Physiological,morphological, and growth responses ofPlatanus occidentalis seedlings to flooding

Summary Flooding ofPlatanus occidentalis L. seedlings for up to 40 days induced several changes including early stomatal closure, greatly accelerated ethylene production by stems, formation of hypertrophied lenticels and adventitious roots on submerged portions of stems, and marked growth inhibition. Poor adaptation ofPlatanus occidentalis seedlings to soil inundation was shown in stomatal closure during the entire flooding period, inhibition of root elongation and branching, and death of roots. Some adaptation to flooding was indicated by (1) production of hypertrophied lenticels which may assist in exchange of dissolved gases in flood water and in release of toxic compounds, and (2) production of adventitious roots on stems which may increase absorption of water. These adaptations appeared to be associated with greatly stimulated ethylene production in stems of flooded plants. The greater reduction of root growth over shoot growth in flooded seedlings will result in decreased drought tolerance after the flood waters recede. The generally low tolerance to flooding of seedlings of species that are widely rated as highly flood tolerant is emphasized.     

Psoralen production in hairy roots and adventitious roots cultures of Psoralea coryfolia

Psoralea corylifolia is an endangered plant producing various compounds of medical importance. Adventitious roots and hairy roots were induced in cultures prepared from hypocotyl explants. Psoralen content was evaluated in both root types grown either in suspension cultures or on agar solidified medium. Psoralen content was ~3 mg g⁻¹ DW in suspension grown hairy roots being higher than in solid grown hairy roots and in solid and suspension-grown adventitious roots.     

Urea fertilization effects on nutrient uptake and growth of Platanus occidentalis during plantation establishment

Summary The benefit of fertilizer application during establishment of a tree plantation depends on effective nutrient uptake and the utilization of the nutrients in growth. Five urea treatments (0, 50, 75, 150, and 450 kg N/ha) were applied in a completely randomized plot design to a field planted with American sycamore (Platanus occidentalis L.) seedlings to evaluate growth responses and nitrogen use efficiency during the first season of plantation establishment. The site was in the Oak Ridge Reservation in eastern Tennessee on a highly weathered soil. Harvests were conducted on 3 occasions during a 22 week experimental period, and dry weights of stems, leaves, and large and small roots were measured. Chemical analyses were conducted on plant tissues from the 0, 75, and 450 kg N/ha treatments. Plant dry weight increased with urea application and growth analysis showed that this was mainly associated with increase in leaf area and to a minor extent with increase in net assimilation rate. Root weight increased significantly with urea application. The specific absorption rate of roots for several nutrients was greater at higher urea levels for the first 2 harvest periods, but this pattern reversed during the 3rd growth period. Surprisingly, manganese uptake and the specific absorption rate for manganese were enhanced with higher urea application. The acidifying effect of urea nitrification is a likely explanation for the increased Mn availability, and nitrate leaching and/or nitrogen immobilization contributed to low uptake of urea-N by the seedlings. The proportion of the applied nitrogen incorporated into the seedlings was 1.5 and 0.6% for the 75 and 450 kg N/ha urea treatments, respectively. Broadcast fertilizer application is not an effective way of supplying nutrients to seedlings during plantation establishment.     

Hydrogen peroxide involvement in formation and development of adventitious roots in cucumber

Hydrogen peroxide (H₂O₂), an active oxygen species, is widely generated in many biological systems. The present study demonstrates that H₂O₂ was generated in seedling explants after the primary roots were removed, and it mediates the auxin response prior to adventitious root formation in cucumber (Cucumis sativus L. Ganfeng 8). When compared with the controls, treatment of cucumber seedling explants after primary roots removal with either 20–40 mM H₂O₂ or 10 μM IAA significantly increased the number of adventitious roots, and treatment with 10–50 mM H₂O₂ significantly increased the fresh weight of adventitious roots. The effects of H₂O₂ on promoting the formation and growth of adventitious roots were eliminated by 2 mM ascorbic acid, 100 U CAT or 1 μM DPI, and the effects of IAA were eliminated by 4 mM ascorbic acid, 100 U CAT or 5 μM DPI. Treatment with either 4 mM ascorbic acid or 1–5 μM DPI inhibited the formation and growth of adventitious roots, and these inhibitory effects were partly reversed by exogenous H₂O₂.Furthermore, a higher concentration of endogenous H₂O₂ was detected in seedling explants 3 h after the primary roots were removed. However, in 10 μM DPI-treated seedling explants, the concentration of endogenous H₂O₂ was markedly reduced by DPI. Results obtained suggest that H₂O₂ may function as a signaling molecule, involved in the formation and development of adventitious roots in cucumber.     

