Ethylene-Induced Activation of Xylanase in Adventitious Roots of Maize as a Response to the Stress Effect of Root Submersion

Submersion of roots of ten-day-old maize (Zea maysL.) seedlings was accompanied by a decrease in pO₂and an increase in pCO₂of the medium adjacent to the roots. These changes stimulated ethylene evolution in intact plants. Enhanced biosynthesis of ethylene was accompanied by xylanase activation in adventitious roots. As a result, an enhanced formation of aerenchyma was observed in the cortex of adventitious roots. Therefore, these processes resulted in the development of a ventilation system by which O₂can reach the root system exposed to hypoxia. The volume of aerenchyma was assessed by the volume of gas cavities (porosity). In contrast to the main root, the growth of adventitious roots was not inhibited under these conditions. Enlargement of the stem base and increase in the number of aerenchymatous adventitious roots facilitated the oxygen supply to the submerged organs of the plants.     

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.     

Transduction of an Ethylene Signal Is Required for Cell Death and Lysis in the Root Cortex of Maize during Aerenchyma Formation Induced by Hypoxia

Ethylene has been implicated in signaling cell death in the lysigenous formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays) subjected to hypoxia. Various antagonists that are known to modify particular steps in signal transduction in other plant systems were applied at low concentrations to normoxic and hypoxic roots of maize, and the effect on cell death (aerenchyma formation) and the increase in cellulase activity that precedes the appearance of cell degeneration were measured. Both cellulase activity and cell death were inhibited in hypoxic roots in the presence of antagonists of inositol phospholipids, Ca2+- calmodulin, and protein kinases. By contrast, there was a parallel promotion of cellulase activity and cell death in hypoxic and normoxic roots by contact with reagents that activate G-proteins, increase cytosolic Ca2+, or inhibit protein phosphatases. Most of these reagents had no effect on ethylene biosynthesis and did not arrest root extension. These results indicate that the transduction of an ethylene signal leading to an increase in intracellular Ca2+ is necessary for cell death and the resulting aerenchyma development in roots of maize subjected to hypoxia.     

Roots of willow (Salix viminalis L.) show marked tolerance to oxygen shortage in flooded soils and in solution culture

Responses to soil flooding and oxygen shortage were studied in field, glasshouse and controlled environment conditions. Established stools ofSalix viminalis L., were compared at five field sites in close proximity but with contrasting water table levels and flooding intensities during the preceding winter. There was no marked effect of site on shoot extension rate, time to half maximum length or final length attained. When rooted cuttings were waterlogged for 4 weeks in a glasshouse, soil redox potentials quickly decreased to below zero. Shoot extension was slowed after a delay of 20 d, while, in the upper 100 mm of soil, formation and outgrowth of unbranched adventitious roots with enhanced aerenchyma development was promoted after 7 d. At depths of 100–200 mm and 200–300 mm, extension by existing root axes was halted by soil flooding, while adventitious roots from above failed to penetrate these deeper zones. After 4 weeks waterlogging, all arrested root tips recommenced elongation when the soil was drained; their extension rates exceeding those of roots that were well-drained throughout. Growth in fresh mass was also stimulated. The additional aerenchyma found in adventitious roots in the upper 100 mm of soil may have been ethylene regulated since gas space development was inhibited by silver nitrate, an ethylene action inhibitor. The effectiveness of aerenchyma was tested by blocking the entry of atmospheric oxygen into plants with lanolin applied to lenticels of woody shoots of plants grown in solution culture. Root extension was halved, while shoot growth remained unaffected. H Lambers Section editor     

Aerenchyma formation and radial O2 loss along adventitious roots of wheat with only the apical root portion exposed to O2 deficiency

This study investigated aerenchyma formation and function in adventitious roots of wheat (Triticum aestivum L.) when only a part of the root system was exposed to O₂ deficiency. Two experimental systems were used: (1) plants in soil waterlogged at 200 mm below the surface; or (2) a nutrient solution system with only the apical region of a single root exposed to deoxygenated stagnant agar solution with the remainder of the root system in aerated nutrient solution. Porosity increased two‐ to three‐fold along the entire length of the adventitious roots that grew into the water‐saturated zone 200 mm below the soil surface, and also increased in roots that grew in the aerobic soil above the water‐saturated zone. Likewise, adventitious roots with only the tips growing into deoxygenated stagnant agar solution developed aerenchyma along the entire main axis. Measurements of radial O₂ loss (ROL), taken using root‐sleeving O₂ electrodes, showed this aerenchyma was functional in conducting O_2. The ROL measured near tips of intact roots in deoxygenated stagnant agar solution, while the basal part of the root remained in aerated solution, was sustained when the atmosphere around the shoot was replaced by N₂. This illustrates the importance of O₂ diffusion into the basal regions of roots within an aerobic zone, and the subsequent longitudinal movement of O₂ within the aerenchyma, to supply O₂ to the tip growing in an O₂ deficient zone.     

