Ethylene-promoted adventitious rooting and development of cortical air spaces (aerenchyma) in roots may be adaptive responses to flooding in Zea mays L

Changes in cell wall ultrastructure induced by sudden flooding at 25{degrees}C in Pisum sativum (Fabaceae) primary roots

Cellular degeneration is essential for many developmental and stress acclimation processes. Undifferentiated parenchymatous cells in the central vascular cylinder of pea primary roots degenerate under hypoxic conditions created by flooding at temperatures >15°C, forming a long vascular cavity that seems to provide a conduit for longitudinal oxygen transport in the roots. We show that specific changes in the cell wall ultrastructure accompanied previously detected cytoplasmic and organellar degradation in the cavity-forming roots. The degenerating cells had thinner primary cell walls, less electron-dense middle lamellae, and less abundant cell wall homogalacturonans in altered patterns, compared to healthy cells of roots grown under cold, nonflooded conditions. Cellular breakdown and changes in wall ultrastructure, however, remained confined to cells within a 50-μm radius around the root center, even after full development of the cavity. Cells farther away maintained cellular integrity and had signs of wall synthesis, perhaps from tight regulation of wall metabolism over short distances. These observations suggest that the cell degeneration might involve programmed cell death. We also show that warm, nonflooded or cold, flooded conditions that typically do not induce vascular cavity formation can also induce variations in cell wall ultrastructure.

Programmed cell death and tissue remodelling in plants

The use of programmed cell death (PCD) to remodel plants at the cellular, tissue, and organ levels is particularly fascinating and occurs in such processes as tracheary element differentiation, lysigenous aerenchyma formation, development of functionally unisexual flowers from bisexual floral primordia, and leaf morphogenesis. The formation of complex leaf shape through the use of PCD is a rare event across vascular plants and occurs only in a few species of Monstera and related genera, and in the lace plant (Aponogeton madagascariensis). During early development, the lace plant leaf forms a pattern of equidistantly positioned perforations across the surface of the leaf, giving it a lattice-like appearance. Due to the accessibility and predictability of this process, the lace plant provides highly suitable material for the study of developmentally regulated PCD in plants. A sterile lace plant culture system has been successfully established, providing material free of micro-organisms for experimental study. The potential role of ethylene and caspase-like activity in developmentally regulated PCD in the lace plant is currently under investigation, with preliminary results indicating that both may play a role in the cell death pathway.

Cortical Aerenchyma formation in hypocotyl and adventitious roots of Luffa cylindrica subjected to soil flooding

BACKGROUND AND AIMS

Aerenchyma formation is thought to be one of the important morphological adaptations to hypoxic stress. Although sponge gourd is an annual vegetable upland crop, in response to flooding the hypocotyl and newly formed adventitious roots create aerenchyma that is neither schizogenous nor lysigenous, but is produced by radial elongation of cortical cells. The aim of this study is to characterize the morphological changes in flooded tissues and the pattern of cortical aerenchyma formation, and to analyse the relative amount of aerenchyma formed.

METHODS

Plants were harvested at 16 d after the flooding treatment was initiated. The root system was observed, and sections of fresh materials (hypocotyl, tap root and adventitious root) were viewed with a light or fluorescence microscope. Distributions of porosity along adventitious roots were estimated by a pycnometer method.

KEY RESULTS

Under flooded conditions, a considerable part of the root system consisted of new adventitious roots which soon emerged and grew quickly over the soil surface. The outer cortical cells of these roots and those of the hypocotyl elongated radially and contributed to the development of large intercellular spaces. The elongated cortical cells of adventitious roots were clearly T-shaped, and occurred regularly in mesh-like lacunate structures. In these positions, slits were formed in the epidermis. In the roots, the enlargement of the gas space system began close to the apex in the cortical cell layers immediately beneath the epidermis. The porosity along these roots was 11-45 %. In non-flooded plants, adventitious roots were not formed and no aerenchyma developed in the hypocotyl or tap root.

CONCLUSIONS

Sponge gourd aerenchyma is produced by the unique radial elongation of cells that make the expansigeny. These morphological changes seem to enhance flooding tolerance by promoting tissue gas exchange, and sponge gourd might thereby adapt to flooding stress.

