Ethylene Evolution from Maize (Zea mays L.) Seedling Roots and Shoots in Response to Mechanical Impedance

The effect of mechanical impedance on ethylene evolution and growth of preemergent maize (Zea mays L.) seedlings was investigated by pressurizing the growth medium in triaxial cells in a controlled environment. Pressure increased the bulk density of the medium and thus the resistance to growth. The elongation of maize primary roots and preemergent shoots was severely hindered by applied pressures as low as 10 kilopascals. Following a steep decline in elongation at low pressures, both shoots and roots responded to additional pressure in a linear manner, but shoots were more severely affected than roots at higher pressures. Radial expansion was promoted in both organs by mechanical impedance. Primary roots typically became thinner during the experimental period when grown unimpeded. In contrast, pressures as low as 25 kilopascals caused a 25% increase in root tip diameter. Shoots showed a slight enhancement of radial expansion; however, in contrast to roots, the shoots increased in diameter even when growing unimpeded. Such morphological changes were not evident until at least 3 hours after initiation of treatment. All levels of applied pressure promoted ethylene evolution as early as 1 hour after application of pressure. After 1 hour, ethylene evolution rates had increased 10, 32, 70, and 255% at 25, 50, 75, and 100 kilopascals respectively, and continued to increase linearly for at least 10 hours. When intact corn seedlings were subjected to a series of hourly cycles of pressure, followed by relaxation, ethylene production rates increased or decreased rapidly, illustrating tight coupling between mechanical impedance and tissue response. Seedlings exposed to 1 microliter of ethylene per liter showed symptoms similar to those shown by plants grown under mechanical impedance. Root diameter increased 5 times as much as the shoot diameter. Pretreatment with 10 micromolar aminoethoxyvinyl glycine plus 1 micromolar silver thiosulfate maintained ethylene production rates of impeded seedlings at basal levels and restored shoot and root extension to 84 and 90% of unimpeded values, respectively. Our results support the hypothesis that ethylene plays a pivotal role in the regulation of plant tissue response to mechanical impedance.     

Factors increasing ethylene production enhance the sensitivity of root growth to auxins

Light inhibits root elongation, increases ethylene production and enhances the inhibitory action of auxins on root elongation of pea ( Pisum sativum L. cv. Weibulls Marma) seedlings. To investigate the role of ethylene in the interaction between light and auxin, the level of ethylene production in darkness was increased to the level produced in light by supplying 1-aminocyclopropane-1-carboxylic acid (ACC) or benzylaminopurine (BAP). Ethylene production was measured in excised root tips after treatment of intact seedlings for 24 h, while root growth was measured after 48 h. Auxin, at a concentration causing a partial inhibition of root elongation, did not increase ethylene production significantly. A 4-fold increase in ethylene production, caused either by light, 0.1 μ M ACC or 0.1 μ M BAP, inhibited root elongation by 40–50%. The auxins 2,4-dichlorophenoxyacetic acid and indolebutyric acid applied at 0.1 μ M inhibited root elongation by 15–25% in darkness but by 50–60% in light. Supply of ACC or BAP in darkness enhanced the inhibitory effects of auxins to about the same extent as in light. The inhibition caused by the auxins as well as by the BAP was associated with swelling of the root tips. ACC and BAP treatment synergistically increased the swelling caused by auxins. We conclude that auxin and ethylene, when applied or produced in partially inhibitory concentrations, act synergistically to inhibit root elongation and increase root diameter. The effect of light on the response of the roots to auxins is mediated by a light-induced increase in ethylene production.     

