Respiratory metabolism in buckwheat seedlings

Young seedlings of buckwheat (Fagopyrum esculentum) respire in air with an RQ of unity. Analysis of respiratory substrates coupled with a study of the utilization of acetate-¹⁴C and glucose-¹⁴C suggest that both the Embden-Meyerhof-Parnas, tricarboxylic acid and pentose phosphate sequences participate in the total respiratory catabolism.

In anoxia CO₂ dropped to one third of the aerobic rate and ethanol accumulated to only about one half the rate of CO₂ output on a molar basis. Smaller amounts of lactate, succinate and free amino acids (particularly alanine and γ-aminobutyric acid) accumulated, carboxylic acids decreased and there were initial increased in pyruvate and α-ketoglutarate. The observed changes are consistent with residual tricarboxylic acid and pentose phosphate cycle activity in anoxia and may account for the excess CO₂ production over ethanol accumulation. CO₂, ethanol and lactate production did not account for all of the carbohydrate consumed in anoxia.

Relative rates of carbon loss were measured in air and in atmospheres containing 3.5%, 2.1%, 1.3% and 0.6% oxygen. The extinction point of anaerobic metabolism was 1.5%.

On return to air from anoxia the CO₂ output increased and the RQ rose from 0.8 to 1.0 over the first 2-hour period. Ethanol, lactate and succinate were consumed and other constituents returned to their previous aerobic level. Some of these changes suggest a rather slow resumption of tricarboxylic acid cycle activity on return to air.

Carbon loss as CO₂ in air was greater than the carbon loss as CO₂ at the extinction point. Carbon loss in anoxia as CO₂, ethanol and lactate was similar to carbon loss at the extinction point. Assessed in this orthodox manner buckwheat seedlings show no Pasteur effect but the complex nature of the changes in levels of metabolic substrates and intermediates do not allow firm conclusions to be drawn on the effects of oxygen on the rates of glycolysis and other respiratory processes.

     

Decay of cytochromes and appearance of a cyanide-insensitive electron transfer pathway in Euglena gracilis grown in anoxia

The effect of anoxia over a 3-week period on the respiratory ability of Euglena gracilis (strains Z and ZC) was studied. Low temperature absorption spectra indicated that comparable alterations of the cytochromes occurred in both cell types studies, leading to the disappearance of cytochrome oxidase and cytochrome c558 in the aplastic strain ZC and in the wild type strain Z. Both types of cells maintained their ability to consume O₂ when they were transferred from the nitrogen atmosphere to air. Resistance of respiration to cyanide and azide, and sensitivity to propyl gallate increased during anoxia, indicating the decreasing role of cytochrome oxidase in this O₂ consumption. Quantitative changes in the O₂ consumption capacity were followed during the course of anoxia; this capacity decreased during the 5 first days of anoxia and then increased to recover its initial value during the second week. This recovery of a high O₂, consumption capacity was linked to the appearance of a cyanide- and azide-resistant, propyl gallate-sensitive O₂ consumption pathway. This paper raises the question of the physiological significance of this electron transfer pathway induced by anoxia.     

Hypoxic Induction of Anoxia Tolerance in Root Tips of Zea mays

When root tips of fully aerobic, intact maize (Zea mays L.) seedlings are made anaerobic, viability normally is only 24 hours or less at 25°C. We find that viability can be extended to at least 96 hours if seedlings are given a hypoxic pretreatment for 18 hours by sparging the solution with 4% O₂ in nitrogen (v/v) before anoxia. Fully aerobic root tips (sparged with 40% O₂) had very low alcohol dehydrogenase (ADH) activity (per gram root fresh weight), and the level remained low under anoxia. In hypoxically pretreated roots, however, high levels of ADH activity were induced, and activity rose further during the initial 24 hours of anoxia, and then remained high at about 20 times that of controls in 40% O₂. ADH activity in roots in solution sparged with air (21% O₂) was about three times that in 40% O₂. Improved viability of hypoxically pretreated root tips was associated with maintenance of a high energy metabolism (ATP concentration, total adenylates, and adenylate energy charge). Roots that were not pretreated lost 94% of the total adenylates and ATP at 24 hours of anoxia. The relation between induced ADH activity, energy metabolism, and improved anoxia-tolerance in acclimated maize root tips is discussed.     

