Roots Of Intact Seedlings Research Articles

Participation of nitric oxide in induction of heat resistance of wheat seedlings by putrescine

Polyamines are stress plant metabolites involved in the formation of adaptive responses. Recently, they have been considered as compounds that are involved in signaling processes. However, the possible role of nitric oxide (NO) and its functional interaction with reactive oxygen species (ROS) in the realization of stress-protective effects of polyamines has been very poorly studied. The participation of NO as a signaling mediator in the induction of the heat resistance of wheat seedlings by the exogenous diamine putrescine is studied. Incubation of intact seedling roots on a 1 mM putrescine solution caused an increase in the activity of diamine oxidase and a transient increase in the content of nitric oxide and hydrogen peroxide, respectively, with peaks at 1 and 2 hours from the start of the treatment. The increase in the NO content was eliminated by the action of aminoguanidine (an inhibitor of diamine oxidase and NO synthase), but not sodium tungstate (an inhibitor of nitrate reductase). Treatment of seedlings with a scavenger of hydrogen peroxide with dimethylthiourea (DMTU) reduced the effect of increasing the NO content in roots caused by putrescine. At the same time, scavenger NO PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) almost completely eliminated the accumulation of hydrogen peroxide in the roots, which occurred under the action of putrescine. The increase in the heat resistance of seedlings induced by the treatment with putrescine was completely eliminated under the influence of the NO PTIO scavenger, an inhibitor of diamine oxidase and NO synthase aminoguanidine, and the antioxidant DMTU. It is concluded about the functional interaction of ROS and NO as a signal intermediaries, providing the induction of protective responses and the development of plants of heat-resistance in the processing of putrescine.

Potassium Efflux and Cytosol Acidification as Primary Anoxia-Induced Events in Wheat and Rice Seedlings.

Both ion fluxes and changes of cytosolic pH take an active part in the signal transduction of different environmental stimuli. Here we studied the anoxia-induced alteration of cytosolic K+ concentration, [K+]cyt, and cytosolic pH, pHcyt, in rice and wheat, plants with different tolerances to hypoxia. The [K+]cyt and pHcyt were measured by fluorescence microscopy in single leaf mesophyll protoplasts loaded with the fluorescent potassium-binding dye PBFI-AM and the pH-sensitive probe BCECF-AM, respectively. Anoxic treatment caused an efflux of K+ from protoplasts of both plants after a lag-period of 300–450 s. The [K+]cyt decrease was blocked by tetraethylammonium (1 mM, 30 min pre-treatment) suggesting the involvement of plasma membrane voltage-gated K+ channels. The protoplasts of rice (a hypoxia-tolerant plant) reacted upon anoxia with a higher amplitude of the [K+]cyt drop. There was a simultaneous anoxia-dependent cytosolic acidification of protoplasts of both plants. The decrease of pHcyt was slower in wheat (a hypoxia-sensitive plant) while in rice protoplasts it was rapid and partially reversible. Ion fluxes between the roots of intact seedlings and nutrient solutions were monitored by ion-selective electrodes and revealed significant anoxia-induced acidification and potassium leakage that were inhibited by tetraethylammonium. The K+ efflux from rice was more distinct and reversible upon reoxygenation when compared with wheat seedlings.

Growth function and intercellular water transfer in excised roots.

On the example of maize seedling roots, it was shown that segments of the root suction zone excised from intact mother seedlings maintain the function of elongation growth and are able to regulate water transfer. Using the gradient NMR method, the effective intercellular permeability of root suction zone segments was shown to reduce with respect to intact seedling roots. The segment fragmentation into smaller pieces 3mm long resulted in the permeability decrease by 60%. The reduction is associated with the cell defensive response to water loss through cuts and blocking of the additive water transfer along the segment length, resulting from segment cutting.

Reflexions on some aspects of the interactions among ROS, RNS, and Ca2+ in response to a mycorrhizal or a pathogenic fungus

In vivo redox activities in the apoplast of axenically cultured intact seedling roots (superoxide anion generation, and superoxide dismutase and peroxidase activities) in contact with the compatible arbuscular mycorrhizal fungus (AMF) were clearly attenuated in comparison with those in contact with the pathogenic fungus (PF) or treated with MeJA, even at the early stages of treatment. Contact of roots with the AMF did not enhance the biosynthesis of phenolic compounds (total phenolics, flavonoids, and phenylpropanoid glycosides), while contact with the PF significantly enhanced the biosynthesis of all phenolic fractions. Reactive oxygen and nitrogen species both seemed to be involved in these responses from the first moments of contact, but the fluorescence imaging of roots showed that ROS were mainly accumulated in the apoplast while NO was mainly stored in the cytosol. In conclusion, intact olive seedling roots clearly differentiated between AMF and PF.