Factors affecting the initiation and growth of aboveground adventitious roots in a tropical cloud forest tree: an experimental approach

Some of the proximate factors that would induce aboveground stems to produce adventitious roots were investigated experimentally on Senecio cooperi, a tropical cloud forest tree. Stem segments were air-layered with different treatments to promote root formation, and the number of roots initiated and rates of root growth were monitored for 20 weeks. Treatments were the application of wet epiphytes or dry epiphytes plus associated humus, sponges wetted with either water or nutrient solutions, or dry sponges. Controls (stem segments with nothing applied) were also monitored. Numbers of adventitious roots formed and rates of subsequent root growth differed among treatments. Wet epiphyte/humus and nutrient solutions were most effective in producing roots, which suggests that epiphytes and the nutrients they intercept and retain within the canopy may cue adjacent host tissue to exploit this resource.     

Interactions between ethylene, gibberellin and abscisic acid regulate emergence and growth rate of adventitious roots in deepwater rice

Growth of adventitious roots is induced in deepwater rice (Oryza sativa L.) when plants become submerged. Ethylene which accumulates in flooded plant parts is responsible for root growth induction. Gibberellin (GA) is ineffective on its own but acts in a synergistic manner together with ethylene to promote the number of penetrating roots and the growth rate of emerged roots. Studies with the GA biosynthesis inhibitor paclobutrazol revealed that root emergence was dependent on GA activity. Abscisic acid (ABA) acted as a competitive inhibitor of GA activity. Root growth rate on the other hand was dependent on GA concentration and ABA acted as a potent inhibitor possibly of GA but also of ethylene signaling. The results indicated that root emergence and elongation are distinct phases of adventitious root growth that are regulated through different networking between ethylene, GA and ABA signaling pathways. Adventitious root emergence must be coordinated with programmed death of epidermal cells which cover root primordia. Epidermal cell death is also controlled by ethylene, GA and ABA albeit with cell-type specific cross-talk. Different interactions between the same hormones may be a means to ensure proper timing of cell death and root emergence and to adjust the growth rate of emerged adventitious roots.     

Hormones are no causal links in phytochrome-mediated adventitious root formation in mustard seedlings (Sinapis alba L.)

The question of whether or not hormones are causal links in the realization of phytochrome control during photomorphogenesis was investigated using the phytochrome-dependent formation of adventitious roots in hypocotyl cuttings excised from mustard seedlings as a test system. Histological examination of regenerating rest seedlings revealed that phytochrome (operationally, continuous far-red light) mediates the de novo formation of root primordia in the pericycle region of the hypocotyl near the cutting surface withing 12–24 h after excision.Auxin (IAA), gibberellin (GA₃), Cytokinin (kinetin), abscisic acid (ABA), and ethylene had no promotive effect on primordium formation in dark-grown or far-red irradiated rest seedlings. Depending on concentration, the application of these hormones was either ineffective or inhibitory in the rooting response. It is concluded that phytochrome does not operate through changes of hormone (auxin, gibberellin, cytokinin, ABA, ethylene) levels.While externally applied ethylene had no specific effect on primordium formation, the number of primordia produced in darkness could be increased to the far-red light level by removing the endogenously formed ethylene. Since the stimulatory effect of light could not be related to a lower ethylene level, it is concluded that ethylene interferes with primordium formation by modulating the susceptibility of this process to phytochrome control. This ethylene effect takes place in a concentration range below the range that can be manipulated by external application of the hormone.Abbreviations ABA abscisic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid - P_r P_fr red and far-red absorbing forms of phytochrome     

Sugar-induced adventitious roots in<Emphasis Type="Italic"> Arabidopsis</Emphasis> seedlings

The effects of sugars on root growth and on development of adventitious roots were analyzed in Arabidopsis thaliana. Seeds were sown on agar plates containing 0.0–5.0% sugars and placed vertically in darkness (DD) or under long day (LD, 16 h:8 h) conditions, so that the seedlings were constantly attached to the agar medium. In the sucrose-supplemented medium, seedlings showed sustained growth in both DD and LD. However, only dark-grown seedlings developed adventitious roots from the elongated hypocotyl. The adventitious roots began to develop 5 days after imbibition and increased in number until day 11. They could, however, be initiated at any position along the hypocotyl, near the cotyledon or the primary root. They were initiated in the pericycle in the same manner as ordinary lateral roots. Sucrose, glucose and fructose greatly stimulated the induction of adventitious roots, but mannose or sorbitol did not. Sucrose at concentrations of 0.5–2.0% was most effective in inducing adventitious roots, although 5.0% sucrose suppressed induction. Direct contact of the hypocotyl with the sugar-supplemented agar medium was indispensable for the induction of adventitious roots. Electronic Publication     

