Hypoxic Induction of Anoxia Tolerance in Roots of Adh1 Null Zea mays L

Seedlings of alcohol dehydrogenase 1 null mutants (Adh1-) of Zea mays L., which fail to synthesize alcohol dehydrogenase 1 (ADH1) isozymes, were hypoxically acclimated by 18 h of exposure to an atmosphere of 4% (v/v) O2 in N2 at 25[deg]C. Their ability to tolerate subsequent anoxia by exposure to anaerobic (O2-free) conditions was compared with that of unacclimated seedlings that were transferred immediately from an atmosphere of 40% (v/v) O2 to anaerobic conditions. Only 10% of the root tips of unacclimated seminal roots survived 6 h of anoxia, whereas 70% of the hypoxically acclimated root tips were viable at 24 h. During anoxia, acclimated root tips had enhanced ADH activity compared with unacclimated root tips, through induction of Adh2. Despite this, enzyme activity was still only about 5% that of acclimated, wild-type root tips and about half that of unacclimated, wild-type root tips. During anoxia, acclimated Adh1- root tips showed a higher rate of anaerobic respiration and ethanol production, greater concentrations of ATP and total adenylates, and a greater adenylate energy charge compared with unacclimated root tips. These results suggest that although enhanced ADH activity may have raised fermentation rates in acclimated Adh1- tissues and thereby contributed to energy metabolism and viability, the high levels of ADH activity inducible in acclimated, wild-type maize root tips appear to be in excess of that required to increase rates of fermentation.     

Metabolic Acclimation to Anoxia Induced by Low (2-4 kPa Partial Pressure) Oxygen Pretreatment (Hypoxia) in Root Tips of Zea mays

Young intact plants of maize (Zea mays L. cv INRA 508) were exposed to 2 to 4 kilopascals partial pressure oxygen (hypoxic pretreatment) for 18 hours before excision of the 5 millimeter root apex and treatment with strictly anaerobic conditions (anoxia). Hypoxic acclimation gave rise to larger amounts of ATP, to larger ATP/ADP and adenylate energy charge ratios, and to higher rates of ethanol production when excised root tips were subsequently made anaerobic, compared with root tips transferred directly from aerobic to anaerobic media. Improved energy metabolism following hypoxic pretreatment was associated with increased activity of alcohol dehydrogenase (ADH), and induction of ADH-2 isozymes. Roots of Adh1⁻ mutant plants lacked constitutive ADH and only slowly produced ethanol when made anaerobic. Those that were hypoxically pretreated acclimated to anoxia with induction of ADH2 and a higher energy metabolism, and a rate of ethanol production comparable to that of nonmutants. All these responses were insensitive to the presence or absence of NO₃⁻. Additionally, the rate of ethanol production was about 50 times greater than the rate of reduction of NO₃⁻ to NO₂⁻. These results indicate that nitrate reductase does not compete effectively with ADH for NADH, or contribute to energy metabolism during anaerobic respiration in this tissue through nitrate reduction. Unacclimated root tips of wild type and Adhl⁻ mutants appeared not to survive more than 8 to 9 hours in strict anoxia; when hypoxically pretreated they tolerated periods under anoxia in excess of 22 hours.     

Enhancement of Anaerobic Respiration in Root Tips of Zea mays following Low-Oxygen (Hypoxic) Acclimation