Evidence that ethylene signalling is not involved in selective root placement by tobacco plants in response to nutrient-rich soil patches

Ethylene is a strong controller of root development, and it has been suggested that it is involved in the increase of lateral root development in nutrient-rich soil patches (selective root placement). Here, this contention was tested by comparing selective root placement of an ethylene-insensitive transgenic tobacco (Nicotiana tabacum) genotype (Tetr) with that of wild-type (Wt) plants. Wt and Tetr plants were grown in pots with locally increased nitrate or phosphate concentrations, after which the root growth patterns were compared with those of plants grown in pots with homogeneous nutrient distribution. Tetr plants responded to nutrient patches in a similar way to Wt plants, by placing their roots preferentially at locations with higher nutrient content, and other aspects of plant morphology were also not greatly influenced by ethylene insensitivity. The response of both Wt and Tetr plants to high-nitrate patches was considerably stronger than to locally high phosphate, suggesting that the two responses are not linked in any functional or regulatory way. As the response to nutrient patches was similar in ethylene-sensing and ethylene-insensitive plants, it is concluded that selective root placement in response to nitrate or phosphate is, at least in tobacco, not mediated or modified by ethylene action.

Inhibition of root elongation by ethylene in wetland and non-wetland plant species and the impact of longitudinal ventilation

The slow gas diffusion rate in flooded soil not only causes oxygen deficiency, but also favours the accumulation of ethylene in root systems to concentrations that may strongly affect root elongation. Previously published experiments showed that root elongation in rice is much less strongly inhibited by ethylene than in some other species less well adapted to wet conditions. Rice roots have also been reported to produce abnormally little ethylene. We tested if these traits are typical of wetland species and are thus likely to be widespread adaptive traits. Comparisons using 14 species indicated that insensitivity to the inhibiting effects of ethylene on root elongation is unlikely to be a common feature of temperate wetland species. However, resistance to longitudinal gas diffusion within roots of wetland species, which largely depends on diameter and the presence of gas-filled channels, was found to be less than in non-wetland species. We show that this can help maintain low internal ethylene concentrations by venting accumulated gas to the shoot and atmosphere.

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.

Sensing and signalling in response to oxygen deprivation in plants and other organisms

AIMS AND SCOPE

All aerobic organisms require molecular di-oxygen (O2) for efficient production of ATP though oxidative phosphorylation. Cellular depletion of oxygen results in rapid molecular and physiological acclimation. The purpose of this review is to consider the processes of low oxygen sensing and response in diverse organisms, with special consideration of plant cells.

CONCLUSIONS

The sensing of oxygen deprivation in bacteria, fungi, metazoa and plants involves multiple sensors and signal transduction pathways. Cellular responses result in a reprogramming of gene expression and metabolic processes that enhance transient survival and can enable long-term tolerance to sub-optimal oxygen levels. The mechanism of sensing can involve molecules that directly bind or react with oxygen (direct sensing), or recognition of altered cellular homeostasis (indirect sensing). The growing knowledge of the activation of genes in response to oxygen deprivation has provided additional information on the response and acclimation processes. Conservation of calcium fluxes and reactive oxygen species as second messengers in signal transduction pathways in metazoa and plants may reflect the elemental importance of rapid sensing of cellular restriction in oxygen by aerobic organisms.

Sensing and signalling during plant flooding

Flooding is a major issue for plant survival in many regions of the world. Soil inundation induces multiple plant physiological dysfunctions, leading to a decline in plant growth and survival capacity. Some of the most important effects of flooding include a reduction in water and nutrient uptake and a decrease in metabolism. Prolonged soil flooding will also ultimately lead to anoxia conditions with profound effects on plant respiratory metabolism. However, it is still unclear which signals and which sensory mechanisms are responsible for triggering the plant response. In contrast, it is now established that flooding responses are typified by enhanced ethylene production, accompanied by a signalling cascade which includes a network of hormones and other common secondary signalling molecules. In recent years, there has been significant progress in the understanding of some of the signalling pathways involved during plant stress responses. Here, we present an overview of recent hypothesises on sensing and signalling during plant flooding.

Transient exposure to ethylene stimulates cell division and alters the fate and polarity of hypocotyl epidermal cells

After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.