Action spectrum for light-induced formation of adventitious shoots in hairy roots of horseradish

An action spectrum was determined for lightinduced formation of adventitious shoots in hairy roots of horseradish (Armoracia rusticana Gaert., Mey. et Scherb.) cultured in vitro. Near ultraviolet (350–400 nm), blue (440–460 nm) and red light (600–680 nm) were most effective in inducing formation of adventitious buds. Farred light (730 nm) inhibited the promotive effect of all three wavelength regions. These results are consistent with induction by phytochrome(s) of adventitious shoots in hairy roots of horseradish.     

Detection of quantitative trait loci influencing growth trajectories of adventitious roots in Populus using functional mapping

The capacity to root from cuttings is a key factor for the mass deployment of superior genotypes in clonal forestry. We studied the genetic basis of rooting capacity by mapping quantitative trait loci (QTLs) that control growth rate and form of root traits in a full-sib family of 93 hybrids derived from an interspecific cross between two Populus species, P. deltoides and P. euramericana. The hybrid family was typed for different marker systems (including SSRs, AFLPs, RAPDs, ISSRs, and SNPs), leading to the construction of two linkage maps based on the female P. deltoides (D map) and male P. euramericana (E map) with a pseudotestcross mapping strategy. The two maps were scanned by functional mapping to detect QTLs that control early growth trajectories of two rooting traits, maximal single-root length and the total number of roots per cutting, measured at five time points in water culture. Of the six QTLs detected for these two growth traits, only one is segregating in P. deltoides with poor rooting capacity, while the other five are segregating in P. euramericana showing good rooting capacity. Tests with functional mapping suggest different developmental patterns of the genetic effects of these root QTLs in time course. Five QTLs were detected to change their effects on root growth trajectories with time, whereas one detected to affect root growth consistently in time course. Knowledge about the genetic and developmental control mechanisms of root QTLs will have important implications for the genetic improvement of vegetative propagation traits in Populus.     

Water relations,ethylene production,and morphological adaptation ofFraxinus pennsylvanica seedlings to flooding

Summary Fraxinus pennsylvanica Marsh. seedlings that were 150 days old adapted well to flooding of soil with stagnant water for 28 days. Early stomatal closure, followed by reopening as well as hypertrophy of lenticels and formation of adventitious roots on submerged portions of stems appeared to be important adaptations for flood tolerance. Leaf water potential (₁) was consistently higher in flooded than in unflooded seedlings, indicating higher leaf turgidity in the former. This was the result of (1) early reduction in transpiration associated with stomatal closure, and (2) subsequently increased absorption of water by the newly-formed adventitious roots as stomata reopened and transpiration increased. Waterlogging of soil was followed by large increases in ethylene content of stems, both below and above the level of submersion. Formation of hypertrophied lenticels and adventitious roots on flooded plants was correlated with increased ethylene production. However, the involvement of various compounds other than ethylene in inducing morphological changes in flooded plants is also emphasized.Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison, WI, USA     

Adsorption and anchoring of Azospirillumstrains to roots of wheat seedlings

Recent microscopic evidence acquired using strain-specific monoclonal antibodies and specific gene probes confirms earlier claims that some strains of Azospirillum lipoferum and A. brasilense, but not others, are capable of infecting the interior of wheat roots. The present study was performed to determine whether this strain specificity in the infection of the interior of wheat roots was apparent in the first 24 h of adsorption (`anchoring') of Azospirillum cells to the root surface. Strains of A. brasilense, originally isolated from surface-sterilised wheat roots (Sp 245, Sp 107) or with a proven ability to infect the interior of wheat roots (Sp 245), showed no greater ability to anchor to the roots than other Azospirillum strains isolated from the wheat rhizosphere (Sp 246) or from the rhizosphere or rhizosphere soil of other gramineae (Sp 7, Cd, S 82). The SEM images showed that at the root tip the Azospirillum cells were principally located in cracks between epidermal cells. In the root hair zone the bacteria were more numerous but again principally located in the depressions between epidermal cells. In all zones of the roots mucilage was present, and near the tip this appeared to have been partially digested, forming `halos' around the bacteria and revealing fibril-like strands attached to the bacteria. Subsequent studies were conducted using a technique originally developed for investigating competition of rhizobia for adsorption sites on legume roots. In the adaptation of this technique it was found that the presence of any significant concentration of Ca⁺⁺ in the incubation medium reduced bacterial adsorption, as did concentrations of (PO₄)^3- above 50 mM. The influence of the pH of the incubation medium on the adsorption of ten different strains of Azospirillum showed, that with one exception, strains isolated from the roots or rhizosphere of wheat showed optimum adsorption at pH 6.0, and all other strains pH 7.0. Apart from this effect of pH no differences in adsorption were detected between strains with a proven capacity to infect wheat roots and those unable to do so. However, strains varied in their capability to compete for adsorption sites, there being a tendency for strains with a proven capacity to invade the internal tissues of wheat roots to be more competitive for adsorption sites.     