Effects of NO-Status modification, heat hardening, and hydrogen peroxide on the activity of antioxidant enzymes in wheat seedlings

In order to determine the role of nitric oxide (NO) as a potential mediator in the induction of the antioxidant system, effects of the following treatments on the activity of antioxidant enzymes in roots of etiolated wheat (Triticum aestivum L.) seedlings were investigated: a scavenger of nitric oxide, 100 µM PTIO (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide); an inhibitor of the enzyme similar to animal NO synthase, 2 mM L-NAME (NG-nitro-L-arginine methyl ester); an NO donor, 2 mM sodium nitroprusside (SNP); a 1-min heat hardening at 42°C; and 10 mM hydrogen peroxide. The activities of superoxide dismutase (SOD), catalase, and guaiacol peroxidase increased under the influence of PTIO and L-NAME. A similar effect was caused by the treatment with SNP and heat hardening; the increased activities of SOD and catalase were observed under the influence of hydrogen peroxide. The pretreatment of intact seedling roots with PTIO and L-NAME did not significantly alter the influence of heat hardening and hydrogen peroxide on the activity of studied enzymes. It is concluded that both the increase and decrease in intracellular NO content can induce the enzymatic antioxidant system.

Redox activities and ROS, NO and phenylpropanoids production by axenically cultured intact olive seedling roots after interaction with a mycorrhizal or a pathogenic fungus.

Roots of intact olive seedlings, axenically cultured, were alternatively placed in contact with Rhizophagus irregularis (mycorrhizal) or Verticillim dahliae (pathogenic) fungi. MeJA treatments were also included. In vivo redox activities in the apoplast of the intact roots (anion superoxide generation, superoxide dismutase and peroxidase activities) were measured. All our results showed that apoplastic redox activities of intact seedling roots in contact with the compatible mycorrhizal fungus were clearly attenuated in comparison with the pathogenic fungus or treated with MeJA, even at the early stages of treatment used. Total phenolics, flavonoids and phenylpropanoid glycosides were also quantified. Roots in contact with the mycorrhizal fungus did not enhance the biosynthesis of phenolic compounds with respect to controls, while those in contact with the pathogenic one significantly enhanced the biosynthesis of all phenolic fractions measured. Reactive oxygen species and nitric oxid accumulation in roots were examined by fluorescence microscopy. All of them presented much higher accumulation in roots in contact with the pathogenic than with the mycorrhizal fungus. Altogether these results indicate that intact olive seedling roots clearly differentiated between mycorrhizal and pathogenic fungi, attenuating defense reactions against the first to facilitate its establishment, while inducing a strong and sustained defense reaction against the second. Both reactive oxygen and nitrogen species seemed to be involved in these responses from the first moments of contact. However, further investigations are required to clarify the proposed crosstalk between them and their respective roles in these responses since fluorescence images of roots revealed that reactive oxygen species were mainly accumulated in the apoplast (congruently with the measured redox activities in this compartment) while nitric oxid was mainly stored in the cytosol.

High frequency shoot organogenesis and somatic embryogenesis in juvenile and adult tissues of seabuckthorn (Hippophae rhamnoides L.)