Enhanced ethylene production by primary roots of Zea mays L. in response to sub-ambient partial pressures of oxygen

Ethylene production by primary roots of 72–h-old intact seedlings of Zea mays L. cv. LG11 was studied under ambient and sub-ambient oxygen partial pressures (pO₂) using a gas flow-through system linked to a photoacoustic laser detector. Despite precautions to minimize physical perturbation to seedlings while setting-up, ethylene production in air was faster during the first 6h than later, in association with a small temporary swelling of the roots. When roots were switched from air (20–8kPa O₂) to 3 or 5kPa O₂ after 6h, ethylene production increased within 2—3 h. When, the roots were returned to air 16 h later, ethylene production decreased within 2—3 h. The presence of 10kPa CO₂ did not interfere with the effect of 3kPa O₂. Transferring roots from air to 12–5kPa did not change ethylene production, while a reduction to 1 kPa O₂ induced a small increase. The extra ethylene formed in 3 and 5 kPa O₂ was associated with plagiotropism, swelling, root hair production, and after 72 h, increased amounts of intercellular space (aerenchyma) in the root cortex. Root extension was also slowed down, but the pattern of response to oxygen shortage did not always match that of ethylene production. On return to air, subsequent growth patterns became normal within a few hours. In the complete absence of oxygen, no ethylene production was detected, even when anaerobic roots were returned to air after 16 h.     

Effect of hypobaric conditions on ethylene evolution and growth of lettuce and wheat

Elevated levels of ethylene occur in enclosed crop production systems and in spaceflight environments, leading to adverse plant growth and sterility. There are engineering advantages in growing plants at hypobaric (reduced atmospheric pressure) conditions in biomass production for extraterrestrial base or spaceflight environments. Objectives of this research were to characterize the influence of hypobaria on growth and ethylene evolution of lettuce (Lactuca sativa) and wheat (Triticum aestivum). Plants were grown under variable total gas pressures [from 30 to 101 kPa (ambient)]. In one study, lettuce and wheat were direct seeded, germinated and grown in the same chambers for 28 d at 50 or 101 kPa. Hypobaria increased plant growth and did not alter germination rate. During a 10-day study, 28-day-old lettuce and 40-day-old wheat seedlings were transplanted together in the same low and ambient pressure chambers; ethylene accumulated in the chambers, but the rate of production by both lettuce and wheat was reduced more than 65% under 30 kPa compared with ambient pressure (101 kPa). Low O2 concentrations [partial pressure of O2 (pO2) = 6.2 kPa] inhibited ethylene production by lettuce under both low (30 kPa) and ambient pressure, whereas ethylene production by wheat was inhibited at low pressure but not low O2 concentration. There was a negative linear correlation between increasing ethylene concentration and decreasing chlorophyll content of lettuce and wheat. Lettuce had higher production of ethylene and showed greater sensitivity to ethylene than wheat. The hypobaric effect on reduced ethylene production was greater than that of just hypoxia (low oxygen).     

The role of oxygen in submergence-induced petiole elongation in Rumex palustris: in situ measurements of oxygen in petioles of intact plants using micro-electrodes

In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c . 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c . 8–12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5–4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1- aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.     

Soil physical strength rather than excess ethylene reduces root elongation of Eucalyptus seedlings in mechanically impeded sandy soils

Seedling establishment in heavily compact soils is hampered by poor root growth caused by soil chemical or physical factors. This study aims to determine the role of ethylene in regulating root elongation through mechanically impeded sandy soils using Eucalyptus todtiana F. Muell seedlings. Concentrations of ethephon (1, 10, and 100???M) were added to non-compact soils, and endogenous ethylene production from seedling roots was compared to ethylene production of roots grown in physically compacted field soils (98.6?% sand). The ethylene-inhibitor 3,5-diiodo-4-hydroxybenzoic acid (DIHB) (0.1???M) was included for each treatment to counteract the negative effects of excess ethylene or compact soils on root elongation. Root elongation was reduced in high ethylene soils by 49?% and high bulk density soils by 44?%. Root ethylene production increased ninefold in roots grown in the high ethylene environment (100???M), but decreased 80?% in compact soils. The use of DIHB did not alter root length and produced varying results with respect to ethylene production, suggesting an interaction effect involving high amounts of soil ethylene. While ethylene regulates root growth, the physical strength of sandy soils is the major factor limiting root elongation in mechanically impeded soils.     

