whereas roots from plants grown in freely drained soil showed diffusion of oxygen only from the upper submerged portion during the same treatment period. Oxygen diffusion over distances of more than 30 cm was detected in certain experiments.Observations on the movement of gas supplied under pressure to Lodgepole pine root segments showed that the rate of gas flow was much greater in roots grown in waterlogged soil than in those grown in freely drained soil and that the gas movement occurred in the stelar tissues. The regions of oxygen entry included the needles, and lenticels on the roots.