狐尾藻断枝上不定根与芽发生的初步研究

本文比较研究了不同节位和不同长度狐尾藻断枝的不定根和芽的发生时间及其形成幼苗的频率(本文中的幼苗指最终形成了不定根和芽的断枝)。结果表明:在狐尾藻顶芽以下叶已完全展开的茎段部分,不定根和芽的发生时间呈现出随着节位下降而逐渐缩短的趋势,而幼苗形成频率呈现出随着节位下降而增高的趋势;断枝的长度(用断枝所含的茎节数表示)对不定根和芽的发生时间及幼苗形成频率也有明显的影响。断枝长度增加,不定根和芽的发生时间缩短,形成幼苗的频率升高。另外与抛掷方式相比,扦插延长多节断枝的不定根和芽形成时间,但提高幼苗的形成频率。这些研究结果为制定水体生态系统中狐尾藻的恢复和管理措施提供了参考。     

Stimulation of adventitious root formation in non-woody stem cuttings by uridine

Uridine strongly stimulated adventitious root formation in stem cuttings of sunflower (Helianthus annuus L.), mung bean (Vigna radiata L.) and common bean (Phaseolus vulgaris L.). A dose response curve of uridine induced rooting showed that the optimum concentration of uridine was 0.1 µM. At all concentrations employed, uridine had no significant effect on root elongation. The rooting response of stem cuttings to the optimal concentration of indole-3-butyric acid (10 µM) in combination with 0.1 µM uridine did not significantly differ from their response to either of these compounds when applied alone. However, the rooting response of the cuttings to sub-optimal IBA (0.01 µM) was significantly stimulated by uridine. These findings suggested that uridine may have stimulated rooting by increasing the sensitivity of the rooting tissue to auxin.     

Production of adventitious root biomass and secondary metabolites of Hypericum perforatum L. in a balloon type airlift reactor

The effects of inoculum density, aeration volume and culture period on accumulation of biomass and secondary metabolites in adventitious roots of Hypericum perforatum in balloon type airlift bioreactors (3 l capacity) were investigated. The greatest increment of biomass as well as metabolite content occurred at an inoculum density of 3 g l⁻¹ and an aeration volume of 0.1 vvm. After 6 weeks of culture, an approximately 50-fold increase in biomass was recorded containing 60.11 mg g⁻¹ dry weight (DW) of phenolics, 42.7 mg g⁻¹ DW of flavonoids and 0.80 mg g⁻¹ DW of chlorogenic acid. Liquid chromatography–mass spectroscopy/mass spectroscopy demonstrated that the presence of quercetin and hyperoside in adventitious roots at a level of 1.33 and 14.01 μg g⁻¹ DW, respectively after 6 weeks of culture. The results suggest scale-up of adventitious root culture of H. perforatum for the production of chlorogenic acid, quercetin and hyperoside is feasible.     

Effect of nitrate on water transfer across roots of nitrogen pre-starved maize seedlings

The addition of 10 mM KNO₃ to the solution bathing the roots of young nitrogen-starved seedlings of Zea mays L. enhanced root water transfer within 15 h, compared with 10 mM KCl addition. The free exudation flux was 2.2–3.9 times higher in excised KNO₃-treated roots than in KCl-treated ones. Cryo-osmometry data for xylem sap suggested that, compared with chloride, nitrate treatment increased the steady solute flux into the xylem, but did not modify the osmotic concentration of sap. Root growth was not significantly modified by nitrate within 15 h. Root hydraulic conductances were measured by using either hydrostatic-pressure or osmotic-gradient methods. During hydrostatic experiments, the conductance (k_p), which is thought to refer mainly to the apoplasmic pathway, was 1.6 times larger in KNO₃-than in KCl-treated plants. From experiments in which polyethylene glycol (PEG) 8000 was used as external osmolyte, osmotic conductances (k_s) were found to be smaller by 5–20 times than k_p for the two kinds of plants. The KCl-treated roots were characterized by a low k_s which was the same for influx or efflux of water. By contrast, KNO₃-treated roots exhibited two distinct conductances k_s1 and k_s2, indicating that influx of water was easier than efflux when the water flow was driven by the osmotic pressure gradient. Infiltration of roots with KNO₃ solution supported the idea that nitrate might enhance the efficiency of the cell-to-cell pathway. The low k_s value of KCl-treated roots and the existence of two contrasting k_s values (k_s1 and k_s2) for KNO₃-treated roots are discussed in terms of reversible closing of water channels.     

