Effects of applying etnylene to the root system of Zea mays on growth and nutrient concentration in relation to flooding tolerance

Previous studies have shown increases in the concentration of ethylene in the soil and roots of plants when the soil is water saturated (flooded). In Zea mays L. this occurs in association with an overall reduction in growth but without extensive foliar senescence and in conjunction with the development of an adventitious root system. We have assessed the possibility that ethylene may be involved in these responses to flooding. Mixtures of the gas in air were therefore supplied to the roots and stem-base of Z. mays growing in nutrient solution.
Seven or 14 d exposure to ethylene (1 or 5 νl 1⁻¹) inhibited seminal root elongation and growth in dry weight and accelerated the emergence of adventitious roots, although their final length and dry weight were depressed. Leaf extension was inhibited by 0.1,1.0 or 5.0 μl 1⁻¹ ethylene around the roots; leaves extending rapidiy at the start of treatment were the most sensitive. Final shoot fresh and dry weights were depressed by the gas but tie shootrroot dry weighl ratio and percentage dry matter were not affected greatly. Leaf chlorosis was not observed but the concentration of phosphorus in the shoots was 26 to 31% below normal.
When aeration of the nutrient solution was stopped, the concentration of dissolved oxygen declined and the concentration of ethylene in the roots increased. Similar changes occur in response to soil flooding. Root and shoot growth was slowed by non-aeration although the shootroot dry weight ratio remained unchanged. The phosphorus concentration of the shoots was depressed but there was little chlorosis or leaf death. The similarity in these respects between the effects of ethylene and non-aeration suggests that in flooded Z. mays , ethylene contributes to their development by accelerating the emergence of adventitioos roots, inhibiting phosphorus accumulation in the shoots and by a non-toxic inhibition of plant growth.     

Molecular and Cellular Adaptations of Maize to Flooding Stress

Anaerobic treatment dramatically alters the patterns of gene expression in maize (Zea mays L.) seedlings. During anaerobiosis there is an immediate repression of pre‐existing protein synthesis, with the concurrent initiation of a selective synthesis of approx. 20 proteins. Among these anaerobic proteins are enzymes involved in glycolysis and related processes. However, inducible genes that have different functions were also found; these may function in other, perhaps more long‐term, processes of adaptations to flooding, such as aerenchyma formation and root‐tip death. In this article we review our recent work on maize responses to flooding stress, which has addressed two questions: how are these gene expression changes initiated and how do they lead to adaptation to flooding stress? Our results indicate that an early rise in cytosolic Ca²⁺, as well as a quick establishment of ionic homeostasis, may be essential for the induction of adaptive changes at the cellular as well as organismal level.     

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.     

Photosynthesis and Dark Respiration in Leaves of Different Ages of Partly Flooded Maize Seedlings

Maize seedlings were flooded for periods from 1 to 15 days, and the leaves of different ages were then taken to examine photosynthesis, dark respiration, transpiration, chlorophyll content, and some morphometric parameters. The responses of leaves to root submergence essentially depended on the leaf layer and the treatment duration. A short-term flooding (1–24 h) induced primary stress responses in the first leaf. Photosynthesis and respiration in this leaf oscillated around the control levels with amplitudes of ±15–25% and ±40–60%, respectively. After a longer flooding, the CO₂ exchange in the second leaf was suppressed, while oxygen uptake was stimulated. In the third leaf, which was formed during submergence, the photosynthetic rate increased and the respiratory activity decreased. The transpiration rate did not change in these leaves for 15 days of flooding. The hypoxic treatment, at its early stages, retarded growth and disturbed the source–sink relations. At later stages the plants adapted to hypoxic environment: the seedling growth was restored, which elevated the demand for assimilates and stimulated photosynthesis. It is concluded that plants overcome negative impact of the root hypoxia at the systemic level.     