Ethylene as a factor regulating the growth of pea epicotyls subjected to physical stress

Pea epicotyls (Pisum sativum, cv. Alaska) were enclosed in chambers in which their elongation was restricted by means of a foam neoprene stopper or by a medium of glass beads. These treatments increased evolution of ethylene and resulted in reduced length and increased diameter of both the internodes and the cells of the internodes. These responses increased with increasing degrees of restriction. A time-sequence study of the emergence of epicotyls through 90 mm of glass beads showed that an accelerated evolution of ethylene preceded a reduction in elongation. As the epicotyls elongated through the glass bead medium and less resistance was encountered, evolution of ethylene declined and rapid elongation was resumed. The morphological and anatomical effects of a 120-mm column of glass beads were duplicated by applied ethylene concentrations of 0.2 ppm or less. Evolution of CO₂ was inhibited slightly by the ethylene treatments. The data indicate that production of ethylene by pea epicotyls is increased by nonwounding physical stress, and that the ethylene acts as an endogenous growth regulator, decreasing elongation and increasing diameter in response to increasing increments of stress.     

How do roots penetrate strong soil?

The mechanical and physiological bases for root growth against high mechanical impedance are reviewed. The best estimates of maximum axial root growth pressure (_max) in completely impeded pea roots appear to be from 0.5 to 0.6 MPa, which results from a turgor pressure of about 0.8 MPa. When roots are incompletely impeded, a range of responses has been reported. Roots do not change elongation rate in a simple mechanical way in response to changes in mechanical impedance. Instead, ethylene might play a key role in mediating an increase in root diameter and a decrease in elongation rate. These changes persist for some hours or days after impedance is removed. Differences between species in their ability to penetrate strong soil layers are not related to differences in _max, but appear to be due to differences in root diameter. In rice, differences between cultivars in the ability of their roots to penetrate strong wax layers are not related to their elongation rates through uniformly strong media. Differences between species or cultivars in their ability to penetrate strong layers may be due to differences in the tendency of roots to deflect or buckle when they grow from a weak to a strong environment.     

The influence of mechanical impedance on the growth of maize roots

Summary Maize roots were grown between 1 mm glass beads on which a pressure of 40 kPa was applied. The roots were supplied with a constant flow of aerated nutrient solution. Compared with roots grown in a nutrient solution, the impeded crown roots showed a reduction in length of about 75%, whereas the diameter was about 50% increased.These changes in root morphology have been attributed to changes in cell wall structure of the cortex cells, which also occur as a result of the influence of ethylene.It is suggested that ethylene acts as an intermediate factor in the effect of mechanical impedance on root growth.     

Ethylene Biosynthesis during Aerenchyma Formation in Roots of Maize Subjected to Mechanical Impedance and Hypoxia

Germinated maize (Zea mays L.) seedlings were enclosed in modified triaxial cells in an artificial substrate and exposed to oxygen deficiency stress (4% oxygen, hypoxia) or to mechanical resistance to elongation growth (mechanical impedance) achieved by external pressure on the artificial substrate, or to both hypoxia and impedance simultaneously. Compared with controls, seedlings that received either hypoxia or mechanical impedance exhibited increased rates of ethylene evolution, greater activities of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, ACC oxidase, and cellulase, and more cell death and aerenchyma formation in the root cortex. Effects of hypoxia plus mechanical impedance were strongly synergistic on ethylene evolution and ACC synthase activity; cellulase activity, ACC oxidase activity, or aerenchyma formation did not exhibit this synergism. In addition, the lag between the onset of stress and increases in both ACC synthase activity and ethylene production was shortened by 2 to 3 h when mechanical impedance or impedance plus hypoxia was applied compared with hypoxia alone. The synergistic effects of hypoxia and mechanical impedance and the earlier responses to mechanical impedance than to hypoxia suggest that different mechanisms are involved in the promotive effects of these stresses on maize root ethylene biosynthesis.     

