Pyruvate metabolism in rice coleoptiles under anaerobiosis

Relative importance of ethanolic, lactate and alanine fermentation pathways was estimated in coleoptiles of rice seedlings (Oryza sativa L.) subjected to anoxic stress. The in vitro activities of alcohol dehydrogenase (ADH, EC 1.1.1.1), pyruvate decarboxylase (PDC, EC 4.1.1.1) and alanine aminotransferase (AlaAT, EC 2.6.1.2) in the coleoptiles increased in anoxia, whereas no significant increase was measured in lactate dehydrogenase (LDH, EC 1.1.1.27) activity. At 48 h, the ADH, PDC and AlaAT activities in anoxic coleoptiles were 62-, 15- and 7.6-fold greater, respectively, than those in the presence of oxygen. Ethanol and alanine in the coleoptiles accumulated rapidly under anoxia, increasing by 48 h, 57- and 5.6-fold compared with those in the presence of oxygen, respectively. However, lactate concentration did not increase and no initial burst of lactate production was detected. The relative ratio of carbon flux from pyruvate to ethanol, lactate and alanine in anoxic coleoptiles was estimated to be 92, 1 and 7% of the total carbon flux, respectively. These results suggest that the potential carbon flux from pyruvate to ethanol may be much greater than the potential flux from pyruvate to lactate and alanine in rice coleoptiles during anoxia.     

Anoxia tolerance in tobacco roots: effect of overexpression of pyruvate decarboxylase

Plant survival during flooding relies on ethanolic fermentation for energy production. The available literature indicates that the first enzyme of the ethanolic fermentation pathway, pyruvate decarboxylase (PDC), is expressed at very low levels and is likely to be rate-limiting during oxygen deprivation. The authors expressed high levels of bacterial PDC in tobacco to study the modulation of PDC activity in vivo, and assess its impact on the physiology of ethanolic fermentation and survival under oxygen stress. In contrast to leaves, wild-type normoxic roots contained considerable PDC activity, and overexpression of the bacterial PDC caused only a moderate increase in acetaldehyde and ethanol production under anoxia compared to wild-type roots. No significant lactate production could be measured at any time, making it unlikely that lactate-induced acidification (LDH/PDC pH-stat) triggers the onset of ethanol synthesis. Instead, the authors favour a model in which the flux through the pathway is regulated by substrate availability. The increased ethanolic flux in the transgenics compared to the wild-type did not enhance anoxia tolerance. On the contrary, rapid utilisation of carbohydrate reserves enhanced premature cell death in the transgenics while replenishment of carbohydrates improved survival under anoxia.     

淹水对两种甜樱桃砧木根系无氧呼吸酶及发酵产物的影响

以美早/东北山樱桃、美早/马哈利为试材,研究了淹水过程中两种甜樱桃砧木生长根、褐色木质根中无氧呼吸酶——丙酮酸脱羧酶(PDC)、乙醇脱氢酶(ADH)和乳酸脱氢酶(LDH)活性及褐色木质根的发酵产物——乙醛、乙醇和乳酸含量变化,结果表明:两类根系PDC、LDH活性均呈先升后降趋势,ADH活性变化在生长根中亦先升后降,而在褐色木质根中为上升趋势,三种酶活性变化幅度表现为生长根大于褐色木质根;美早/东北山樱桃两类根系中ADH和LDH活性增加幅度大于美早/马哈利,PDC则相反;两种砧木褐色木质根乙醛、乙醇含量呈升高趋势,乳酸含量先升后降;最终美早/东北山樱桃褐色木质根中乙醛含量低于美早/马哈利,乙醇含量则相反,而乳酸含量前者较早达峰值且高于后者峰值。     