Flooding tolerance of Carex species in relation to field distribution and aerenchyma formation

The flooding tolerance of Carex species was studied in relation to their field distribution and their capacity to form root aerenchyma under controlled conditions. In an alpine meadow, six Carex species were selected which were distributed in a clear zonation correlating with water content of the soil. Carex sempervirens and C. ferruginea were only found on nonflooded soil, the latter species preferring moister conditions. Carex davalliana and C. nigra were both associated with water-saturated soil, whereas C. limosa and C. rostrata preferred partially submerged conditions. Carex davalliana and C. limosa were bound to flooded soils with a relatively high redox potential and horizontally flowing groundwater. Carex rostrata and C. nigra grew in stagnant soil-flooded conditions with low soil redox potentials. The amount of aerenchyma in the roots of all species increased when grown in oxygen-deficient stagnant agar. This increase in root porosity, combined with increased root diameter, presumably improved internal aeration of the roots. Although all species survived experimental soil flooding, partial submergence was lethal to C. sempervirens and, surprisingly, also to the wetland species C. davalliana. Carex ferruginea showed a reduced growth rate during partial submergence. The three other species, all wetland plants, reached highest biomass production under soil-flooded and partially submerged conditions, with slower growth on free-draining soil. It is concluded that aerenchyma is not constitutive in the Carex species under study, and is best developed in Carex species from wetlands. Species with less aerenchyma perform poorly when soil-flooded, but conditions of partial submergence could even affect species with a considerable amount of root aerenchyma.     

水杨酸诱导的玉米幼苗适应高温和低温胁迫的能力与抗氧化酶系统的关系

玉米种子经水杨酸(SA)预处理后其幼苗的耐热性与耐冷性提高.其中以300μmol·L-1SA预处理的玉米幼苗对46℃高温胁迫2 d的耐热性提高最大,150μmol·L-1SA预处理的玉米幼苗对1℃低温胁迫5 d的耐冷性提高最大.在高温和低温胁迫过程中,SA预处理过的玉米幼苗中过氧化氢酶(CAT)、抗坏血酸过氧化物酶(APX)、过氧化物酶(GPX)、超氧化物歧化酶(SOD)和谷胱甘肽还原酶(GR)的活性水平均高于未经SA处理的.     

Proline Metabolism and Transport in Maize Seedlings at Low Water Potential

The growing zone of maize seedling primary roots accumulatesproline at low water potential. Endosperm removal and excisionof root tips rapidly decreased the proline pool and greatlyreduced proline accumulation in root tips at low water potential.Proline accumulation was not restored by exogenous amino acids.Labelling root tips with [¹⁴C]glutamate and [¹⁴C]proline showedthat the rate of proline utilization (oxidation and proteinsynthesis) exceeded the rate of biosynthesis by five-fold athigh and low water potentials. This explains the reduction inthe proline pool following root and endosperm excision and theinability to accumulate proline at low water potential. Theendosperm is therefore the source of the proline that accumulatesin the root tips of intact seedlings. Proline constituted 10% of the amino acids released from the endosperm. [¹⁴C]Prolinewas transported from the scutellum to other parts of the seedlingand reached the highest concentration in the root tip. Less[¹⁴C]proline was transported at low water potential but becauseof the lower rate of protein synthesis and oxidation, more accumulatedas proline in the root tip. Despite the low biosynthesis capacityof the roots, the extent of proline accumulation in relationto water potential is precisely controlled by transport andutilization rate.     

Phytohormones in Seedlings of Maize Hybrids Differing in Their Tolerance to High Temperatures

The contents of phytohormones (IAA, ABA, cytokinins, and gibberellin-like compounds) were measured in shoots and roots of eight-day-old seedlings of two maize (Zea mays L.) hybrids differing in their tolerance to elevated temperatures. More tolerant seedlings initially contained more ABA and cytokinins, and the contents of these hormones changed less after a temperature increase than in seedlings of the sensitive hybrid. Hyperthermia induced a destruction of chloroplast lamellar structure in the leaf sheath cells of the sensitive but not of the tolerant hybrid.     

Exploring the Radial and Longitudinal Aeration of Primary Maize Roots by Means of Clark-Type Oxygen Microelectrodes

Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distribution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen demand was met internally by transport from the shoots, but a little oxygen was received by radial inward diffusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally significant. In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low (< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very significant resistance to oxygen transport to the root.     