Oxygen Transport and Root Respiration of Maize Seedlings: A Quantitative Approach Using the Correlation between ATP/ADP and the Respiration Rate Controlled by Oxygen Tension

Oxygen uptake and ATP/ADP ratio were simultaneously monitored during incubation of excised maize (Zea mays L. INRA 508) root tips under varying O₂ partial pressure. Both variables were independent of O₂ tension until a critical O₂ pressure was reached. Below this pressure, ATP/ADP ratio and respiratory rate declined. However, in tissues having a high glycolytic capacity, the correlation between the ATP/ADP ratio and the respiratory rate breaks down as O₂ tension decreases, due to the increasing contribution of fermentative processes.

In presence of 2 millimolar NaF, the ATP/ADP ratio varied solely as a function of the O₂ tension, without interference by fermentative activity, and a close correlation links the ATP/ADP ratio and the respiratory rate of excised maize root tips over the whole range of O₂ tensions tested.

Using this correlation, a method is proposed for the quantitative determination of the relative cellular respiratory rate permitted by O₂ transport from the aerial part of young maize seedlings along the seminal root placed in an anoxic environment.

Data are presented which demonstrate the preeminent part played by the cortical air spaces in O₂ transport. Their contribution to respiration was high in the first few centimeters nearest the seed and decreased rapidly as the distance from the aerated source increased. It is concluded that O₂ transport might contribute to the survival or to adaptive responses of root tissues in flooded soils but that the ventilation of the apical growing zone was inadequate to sustain the growth.

     

Regulation of Respiration in the Leaves and Roots of Two Lolium perenne Populations with Contrasting Mature Leaf Respiration Rates and Crop Yields

Measurements of O₂ uptake were made on leaves and roots of two populations of Lolium perenne L. cv S23 (GL66 and GL72), previously shown to have contrasting rates of CO₂ evolution and yields of dry matter. O₂ uptake was faster in the mature leaves of GL66 than those of GL72, but no difference was observed in the respiratory rates of meristematic leaf bases or mature roots. The growth rate of GL72 was faster than that of GL66. Cyanide resistance was substantial in mature leaves but the alternative path did not contribute to O₂ uptake in the dark. In both populations, adding malate and glycine stimulated O₂ uptake, but exogenous sucrose only stimulated when uncoupler was also present. The difference between the respiratory rates of the two populations was maintained under all investigated conditions. We conclude that the rate of mature leaf respiration in the dark in L. perenne is limited by adenylate control of glycolysis. The difference between the fast (GL66) and slow (GL72) respiring populations reflected a greater respiratory capacity and higher turnover of ATP in GL66. Alternative path capacity was also high in the roots of both and contributed substantially to O₂ uptake, as indicated by inhibition by salicylhydroxamic acid in the absence of KCN. The alternative path capacity of meristematic leaf bases was considerably less than that in mature leaves.

Transverse and cross-sections were made of mature leaves of both populations to study anatomical features which might explain the differences in ATP turnover, suggested by the biochemical experiments. Leaves of GL72 were thicker but did not show a different anatomy when compared with GL66. The increased thickness was not due to more or larger cells but entirely to a larger intercellular volume.

     

Anaerobiosis in Echinochloa crus-galli (Barnyard Grass) Seedlings : Intermediary Metabolism and Ethanol Tolerance

Tolerance to ethanol and the ability to metabolize key intermediary substrates under anaerobiosis were studied in Echinochloa crus-galli (L.) Beauv. var oryzicola seeds to further characterize the mechanisms which enable it to germinate and grow without O₂.

Our results indicate that E. crus-galli var oryzicola possesses an inherently high tolerance to ethanol and is able to metabolize low levels of ethanol in the absence of O₂. Concentrations of ethanol 45-fold greater than endogenous levels did not prove toxic to germinating seeds.

Five-day anaerobically grown seedlings of E. crus-galli var oryzicola metabolized added [¹⁴C]sucrose primarily to CO₂ and ethanol. Of the soluble compounds labeled, the phosphorylated intermediates of glycolysis and the oxidative pentose phosphate pathway predominated more under anaerobiosis than in air. In addition, organic acids and lipids were labeled from [¹⁴C]sucrose, the latter indicating that metabolism of carbohydrate via acetyl-CoA occurred in the absence of O₂. Lipids were also labeled when seeds were supplied with [¹⁴C]ethanol or [¹⁴C]acetate. Labeling experiments using the above compounds plus [¹⁴C]NaHCO₃, showed further labeling of organic acids; succinate and citrate being labeled under nitrogen, while fumarate was formed in air.