Root tips (10-millimeter length) were excised from hypoxically pretreated (HPT, 4% [v/v] oxygen at 25°C for 16 hours) or nonhypoxically pretreated (NHPT, 40% [v/v] oxygen) maize (Zea mays) plants, and their rates of respiration were compared by respirometry under aerobic and anaerobic conditions with exogenous glucose. The respiratory quotient under aerobic conditions with 50 millimolar glucose was approximately 1.0, which is consistent with glucose or other hexose sugars being utilized as the predominant carbon source in glycolysis. Under strictly anaerobic conditions (anoxia), glycolysis was accelerated appreciably in both HPT and NHPT root tips, but the rate of anaerobic respiration quickly declined in NHPT roots. [U-¹⁴C]Glucose supplied under anaerobic conditions was taken up and respired by HPT root tips up to five times more rapidly than by NHPT roots. When anaerobic ethanol production was measured with excised root tips in 50 millimolar glucose, HPT tissues consistently produced ethanol more rapidly than NHPT tissues. These data suggest that a period of low oxygen partial pressure is necessary to permit adequate acclimation of the root tip of maize to subsequent anoxia, resulting in more rapid rates of fermentation and generation of ATP.     

Hypoxia induces anoxia tolerance in roots and shoots of lupine seedlings

Lupine seedlings were exposed to 4 kPa partial pressure oxygen (hypoxically pretreated) for 18 hours before treatment with strictly anaerobic conditions (anoxia). Seedlings previously exposed to hypoxia were more tolerant than the controls (not hypoxically pretreated) to anoxic stress in both roots and shoots. Hypoxic pretreatment induced roots and shoots survival in anoxia. Improved viability of roots, following hypoxic pretreatment, was associated with increased activity of ADH. In nonacclimated roots and shots significant increase in LDH activity occurd during the first hours under anoxia but the in vitro activity of LDH was two orders of magnitude lower than that of ADH. The results are discussed in relation to the ability of lupine seedlings to survive anoxia.     

Effects of Anoxia on Wheat Seedlings: II. INFLUENCE OF O2 SUPPLY PRIOR TO ANOXIA ON TOLERANCE TO ANOXIA, ALCOHOLIC FERMENTATION, AND SUGAR LEVELS

Prior exposure of roots of intact wheat seedlings for 15–30h to hypoxia (0016-006 mol m 02) greatly increased their toleranceto subsequent anoxia, as assessed by the ability of the rootsto elongate upon return to air. Such hypoxically pretreatedroots had 2–4-fold higher activities of pyruvate decarboxylase(PDC) and 35–l7-fold higher activities of alcohol dehydrogenase(ADH) in their 0–1 mm apices and 0–5 mm root tipsthan in apices and tips of roots pretreated in air (026–031mol m3 02). The ADH/ PDC ratio increased I 3–5-fold duringhypoxic pretreatment. Furthermore, the rate of alcoholic fermentationby 0–5 mm tips of the hypoxically pretreated roots was14-4-fold faster than in tips from aerobically pretreated roots.No consistent difference between 02 pretreatment was found foralcoholic fermentation by tissues taken between 10 and 20 mmfrom the root tip. The observed activities of PDC and rates of alcoholic fermentationindicate that alcoholic fermentation is usually rate-limitedby PDC in 0–1 mm apices and 0–5 mm tips of wheatroots. Comparisons with data in the literature indicate thatwheat has at most a small Pasteur effect, which may explainwhy wheat is more intolerant to anoxia than rice. Exogenous glucose delayed the loss of elongation potential inboth aerobically and hypoxically pretreated roots. In the absenceof glucose, more than 85% of aerobically pretreated roots hadlost their elongation potential after 9 h anoxia, compared with30% in the presence of glucose. After 21 h anoxia nearly allaerobically pretreated roots had lost their elongation potential,compared with 10% and 0% of hypoxically pretreated roots inthe absence and presence, of glucose, respectively. The protective effect of glucose was presumably not due to anendogenous sugar deficiency; at the start of anoxia, 0–1mm apices of aerobically pretreated roots contained sufficientsugar for 23 h of their measured rate of ethanol synthesis yet,85% of these apices had lost their elongation potential afteronly 9 h of anoxia. It is suggested that in wheat roots, lowrates of synthesis of ethanol and hence of ATP, lead to injuryof cells, in turn generating a requirement for exogenous glucose,despite high endogenous sugar concentrations. Key words: Wheat seedlings, anoxia, glucose, O₂ pretreatment, alcoholic fermentation     