Adaptations of Central Amazon Tree Species to Prolonged Flooding: Root Morphology and Leaf Longevity

Abstract: Várzeas are species-rich forest communities of the Central Amazon floodplains, inhabited by highly adapted tree species that can withstand long flooding periods. The leaf shedding behaviour and morphological traits that may contribute to adaptation to low oxygen levels were studied at the Ilha de Marchantaria on the lower Solimoes-Amazonas river, Brazil, and in greenhouse experiments with cuttings of six tree species typical of the Amazon floodplain. Comparison of deciduousness in situ revealed that four of the species under investigation, Salix martiana , Tabernaemontana juruana , Laetia corymbulosa and Pouteria glomerata , are able to maintain their leaf system during the aquatic period. Adventitious roots were formed by S. martiana and T. juruana , but anatomical characteristics differed between the species. Whereas S. martiana developed lysigenous aerenchyma in its roots during aerobic and anaerobic growth, only small intercellular spaces of schizogenous origin were formed in the root cortex of T. juruana . Similar to the latter species, such spaces were constitutively formed in the deciduous species Crateva benthami and Vitex cymosa . Suberin deposits were observed in tangential and radial cell walls of the hypodermis of roots from T. juruana , L. corymbulosa and P. glomerata . Suberin deposits were less pronounced in roots of S. martiana and absent in V. cymosa and C. benthami . The data show that different, almost contrasting, survival mechanisms have evolved in roots of plants with similar life forms in the same habitat. The results further suggest that the morphological traits of the investigated trees are causally linked with the in situ leaf shedding behaviour.     

Calcium-Mediated Responses of Maize to Oxygen Deprivation

Oxygen limitation dramatically alters the patterns of gene expression as well as development of plants. Complete removal of O₂ leads to an immediate cessation of protein synthesis followed by a selective synthesis of about twenty anaerobic proteins in maize (Zea mays L.) seedlings. Among these are enzymes involved in glycolysis and related processes. However, inducible genes that have different functions were also found; they may function in other, perhaps more long-term, processes of adaptations to flooding, such as aerenchyma formation and root-tip death. Our recent research has addressed two questions: how these gene expression changes are initiated and how do these responses culminate in the overall adaptation of plants to flooding-stress. The results obtained indicate that an early rise in cytosolic Ca²⁺ as well as a quick establishment of ionic homeostasis may be essential for the induction of adaptive changes at the cellular as well as organismal level.     

离体蒜苔衰老过程中乙烯产生和纤维素酶活力的变化

自Horton和Osborne(1967)报道衰老、脱落中激素和纤维素酶活力相关性以来,许多实验证明,乙烯和纤维素酶与衰老和脱落有密切关系。关颖谦等(1981)指明离体水稻叶片衰老时,纤维素酶的活力     

水杨酸对镉胁迫下玉米幼苗质膜透性和保护酶活性的影响

在Cd^2＋胁迫下,添加外源水杨酸（SA）的培养液中生长的玉米幼苗叶中超氧化物歧化酶（SOD）、过氧化物酶（POD）和过氧化氢酶（CAT）的活性均提高,质膜透性降低,丙二醛（MDA）的积累减少,显示SA对Cd^2＋胁迫具有一定的缓解效应。     

Formation of aerenchyma in roots of Zea mays in aerated solutions, and its relation to nutrient supply

The formation of lysigenous cavities (aerenchyma) in the root cortex of maize, Zea mays L. cv. Capella, under well-aerated conditions has been studied in relation to the composition of the nutrient solutions. Nitrogen, either supplied as nitrate or as ammonium, reduced the cavity formation by the roots. This reduction was most apparent at nitrate concentrations above 2 mM. Cavities were increasingly formed when the nitrate concentration was decreased and they reached their largest dimensions in roots growing in water. Thus, inadequate availability of nitrogen leads, under acrated conditions, to deterioration of cortex cells and cavity formation in the maize roots. It is suggested that cavity formation in these roots is connected with reduced nitrogen assimilation.     