Seabuckthorn (Hippophae rhamnoides) is a multipurpose small tree with unique berries of high nutritional and pharmaceutical values. A clonally propagated plant originating from a 20-year-old tree of H. r. rhamnoides × mongolica hybrid cultivar Julia and seedling offspring of this cultivar were investigated regarding induction of shoot organogenesis in leaf explants and in roots of intact seedlings, and induction of direct somatic embryogenesis in explants from shoot tissue. The highest percentage of leaf explants showing shoot organogenesis was achieved (juvenile explants, 65%; adult explants, 75%) when incubated in Murashige and Skoog (MS) medium supplemented with either 4.5 μM of the phenylurea cytokinin thidiazuron (TDZ) or 2.25 μM TDZ plus 2.2 μM 6-benzyladenine (BA), for juvenile and adult explants, respectively, both supplemented with 0.53 μM α-naphthaleneacetic acid (NAA). Juvenile explants developed on average 18 shoots per explant in the MS medium supplemented with 4.5 μM TDZ, a four fold increase over those incubated on the medium supplemented with 2.25 μM TDZ and 2.2 μM BA. Adult leaf explants grown on medium containing 2.25 μM TDZ and 2.2 μM BA medium produced 12 shoots per explant, while those grown on medium containing 4.5 μM TDZ produced 5 shoots per explant. Shoot organogenesis was observed in roots of intact seedlings pre-cultured on plain medium lacking nutrients (PM) or woody plant medium (WPM) salts and then grown on WPM salts supplemented with 4.4 μM BA, 0.29 μM gibberrelic acid (GA3), and 57.0 μM indoleacetic acid (IAA). The number of shoots formed on each seedling root system was ten fold higher when the pre-culture was in WPM medium indicating a promoting effect of mineral nutrients in the pre-culture medium. Somatic embryogenesis was induced in both juvenile and adult leaf explants in 65 and 78% of the explants, respectively, in MS-based medium supplemented with 2.0 μM N-(2-Chloro-4-pyridyl)-N1-phenylurea (CPPU), 0.53 μM NAA and varying concentrations of BA. There was an interaction effect between MS salt strength and BA concentration. The most effective medium for inducing somatic embryogenesis in juvenile explants contained half strength MS salts and 2.2 μM BA and full strength MS salts and 13.2 μM BA for adult explants.

Colloidal suspensions of clay or titanium dioxide nanoparticles can inhibit leaf growth and transpiration via physical effects on root water transport

A laboratory investigation was conducted to determine whether colloidal suspensions of inorganic nanoparticulate materials of natural or industrial origin in the external water supplied to the primary root of maize seedlings (Zea mays L.) could interfere with water transport and induce associated leaf responses. Water flow through excised roots was reduced, together with root hydraulic conductivity, within minutes of exposure to colloidal suspensions of naturally derived bentonite clay or industrially produced TiO2 nanoparticles. Similar nanoparticle additions to the hydroponic solution surrounding the primary root of intact seedlings rapidly inhibited leaf growth and transpiration. The reduced water availability caused by external nanoparticles and the associated leaf responses appeared to involve a rapid physical inhibition of apoplastic flow through nanosized root cell wall pores rather than toxic effects. Thus: (1) bentonite and TiO2 treatments also reduced the hydraulic conductivity of cell wall ghosts of killed roots left after hot alcohol disruption of the cell membranes; and (2) the average particle exclusion diameter of root cell wall pores was reduced from 6.6 to 3.0 nm by prior nanoparticle treatments. Irrigation of soil-grown plants with nanoparticle suspensions had mostly insignificant inhibitory effects on long-term shoot production, and a possible developmental adaptation is suggested.

Does nitrogen concentration affect relative uptake rates of nitrate, ammonium, and glycine?

AbstractPlant roots are exposed to a variety of nitrogen forms (e.g., nitrate, ammonium, amino acids) and take up these forms at different rates. Many studies have investigated whether plants prefer nitrate, ammonium, or amino acids; but studies may not be comparable because they used substrate concentrations between 100 and 2000 μmol L–1. This study tests the hypothesis that substrate concentrations from 10 to 1750 μmol L–1 affect plant preference for N forms. Nitrogen uptake by the herb Ocimum basilicum and the evergreen tree Eucalyptus regnans was examined by placing roots of intact seedlings in equimolar mixtures of nitrate, ammonium, and glycine in which one of the N forms was 15N‐labelled (and 13C‐labelled in the case of glycine). In both species, preference for N forms was affected by substrate concentration. At 10 μmol L–1 (O. basilicum) or 10 and 50 μmol L–1 (E. regnans), rates of N uptake did not differ among N forms. At substrate concentrations of 50 μmol L–1 and greater O. basilicum took up ammonium the fastest, glycine the slowest, and nitrate at an intermediate rate. At substrate concentrations from 100 to 1750 μmol L–1, E. regnans took up ammonium the fastest with glycine and nitrate taken up at slower rates. The absence of significant differences at lower concentrations was a true biological effect rather than a function of larger relative errors. This study demonstrates that substrate concentration has a large effect on plant preference for N forms, and sounds a warning for studies of N nutrition that do not consider the concentration‐dependence of plant preference for N forms.