Ethanolic fermentation and anoxia tolerance in four rice cultivars

The relationship between coleoptile elongation and ethanolic fermentation was investigated in rice (Oryza sativa L.) coleoptiles of four cultivars subjected to a 48-h anoxic stress. The coleoptile elongation of all cultivars was suppressed by anoxic stress; however, the elongation of cvs Yukihikari and Nipponbare was much greater than that of cvs Leulikelash and Asahimochi. The stress did not significantly increase lactate dehydrogenase (LDH) activity or lactate concentration, but increased alcohol dehydrogenase (ADH) and pyruvate decarboxylase (PDC) activities, as well as ethanol concentration in the coleoptiles of all cultivars. The elevated ADH and PDC activities and ethanol concentration in cvs Yukihikari and Nipponbare were much greater than those of cvs Leulikelash and Asahimochi, suggesting that ethanolic fermentation is likely more active in cvs Yukihikari and Nipponbare than in cvs Leulikelash and Asahimochi. ATP concentration in cvs Yukihikari and Nipponbare in anoxia was also greater than that in cvs Leulikelash and Asahimochi in anoxia. The ethanol concentration in the coleoptiles was correlated with anoxia tolerance with respect to the ATP concentration and coleoptile elongation. These results suggest that the ability to increase ethanolic fermentation may be one of the determinants in anoxia tolerance of rice coleoptiles.     

Effect of low temperature on ethanolic fermentation in rice seedlings

Rice seedlings (Oryza sativa L.) were incubated at 5-30 degrees C for 48 h and the effect of temperature on ethanolic fermentation in the seedlings was investigated in terms of low-temperature adaptation. Activities of alcohol dehydrogenase (ADH, EC 1.1.1.1) and pyruvate decarboxylase (PDC, EC 4.1.1.1) in roots and shoots of the seedlings were low at temperatures of 20-30 degrees C, whereas temperatures of 5, 7.5 and 10 degrees C significantly increased ADH and PDC activities in the roots and shoots. Temperatures of 5-10 degrees C also increased ethanol concentrations in the roots and shoots. The ethanol concentrations in the roots and shoots at 7.5 degrees C were 16- and 12-times greater than those in the roots and shoots at 25 degrees C, respectively. These results indicate that low temperatures (5-10 degrees C) induced ethanolic fermentation in the roots and shoots of the seedlings. Ethanol is known to prevent lipid degradation in plant membrane, and increased membrane-lipid fluidization. In addition, an ADH inhibitor, 4-methylpyrazole, decreased low-temperature tolerance in roots and shoots of rice seedlings and this reduction in the tolerance was recovered by exogenous applied ethanol. Therefore, production of ethanol by ethanolic fermentation may lead to low-temperature adaptation in rice plants by altering the physical properties of membrane lipids.     

The catalytic direction of pyrophosphate:fructose 6-phosphate 1-phosphotransferase in rice coleoptiles in anoxia

The catalytic direction of pyrophosphate:fructose-6-phosphate 1-phosphotransferase (PFP; EC 2.7.1.90) in coleoptiles of rice ( Oryza sativa L.) seedlings subjected to anoxia stress is discussed. The stress greatly induced ethanol synthesis and increased activities of alcohol dehydrogenase (ADH; EC 1.1.1.1) and pyruvate decarboxylase (PDC; EC 4.1.1.1) in the coleoptiles, whereas the elevated PDC activity was much lower than the elevated ADH activity, suggesting that PDC may be one of the limiting factors for ethanolic fermentation in rice coleoptiles. Anoxic stress decreased concentrations of fructose 6-phosphate (Fru-6-P) and glucose 6-phosphate, and increased concentration of fructose 1,6-bisphosphate (Fru-1,6-bisP) in the coleoptiles. PFP activity in rice coleoptiles was low in an aerobic condition and increased during the stress, whereas no significant increase was found in ATP:fructose-6-phosphate 1-phosphotransferase (PFK; EC 2.7.1.11) activity in stressed coleoptiles. Fructose 2,6-bisphosphate concentration in rice coleoptiles was increased by the stress and pyrophosphate concentration was above the K_m for the forward direction of PFP and was sufficient to inhibit the reverse direction of PFP. Under stress conditions the potential of carbon flux from Fru-6-P toward ethanol through PFK may be much lower than the potential of carbon flux from pyruvate toward ethanol through PDC. These results suggest that PFP may play an important role in maintaining active glycolysis and ethanolic fermentation in rice coleoptiles in anoxia.     