Dependence of Ethanolic Fermentation, Cytoplasmic pH Regulation, and Viability on the Activity of Alcohol Dehydrogenase in Hypoxic Maize Root Tips

We examined the role of alcohol dehydrogenase (ADH) in the metabolism and survival of hypoxic maize (Zea mays L.) root tips. The dependence of the rate of ethanolic fermentation, cytoplasmic pH, and viability on the activity of ADH in maize root tips during extreme hypoxia was determined. Maize lines with ADH activities differing over about a 200-fold range were studied. Effects of genetic background were controlled by comparing pairs of F4 progeny of crosses between mutant (low ADH activity) and reference inbred lines. The capacity of hypoxic root tips to perform ethanolic fermentation exhibited a dependence on ADH activity only at activities found in Adh 1 nulls. The ability of maize root tips to withstand prolonged and extreme hypoxia was like-wise independent of ADH activity, except at the lowest activities. Root tips that exhibited lower tolerance of hypoxia had more acidic cytoplasm during extreme hypoxia. We conclude that the activity of ADH in normal maize root tips does not limit the capacity for energy production via fermentation, and does not determine viability under extreme hypoxia. The significance of the induction of ADH activity in plants by hypoxia is discussed.     

Lactic Acid efflux as a mechanism of hypoxic acclimation of maize root tips to anoxia

Hypoxic pretreatment (3 kPa oxygen) of maize (Zea mays L.) root tips improved their survival time in a subsequent anoxic incubation from 10 h to more than 3 d, provided that glucose was added to the medium to sustain metabolism. The glycolytic flux (lactate + ethanol) was the same in both pretreated and untreated root tips during the 1st h after transfer to anoxia. It was only after 2 h that it declined sharply in untreated tips, but was sustained in pretreated ones. Right after the transition from normoxia to anoxia of untreated root tips, the only fermentative product detected was lactic acid, which accumulated in a 7:1 proportion after 30 min in tissue and medium, respectively. It took 10 min before ethanol could be detected and 20 min for it to be produced at its maximum rate at the expense of lactate production, which slowed down. In contrast, in hypoxically pretreated root tips, ethanol was produced at a maximum rate right after the transfer to anoxia. Concurrently, low amounts of lactic acid were produced that accumulated in a 1:1 proportion after 30 min in tissue and medium, respectively. This large efflux of lactic acid could account for the higher cytoplasmic pH values always found in pretreated tissues. The presence of cycloheximide during pretreatment abolished this difference, suggesting that the greater efficiency of lactate efflux was linked to protein synthesis. The role of lactate in cytosolic pH regulation and in sensitivity to anoxia is discussed.     

Further Evidence that Cytoplasmic Acidosis Is a Determinant of Flooding Intolerance in Plants

We present two pieces of evidence that regulation of cytoplasmic pH near neutrality is a prerequisite for survival of root tips during hypoxia. First, blackeye peas and navy beans show earlier cytoplasmic acidosis under hypoxia than soybeans or pumpkin or maize, and die earlier. Second, when cytoplasmic acidosis in maize root tips is greatly retarded by treatment with 25 millimolar Ca(NO₃)₂, they remain viable under hypoxia for a much longer period of time than untreated hypoxic root tips. We also show that viability of maize root tips is unaffected by the supply of exogenous sugar (and so on the rate of ethanolic fermentation) for at least 16 hours of hypoxia.     

Regulated expression of three alcohol dehydrogenase genes in barley aleurone layers

Three genes specify alcohol dehydrogenase (EC 1.1.1.1.; ADH) enzymes in barley (Hordeum vulgare L.) (Adh 1, Adh 2, and Adh 3). Their polypeptide products (ADH 1, ADH 2, ADH 3) dimerize to give a total of six ADH isozymes which can be resolved by native gel electrophoresis and stained for enzyme activity.