The ethylene biosynthetic and perception machinery is differentially expressed during endosperm and embryo development in maize

The maize endosperm undergoes programmed cell death late in its development so that, with the exception of the aleurone layer, the tissue is dead by the time the kernel matures. Although ethylene is known to regulate the onset of endosperm cell death, the temporal and spatial control of the ethylene biosynthetic and perception machinery during maize endosperm development has not been examined. In this study, we report the isolation of the maize gene families for ACC synthase, ACC oxidase, the ethylene receptor, and EIN2 and EIL, which act downstream of the receptor. We show that ACC oxidase is expressed primarily in the endosperm, and only at low levels in the developing embryo late in its development. ACC synthase is expressed throughout endosperm development but, in contrast to ACC oxidase, it is transiently expressed to a significantly higher level in the developing embryo at a time that corresponds with the onset of endosperm cell death. Only two ethylene receptor gene families were identified in maize, in contrast to the five types previously identified in Arabidopsis. Members of both ethylene receptor families were expressed to substantially higher levels in the developing embryo than in the endosperm, as were members of the EIN2 and EIL gene families. These results suggest that the endosperm and embryo both contribute to the synthesis of ethylene, and they provide a basis for understanding why the developing endosperm is especially sensitive to ethylene-induced cell death while the embryo is protected.

Apoptosis in plants: specific features of plant apoptotic cells and effect of various factors and agents

Apoptosis is an integral part of plant ontogenesis; it is controlled by cellular oxidative status, phytohormones, and DNA methylation. In wheat plants apoptosis appears at early stages of development in coleoptile and initial leaf of 5- to 6-day-old seedlings. Distinct ultrastructural features of apoptosis observed are (1). compaction and vacuolization of cytoplasm in the apoptotic cell, (2). specific fragmentation of cytoplasm and appearance in the vacuole of unique single-membrane vesicles containing active organelles, (3). cessation of nuclear DNA synthesis, (4). condensation and margination of chromatin in the nucleus, (5). internucleosomal fragmentation of nuclear DNA, and (6). intensive synthesis of mitochondrial DNA in vacuolar vesicles. Peroxides, abscisic acid, ethylene releaser ethrel, and DNA methylation inhibitor 5-azacytidine induce and stimulate apoptosis. Modulation of the reactive oxygen species (ROS) level in seedling by antioxidants and peroxides results in tissue-specific changes in the target date for the appearance and the intensity of apoptosis. Antioxidant butylated hydroxytoluene (BHT) reduces the amount of ROS and prevents apoptosis in etiolated seedlings, prolongs coleoptile life span, and prevents the appearance of all apoptotic features mentioned. Besides, BHT induces large structural changes in the organization of all cellular organelles and the formation of new unusual membrane structures in the cytoplasm. BHT distorts mitosis and this results in the appearance of multiblade polyploid nuclei and multinuclear cells. In roots of etiolated wheat seedlings, BHT induces differentiation of plastids with the formation of chloro(chromo)plasts. Therefore, ROS controlled by BHT seems to regulate mitosis, trigger apoptosis, and control plastid differentiation and the organization of various cellular structures formed by endocytoplasmic reticulum.

Growth and physiological responses of six barley genotypes to waterlogging and subsequent recovery

In this study, the growth response of 6 barley genotypes of different origin (3 from China, 2 from Australia, 1 from Japan) to waterlogging and subsequent recovery was evaluated in 2 different soil types, an artificial potting mix and a Vertosol. A range of physiological measurements was assessed, to develop a method to aid selection for waterlogging tolerance. Plants at the 3 or 4 expanded leaf stages were subjected to waterlogging for 3 weeks followed by 2 weeks of recovery. Both shoot and root growth was negatively affected by waterlogging. As waterlogging stress developed, chlorophyll content, CO2 assimilation rate, and maximal quantum efficiency of photosystem II (Fv/Fm) decreased significantly. The adverse effect of waterlogging was most severe for genotype Naso Nijo, intermediate for ZP, Gairdner, DYSYH, and Franklin, and least for TX9425 in both trials. Studies of the root anatomy suggested that such a contrasting behaviour may be partially due to a significant difference in the pattern of aerenchyma formation in barley roots. The adverse effects in stressed plants were alleviated after 2 weeks of drainage for all genotypes. In general, TX9425 continued to grow better than other varieties, whereas recovery of Naso Nijo was extremely slow. It is suggested that screening a small number of lines for waterlogging tolerance could be facilitated by selecting genotypes with least pronounced reduction of photosynthetic rate or total chlorophyll content, and for a larger number of lines, chlorophyll fluorescence is the most appropriate tool.