Origin of cadmium‐induced reactive oxygen species production: mitochondrial electron transfer versus plasma membrane NADPH oxidase

* Cadmium (Cd(2+)) is an environmental pollutant that causes increased reactive oxygen species (ROS) production. To determine the site of ROS production, the effect of Cd(2+) on ROS production was studied in isolated soybean (Glycine max) plasma membranes, potato (Solanum tuberosum) tuber mitochondria and roots of intact seedlings of soybean or cucumber (Cucumis sativus). * The effects of Cd(2+) on the kinetics of superoxide (O2*-), hydrogen peroxide (H(2)O(2)) and hydroxyl radical ((*OH) generation were followed using absorption, fluorescence and spin-trapping electron paramagnetic resonance spectroscopy. * In isolated plasma membranes, Cd(2+) inhibited O2*- production. This inhibition was reversed by calcium (Ca(2+)) and magnesium (Mg(2+)). In isolated mitochondria, Cd(2+) increased and H(2)O(2) production. In intact roots, Cd(2+) stimulated H(2)O(2) production whereas it inhibited O2*- and (*)OH production in a Ca(2+)-reversible manner. * Cd(2+) can be used to distinguish between ROS originating from mitochondria and from the plasma membrane. This is achieved by measuring different ROS individually. The immediate (<or= 1 h) consequence of exposure to Cd(2+) in vivo is stimulation of ROS production in the mitochondrial electron transfer chain and inhibition of NADPH oxidase activity in the plasma membrane.

Accumulation of sense–antisense transcripts of the rice catalase gene CatB under dark conditions requires signals from shoots

The amount of mRNA of the Oryza sativa L., cv. Nipponbare (rice) catalase gene, CatB, was decreased in the roots of intact seedlings kept in continuous darkness (DD). In contrast, sense and antisense unspliced CatB transcripts accumulated in the same tissue. Both strands cover the entire CatB-coding region, and form double-stranded RNA (dsRNA). The results of RNA dot-blot hybridization analysis using low molecular weight RNAs suggested that the sense and antisense CatB transcripts were more stable under DD conditions than under a light–dark regimen (LD). After shifting the lighting conditions from DD to LD, both the sense and antisense CatB transcripts were hardly detected, and the amount of CatB mRNA was restored. From these results, the antisense CatB transcripts might play a role in suppressing the normal processing of sense CatB transcript and also CatB protein synthesis by dsRNA formation, under conditions unsuitable for plant growth such as DD. This study indicates that signals transmitted from shoots are associated with the accumulation of sense and antisense CatB transcripts in roots under DD conditions, and that auxin is one of the possible signals.

Fast water uptake and outflow and electrophysiological responses of roots induced by changes in salinity, osmotic pressure, and pH of external solution

Fast reversible water losses by roots of intact seedlings of tomato (Lycopersicon esculentum L.) and sunflower (Helianthus annuus L.) in response to changes in composition and concentration of external medium were studied with a highly sensitive fast-response gravimetric method. The dehydrating effect of solutes, applied at low and moderate concentrations (5–100 mM), increased in the following row: ethanol < glycerol < sucrose ≪ neutral salts < base < acid. At concentrations below 10 mM, nonelectrolytes did not cause significant water losses from roots. Neutral salts had a characteristic gradual effect in a wide range of concentrations (0.3–500 mM NaCl). Amplitudes of gravimetric responses to treatments with bases and acids were 1.5–2 times higher (NaOH) and more than 3 times higher (HCl) than the response to equimolar concentration (5 mM) of a neutral salt. In all cases the water loss from roots was fast, reversible, and well reproducible. The presence of electrical charge (ions vs. neutral molecules) was crucial for the strength of the solute effect, especially at low concentrations. In parallel experiments with tomato seedlings, fast kinetics of electric potential difference between the root and the hypocotyle (electrophysiological response) was measured after a change in the composition and concentration of external solution. Possible mechanisms of observed phenomena are discussed.

Role of basipetal auxin transport and lateral auxin movement in rooting and growth of etiolated lupin hypocotyls.