Aerenchyma formation in the wetland plant Juncus effusus is independent of ethylene

Flooded plant roots commonly form aerenchyma, which allows gas diffusion between shoots and roots. The programmed cell death involved in this induced aerenchyma formation is controlled by the plant hormone ethylene, as has been shown for maize (Zea mays). However, the role of ethylene is uncertain in wetland species that form constitutive aerenchyma (also under nonflooded conditions). The aim of this study is to shed light on the involvement of ethylene in constitutive aerenchyma formation in Juncus effusus. Plants of J. effusus and maize were treated with ethylene and inhibitors of ethylene action to determine the consequences for aerenchyma formation. Neither treatment with high ethylene concentrations nor with ethylene inhibitors resulted in changes in root aerenchyma in J. effusus. By contrast, ethylene increased aerenchyma development in maize unless ethylene action inhibitors were applied simultaneously. Similarly, root elongation was insensitive to ethylene treatment in J. effusus, but was affected negatively in maize. The data show that aerenchyma in J. effusus is highly constitutive and, in contrast to the inducible aerenchyma in maize, is not obviously controlled by ethylene.     

Root aeration in rice (Oryza sativa): evaluation of oxygen, carbon dioxide, and ethylene as possible regulators of root acclimatizations

Adventitious roots of rice (Oryza sativa) acclimatize to root-zone O(2) deficiency by increasing porosity, and induction of a barrier to radial O(2) loss (ROL) in basal zones, to enhance longitudinal O(2) diffusion towards the root tip. Changes in root-zone gas composition that might induce these acclimatizations, namely low O(2), elevated ethylene, ethylene-low O(2) interactions, and high CO(2), were evaluated in hydroponic experiments. Neither low O(2) (0 or 0.028 mol m(-3) O(2)), ethylene (0.2 or 2.0 microl l(-1)), or combinations of these treatments, induced the barrier to ROL. This lack of induction of the barrier to ROL was despite a positive response of aerenchyma formation to low O(2) and elevated ethylene. Carbon dioxide at 10 kPa had no effect on root porosity, the barrier to ROL, or on growth. Our findings that ethylene does not induce the barrier to ROL in roots of rice, even though it can enhance aerenchyma formation, shows that these two acclimatizations for improved root aeration are differentially regulated.     

Physiological and Anatomical Basis of Differential Tolerance to Soil Flooding of Lotus corniculatus L. and Lotus glaber Mill

Lotus corniculatus L. and Lotus glaber Mill. are warm-season legume species adapted to many kinds of environmental stress, including flooding conditions, whereas other popular forage legumes, like alfalfa or white clover, cannot thrive. This study evaluates the relationship between root aerenchyma, water relations and leaf gas exchange and the differential tolerance to soil flooding of L. corniculatus and L. glaber. Adult plants of these species, established independently in grasslands mesocosms, were subjected to 40 days of early spring flooding at a water depth of 6 cm. Both species presented constitutive aerenchyma tissue in the roots. Under flooding conditions, this parameter was 26.2% in L. glaber and 15.3% in L. corniculatus. In addition, flooded plants of L. glaber presented a leaf biomass 47.5% higher above water while L. corniculatus showed a leaf biomass 59.6% lower in the same layer, in comparison to control plants. Flooded plants of L. glaber maintained the stomatal conductance (g _s) and transpiration rate (E) for 25 days, although these parameters reduce slightly to 40–60% in comparison to controls after 40 days of flooding. In this species, a reduction in photosynthesis (A) in flooding conditions was detected only on the last day of measurement. In L. corniculatus, the same parameters (g _s, E and A) were affected by flooding since day 18 of treatment, and values reached 25–40% in comparison to control plants after 40 days of flooding. Flooding did not affect above-ground biomass in L. glaber; while in L. corniculatus, above-ground biomass was 35% lower than in control plants. Our results confirmed that L. glaber is more able to cope with flooding stress than L. corniculatus, even in the presence of natural competitors. On the whole, this experiment provides information that can aid in the identification of anatomical and physiological parameters associated with flood-tolerance in this forage legume species, with economic potential for the agricultural areas subject to periodic flooding.     