Expression of a stress-induced hemoglobin affects NO levels produced by alfalfa root cultures under hypoxic stress

Transgenic alfalfa root cultures expressing sense and antisense barley hemoglobin transcripts were examined under varying levels of atmospheric oxygen. Root cultures overexpressing the hemoglobin gene (Hb+) maintained root growth when placed under 3% oxygen, whereas control cultures or cultures underexpressing hemoglobin (Hb-) experienced 30-70% declines in growth under the same conditions. ATP levels and ATP/ADP ratios for Hb+ lines did not significantly differ in 40 and 3% oxygen, whereas the ATP levels and ATP/ADP ratios in control and Hb- lines were significantly lower under 3% oxygen. Large increases in the production of nitric oxide (NO) were measured in root cultures grown under hypoxic conditions compared to aerobic conditions. The amount of NO accumulated in an Hb- line was 2.5-fold higher than that in the Hb+ line. Treatment of transgenic root lines under 40% oxygen with NO resulted in significant declines in the ATP levels and ATP/ADP ratio of an Hb- line and the control line, with no significant change in an Hb+ line. The root cell structure of an Hb- line showed evidence of cell breakdown under hypoxic growth, whereas an Hb+ line had no evidence of cell breakdown under similar growth conditions. These results lead us to hypothesize that NO is involved in the response of plants to hypoxia and that hemoglobin modulates the levels of NO in the hypoxic cell.

Changes in growth and structure of pea primary roots (Pisum sativum L. cv. Alaska) as a result of sudden flooding

Pea (Pisum sativum L. cv. Alaska) primary roots were exposed to flooding after growth for 4 or 5 d at 25 degrees C under relatively dry conditions. Flooding after 4 d growth reduced, but did not stop, primary root growth, and cavities caused by degradation of central vascular cells were typically found from 10-60 mm from the tips. Flooding after 5 d stopped primary root growth and caused cell death in the tips, and vascular cavities formed that typically were 20-60 mm from the tips of the roots. Degradation of root tip cells in 5-day-roots was very rapid and began in the elongation zone and later in the apical zone. Root tips discolored, narrowed or curled before growth arrest. The mitotic indices of 5-day-root tips were suppressed by the flooding treatment. A few mitotic figures were observed in roots treated with flooding after 4 d growth. Affected cells had condensed nuclei, but cytoplasms appeared to be normal in the early stages of cell degradation. Later these cells became very vacuolated. The relationship of flooding to root growth, vascular cavity formation, and the morphology of pea primary roots is described with regard to the ability to resist flooding stress.

Metabolic inhibition of root water flow in red-osier dogwood (Cornus stolonifera) seedlings

The short-term effects of sodium azide (NaN(3)) on water flow in red-osier dogwood (Cornus stolonifera Michx.) seedlings were examined in excised roots at a constant pressure of 0.3 MPa. NaN(3) significantly decreased root water flow rates (Q(v)). It also induced a significant reduction in root respiration and reduced stomatal conductance to a greater extent in intact seedlings than in excised shoots. Apoplastic flow of water increased with the NaN(3)-induced decreases in Q(v). Mercuric chloride (HgCl(2)) was also used to characterize the water flow responses and respiration of dogwood roots. Similarly to NaN(3), 0.1 and 0.3 mM HgCl(2) decreased root respiration rates and Q(v). The lower, 0.05 mM HgCl(2) treatment, reduced Q(v), but had no significant effect on root oxygen uptake. The reduction of Q(v) in HgCl(2)-treated plants was only partly reversed by 50 mM mercaptoethanol. The mercurial inhibition of Q(v) suggested the presence of Hg-sensitive water channels in dogwood roots. The results indicate that root-absorbed NaN(3) metabolically inhibited water channel activities in roots and in shoots and resulted in stomatal closure. It is suggested that the inhibition of respiration that occurs in plants stressed with environmental factors such as flooding, cold soils, and drought may be responsible for the closure of water channels in root cells and inhibition of root water flow.