The involvement of polar auxin transport (PAT) on the growth of light-grown seedlings and rooting is generally accepted, while the role of auxin and PAT on the growth of dark-grown seedlings is subject to controversy. To further investigate this question, we have firstly studied the influence of NPA, a known inhibitor of PAT, on the rooting and growth of etiolated Lupinus albus hypocotyls. Rooting was inhibited when the basal ends of de-rooted seedlings were immersed in 100 micro m NPA but was partially restored after immersion in NPA + auxin. However, NPA applied to de-rooted seedlings or the roots of intact seedlings did not inhibit hypocotyl growth. It was taken up and distributed along the organ, and actually inhibited the basipetal transport of ((3)H)-IAA applied to isolated hypocotyl sections. Since the apex is the presumed auxin source for hypocotyl growth and rooting, and the epidermis is considered the limiting factor in auxin-induced growth, the basipetal and lateral auxin movement (LAM) after application of ((3)H)-IAA to decapitated seedlings were studied, in an attempt to evaluate the role of PAT and LAM in the provision of auxin to competent cells for growth and rooting. Local application of ((3)H)-IAA to the stele led to the basipetal transport of auxin in this tissue, but the process was drastically reduced when roots were immersed in NPA since no radioactivity was detected below the apical elongation region of the hypocotyl. LAM from the stele to the cortex and the epidermis occurred during basipetal transport, since radioactivity in these tissues increased as transport time progressed. Radioactivity on a per FW basis in the epidermis was 2-4 times higher than in the cortex, which suggests that epidermal cells acted as a sink for LAM. NPA did not inhibit LAM along the elongation region. These results suggest that while PAT was essential for rooting, LAM from the PAT pathway to the auxin-sensitive epidermal cells could play a key role in supplying auxin for hypocotyl elongation in etiolated lupin seedlings.

Thidiazuron-Induced High-Frequency Shoot Regeneration from Root Region of Robinia pseudoacacia L. Seedlings

High-frequency regeneration of shoots was achieved at root region of seedlings of Robinia pseudoacacia L. cultured from seeds on medium supplemented with thidiazuron (TDZ, 1.0 &mu;M). The roots of intact seedlings proliferated and formed a compact callus followed by differentiation of numerous shoots. Corresponding cultures on benzylaminopurine-containing medium exhibited much weaker response. Hypocotyl segments also formed shoots at a lower concentration of TDZ (0.1 &mu;M). The shoots formed on TDZ-containing medium were well-developed and readily rooted on hormone-free medium. The obtained plants after acclimation in culture room survived after transfer to soil.

Occurrence of osmiophilic particles is correlated to elongation growth of higher plants

The occurrence of elongation growth-related osmiophilic particles (OPs) was investigated in hypocotyls of sunflower, bean, and spruce as well as in pea epicotyls and in cress roots of intact seedlings. In all analyzed species, OPs were found to occur specifically within the periplasmic space between plasma membrane and the outer epidermal cell walls of elongating parts of hypocotyls, epicotyls, and roots, whereas cells of nonelongating parts were devoid of OPs. Auxin (IAA) markedly increased the number of OPs in epicotyl and hypocotyl segments. Treatment of pea epicotyl segments with the lectin concanavalin A inhibited their elongation growth in the presence of IAA. At a subcellular level this effect was characterized by the occurrence of a pronounced osmiophilic layer in the periplasmic space of the outer periclinal and the outer part of the anticlinal epidermal cell walls. Treatment of IAA-incubated segments with the secretion inhibitor brefeldin A inhibited both elongation growth and periplasmic occurrence of OPs. This effect was accompanied by complementary accumulation of OPs in the peripheral cytoplasm of epidermal cells. Together the results indicate that IAA-induced epidermis-specific secretion of OPs is closely related to cell elongation growth not only in organs of monocotyledonous species, but also in dicotyledonous angiosperms as well as in gymnosperms.

Slow wave potentials in cucumber differ in form and growth effect from those in pea seedlings

A positive hydraulic signal in the form of a xylem pressure step was applied to the roots of intact seedlings of Cucumis sativus L. and Pisum sativum L. Surface electrodes at three positions along the epicotyl/hypocotyl recorded a propagating depolarization which appeared first in the basal, then the central and sometimes the apical electrode positions and fitted the characteristics of a slow wave potential (SWP). This depolarization differed between pea and cucumber. It was transient in cells of pea epicotyls but sustained in cucumber hypocotyls. It was not associated with a change in cell input resistance in pea epicotyls but preceded an increase in the input resistance of cucumber hypocotyl cells. With the increased xylem pressure the growth rate (GR) of cucumber hypocotyls and pea epicotyls underwent a transient increase, peaking after 5 min. If the depolarization reached the growing upper region, it preceded a sustained decrease in the GR of cucumber hypocotyls but only a transient decrease in the GR of pea epicotyls. A temperature jump in the root medium (heat treatment) induced a steep pressure spike in the xylem of the cucumber hypocotyl which showed similar electric and growth effects as the previously applied, non‐injurious pressure steps. We suggest that the observed differences in the electric and growth responses between the species were caused by the closure of ion channels in depolarized cells of cucumber but not pea seedlings.