Survival and growth responses of Myricaria laxiflora seedlings to summer flooding

Myricaria laxiflora, an endangered shrub species distributed along the banks of the Yangtze River in the Three Gorges area, is completely submerged from June to October every year. It is generally assumed that summer flooding has a strong impact on the survival and growth of seedlings. We designed an outdoor randomized block experiment on the responses of seedling survival and growth to different flooding depth and flooding duration treatments during the flood season in the Three Gorges area. Seedling survival rate, aboveground biomass, belowground biomass, total biomass, root depth, length of primary branch and the number of primary and secondary branches were examined.M. laxiflora was found to acclimate to summer flooding by becoming dormant and losing biomass. Seedlings of M. laxiflora ceased growing during the summer flooding season, regardless of the flooding depth and flooding duration they were subjected to. The number of primary and secondary branches, aboveground biomass and total biomass of seedlings was reduced with prolonged flooding. The length of primary branches and aboveground biomass were more sensitive to flooding than other measured parameters and differed significantly between the onset and the end of flooding.In each flooding treatment most seedlings of M. laxiflora survived a flooding period of 2 months and recovered rapidly after the flooding was terminated in September. After 3 months of recovery, aboveground biomass, total biomass and the number of the primary branches increased significantly. Furthermore, seedling survival and growth in the flooding treatments were not significantly different from the controls both during the summer flooding stage and in the recovery stage. All of these results suggest that summer flooding does not affect seedling survival and growth in this species. On the contrary, flooding released seedlings from the stress of drought during summer and facilitated seedling establishment. M. laxiflora appears to cope adaptively with the flooding cycle by going into a state of dormancy during the flood season.     

Effect of spring flooding on endophyte differentiation,nitrogenase activity,root growth and shoot growth inMyrica gale

Summary Spring flooding was investigated as a possible limiting factor in the development of nitrogenase activity, root growth, and shoot growth inMyrica gale. Dormant, one year oldMyrica gale plants were placed in a greenhouse in early April and given three treatments: control (not flooded), flooded-water (flooded with water to 2.5 cm above the soil level) and flooded-peat (flooded with water-saturated peat to 4.0 cm above the soil level). Nitrogenase activity was absent at budbreak but appeared concurrently with the differentiation of vesicles by theFrankia sp. endophyte. Flooding delayed the onset of nitrogenase activity, substantially reduced the specific nitrogenase activity of the nodules, and also severely limited the production of the new nodule biomass. Consequently by 67 days past budbreak nitrogenase activity was much greater in the control plants (5.55±0.42 mol C₂H₄/plant.h; ± SE; N=9) than in the flooded-water (1.18±0.29) and flooded-peat (0.15±0.05) plants. Production of new secondary roots was substantially reduced in the flooded plants but adventitious roots were rapidly produced along the flooded portion of the stem in the better aerated zone near the surface. New nodules formed on several adventitious roots by 67 days indicating that the plants are able to replace their largely nonfunctional deeply flooded nodules with new nodules in the aerobic zone. Initially shoot growth was unaffected by flooding but by 67 days the flooded plants had substantially less leaf biomass, lower leaf and stem nitrogen concentrations, and less total shoot nitrogen content than the control plants.     

Development of the adhesive pad on climbing fig (Ficus pumila) stems from clusters of adventitious roots

Vines have different climbing strategies; one type, called clinging vines, has developed a specialized structure, the adhesive pad that secretes a sticky substance that adheres to almost any substrate. In this study on the climbing fig, Ficus pumila L., we report on the developmental anatomy of clusters of adventitious roots that become transformed into this unique structure. Clustered adventitious roots in F. pumila are initiated in pairs on either side of a vascular bundle at the 2nd to 3rd internodes of young stems. After emergence through the cortex and epidermis, root hairs form, which secrete a substance that stains positively for polysaccharide and protein. The adventitious roots and root hairs stick together forming the adhesive pad. Pads stick to almost any substrate. If the adventitious roots do not touch a substrate they usually dry up, if they touch moist soil they tend to branch and change to a terrestrial form. We discuss experiments on the shoot developmental behavior leading to adhesive pad formation and the possible roles of auxin.