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 l l^–1), or allowed to become oxygen-deficient in nonaerated (but not completely anaerobic) solution. Additions of 0.1 l 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 M), 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.     

洪水条件下湿地植物的生存策略

洪水是自然界存在的一种普遍现象。湿地植物由于所处生境的特殊性,会经常受到周期性或永久性的洪水胁迫。在长期的适应进化过程中,湿地植物形成了一些特殊的生存策略,以适应水文条件的大幅度变化。主要的生存策略如下:1)生活史方面,植物可通过改变生长时间、繁殖方式、种子特征等避免洪水的直接伤害或利用洪水的流动起到传播扩散的作用;2)形态学特征方面,植物可通过调整根系形态、分布等将根系生长到氧气相对充足的土壤表层或形成不定根增强根系通气功能;3)解剖学方面,植物可通过改善组织孔隙度形成通气组织等改善空气传导到根系的"气体通道";4)生理生化方面,植物可通过增加碳水化合物含量以延长生存时间,释放出一些生长激素(乙烯等)以调节植物缺氧条件下的生理活动或形态、解剖方面的变化。在今后的研究中,不定根的形成机理、乙烯在通气组织形成中的作用及其过程、放射氧损失(ROL)的形成机理及其释放速率的调控等一些机理性的工作还需进一步加强。     

Dynamics of Aerenchyma distribution in the cortex of sulfate-deprived adventitious roots of maize

BACKGROUND AND AIMS: Aerenchyma formation in maize adventitious roots is induced in nutrient solution by the deprivation of sulfate (S) under well-oxygenated conditions. The aim of this research was to examine the extent of aerenchyma formation in the cortex of sulfate-deprived adventitious roots along the root axis, in correlation with the presence of reactive oxygen species (ROS), calcium levels and pH of cortex cells and root lignification. METHODS: The morphometry of the second whorl of adventitious (W2) roots, subject to S-deprivation conditions throughout development, was recorded in terms of root length and lateral root length and distribution. W2 roots divided into sectors according to the mean length of lateral roots, and cross-sections of each were examined for aerenchyma. In-situ detection of alterations in ROS presence, calcium levels and pH were performed by means of fluorescence microscopy using H(2)DCF-DA, fluo-3AM and BCECF, respectively. Lignification was detected using the Wiesner test. KEY RESULTS: S-deprivation reduced shoot growth and enhanced root proliferation. Aerenchyma was found in the cortex of 77 % of the root length, particularly in the region of emerging or developing lateral roots. The basal and apical sectors had no aerenchyma and no aerenchyma connection was found with the shoot. S-deprivation resulted in alterations of ROS, calcium levels and pH in aerenchymatous sectors compared with the basal non-aerenchymatous region. Lignified epidermal layers were located at the basal and the proximal sectors. S-deprivation resulted in shorter lateral roots in the upper sectors and in a limited extension of the lignified layers towards the next lateral root carrying sector. CONCLUSIONS: Lateral root proliferation is accompanied by spatially localized induced cell death in the cortex of developing young maize adventitious roots during S-deprivation.     

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).Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - AVG aminoethoxyvinylglycine     

淹水对玉米不定根形态结构和ATP酶活性的影响

淹水２天后,玉米苗基节内即有不定根原基一进于正常植株。淹水１６天后,从基节部长出的不定根数多于正常植株,但淹水导致根系生长和干物质积累大幅度下降。淹水幼苗不定根伸长区内有发达的通气组织形成,使根内部组织孔隙度大幅提高。电镜细胞化学研究表明,经１５天淹一,不定根根尖细胞内ＡＴＰ酶的分布与正常功苗基本相同,酶活性尽管有一定的下降,但仍保持较高水平。根据实验结果,本文重点讨论了不定根的发生及其内部通气组     

Decreased Ethylene Biosynthesis, and Induction of Aerenchyma, by Nitrogen- or Phosphate-Starvation in Adventitious Roots of Zea mays L