Cellular Dimorphism in the Maize Root Cortex: Involvement of Microtubules,Ethylene and Gibberellin in the Differentiation of Cellular Behaviour in Postmitotic Growth Zones

Detailed morphometric analysis of cell shapes and an immunofluorescent study of microtubules were carried out on primary roots of Zea mays L. Two types of cells were found to be formed within the postmitotic isodiametric growth (PIG) region of the root cortex that were differentially responsive to low level of exogenous ethylene. The innermost and central cell rows of the cortex were sensitive to ethylene treatment and showed a disturbed distribution of cortical microtubules (CMTs) as well as changed polarity of cell growth, whereas the 2–3 outermost cell rows were less sensitive in this respect. This suggests that post-mitotic cells of the inner cortex are specific targets for ethylene action. These properties of the inner cortex are compatible with its cells being involved in the formation of aerenchyma; they may also favour root growth in compacted soil. By contrast, the specific properties of the outer cortex indicate that this tissue domain is necessary for the gaseous impermeability and the mechanical strengthening of subjacent aerenchymatous cortex, especially in the mature region of the root. Ethylene affected neither the pattern of cortical cell expansion in the meristem nor the position of the PIG region with respect to the root tip. This contrasts with gibberellin-deficiency which affected these parameters in both parts of the cortex. These observations indicate a fundamental difference between the role of these two phytohormones in the morphogenesis and development of maize roots.     

Ca2+-CaM对过氧化氢诱导玉米幼苗耐冷性的影响

H2O2预处理可提高玉米幼苗的耐冷性及其体内钙调素(CaM)活性。阻断胞内Ca2 库的动员(钌红处理)、降低细胞中Ca2 水平(EGTA处理)及抑制CaM活性(TFP和CPZ处理)均可完全消除H2O2诱导的玉米幼苗的耐冷性。阻止胞外Ca2 跨膜进入胞内(La3 处理)并不抑制、甚至还能轻微地提高H2O2诱导的耐冷性。高Ca2 (20mmol.L^-1)处理削弱H2O2诱导的耐冷性。这些结果表明,CaM及胞内Ca2 库在H2O2诱导的玉米幼苗耐冷性的形成过程中起重要作用,而质外体中高浓度Ca2 和跨膜进入胞内会削弱H2O2诱导的耐冷性。     

The influence of oxygen deficiency on ethylene synthesis, 1-aminocyclopropane-1-carboxylic acid levels and aerenchyma formation in roots of Zea mays

The relationship between ethylene production, 1-aminocyclopropane-l-carboxylic acid (ACC) concentration and aerenchyma formation (ethylene-promoted cavitation of the cortex) was studied using nodal roots of maize (Zea mays L. cv. LG11) subjected to various O₂ treatments. Ethylene evolution was 7–8 fold faster in roots grown at 3 kPa O₂ than in those from aerated solution (21 kPa O₂), and transferring roots from aerated solution to 3 kPa O₂ enhanced ethylene synthesis within less than 2 h. Ethylene production and ACC accumulation were closely correlated in different zones of hypoxic roots, regardless of whether O₂ was furnished to the roots through aerenchyma or external solution. Both ethylene production and ACC concentrations (fresh weight basis) were more than 10-fold greater in the distal 0–10 mm than in the fully expanded zone of roots at 3 kPa O₂. Aerenchyma formation occurred in the apical 20 mm of these roots. Roots transferred from air to anoxia accumulated less than 0. 1 nmol ACC (mg protein)^-1 for the first 1.75 h; no ethylene was produced in this time. The subsequent rise in ACC levels shows that ACC can reach high concentrations even in the absence of O₂, presumably due to a de-repression of ACC synthase. The hypothesis was therefore tested that anoxia in the apical region of the root caused enhanced synthesis of ACC, which was transported to more mature regions (10–20 mm behind the apex), where ethylene could be produced and aerenchyma formation stimulated. Surprisingly, exposure of intact root tips to anoxia inhibited aerenchyma formation in the mature root axis. High osmotic pressures around the growing region or excision of apices had the same effect, demonstrating that a growing apex is required for high rates of aerenchyma formation in the adjacent tissue.