The above metabolic characteristics would allow for the maintenance of an active alcoholic fermentation system which, along with high alcohol dehydrogenase activity, would continue to recycle NAD and result in continued energy production without O₂. In addition, Echinochloa's ability to metabolize carbohydrate intermediates and to synthesize lipids indicates that mechanisms exist for providing the carbon intermediates for biosynthesis, particularly membrane synthesis for growth, even in the absence of O₂.

     

Lipid biochemistry of echinochloa crus-galli during anaerobic germination

Seven day old seedlings of Echinochloa crus-galli var. oryzicola (Vasing) had a higher total lipid content when germinated under N₂ than in air, although ungerminated seeds contained more lipid than either seedling. The triacylglycerol pool was not depleted under anaerobiosis as it was in air and only air-grown seedlings showed a net increase in free fatty acids and polar lipids. Concentrations of most of the individual acids of the total fatty acid profile declined during germination in air and in the free acid and polar lipid fractions of these seedlings the relative proportion of polyunsaturated fatty acids increased. Compared to air-grown seedlings, ungerminated seeds and N₂-grown seedlings had a similar qualitative and quantitative lipid composition. Our results show that mobilization of storage lipids was apparently severely inhibited under anoxia. The importance of lipid metabolism to the germination and growth of Echinochloa during anoxia is discussed in terms of maintaining membrane integrity and serving (indirectly) to reoxidize pyridine nucleotides.     

Effects of o(2) concentration on rice seedlings

The ability of rice, wheat, and oat seedlings to germinate and grow as the O₂ concentration was lowered to zero was compared. The germination of rice was completely unaffected by O₂ supply, whereas that of oats and wheat was strongly retarded at levels below 5% O₂. In contrast to the coleoptiles of oats and wheat and to roots of all three species where growth was progressively diminished as the O₂ concentration was lowered, that of the rice coleoptile was progressively increased. However, the dry weight and content of protein, sugars, and cellulose were all depressed in the rice coleoptile in anoxia, and the levels of several respiratory enzymes, particularly those of mitochondria, were also much lower than those of the coleoptiles grown in air. In 1% O₂, the growth of the rice coleoptile was similar to that in air. The effect of ethanol concentration on germination and growth of rice was measured. Coleoptile growth was reduced when the ethanol concentration exceeded 40 millimolarity, and root growth was somewhat more sensitive. Coleoptiles of all three species grown in air were transferred to N₂, and ethanol accumulation was measured over 24 hours. The rate of ethanol accumulation in oats was close to that in rice, and in all three species the amounts of ethanol lost to the surrounding medium were those expected from simple diffusion from the tissue. The ability of the rice coleoptile to grow in anoxia is apparently not due to a particularly low rate of ethanol formation or to unusual ethanol tolerance. Any explanation of the success of rice in anoxia must encompass the much lower rate of ATP synthesis than that in air and account for the biochemical deficiencies of the coleoptile.     

Changes in Hexokinase Activity in Echinochloa phyllopogon and Echinochloa crus-pavonis in Response to Abiotic Stress

Hexokinase (HXK; EC 2.7.1.1) regulates carbohydrate entry into glycolysis and is known to be a sensor for sugar-responsive gene expression. The effect of abiotic stresses on HXK activity was determined in seedlings of the flood-tolerant plant Echinochloa phyllopogon (Stev.) Koss and the flood-intolerant plant Echinochloa crus-pavonis (H.B.K.) Schult grown aerobically for 5 d before being subjected to anaerobic, chilling, heat, or salt stress. HXK activity was stimulated in shoots of E. phyllopogon only by anaerobic stress. HXK activity was only transiently elevated in E. crus-pavonis shoots during anaerobiosis. In roots of both species, anoxia and chilling stimulated HXK activity. Thus, HXK is not a general stress protein but is specifically induced by anoxia and chilling in E. phyllopogon and E. crus-pavonis. In both species HXK exhibited an optimum pH between 8.5 and 9.0, but the range was extended to pH 7.0 in air-grown E. phyllopogon to 6.5 in N₂-grown E. phyllopogon. At physiologically relevant pHs (6.8 and 7.3, N₂ and O₂ conditions, respectively), N₂-grown seedlings retained greater HXK activity at the lower pH. The pH response suggests that in N₂-grown seedlings HXK can function in a more acidic environment and that a specific isozyme may be important for regulating glycolytic activity during anaerobic metabolism in E. phyllopogon.     