Plants of Zea mays L. cv TX5855 were grown in a complete, well oxygenated nutrient solution then subjected to nutrient starvation by omitting either nitrate and ammonium or phosphate from the solution. These treatments induced the formation of aerenchyma close to the apex of the adventitious roots that subsequently emerged from the base of the shoot, a response similar to that shown earlier to be induced by hypoxia. Compared with control plants supplied with all nutrients throughout, N- or P-starvation consistently depressed the rates of ethylene release by excised, 25 mm apical segments of adventitious roots. Some enzymes and substrates of the ethylene biosynthetic pathway were examined. The content of 1-amino cyclopropane-1-carboxylic acid (ACC) paralleled the differences in ethylene production rates, being depressed by N or P deficiency, while malonyl-ACC showed a similar trend. Activity of ACC synthase and of ethylene forming enzyme (g⁻¹ fresh weight) was also greater in control roots than in nutrient starved ones. These results indicate that much of the ethylene biosynthetic pathway is slowed under conditions of N- or P-starvation. Thus, by contrast to the effects of hypoxia, the induction of aerenchyma in roots of Zea mays by nutrient starvation is not related to an enhanced biosynthesis and/or accumulation of ethylene in the root tips.     

Formation and function of secondary aerenchyma in hypocotyl, roots and nodules of soybean (Glycine max) under flooded conditions

Flooding is a major problem in many areas of the world and soybean is susceptible to the stress. Understanding the morphological mechanisms of flooding tolerance is important for developing flood-tolerant genotypes. We investigated secondary aerenchyma formation and function in soybean (Glycine max) seedlings grown under flooded conditions. Secondary aerenchyma, a white and spongy tissue, was formed in the hypocotyl, tap root, adventitious roots and root nodules after 3 weeks of flooding. Under irrigated conditions aerenchyma development was either absent or rare and phellem was formed in the hypocotyl, tap root, adventitious roots and root nodules. Secondary meristem partially appeared at the outer parts of the interfascicular cambium and girdled the stele, and then cells differentiated to construct secondary aerenchyma in the flooded hypocotyl. These morphological changes proceeded for 4 days after the initiation of the flooding. After 14 days of treatment, porosity exceeded 30% in flooded hypocotyl with well-developed secondary aerenchyma, while it was below 10% in hypocotyl of irrigated plants that had no aerenchyma. When Vaseline was applied to the hypocotyl of plants from a flooded treatment to prevent the entry of atmospheric oxygen into secondary aerenchyma, plant growth, especially that of roots, was sharply inhibited. Thus secondary aerenchyma might be an adaptive response to flooding.     

Aerenchyma (Gas-space) Formation in Adventitious Roots of Rice (Oryza sativa L.) is not Controlled by Ethylene or Small Partial Pressures of Oxygen

Jackson, M. B., Fenning, T. M., and Jenkins, W. 1985 Aerenchyma(gas-space) formation in adventitious roots of rice (Oryza sativaL.) is not controlled by ethylene or small partial pressuresof oxygen.—J. exp. Bot. 36: 1566–1572. The extent of gas-filled voids (aerenchyma) within the cortexof adventitious roots of vegetative rice plants (Oryza sativaL. cv. RB3) was estimated microscopically from transverse sectionswith the aid of a computer-linked digitizer drawing board. Gas-spacewas detectable in 1-d-old tissue and increased in extent withage. After 7 d, approximately 70% of the cortex had degeneratedto form aerenchyma. The extent of the voids in 1-4-d-old tissuewas not increased by stagnant, poorly-aerated external environmentscharacterized by sub-ambient oxygen partial pressures and accumulationsof carbon dioxide and ethylene. Treatment with small oxygenpartial pressures, or with carbon dioxide or ethylene appliedin vigorously stirred nutrient solution also failed to promotethe formation of cortical gas-space. Furthermore, ethylene productionby rice roots was slowed by small oxygen partial pressures typicalof stagnant conditions. Silver nitrate, an inhibitor of ethylene action, did not retardgas-space formation; similarly when endogenous ethylene productionwas inhibited by the application of aminoethoxyvinylglycine(A VG), aerenchyma development continued unabated. Cobalt chloride,another presumed inhibitor of ethylene biosynthesis, did notimpair formation of the gas in rice roots nor did it decreasethe extent of aerenchyma even if A VG was supplied simultaneously.These results contrast with those obtained earlier using rootsof Zea mays L. We conclude that in rice, aerenchyma forms speedily even inwell-aerated environments as an integral part of ordinary rootdevelopment There seems to be little or no requirement for ethyleneas a stimulus in stagnant root-environments where aerenchymais likely to increase the probability of survival. Key words: Rice (Oryza sativa L.), ethylene, flooding, aeration, aerenchyma, environmental stress     