Under fully aerobic conditions, aleurone layers of cv Himalaya had a high titer of a single isozyme, the homodimer containing ADH 1 monomers. This isozyme was accumulated by the aleurone tissue during the later part of seed development, and survived seed drying and rehydration. The five other possible ADH isozymes were induced by O₂ deficit. The staining of these five isozymes on electrophoretic gels increased progressively in intensity as O₂ levels were reduced below 5%, and were most intense at 0% O₂.

In vivo³⁵S labeling and specific immunoprecipitation of ADH peptides, followed by isoelectric focusing of the ADH peptides in the presence of 8 molar urea (urea-IEF) demonstrated the following. (a) Aleurone layers incubated in air synthesized ADH 1 and a trace of ADH 2; immature layers from developing seeds behaved similarly. (b) At 5% O₂, synthesis of ADH 2 increased and ADH 3 appeared. (c) At 2% and 0% O₂, the synthesis of all three ADH peptides increased markedly.

Cell-free translation of RNA isolated from aleurone layers, followed by immunoprecipitation and urea-IEF of in vitro synthesized ADH peptides, showed that levels of mRNA for all three ADH peptides rose sharply during 1 day of O₂ deprivation. Northern hybridizations with a maize Adh 2 cDNA clone established that the clone hybridized with barley mRNA comparable in size to maize Adh 2 mRNA, and that the level of this barley mRNA increased 15- to 20-fold after 1 day at 5% or 2% O₂, and about 100-fold after 1 day at 0% O₂.

We conclude that in aleurone layers, expression of the three barley Adh genes is maximal in the absence of O₂, that regulation of mRNA level is likely to be a major controlling factor, and that whereas the ADH system of barley has strong similarities to that of maize, it also has some distinctive features.

     

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).     

Improved Cytoplasmic pH Regulation,Increased Lactate Efflux,and Reduced Cytoplasmic Lactate Levels Are Biochemical Traits Expressed in Root Tips of Whole Maize Seedlings Acclimated to a Low-Oxygen Environment

We tested the hypothesis (J.-H. Xia and P.H. Saglio [1992] Plant Physiol 100: 40-46) that the enhanced ability of maize (Zea mays) root tips to survive anoxia, elicited by a 4-h exposure to 3% O2 ("acclimation"), is due to less cytoplasmic acidosis early in anoxia. Cytoplasmic pH and fermentation reactions were monitored in excised and intact (attached) maize root tips by simultaneous in vivo 13C- and 31P-NMR spectroscopy. We demonstrate that both excised and intact acclimated root tips have significantly higher cytoplasmic pH values under anoxia. This reduction in cytoplasmic acidosis is greater in intact root tips. Remarkably, cytoplasmic pH does not change when root tips are transferred from 3% O2 to anoxia. The earlier observation of considerable lactate efflux and lowered intracellular lactate in excised, acclimated root tips (ibid.) was extended to intact seedlings. The predominant fermentation end product retained in the cells of acclimated root tips is alanine. We discuss the relationship between cytoplasmic pH and levels of intracellular lactate and alanine in sugar-replete roots, and the role of cytoplasmic pH in determining survival under anoxia.     

Protein Synthesis in Maize during Anaerobic and Heat Stress

Protein accumulation and protein synthesis were investigated during anaerobic stress and heat shock in maize seedlings (Zea mays L.). Antibodies against alcohol dehydrogenase (ADH) and cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC) were used to investigate the expression of the genes encoding these proteins during stress treatment. ADH1 protein accumulation is shown to increase about 10-fold in the root after 24 hours of anaerobic treatment. The Gpc gene products are separable into two size classes: the slow mobility GAPC1 and GAPC2 (GAPC1/2), and the faster GAPC3 and GAPC4 (GAPC3/4). The GAPC1/2 antigen did not increase at all, whereas the GAPC3/4 antigen increased less than fourfold. The proteins synthesized in the root during aerobic and anaerobic conditions were compared, and GAPC3/4 was identified as an anaerobic polypeptide. In vitro translations were used to estimate the levels of different mRNAs in roots following anaerobiosis, recovery from anaerobiosis, and heat shock. This was compared with the in vivo protein synthesis rates in roots labeled under identical conditions. In vivo labeling indicates that GAPC and ADH are not heat shock proteins. Although both GAPC3/4- and ADH1-translatable mRNA levels increase about 10-fold during anaerobiosis, in vivo labeling of these proteins (relative to total protein synthesis) is further enhanced, leading to a selective translation effect for ADH1 of threefold, and for GAPC3/4 of sixfold. In contrast, anoxia causes no change in GAPC1/2-translatable mRNA levels or in vivo labeling. As an additional comparison, β-glucosidase mRNA levels are found to be constant during anoxia, but in vivo synthesis decreases.     