Ethylene is differentially regulated during sugar beet germination and affects early root growth in a dose-dependent manner

Main conclusion

By integrating molecular, biochemical, and physiological data, ethylene biosynthesis in sugar beet was shown to be differentially regulated, affecting root elongation in a concentration-dependent manner. There is a close relation between ethylene production and seedling growth of sugar beet (Beta vulgaris L.), yet the exact function of ethylene during this early developmental stage is still unclear. While ethylene is mostly considered to be a root growth inhibitor, we found that external 1-aminocyclopropane-1-carboxylic acid (ACC) regulates root growth in sugar beet in a concentration-dependent manner: low concentrations stimulate root growth while high concentrations inhibit root growth. These results reveal that ethylene action during root elongation is strongly concentration dependent. Furthermore our detailed study of ethylene biosynthesis kinetics revealed a very strict gene regulation pattern of ACC synthase (ACS) and ACC oxidase (ACO), in which ACS is the rate liming step during sugar beet seedling development.     

Effects of antagonists and inhibitors of ethylene biosynthesis on maize root elongation

During the first days of development, maize roots showed considerable variation in the production of ethylene and the rate of elongation. As endogenous ethylene increases, root elongation decreases. When these roots are treated with the precursor of ethylene aminocyclopropane- 1-carboxylic acid (ACC), or inhibitors of ethylene biosynthesis 2-aminoethoxyvinyl glycine (AVG) or cobalt ions, the root elongation is also inhibited. Because of root growth diminishes at high or reduced endogenous ethylene concentrations, it appears that this phytohormone must be maintained in a range of concentrations to support normal root growth. In spite of its known role as inhibitor of ethylene action, silver thiosulphate (STS) does not change significantly the root elongation rate. This suggests that the action of ethylene on root elongation should occur, at least partially, by interaction with other growth regulators.Key words: 2-aminoethoxyvinyl glycine, cobalt, ethylene, root elongation, silver thiosulphate, Zea mays     

Effects of Mechanical Impedance on Root Growth in Barley (Hordeum vulgare L. ): I. EFFECTS ON THE ELONGATION AND BRANCHING OF SEMINAL ROOT AXES

Apparatus is described in which the root system of plants canbe exposed to accurately known external stresses in beds ofballotini for periods up to 3 weeks. A constant flow of aeratednutrient solution was supplied. Modifications of the apparatusallowed the emerging coleoptile also to be subjected to mechanicalstress. Earlier observations that roots are unable to decrease in diameterto enter small pores were confirmed. The effect of externalpressure on the rate of root elongation was independent of thediameter of pores provided this was smaller than the penetratingroot. Pressures of 20 and 50 kPa reduced the elongation of theseminal axes of barley by 50 and 80 per cent respectively. Significantreductions in root extension were also caused by 50 kPa or lessin more limited experiments with a number of other crop plants.The extension of the coleoptile and the first leaf respondedsimilarly to that of the roots. When the diameter of pores inthe rooting medium was intermediate between those of axes andlaterals, the former being impeded, an enhanced developmentof laterals led to a dense superficial root system. This considerablemodification of root form was without effect on nutrient uptakeprovided that an adequate supply of nutrients was availablein the restricted rooting zone. Attention is directed to some of the implications of these responsesfor plant growth under field conditions. However, considerationof the physiological mechanisms responsible for the effectsof low external pressures and the changes in cell structureto which they lead are deferred to subsequent papers.     