Root Formation in Ethylene-Insensitive Plants

Experiments with ethylene-insensitive tomato (Lycopersicon esculentum) and petunia (Petunia x hybrida) plants were conducted to determine if normal or adventitious root formation is affected by ethylene insensitivity. Ethylene-insensitive Never ripe (NR) tomato plants produced more below-ground root mass but fewer above-ground adventitious roots than wild-type Pearson plants. Applied auxin (indole-3-butyric acid) increased adventitious root formation on vegetative stem cuttings of wild-type plants but had little or no effect on rooting of NR plants. Reduced adventitious root formation was also observed in ethylene-insensitive transgenic petunia plants. Applied 1-aminocyclopropane-1-carboxylic acid increased adventitious root formation on vegetative stem cuttings from NR and wild-type plants, but NR cuttings produced fewer adventitious roots than wild-type cuttings. These data suggest that the promotive effect of auxin on adventitious rooting is influenced by ethylene responsiveness. Seedling root growth of tomato in response to mechanical impedance was also influenced by ethylene sensitivity. Ninety-six percent of wild-type seedlings germinated and grown on sand for 7 d grew normal roots into the medium, whereas 47% of NR seedlings displayed elongated tap-roots, shortened hypocotyls, and did not penetrate the medium. These data indicate that ethylene has a critical role in various responses of roots to environmental stimuli.     

Elongation by primary lateral roots and adventitious roots during conditions of hypoxia and high ethylene concentrations

Soil flooding results in unusually low oxygen concentrations and high ethylene concentrations in the roots of plants. This gas composition had a strongly negative effect on root elongation of two Rumex species. The effect of low oxygen concentrations was less severe when roots contained aerenchymatous tissues, such as in R. palustris Sm. R. thyrsiflorus Fingerh., which has little root porosity, was much more affected. Ethylene had an even stronger effect on root elongation than hypoxia, since very small concentrations (0.1 cm³ m^?3) reduced root extension in the two species, and higher concentrations inhibited elongation more severely than did anoxia in the culture medium. Thus, ethylene contributes strongly to the negative effects of flooding on root growth. An exception may be the highly aerenchymatous, adventitious roots of R. palustris. Aerenchyma in these roots provides a low-resistance diffusion pathway for both endogenously produced ethylene and shoot-derived oxygen. This paper shows that extension by roots of R. palustris in flooded soil depends almost completely on this shoot-derived oxygen, and that aerenchyma prevents accumulation of growth-inhibiting levels of ethylene in the root.     

Aerenchyma formation and recovery from hypoxia of the flooded root system of nodulated soybean

BACKGROUND AND AIMS: Flooding results in hypoxia of the root system to which N2 fixation of nodulated roots can be especially sensitive. Morphological adaptions, such as aerenchyma formation, can facilitate the diffusion of oxygen to the hypoxic tissues. Using soybean, the aim of the study was to characterize the morphological response of the nodulated root system to flooding and obtain evidence for the recovery of N metabolism. METHODS: Sections from submerged tissues were observed by light microscopy, while sap bleeding from the xylem was analysed for nitrogenous components. KEY RESULTS: Flooding resulted in the rapid formation of adventitious roots and aerenchyma between the stem (immediately above the water line), roots and nodules. In the submerged stem, taproot, lateral roots and adventitious roots, lysigenous aerenchyma arose initially in the cortex and was gradually substituted by secondary aerenchyma arising from cells derived from the pericycle. Nodules developed aerenchyma from cells originating in the phellogen but nodules situated at depths greater than 7-8 cm showed little or no aerenchyma formation. As a result of aerenchyma formation, porosity of the taproot increased substantially between the 4th and 7th days of flooding, coinciding with the recovery of certain nitrogenous products of N metabolism of roots and nodules transported in the xylem. Thus, on the first day of flooding there was a sharp decline in xylem ureides and glutamine (products of N2 fixation), together with a sharp rise in alanine (product of anaerobic metabolism). Between days 7 and 10, recovery of ureides and glutamine to near initial levels was recorded while recovery of alanine was partial. CONCLUSIONS: N metabolism of the nodulated soybean root system can recover at least partially during a prolonged period of flooding, a process associated with aerenchyma formation.     

An Ethylene-Mediated Increase in Sensitivity to Auxin Induces Adventitious Root Formation in Flooded Rumex palustris Sm

The hormonal regulation of adventitious root formation induced by flooding of the root system was investigated in the wetland species Rumex palustris Sm. Adventitious root development at the base of the shoot is an important adaptation to flooded conditions and takes place soon after the onset of flooding. Decreases in either endogenous auxin or ethylene concentrations induced by application of inhibitors of either auxin transport or ethylene biosynthesis reduced the number of adventitious roots formed by flooded plants, suggesting an involvement of these hormones in the rooting process. The rooting response during flooding was preceded by increased endogenous ethylene concentrations in the root system. The endogenous auxin concentration did not change during flooding-induced rooting, but a continuous basipetal transport of auxin from the shoot to the rooting zone appeared to be essential in maintaining stable auxin concentrations. These results suggest that the higher ethylene concentration in soil-flooded plants increases the sensitivity of the root-forming tissues to endogenous indoleacetic acid, thus initiating the formation of adventitious roots.     