Ethylene induces epidermal cell death at the site of adventitious root emergence in rice

In deepwater rice (Oryza sativa), adventitious root primordia initiate at the nodes as part of normal development. Emergence of the roots is dependent on flooding of the plant and is mediated by ethylene action. Root growth was preceded by the induced death of epidermal cells of the node external to the tip of the root primordium. Cell death proceeded until the epidermis split open. Through this crack the root eventually emerged. Induced death was confined to nodal epidermal cells covering the tip of the primordia. Our results suggest that this process facilitates adventitious root emergence and prevents injury to the growing root. Cell death was inducible not only by submergence but also by application of 1-aminocyclopropane-1-carboxylic acid, the natural precursor of ethylene and it was suppressed in the presence of 2,5-norbornadiene (bicyclo[2.2.1]hepta-2,5-diene), an inhibitor of ethylene action. Adventitious root growth and epidermal cell death are therefore linked to the ethylene signaling pathway, which is activated in response to low oxygen stress.

Programmed cell death during endosperm development

The endosperm of cereals functions as a storage tissue in which the majority of starch and seed storage proteins are synthesized. During its development, cereal endosperm initiates a cell death program that eventually affects the entire tissue with the exception of the outermost cells, which differentiate into the aleurone layer and remain living in the mature seed. To date, the cell death program has been described for maize and wheat endosperm, which exhibits common and unique elements for each species. The progression of endosperm programmed cell death (PCD) in both species is accompanied by an increase in nuclease activity and the internucleosomal degradation of nuclear DNA, hallmarks of apoptosis in animals. Moreover, ethylene and abscisic acid are key to mediating PCD in cereal endosperm. The progression of the cell death program in developing maize endosperm follows a highly organized pattern whereas in wheat endosperm, PCD initiates stochastically. Although the essential characteristics of cereal endosperm PCD are now known, the molecular mechanisms responsible for its execution remain to be identified.

Changes in cell structure during the formation of root aerenchyma inSAGITTARIA LANCIFOLIA (Alismataceae)

In many wetland species, root aerenchyma is produced by the predictable collapse of root cortex cells, indicating a programmed cell death (PCD). The objective of this study was to characterize the cellular changes that accompany this PCD in the marsh species Sagittaria lancifolia. Structural changes in membranes and organelles were examined during development of root cortex cells to compare with previous examples of PCD. The organization of cortical microtubule (CMT) arrays in root cells from S. lancifolia was also evaluated as a possible predictor of cell lysis. Nuclear fragmentation and condensation were the earliest changes observed in cells undergoing lysis. Breakdown of the tonoplast and other organelles and disruption of the plasma membrane followed. After loss of cytoplasm, cells collapsed to form gas spaces. These results were compared to collapse of root cortical cells of Zea mays and Oryza sativa during aerenchyma development. Changes in the appearance of the cytoplasm of all three species were similar at later stages of aerenchyma development. The relative timing of disintegration of the tonoplast and middle lamella appeared to differ among the three species. Changes in the organization of CMT arrays did not appear to be a predictor of PCD in S. lancifolia. Aerenchyma production in plants involves a type of PCD that is morphologically distinct from PCD described from many animals.

Development of the endodermis and hypodermis of Typha glauca Godr. and Typha angustifolia L. roots

The development of the endodermis and hypodermis in adventitious roots of Typha angustifolia L. and Typha glauca Godr. was followed from the apical meristem to full maturity. The endodermis was typical, developing a thin Casparian band near the root tip, followed by suberin lamellae and asymmetric, secondary, lignified walls (C-type at maturity). Passage cells were present at an intermediate stage but eventually disappeared when all cells developed lamellae and secondary walls. The hypodermis was multiple (four to six layers at maturity) and began differentiating near the root tip. Here, the radial and transverse walls of the outermost layer did not dissolve in strong acid and the former were wavy in the longitudinal direction, both features characteristic of a Casparian band, but these walls were permeable to berberine. No other indication of a wall modification was seen for 3 weeks, at which time the root had become determinate and aerenchyma was beginning to form in the midcortex. Casparian bands, which were impermeable to berberine, matured in the hypodermis; thus, it proved to be an exodermis. Different forms of Casparian band were detected: one was typical and occupied the radial and transverse walls of the outermost layer, but others were novel and included tangential walls, often forming an H-shaped structure (as seen in cross section of the root). We propose calling the latter type an H-type Casparian band. It functioned as an apoplastic barrier to berberine applied either externally or internally by injection into the cortical aerenchyma. Following maturation of the Casparian band, the outer two layers of the exodermis soon produced suberin lamellae. These continued to be deposited in a centripetal pattern until they were found in all layers of the multiple exodermis. Development of the suberin wall modifications correlates with the development of the aerenchyma and may play a role in preventing gas exchange between the root and the rhizosphere. Later, all exodermal cells produced lignified, secondary walls. These were asymmetric in the outermost and innermost layers of the hypodermis (like the C-type endodermis); eventually, all layers had Casparian wall materials. Thus, the mature hypodermis consisted of two to six layers of exodermis, except at the tips of determinate roots where the exodermis was uniseriate with typical Casparian bands.Key words: cattail, endodermis, exodermis, hypodermis, roots, Typha.