Ethylene as a possible mediator of light-induced inhibition of root growth

Eliasson, L. and Bollmark, M. 1988. Ethylene as a possible mediator of light-induced inhibition of root growth. - Physiol. Plant. 72: 605–609.
Pea seedlings ( Pisum sativum L. cv. Weibull's Marma) were used to investigate the possible role of ethylene in light-induced inhibition of root elongation. Illumination of the roots with white light inhibited root elongation by 40–50% and increased ethylene production by the roots about 4-fold. Our main approach was to use exogenous 1-aminocyclopropane-1-carboxylic acid (ACC), supplied in the growth solution, to monitor ethylene production of the roots independent of light treatment. Ethylene production of excised root tips increased with increasing ACC concentrations. The rate of ethylene production in dark-grown roots treated with 0.1 μ M ACC was similar to that caused by illumination. Low ACC concentrations (0.01–0.1 μ M ) decreased the rate of root elongation, especially in seedlings grown in the dark, and 0.1 μ M ACC inhibited elongation to about the same extent as light. In light the roots curved and grew partly plagiogravitropically. This effect was also simulated by the 0.1 μ M ACC treatment. At 1 μ M and higher concentrations, ACC inhibited root growth almost completely and caused conspicuous curvatures of the root tips both in light and darkness. Inhibitors of ethylene synthesis and action partially counteracted the inhibition of root elongation caused by light. These observations suggest that the increase in ethylene production caused by light is at least partly responsible for the decreased growth of light-exposed roots.     

Growth of tree roots in hostile soil: A comparison of root growth pressures of tree seedlings with peas

Background and Aims

As part of a study on growth of tree roots in hostile soil, we envisaged that establishment and survival of trees on hard, dry soil may depend on their ability to exert axial root growth pressures of similar magnitude to those of the roots of agricultural plants (with significant root thickening when roots grow across an air gap or cracks and biopores). We selected tree species originating from a range of different soil and climatic conditions to evaluate whether their relative success on harsh soil (in an evolutionary sense) might be related to the magnitude of root growth pressures they could exert, or how they performed in the very early stages of growth after germination.

Methods

We measured the maximum axial root growth force (F_max) on single lateral root axes of 3- to 4- month old seedlings of 6 small-seeded eucalypts from 2 different habitats and 2 contrasting soil types. Root growth rate, root diameter and F_max were also measured on the primary root axes of a large-seeded acacia and a domesticated annual (Pisum sativum) seedling for up to 10 days following germination.

Results

The lateral roots of the 6 eucalypts and the primary roots of the acacia were considerably smaller than the primary roots of P. sativum and they exerted average forces of similar magnitude to one another (0.198 to 0.312 N). The maximum axial root growth pressures were all in the range 150 to 250 kPa but E. leucoxylon, E. loxophleba and A. salicina exerted the greatest pressures among the trees, and comparable pressures to those exerted by the primary roots of 2-day-old P. sativum (211-252 kPa). Although the primary roots of acacia seedlings exerted increasing axial root growth pressures over a 10-day period following germination, the pressures were still only slightly greater than those of the domesticated plant, P. sativum.

Conclusions

The lack of any very large differences in axial root growth pressures between trees and domesticated plants suggests that trees that grow well in harsh soil don’t do so by exerting higher root growth pressures alone but by also exploring the network of cracks and pores more effectively than do other plants that are less successful.     

Root elongation is restricted by axial but not by radial pressures: so what happens in field soil?

Root distribution determines largely the zone of soil that roots have access to for water and nutrient uptake, and is of great importance especially if water and fertilizer input is restricted. Mechanical impedance is the major limitation to root elongation in many field soils. Until now, experiments have focused largely on the axial resistance to root growth. In a fascinating study of the radial forces exerted by the roots of chickpea, root extension, diameter change, and the radial forces that axially unimpeded roots exert are reported: Kolb et al. (this volume) record radial stresses of about 0.3?MPa that are broadly consistent with cell turgor pressures, but, interestingly, find no restriction to axial elongation. This result is in marked contrast to large decreases in elongation of pea radicles resulting from much smaller axial pressures reported elsewhere in the literature (e.g., an 85?% decrease in root elongation in response to axial pressures of?<?0.1?MPa). The situation is different also from that in homogeneous soil, where root penetration resistance pressures of 0.4-1.0?MPa are typically required to halt root elongation. Soil structure and strength properties will determine the balance of axial and radial pressures on an individual root tip, and hence the root elongation response. It appears that a degree of radial confinement may help roots to extend axially into hard soil. This result also complements recent findings that in strong field soils the availability of soil macropores has a large influence on regulating the root-elongation rates of seedlings.     