Ethylene insensitivity impedes a subset of responses to phosphorus deficiency in tomato and petunia

The role of ethylene in growth and developmental responses to low phosphorus stress was evaluated using ethylene-insensitive 'Never-ripe' (Nr) tomato and etr1 petunia plants. Low phosphorus increased adventitious root formation in 'Pearson' (wild-type) tomato plants, but not in Nr, supporting a role for ethylene in adventitious root development and showing that ethylene is important for this aspect of phosphorus response. Low phosphorus reduced ethylene production by adventitious roots of both genotypes, suggesting that ethylene perception--not production--regulates carbon allocation to adventitious roots at the expense of other roots under low phosphorus stress. With the exception of its effect on adventitious rooting, Nr had positive effects on growth and biomass accumulation in tomato whereas etr1 tended to have negative effects on petunia. This was particularly evident during the recovery from transplanting, when the effective quantum yield of photosystem II of etr1 petunia grown with low phosphorus was significantly lower than 'Mitchell Diploid', suggesting that etr1 petunia plants may undergo more severe post-transplant stress at low phosphorus availability. Our results demonstrate that ethylene mediates adventitious root formation in response to phosphorus stress and plays an important role for quick recovery of plants exposed to multiple environmental stresses, i.e. transplanting and low phosphorus.     

Enhanced formation of aerenchyma and induction of a barrier to radial oxygen loss in adventitious roots of Zea nicaraguensis contribute to its waterlogging tolerance as compared with maize (Zea mays ssp. mays)

Enhancement of oxygen transport from shoot to root tip by the formation of aerenchyma and also a barrier to radial oxygen loss (ROL) in roots is common in waterlogging‐tolerant plants. Zea nicaraguensis (teosinte), a wild relative of maize (Zea mays ssp. mays), grows in waterlogged soils. We investigated the formation of aerenchyma and ROL barrier induction in roots of Z. nicaraguensis, in comparison with roots of maize (inbred line Mi29), in a pot soil system and in hydroponics. Furthermore, depositions of suberin in the exodermis/hypodermis and lignin in the epidermis of adventitious roots of Z. nicaraguensis and maize grown in aerated or stagnant deoxygenated nutrient solution were studied. Growth of maize was more adversely affected by low oxygen in the root zone (waterlogged soil or stagnant deoxygenated nutrient solution) compared with Z. nicaraguensis. In stagnant deoxygenated solution, Z. nicaraguensis was superior to maize in transporting oxygen from shoot base to root tip due to formation of larger aerenchyma and a stronger barrier to ROL in adventitious roots. The relationships between the ROL barrier formation and suberin and lignin depositions in roots are discussed. The ROL barrier, in addition to aerenchyma, would contribute to the waterlogging tolerance of Z. nicaraguensis.     

Water uptake by roots of Hordeum marinum: formation of a barrier to radial O2 loss does not affect root hydraulic conductivity

The adventitious roots of Hordeum marinum grown in stagnant deoxygenated solution contain a barrier to radial O2 loss (ROL) in basal zones, whereas roots of plants grown in aerated solution do not. The present experiments assessed whether induction of the barrier to ROL influences root hydraulic conductivity (Lpr). Wheat (Triticum aestivum) was also studied since, like H. marinum, this species forms aerenchyma in stagnant conditions, but does not form a barrier to ROL. Plants were grown in either aerated or stagnant, deoxygenated nutrient solution for 21-28 d. Root-sleeving O2 electrodes were used to assess patterns of ROL along adventitious roots, and a root-pressure probe and a pressure chamber to measure Lpr for individual adventitious roots and whole root systems, respectively. Lpr, measured under a hydrostatic pressure gradient, was 1.8-fold higher for individual roots, and 5.6-fold higher for whole roots systems, in T. aestivum than H. marinum. However, there was no difference in Lpr between the two species when measured under an osmotic driving force, when water moved from cell to cell rather than apoplastically. Root-zone O2 treatments during growth had no effect on Lpr for either species (measured in aerobic solution). It is concluded that induction of the barrier to ROL in H. marinum did not significantly affect the hydraulic conductivity of either individual adventitious roots or of the whole root system.