Stimulation of alanine amino transferase (AlaAT) gene expression and alanine accumulation in embryo axis of the model legume Medicago truncatula contribute to anoxia stress tolerance

The efficiency of ethanolic fermentation in anoxia tolerance under sugar-limiting conditions, as in the field is still matter of debate. Due to higher rates of glycolysis and ethanol fermentation, faster depletion of sugar stores leads to decreased survival. In the present work the hypothesis that alanine amino transferase ( AlaAT ) fermentation be involved in anoxia tolerance was explored in Medicago truncatula during germination and seedling establishment. Expression of AlaAT and two low oxygen-responsive genes, alcohol dehydrogenase ( ADH ) and lactate dehydrogenase ( LDH ) were determined by real time quantitative RT-PCR and AlaAT activity was determined by ¹⁵N-Glutamate labelling coupled to amino acids analysis by gas chromatography–mass spectrometry and HPLC. Under anoxia not only ADH and LDH levels of expression increased but also AlaAT expression increased substantially. In parallel in vivo AlaAT activity increased and resulted in an increase in alanine synthesis that accumulated as the major amino acid instead of asparigine. These findings support the hypothesis that AlaAT expression and alanine accumulation contribute efficiently to anoxia tolerance. By competing with ethanolic fermentation for pyruvate, under sugar-limiting conditions alanine synthesis saves C3 skeletons avoiding a shortage in carbon availability and limits accumulation of acetaldehyde, a toxic compound. On another hand, increase in alanine was accompanied by an increase in γ-amino butyric acid, both amino acids may intervene in cytosolic pH regulation. Finally the role of alanine in anoxia tolerance was strengthened by the fact that when alanine synthesis was impaired germination and seedling development failed under anoxia.     

Effect of root-applied spermidine on growth and respiratory metabolism in roots of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis>) seedlings under hypoxia

The effects of exogenous spermidine (Spd) application to hypoxic nutrient solution on the contents of endogenous polyamines (PAs) and respiratory metabolism in the roots of cucumber (Cucumis sativus L.) seedlings were investigated. Cucumber seedlings were grown hydroponically in control and hypoxic nutrient solutions with and without addition of Spd at a concentration of 0.05 mM. The activities of key enzymes involved in the tricarboxylic acid cycle (TCAC), such as succinate dehydrogenase (SDH) and isocitrate dehydrogenase (IDH), were significantly inhibited under root-zone hypoxia with dissolved oxygen (DO) at 1 mg/l. In contrast, the activities of enzymes involved in the process of fermentation, such as pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH), and alanine aminotransferase (AlaAT), were significantly increased. Thus, aerobic respiration was inhibited and fermentation was enhanced in the roots of cucumber seedlings as a result of decreasing ATP content to inhibit the dry weight of seedlings under hypoxic stress. Moreover, the contents of free, soluble conjugated, and insoluble bound putrescine (Put), Spd, and spermine (Spm) in the roots of cucumber seedlings were significantly increased under hypoxia stress. Interestingly, application of Spd to hypoxic roots markedly suppressed the accumulation of free Put and, in contrast, promoted an increase in free Spd and Spm, as well as soluble conjugated and insoluble bound Put, Spd, and Spm contents. From these data, we deduced that exogenous Spd promotes the conversion of free Put into free Spd and Spm, and soluble conjugated and insoluble bound PAs under hypoxia stress. Furthermore, the activities of LDH, PDC, and ADH were suppressed and, in contrast, the activities of SDH and IDH were enhanced by application of exogenous Spd to hypoxic roots. As a result, aerobic respiration was enhanced but fermentation metabolism was inhibited in the roots of cucumber seedlings, leading to an increase in ATP content to alleviate the inhibited dry weight of seedlings due to hypoxia stress. These results suggest that application of Spd to hypoxic nutrient solution promoted conversion of free Put into free Spd and Spm as well as soluble conjugated and insoluble bound PAs, further enhanced IDH and SDH activities, and inhibited ethanol fermentation and lactate fermentation, resulting in increased ATP content and eventually enhanced tolerance of cucumber plants to root-zone hypoxia.     