Causes of root growth retardation induced by ultraviolet-B irradiation of shoots in Barley seedlings

In barley seedlings (Hordeum vulgare L.) during two days after irradiation of shoots with UV-B (0.5 W/m², 6 h), the rate of elongation of primary roots decreased 2–3 times compared to that in control plants. The modulus of elasticity of roots (ε) increased at most twofold in 12 h after the onset of irradiation; the hydraulic conductivity (L _p) diminished by a factor of two in 12 h, and the root osmotic pressure gradually decreased by 0.08 MPa in 24 h. Changes in ε and L _p were shown to be related to oxidative stress in growing roots, which was evidenced from the increase in H₂O₂ level up to 15-fold increase in 6 h and in activity of guaiacol peroxidase (3.5-fold in 12 h). After 48 h, the characteristics of oxidative metabolism and root characteristics ε and L _p became identical in untreated and treated plants. On the third day, the rate of root growth in treated plants reached its initial value. It is concluded that the main causes of retardation of root growth under these conditions were as follows: the increase in cell wall rigidity related to formation of oxidative cross-links in the apoplast and the decrease in root osmotic pressure due to limited transport of assimilates from irradiated leaves. After the intensity of UV-B irradiation applied to shoots was enhanced (1.6 W/m², 4 h), another physiological status of roots was observed on the 2nd day characterized by twofold increase in L _p, tenfold decreased root elongation rate, and by a progressing increase of root diameter in growing roots. The comparison of root responses induced by irradiation of shoots with the root responses to sodium salicylate and ABA suggests that both agents might participate in the transmission of signals from irradiated leaves to roots.     

4,4,4-trifluoro-3-(indole-3-)butyric acid promotes root elongation in Lactuca sativa independent of ethylene synthesis and pH

We studied the mode of action of 4,4,4-trifluoro-3- (indole-3-) butyric acid (TFIBA), a recently described root growth stimulator, on primary root growth of Lactuca sativa L. seedlings. TFIBA (100 µ M ) promoted elongation of primary roots by 40% in 72 h but inhibited hypocotyl growth by 35%. TFIBA induced root growth was independent of pH. TFIBA did not affect ethylene production, but reduced the inhibitory effect of ethylene on root elongation. TFIBA promoted root growth even in the presence of the ethylene biosynthesis inhibitor l - α -(2-aminoethoxyvinyl)glycine. TFIBA and the ethylene-binding inhibitor silver thiosulphate (STS) had a similar effect on root elongation. The results indicate that TFIBA-stimulated root elongation was neither pH-dependent nor related to inhibition of ethylene synthesis, but was possibly related to ethylene action.     

Root elongation, water stress, and mechanical impedance: a review of limiting stresses and beneficial root tip traits

Root elongation in drying soil is generally limited by a combination of mechanical impedance and water stress. Relationships between root elongation rate, water stress (matric potential), and mechanical impedance (penetration resistance) are reviewed, detailing the interactions between these closely related stresses. Root elongation is typically halved in repacked soils with penetrometer resistances >0.8-2?MPa, in the absence of water stress. Root elongation is halved by matric potentials drier than about -0.5?MPa in the absence of mechanical impedance. The likelihood of each stress limiting root elongation is discussed in relation to the soil strength characteristics of arable soils. A survey of 19 soils, with textures ranging from loamy sand to silty clay loam, found that ～10% of penetration resistances were >2?MPa at a matric potential of -10?kPa, rising to nearly 50% >2?MPa at - 200?kPa. This suggests that mechanical impedance is often a major limitation to root elongation in these soils even under moderately wet conditions, and is important to consider in breeding programmes for drought-resistant crops. Root tip traits that may improve root penetration are considered with respect to overcoming the external (soil) and internal (cell wall) pressures resisting elongation. The potential role of root hairs in mechanically anchoring root tips is considered theoretically, and is judged particularly relevant to roots growing in biopores or from a loose seed bed into a compacted layer of soil.