NO和H2O2诱导大豆根尖和边缘细胞耐铝反应的作用

 NO和H₂O₂是参与植物抗非生物胁迫反应的重要信号分子, 为了确定NO和H₂O₂在大豆(Glycine max)根尖和根边缘细胞(root border cells, RBCs)耐铝反应中的作用及其相互关系, 以‘浙春3号’大豆为材料, 研究了铝毒胁迫下大豆根尖内源NO和H₂O₂的变化, 以及外源NO和H₂O₂诱导大豆根尖和RBCs的耐铝反应。结果表明, 50 μmol·L^–1 Al处理48 h显著抑制大豆根的伸长, 提高Al在根尖的积累, 同时显著增加根尖内源NO和H₂O₂含量。施加0.25 mmol·L^–1外源NO供体亚硝基铁氰化钠(Na₂[Fe(CN)₅NO]·2H₂O, sodium nitroprusside, SNP)和0.1 mmol·L^–1H₂O₂, 能有效地缓解Al对大豆根伸长的抑制、根尖Al积累和RBCs 的死亡, 该缓解作用可以被0.05 mmol·L^–1 NO清除剂2-(4- 羧基苯)-4,4,5,5- 四甲基咪唑-1- 氧-3- 氧化物, 钾盐(C₁₄H₁₆N₂O₄·K, carboxy-PTIO, cPTIO)和150 U·mL^–1 H₂O₂清除酶(catalase, CAT)逆转。并且外源NO能够显著促进根尖H₂O₂的积累, 而外源H₂O₂对根尖NO的含量无显著影响。这表明NO和H₂O₂是诱导大豆根尖及RBCs耐铝反应的两种信号分子, NO可能通过调控H₂O₂的形成, 进而诱导大豆根尖及RBCs的耐铝反应。     

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.     

Increased Fatty Acid beta-Oxidation after Glucose Starvation in Maize Root Tips

The effects of glucose starvation on the oxidation of fatty acids were studied in excised maize (Zea mays L.) root tips. After 24 hours of glucose starvation, the rate of oxidation of palmitic acid to CO₂ by the root tips was increased 2.5-fold. Different enzyme activities were tested in a crude particulate fraction from nonstarved root tips and those starved for 24 hours. The activities of the β-oxidation enzymes crotonase, hydroxyacyl-coenzyme A (CoA) dehydrogenase, and thiolase and those of catalase, malate synthase, and peroxisomal citrate synthase were higher after starvation. However, no isocitrate lyase activity was detected, thus suggesting that the glyoxylate cycle does not operate. The overall β-oxidation activity was assayed as the formation of [¹⁴C]acetyl-CoA from [¹⁴C]palmitic acid after high-performance liquid chromatography analysis of the CoA derivatives. An activity was detected in sugar-fed root tips, and it was increased by two-to fivefold in starved roots. Because the recovery of enzyme activities is only marginally better in starved roots compared with nonstarved roots, these results indicate that the β-oxidation activity in the tissues is increased during sugar starvation. This increase is probably an essential part of the response to a situation in which lipids and proteins replace carbohydrates as the major respiratory substrates. These results are discussed in relation to the metabolic changes observed in senescing plant tissues.     