Respiratory metabolism in mangrove seedlings

The respiratory gaseous exchanges of detached whole mangrove seedlings (Avicennia, Bruguiera, Rhizophora) in a range of O₂ concentrations from 0 to 21% (air) were markedly reduced by the presence of external CO₂. Aerobic respiration decreased steadily for 16 days but the RQ remained at unity.     

Effect of photosynthesis and carbohydrate status on respiratory rates and the involvement of the alternative pathway in leaf respiration

In spinach (Spinacia oleracea Hybrid 102 [New World seeds]) and wheat (Triticum aestivum L. cv Gabo) leaves, O₂ uptake rates in the dark were faster after the plants had been allowed to photosynthesize for a period of several hours. Alternative path activity also increased following a period of photosynthesis in these leaves. No such effects were observed with isolated mitochondria. In spinach and wheat leaves, the level of fructose plus glucose decreased during a period of darkness. In pea (Pisum sativum cv Alaska) leaves, the level of these sugars did not vary significantly during the day, and respiratory rates were also constant. In slices cut from wheat leaves harvested at the end of the night, addition of sugars increased the rate of respiration and engaged the previously latent alternative oxidase. In pea leaves, O₂ uptake in the first few minutes following illumination was faster than that observed before illumination, but declined during the next 15 to 20 minutes. Adding the alternative oxidase inhibitor salicylhydroxamic acid, or imposing high bicarbonate concentrations during the period of photosynthesis, prevented the rise in O₂ uptake rate during the immediate post illumination period.

We conclude that the level of respiratory substrate in leaves determines their rate of O₂ uptake, and the degree to which the alternative path contributes to that O₂ uptake.

     

Time requirement for hypoxic acclimation to anoxia tolerance in rice coleoptiles

Rice (Oriza sativa L.) seedlings were subjected tohypoxic pretreatment (H-PT; incubated in 5% O₂ atmosphere) forvarious lengths of time followed by a 24-h anoxic stress. Anoxiatolerance of rice coleoptiles was improved with increasing duration of H-PT, butH-PT longer than 6 h gave no additional improvement. ATP andethanol concentrations in the coleoptiles were increased by H-PT, and the timeand pattern of increase in ATP level and ethanol production rate were similar tothose of increase in the anoxia tolerance. These results suggest that the H-PTmay increase anoxia tolerance due to maintenance of anaerobic glycolysis withinduction of ethanolic fermentation to generate ATP, and hypoxic acclimation toanoxic stress in rice coleoptiles may occur within 6 h.     

Activities of isolated mitochondria and mitochondrial enzymes from aerobically and anaerobically germinated barnyard grass (echinochloa) seedlings

Activity of mitochondria isolated from whole seedlings of Echinochloa crus-galli (L.) Beauv. var oryzicola germinated under aerobic and anaerobic conditions for 5 to 7 days was investigated. Mitochondria from both treatments exhibited good respiratory control and ADP/O ratios. Although O₂ uptake was low in anaerobic mitochondria, activity rapidly increased when the seedlings were transferred to air. Mitochondria from both aerobically and anaerobically grown seedlings of E. crus-galli var oryzicola maintained up to 66% of their initial respiration rate in the presence of both cyanide and salicylhydroxamic acid, and the inhibitory effects of cyanide and azide were additive. In addition, antimycin A was not an effective inhibitor of respiration. Reduced-minus-oxidized absorption spectra revealed that cytochromes a, a₃, and b were reduced to a greater extent and cytochrome c was reduced to a lesser extent in anaerobically germinated seedlings relative to that in aerobically germinated seedlings. An absorption maximum in the cytochrome d region of the spectrum was reduced to the same extent under both germination conditions and an absorption maximum at 577 nm was present only in anaerobically germinated seedlings. Anaerobically germinated seedlings contained 70% of the cytochrome c oxidase activity found in air grown seedlings. Upon exposure to air, the developmental pattern of this enzyme in anaerobically germinated seedlings was similar to air controls. Succinate dehydrogenase activity in anaerobic seedlings was only 45% of the activity found in aerobically germinated seeds, but within 1 hour of exposure to air, the activity had increased to control levels. The results suggest that mitochondria isolated from E. crus-galli var oryzicola differ from other plants studied and that the potential for mitochondrial function during anaerobiosis exists.     