Abscisic acid and hypoxic induction of anoxia tolerance in roots of lettuce seedlings

Lettuce (Lactuca sativa L.) seedlings were subjected to anoxic stress after ABA-pretreatment (ABA-PT) or hypoxic-pretreatment (H-PT). The H-PT increased the survivability of the anoxia in roots of the seedlings by 5.2-fold compared to that of non-pretreated (N-PT) seedlings. ABA-PT also increased the survivability at concentrations greater than 1 microM, and the survivability increased with increasing ABA doses. At 100 microM ABA, the survivability was 4.5-fold greater than that of N-PT seedlings. During pretreatment periods, alcohol dehydrogenase (ADH, EC 1.1.1.1) activity in the roots became 3.1- and 3.4-fold greater than that of N-PT seedlings following 100 microM ABA-PT and H-PT seedlings, respectively. After the onset of anoxic stress, ADH activities in all roots increased, but the activities in H-PT and ABA-PT roots remained much greater than that in N-PT roots, and the average ethanol production rate for the initial 6 h was 5.3, 4.0 and 1.4 micromol g(-1) FW h(-1) for H-PT, ABA-PT and N-PT roots, respectively. Roots of the seedlings lost ATP rapidly under anoxic stress; however, the decrease in ATP was much slower in the ABA-PT and H-PT seedlings than in the N-PT seedlings. These results suggest that the ABA-PT and H-PT may maintain ATP levels due to activation of ethanolic fermentation, which may be one of the causes of the increasing anoxia tolerance in the seedling roots. Measurement of endogenous ABA levels, however, showed that ABA levels did not increase during the H-PT, suggesting that the H-PT does not increase tolerance through an increase in ABA levels.     

Activity of biochemical pH-stat enzymes in cereal root tips under oxygen deficiency

The activity of the enzymes of alcoholic and lactic-acid fermentation: pyruvate decarboxylase (PDC, EC 4.1.1.1), alcohol dehydrogenase (ADH, EC 1.1.1.1), lactate dehydrogenase (LDH, EC 1.1.1.27) and the enzymes of malic acid metabolism: phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.23), NAD-dependent malate dehydrogenase (NAD-MDH, EC 1.1.1.37), and NADP-dependent malic enzyme (NADP-ME, EC 1.1.1.40) involved in the operation of biochemical pH-stat was investigated in the root tips of wheat (Triticum aestivum L.) and rice (Oryza sativa L.) under hypoxia and anoxia. Exposures lasted for 6, 12, and 18 h. The most pronounced response was detected for the enzymes of alcoholic fermentation. The activation of ADH and PDC in wheat occurred only under hypoxia, whereas in rice it was detected both under hypoxia and anoxia. The activation of LDH in wheat occurred under hypoxia, and in rice, the activity of this enzyme was slightly enhanced. The activity of the enzymes of malic acid metabolism did not change except in wheat root tips under hypoxia when PEPC activity decreased and NADP-ME activity simultaneously rose. The role of biochemical pH-stat in the regulation of cytoplasmic pH in plant cells under oxygen deficit and the mechanisms for regulating the activities of enzymes involved in biochemical pH-stat are discussed as well as the interaction between biochemical pH-stat and other mechanisms maintaining pH of plant cells. The results are analyzed within a context of intracellular pH regulation.     