Intracellular pH in rice and wheat root tips under hypoxic and anoxic conditions

The influence of anoxia and hypoxia on dynamic of intracellurar pH and ATP content in rice and wheat root tips was investigated with ³¹P-NMR spectroscopy. Both cereals responded to hypoxia similarly, by rapid cytoplasmic acidification (from pH 7.6–7.7 to 7.1), which was followed by slow partial recovery (0.3 units). Anoxia led to a dramatic pH_cyt drop in tissues of both species (from pH 7.6–7.7 to less than 7.0) and partial recovery took place in rice only. In wheat, the acidification continued to pH 6.8 after 6 h of exposure. Anoxic wheat root tips were deficient in ADH induction, whereas increased activity of alcoholic fermentation enzymes took place in anoxic rice root tips, as well as in both species after hypoxic treatment. In both plants, NTP content followed the dynamics of pH_cyt. There was a strong correlation between NTP content and cytoplasmic H⁺ activity ([H⁺]_cyt = 10^−pHcyt) for both hypoxic and anoxic conditions. In this addendum we want to focus the reader''s attention on the importance of adequate experimental design when hypoxia is under investigation and on some further perspectives of intracellular pH regulation in plants under anaerobic conditions.Key words: anoxia, hypoxia, rice, wheat, cytoplasmic pH regulation     

Regulation of H+ Extrusion and Cytoplasmic pH in Maize Root Tips Acclimated to a Low-Oxygen Environment

We tested the hypothesis that H+ extrusion contributes to cytoplasmic pH regulation and tolerance of anoxia in maize (Zea mays) root tips. We studied root tips of whole seedlings that were acclimated to a low-oxygen environment by pretreatment in 3% (v/v) O2. Acclimated root tips characteristically regulate cytoplasmic pH near neutrality and survive prolonged anoxia, whereas nonacclimated tips undergo severe cytoplasmic acidosis and die much more quickly. We show that the plasma membrane H+-ATPase can operate under anoxia and that net H+ extrusion increases when cytoplasmic pH falls. However, at an external pH near 6.0, H+ extrusion contributes little to cytoplasmic pH regulation. At more acidic external pH values, net H+ flux into root tips increases dramatically, leading to a decrease in cytoplasmic pH and reduced tolerance of anoxia. We present evidence that, under these conditions, H+ pumps are activated to partly offset acidosis due to H+ influx and, thereby, contribute to cytoplasmic pH regulation and tolerance of anoxia. The regulation of H+ extrusion under anoxia is discussed with respect to the acclimation response and mechanisms of intracellular pH regulation in aerobic plant cells.     

Growth and energy status of arrowhead tubers,pondweed turions and rice seedlings under anoxic conditions

The responses of two aquatic plants, arrowhead (Sagittaria pygmaea Miq.) and pondweed (Potamogeton distinctus A. Benn), to anoxia were compared with those of rice (Oryza sativa L.). Shoot elongation of arrowhead tubers was enhanced at around 1 kPa O₂, whereas that of pondweed turions was slight in air and reached a maximum in the absence of O₂. Anaerobic enhancement of alcohol dehydrogenase (ADH) activity took place in rice coleoptiles but not in arrowhead and pondweed shoots. Shoots of both arrowhead and pondweed maintained a more stable energy status than did the rice coleoptile under anaerobic conditions. Total adenylate nucleotide contents of arrowhead and pondweed shoots were constant under anaerobic conditions. Adenylate energy charge in both shoots remained at a high and stable level of more than 0·8 for at least 8 d. Three forms of ADH from arrowhead shoots were separated by starch gel electrophoresis, showing that the activity of each ADH form was different under aerobic and anaerobic conditions. The incorporation of ³⁵S-labelled Cys and Met into soluble proteins in arrowhead shoots showed active protein biosynthesis and an involvement of a special set of polypeptides in the anaerobiosis.     

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.