The Propagation of Slow Wave Potentials in Pea Epicotyls

Slow wave potentials are considered to be electric long-distance signals specific for plants, although there are conflicting ideas about a chemical, electrical, or hydraulic mode of propagation. These ideas were tested by comparing the propagation of hydraulic and electric signals in epicotyls of pea (Pisum sativum L). A hydraulic signal in the form of a defined step increase in xylem pressure (Px) was applied to the root of intact seedlings and propagated nearly instantly through the epicotyl axis while its amplitude decreased with distance from the pressure chamber. This decremental propagation was caused by a leaky xylem and created an axial Px gradient in the epicotyl. Simultaneously along the epicotyl surface, depolarizations appeared with lag times that increased acropetally with distance from the pressure chamber from 5 s to 3 min. When measured at a constant distance, the lag times increased as the size of the applied pressure steps decreased. We conclude that the Px gradient in the epicotyl caused local depolarizations with acropetally increasing lag times, which have the appearance of an electric signal propagating with a rate of 20 to 30 mm min-1. This static description of the slow wave potentials challenges its traditional classification as a propagating electric signal.

A reduced xylem pressure altered the electric and growth responses in cucumber hypocotyls

ABSTRACTThis study measured the electric and growth responses in excised cucumber hypocotyls and compared them with those in intact seedlings. Root excision (first severing cut) eliminated most of the positive xylem pressure (Px) in the hypocotyl, caused a rapid, transient drop in the hypocotyl growth rate (GR) and some small, local depolarization near the cut site. Although accompanied by a smaller decrease in Px, a second, severing cut in the basal hypocotyl caused a decrease in GR which was no longer transient and a depolarization which was increased in both size and extent. These changes were not wound effects because they could be simulated by root incubation in mannitol. The reduced GR recovery occurred also in the absence of electric changes after a second increase in the mannitol concentration incubating the root of intact seedlings. Increased electric sensitivity and altered growth response therefore appear to be two independent examples of physiological changes resulting from a lowered Px.

Root‐to‐Shoot Communication in Flooded Plants: Involvement of Abscisic Acid, Ethylene, and 1‐Aminocyclopropane‐1‐carboxylic Acid

AbstractNonhydraulic communication between roots and shoots of flooded plants is reviewed. Contrary to reports for roots of droughted plants, those of flooded tomato plants (Lycopersicon esculentum L.) did not export increased amounts of abscisic acid (ABA) to the shoots, even though stomata closed in association with increased foliar ABA, and in the absence of marked losses in leaf hydration. Flooded tomato root systems did export more of the ethylene precursor 1‐aminocyclopropane‐1‐carboxylic acid (ACC). Ethylene formed in the leaves by oxidation of the ACC to ethylene and the epinastic leaf curvatures the ethylene induced were found to be inhibited in tomato plants transformed with an antisense gene to the ethylene‐forming enzyme ACC oxidase. Our estimates of the concentration and delivery of ABA and ACC in the xylem sap as it passes into the shoot from the roots avoided artifacts associated with variable sap flow rates and temporary contamination from wounded tissue. Measurements of ethylene production by individual roots of intact seedlings of maize (Zea mays L.) by highly sensitive photoacoustic laser detection confirmed earlier work linking enhanced ethylene evolution with more extensive aerenchyma development in roots that are partially deficient in oxygen. The development of aerenchyma enhances oxygen movement from shoots to roots.

Thidiazuron-induced in vitro shoot formation from roots of intact seedlings of Albizzia julibrissin

Seedlings of silktree (Albizzia julibrissin Durrazz.) were grown in vitro on MS-media containing B5 vitamins, 3% sucrose, 0.25% phytagel and various concentrations (0.1–10 μM) of thidiazuron (TDZ). Addition of TDZ to the culture medium greatly reduced shoot and root elongation but did not influence shoot production from the cotyledonary node or apex. Within 8–10 days the seedling roots split open, formed large masses of callus, and developed green patches which eventually grew into normal shoots while still within the culture medium containing TDZ at 0.1–1.0 μM. Such callus and shoot formation did not occur in control cultures lacking TDZ. At higher TDZ concentrations (2.5–10 μM), the green patches formed in the callus did not further develop into shoots. Addition of other cytokinins (kinetin, benzylaminopurine, zeatin) to the culture medium also induced some shoot formation from the roots, but higher concentrations than TDZ were required to induce regeneration. Isopentenyladenine failed to induced shoot formation. Following excision and transfer to MS media with or without 4.9 μM IBA, the shoots induced by kinetin or benzylaminopurine rooted 4–7 days earlier than those induced by TDZ, but all excised shoots developed into normal rooted plantlets within 3 weeks.