Waterlogging responses of Sporobolus virginicus (L.) Kunth

Flooding responses in Sporobolus virginicus (L.) Kunth., a perennial C₄ grass, propagated from plants collected on the fringes of a mangrove swamp, were examined in a glasshouse study over 42 days. Flooding significantly reduced soil redox potential, induced adventitious root development, shifted resource allocation from below- to above-ground components without affecting total biomass accumulation and significantly decreased below-ground/above-ground biomass ratios. Although soil waterlogging significantly increased alcohol dehydrogenase activity (ADH) after 30 h, significant increase in central air space by 45-50% of the cross-sectional stem area eliminated root hypoxia, and ADH activity decreased to levels equivalent to drained controls after 42 days. In addition, flooded plants exhibited significantly higher carbon dioxide assimilation rates but similar relative growth rates (RGR) to drained controls. The results indicate that S. virginicus responds to water-logging by a combination of metabolic, morphological and anatomical mechanisms, which may account for its widespread distribution in coastal lagoons, estuaries and marshes.

PHENOTYPIC FLEXIBILITY OF CAREX FLACCA SCHREB.: TOLERANCE OF SOIL FLOODING BY Populations FROM CONTRASTING HABITATS

Nine populations of Carex flacca Schreb., collected from sites which vary widely in their liability to soil flooding, were grown on an experimental soil with free drainage, transient flooding or continuous flooding for 26 weeks. Although the flooding treatments resulted in low redox potentials and high concentrations of reduced iron and manganese in the soil, the growth of C. flacca plants was little affected by flooding. All populations grew equally well on the flooded soils. All populations produced adventitious roots at the soil surface in the flooded treatments; these were absent from plants grown on the freely drained soil. Flooding produced an increase in the manganese concentration of roots and shoots but, although the iron content of roots was markedly increased by flooding, there was no increased transport to the shoots. In view of the absence of detectable phenotypic differentiation between these populations from widely contrasting sites and their unimpaired growth on severely waterlogged soils, it is argued that C. flacca plants have a broad tolerance (phenotypic flexibility) of soil flooding. This may be important in the wide ecological amplitude of this species.

Suberin lamellae in the hypodermis of maize (Zea mays) roots; development and factors affecting the permeability of hypodermal layers

The development of suberin lamellae in the hypodermis of Zea mays cv. LG 11 was observed by electron microscopy and the presence of suberin inferred from autoliuorescence and by Sudan black B staining in nodal (adventitious) and primary (seminal) root axes. Suberin lamellae were evident at a distance of 30-50 mm from the tip of roots growing at 20°C and became more prominent with distance from the tip. Both oxygen deficiency and growth at 13°C produced shorter roots in which the hypodermis was suberized closer to the root tip. There were no suberin lamellae in epidermal cells or cortical collenchyma adjacent to the hypodermis. Plasmodesmata were not occluded by the suberin lamellae: there were twice as many of them in the inner tangential hypodermal wall (1,14 μn^-2 ) as in the junction between the epidermis and hypodermis (0.54 μm^-2 ). Water uptake by seminal axes (measured by micropotometry) was greater at distances more than 100 mm from the root lip than in the apical zone where the hypodermis was unsuberized. In the more mature zones of roots grown at 13°C rates of water uptake were greater than in roots grown at 20°C even though hypodermal suberization was more marked. Sleeves of epidermal/hypodermal cells (plus some accessory collenchyma) were isolated from the basal 60 mm of nodal axes by enzymatic digestion (drisclase). The roots were either kept totally immersed in culture solution or had the basal 50 mm exposed to moist air above the solution surface. In both treatments the permeabilities to tritiated water and ₈₆ Rb were low (circa 10^-5 mms^-1 ) in sleeves isolated from the extreme base. In roots grown totally immersed, however, the permeability of sleeves increased 10 to 50-fold over a distance of 40 mm. In roots exposed to moist air the permeability remained at a low level until the point where the root entered the culture solution and then increased rapidly (> 50-fold in a distance of 8 mm). Growth of roots in oxygen depleted (5% O₂ ) solutions promoted the development of extensive cortical aerenchymas. These developments were not associated with any reduction in permeability of sleeves isolated from the basal 40 mm of the axis. It was concluded that the presence of suberin lamellae in hypodermal walls does not necessarily indicate low permeability of cells or tissues to water or solutes. The properties of the walls (lamellae?) can be greatly changed by exposure to moist air, perhaps due to increased oxygen availability.