Impact of oxygen stress and energy availability on membrane stability of plant cells

This article reviews the relationship between the energy status of plant cells under O₂ stress (e.g. waterlogging) and the maintenance of membrane intactness, using information largely derived from suspension cultures of anoxia‐intolerant potato cells. Energy‐related parameters measured were fermentation end‐products (ethanol, lactate, alanine), respiratory rate, ATP, adenylate energy charge, nitrate reductase activity and biomass. ATP synthesis rates were calculated from the first four parameters. Reactive oxygen species were estimated from H₂O₂ and superoxide levels, and the enzymatic detoxification potential from the activity levels of catalase and superoxide dismutase. Structure‐related parameters were total fatty acids, free fatty acids (FFAs), lipid hydroperoxides, total phospholipids, N‐acylphosphatidylethanolamine (NAPE) and cell viability. The following issues are addressed in this review: (1) what is the impact of anoxia on membrane lipids and how does this relate to energy status; (2) does O₂ per se play a role in these changes; (3) under which conditions and to what extent does lipid peroxidation occur upon re‐aeration; and (4) can the effects of re‐aeration be distinguished from those of anoxia? The emerging picture is a reappraisal of the relative contributions of anoxia and re‐aeration. Two successive phases (pre‐lytic and lytic) characterize potato cells under anoxia. They are connected by a threshold in ATP production rate, below which membrane lipids are hydrolysed to FFAs, and NAPE increases. Since lipid peroxidation occurs only when cells are reoxygenated during the lytic phase, its biological relevance in an already damaged system is questionable.Key words: Acorus calamus L., energy shortage, free fatty acids, lipid peroxidation, lipolytic acyl hydrolase, lipoxygenase, membrane intactness, N‐acylphosphatidylethanolamine, O₂ stress, reactive oxygen species, Solanum tuberosum L.     

Oxygen distribution and movement,respiration and nutrient loading in banana roots (Musa spp. L.) subjected to aerated and oxygen-depleted environments

Excessive soil wetness is a common feature where bananas (Musa spp.) evolved. Under O₂ deficiency, a property of wet soils, root growth and functions will be influenced by the respiratory demand for O₂ in root tissues, the transport of O₂ from the shoot to root and the supply of O₂ from the medium. In laboratory experiments with nodal roots of banana, we examined how these features influenced the longitudinal and radial distributions of O₂ within roots, radial O₂ loss, solute accumulation in the xylem, root hydraulic conductivity, root elongation and root tip survival. In aerated roots, the stele respired about 6 times faster than the cortex on a volume basis. Respiratory O₂ consumption decreased substantially with distance from the root apex and at 300–500 mm it was 80% lower than at the apex. Respiration of lateral roots constituted a sink for O₂ supplied via aerenchyma, and reduced O₂ flow towards the tip of the supporting root. Stelar anoxia could be induced either by lowering the O₂ partial pressure in the bathing medium from 21 to 4 kPa (excised roots) or, in the case of intact roots, by reducing the O₂ concentration around the shoot. The root hair zone sometimes extended to 1.0 mm from the root surface and contributed up to a 60% drop in O₂ concentration from a free-flowing aerated solution to the root surface. There was a steep decline in O₂ concentration across the epidermal-hypodermal cylinder and some evidence of a decline in the O₂ permeability of the epidermal-hypodermal cylinder with increasing distance from the root apex. The differences in O₂ concentration between cortex and stele were smaller than reported for maize and possibly indicated a substantial transfer rate of dissolved O₂ from cortex to stele in banana, mediated by a convective water flow component. An O₂ partial pressure of 4 kPa in the medium reduced net nutrient transfer into the vascular tissue in the stele within 1 or 2 h. Hypoxia also caused a temporary decrease in radial root hydraulic conductivity by an order of magnitude. In O₂ deficient environments, the stele would be among the first tissues to suffer anoxia and O₂ consumption within the root hair zone might be a major contributor to root anoxia/hypoxia in banana growing in temporarily flooded soils.     

Effects of Oxygen Concentration on the Respiration of Excised Root-Tip Segments of Maize and Rice, and of Germinating Grains of Rice and Buckwheat

In view of the known tolerance of rice (Oryza sativa) to wet soils, in contrast to maize (Zea mays), a comparison of the respiratory responses of the two plants to different O₂ concen trations was made. It was found that if the O₂ concentration was raised to 100%, an increase was observed of the O₂ input and the CO₂ output in maize root-tip segments and in germinating grains of rice and also in buckwheat (Fagopyrum sp.). In marked contrast rice root-tip segments, when treated with 100% O₂, exhibited no such increase over the air controls. Increased O₂ concentrations decreased RQ values ranging from 2.0 (with 5% O₂) to 0.86 (with 100% O₂). Air-control RQ values were lowest with maize roots (0.9) and highest with germinating rice grains (1.0). Rice grains also exhibited the highest F/R ratio (0.85), and buckwheat grains the lowest (0.45). The addition of sucrose to maize root tips tended to promote fermentation (CO₂ output) rather than aerobic respiration (O₂ input).     

Ultrastructure of wheat coleoptile mitochondria at short-term anoxia and post-anoxia

Abstract. Mitochondrial ultrastructure in the cells of coleoptiles of 4 d seedlings was investigated under conditions of a 1.5, 3, and 36 h anoxia and with subsequent transfer of the seedlings, after a 1.5 h anoxia, from the anaerobic into an aerobic medium. Even with short-term anoxia (1.5 h) destructive changes take place in the ultrastructure of mitochondria, which are reversible not only following the transfer of these seedlings from the anaerobic into aerobic conditions, but also with their continued maintenance under strict anoxia. Irreversible changes in the ultrastructure of mitochondria were seen only with a more prologed (36 h and longer) anoxia. The observed phenomena are discussed from the viewpoint of energy provision of the seedling cells in anoxia and post-anoxia.     

Changes in k, rb, and na transport to shoots after anoxia

The effect of anoxia on subsequent uptake and transport of K, Rb, and Na was examined with seedlings of barley (Hordeum vulgare L.), corn (Zea mays L.), and tall fescue (Lolium × Festuca hybrid derivative) to further our understanding of xylem loading. Roots were incubated in solutions depleted of O₂ by flushing with N₂ gas. After 1 hour exposure, plants were returned to aerated solutions for 16 hours prior to measuring uptake and transport. For each species, anoxia pretreatment significantly enhanced Na transport to the shoot. The rate of Na accumulation into roots, however, was not affected. There was no enhancement of either K or Rb accumulation in shoots, indicating specificity for Na transport. A minimum exposure to anoxia of 30 minutes and a minimum of 12 hours elapsed time was necessary to achieve the maximum rate of Na transport to the shoot in barley seedlings. Accumulation of Na in the shoot of both the control and anoxia pretreated barley plants was inhibited by anoxia and by addition of the proline analog, l-azetidine-2-carboxylic acid, during the uptake period. Enhancement of Na transport was associated with a proportional increase in the rate of synthesis of a membrane bound protein with a molecular weight of 78,000 daltons.     

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.     

Potentiating effect of pure oxygen on the enhancement of respiration by ethylene in plant storage organs: a comparative study

A number of fruits and bulky storage organs were studied with respect to the effect of pure O₂ on the extent and time-course of the respiratory rise induced by ethylene. In one group, of which potato (Solanum tuberosum var. Russet) and carrot (Daucus carota) are examples, the response to ethylene in O₂ is much greater than in air. In a second group, of which avocado (Persea americana Mill. var. Hass) and banana (Musa cavendishii Lambert var. Valery) are examples, air and O₂ are equally effective. When O₂-responsive organs are peeled, air and O₂ synergize the ethylene response to the same extent in parsnip (Pastinaca sativa), whereas O₂ is more stimulatory than air in carrots. In the latter instance, carrot flesh is considered to contribute significantly to diffusion resistance. The release of CO₂, an ethylene antagonist, is recognized as another element in the response to peeling.