EARLY SENESCENCE OF THE ROOT CORTEX OF AGRICULTURAL GRASSES, AND OF WHEAT FOLLOWING ROOT AMPUTATION OR INFECTION BY THE TAKE-ALL FUNGUS

Nuclear fluorescence following acridine orange staining was used to assess patterns and rates of death of the root cortex (RCD) in Lolium perenne L., L.×hybridum Hausskn. and Dactylis glomerata L. grown in pathogen-free soil in a glasshouse. The pattern of RCD was as previously described for cereals. The rate of RCD differed significantly between grasses and between cultivates of L. pefenne, but in most instances the cortex was anucleate, except for the innermost cell layer next to the endodermis, in 26 to 27 d old regions of seminal root axes. Root impedance caused by a nylon gauze barrier in soil significantly increased the rate of RCD in one tested cultivar of L. perenne. RCD was more rapid in wheat than in grasses. In 8 d old regions of wheat seminal root axes the cortex contained only 54% of the nuclei initially present. Infection of wheat roots by the take-all fungus, Gaeumannomyces graminis (Sacc.) Arx & Olivier var tritici Walker, caused vascular disruption followed by more rapid RCD than in uninfected roots. Similarly rapid RCD occurred in amputated roots, though in all instances the pattern of RCD was unchanged. The rate of RCD was slower in long than in short lengths of amputated root, perhaps because of remobilization of nutrients from dying cells. Nuclei persisted for at least 10 d in the inner cortex of 2 cm lengths of young roots of wheat, barley and L. perenne buried in soil at 20 °C. The pattern of nuclear deletion from the root pieces was the same as in whole roots, and the rate of RCD was faster in wheat than in barley, as also found for roots attached to plants. All these results demonstrate a consistent pattern of cortical senescence in graminaceous roots and suggest that it is a programmed phenomenon.

Stimulation of ethylene production and gas-space (aerenchyma) formation in adventitious roots of Zea mays L. by small partial pressures of oxygen

Adventitious roots of two to four-weekold intact plants of Zea mays L. (cv. LG11) were shorter but less dense after extending into stagnant, non-aerated nutrient solution than into solution continuously aerated with air. Dissolved oxygen in the non-aerated solutions decreased from 21 kPa to 3-9 kPa within 24 h. When oxygen partial pressures similar to those found in non-aerated solutions (3, 5 and 12 kPa) were applied for 7 d to root systems growing in vigorously bubbled solutions, the volume of gas-space in the cortex (aerenchyma) was increased several fold. This stimulation of aerenchyma was associated with faster ethylene production by 45-mm-long apical root segments. When ethylene production by roots exposed to 5 kPa oxygen was inhibited by aminoethoxyvinylglycine (AVG) dissolved in the nutrient solution, aerenchyma formation was also retarded. The effect of AVG was reversible by concomitant applications of 1-aminocyclopropane-1-carboxylic acid, an immediate precursor of ethylene. Addition of silver nitrate, an inhibitor of ethylene action, to the nutrient solution also prevented the development of aerenchyma in roots given 5 kPa oxygen. Treating roots with only 1 kPa oxygen stimulated ethylene production but failed to promote gas-space formation. These severely oxygen-deficient roots seemed insensitive to the ethylene produced since a supplement of exogeneous ethylene that promoted aerenchyma development in nutrient solution aerated with air (21 kPa oxygen) failed to do so in nutrient solution supplied with 1 kPa oxygen. Both ethylene production and aerenchyma formation were almost completely halted when roots were exposed to nutrient solutions devoid of oxygen. Thus both processes require oxygen and are stimulated by oxygen-deficient surroundings in the 3-to 12-kPa range of oxygen partial pressures when compared with rates observed in air (21 kPa oxygen).

Larger adenylate energy charge and ATP/ADP ratios in aerenchymatous roots of Zea mays in anaerobic media as a consequence of improved internal oxygen transport

Internal transport of O2 from the aerial tissues along the adventitious roots of intact maize plants was estimated by measuring the concentrations of adenine nucleotides in various zones along the root under an oxygen-free atmosphere. Young maize plants were grown in nutrient solution under conditions that either stimulated or prevented the formation of a lysigenous aerenchyma, and the roots (up to 210 mm long) were then exposed to an anaerobic (oxygen-free) nutrient solution. Aerenchymatous roots showed higher values than non-aerenchymatous ones for ATP content, adenylate energy charge and ATP/ADP ratios. We conclude that the lysigenous cortical gas spaces help maintain a high respiration rate in the tissues along the root, and in the apical zone, by improving internal transport of oxygen over distances of at least 210 mm. This contrasted sharply with the low energy status (poor O2 transport) in non-aerenchymatous roots.

Electron microscopy of gas space (aerenchyma) formation in adventitious roots of Zea mays L. subjected to oxygen shortage

This paper examines the ultrastructure of cortical cells in maize root tips during the early stages in lysigenous aerenchyma formation, promoted by oxygen-deficient nutrient solution. The aim was to determine whether changes in fine structure were compatible with oxygen starvation as the primary cause of cell degeneration and death. There was an initial collapse of some cortical cells, indicating loss of turgor, and the cytoplasm became more electron dense. Mitochondria and endoplasmic reticulum appeared normal at this early stage though the tonoplast lost its integrity. Subsequently the cytoplasm became less electron dense than surrounding healthy cells, and underwent further degeneration while the plasmalemma retracted from the cell wall. Cell walls remained unaltered until this stage, but some then became thin and electron transparent. No cells of the stele were found to degenerate. These observations, which do not readily accord with the hypothesis that oxygen starvation was the cause of cell death, are compared with detailed studies of cell degeration in other cell types. An alternative mechanism for the stimulation of cortical cell lysis in poorly oxygenated roots involving the hormone ethylene, is discussed.

Inhibition by silver ions of gas space (aerenchyma) formation in adventitious roots of Zea mays L. subjected to exogenous ethylene or to oxygen deficiency

We have studied the role of ethylene in accelerating the lytic formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays L.) growing in poorly aerated conditions. Such roots had previously been shown to contain increased concentrations of ethylene. Ten day-old maize plants bearing seminal roots and one whorl of emerging adventitious roots were grown in nutrient solution bubbled with air, ethylene in air (0.1 to 5.0 μl l(-1)), or allowed to become oxygen-deficient in nonaerated (but not completely anaerobic) solution. Additions of 0.1 μl l(-1) ethylene or more promoted the formation of aerenchyma, with lysis of up to 47% of the cortical cells. The effects of non-aeration were similar to those of exogenous ethylene. When silver ions, an ethylene antagonist, were present at low, non-toxic concentrations (circa 0.6 μM), aerenchyma formation was prevented in ethylene treated roots and in those exposed to oxygen deficiency. Silver ions also blocked the inhibiting effect of exogenous ethylene on root extension. By contrast, the suppression of aerenchyma formation by silver ions under oxygendeficient conditions was associated with a retardation of root extension, indicating the importance of aerenchyma for root growth in poorly aerated media. Rates of production of ethylene by excised roots were stimulated by a previous non-aeration treatment. The effectiveness of Ag(+) in inhibiting equally the action on cortical cells of exogenous ethylene and of non-aeration, supports the view that gas space (aerenchyma) formation in adventitious roots 'adpted' to oxygendeficient environments is mediated by increased concentrations of endogenous ethylene. The possibility that extra ethylene could arise from increased biosynthesis of a precursor in root tissues with a restricted oxygen supply is discussed.