Sugar utilization and anoxia tolerance in rice roots acclimated by hypoxic pretreatment

Although most cereal roots cannot elongate under anoxic conditions, primary roots of three-day-old rice (Oryza sativa L.) seedlings were able to elongate during a 24-h period of anoxia. Hypoxic pretreatment (H-PT) increased the elongation of their roots. Sucrose synthase (EC 2.4.1.13), glucokinase (EC 2.7.1.2), fructokinase (EC 2.7.1.4), pyruvate decarboxylase (EC 4.1.1.1) and alcohol dehydrogenase (EC 1.1.1.1) activities were increased by anoxia in both H-PT and non-pretreated (N-PT) roots. However, these activities were greater in the H-PT roots than in the N-PT roots. The average rate of production of ethanol for the initial 6h after the onset of anoxia was 3.7 and 1.4 micromolg(-1) fresh weight h(-1) for the H-PT and N-PT roots, respectively, suggesting that ethanolic fermentation may increase more quickly in the H-PT roots than in the N-PT roots. Roots of the seedlings lost ATP and total adenine nucleotides in anoxia, however, the H-PT roots maintained higher levels of ATP and total adenine nucleotides compared to the N-PT roots. These results show that rice roots are able to utilize the set of enzymes involved in the metabolism of soluble sugars under anoxia. The ability to maintain an active fermentative metabolism for production of ATP by fueling the glycolytic pathway with fermentable carbohydrate is probably greater in H-PT than in N-PT roots.     

Hypoxic induction of alcohol and lactate dehydrogenases in lupine seedlings

Seedlings of lupine (Lupinus luteus L. cv. Juno) were exposed for up to 96 hours to 1 to 2 kPa partial pressure oxygen (hypoxic treatment) and activities of alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH) and their isoform profiles were determined. Roots of lupine seedlings were grown in a nitrogen flushed nutrient solution while their shoots were in air. Prolonged hypoxia led to a reduction of root elongation. This was accompanied by reduced increase in dry weight suggesting that insufficient carbohydrate supply was the cause of retarded growth of lupine roots. Hypoxically treated roots showed induction of ADH and LDH acivities. The maximum increase in LDH activity was low (2-fold) in contrast to ADH activity, which increased up to 7-fold. Hypoxic treatment of roots did not affect the activities of ADH and LDH in hypocotyls and cotyledons. Analysis of ADH and LDH activity gels indicated in roots 1 and 2 isoforms, respectively. The level of isozymes of both enzymes increased in roots upon exposure to hypoxic stress. Differences in isoenzymatic spectrum of ADH and LDH between roots, hypocotyls and cotyledons indicate organ specificity of isozymes of both enzymes. The importance of alcohol and lactate fermentation in roots to cope with hypoxic stress is discussed.     

根际低氧胁迫对黄瓜幼苗根系呼吸代谢的影响

采用营养液栽培方法,研究了低氧胁迫对两个耐低氧能力不同的黄瓜品种根系呼吸代谢的影响.结果表明：低氧胁迫下,两个黄瓜品种根系三羧酸循环显著受阻,无氧呼吸代谢被促进.与耐低氧能力较弱的中农8号相比,耐低氧能力较强的绿霸春4号根系琥珀酸脱氢酶和异柠檬酸脱氢酶活性的降低幅度较小,乳酸脱氢酶活性、乳酸和丙酮酸含量的增加幅度较小,而丙酮酸脱羧酶、乙醇脱氢酶活性及乙醇、丙氨酸含量的增加幅度较大;低氧胁迫8 d时,与相应对照相比,绿霸春4号根系乙醇脱氢酶活性及乙醇和丙氨酸含量分别增加了409.30%、112.13%和30.64%,中农8号根系分别增加了110.42%、31.84%和4.78%,这是两个黄瓜品种耐低氧能力差异的主要生理原因.两品种幼苗根系丙氨酸氨基转移酶活性和乙醛含量没有显著差异.表明低氧胁迫下黄瓜根系乙醇发酵代谢途径的增强和丙氨酸的积累有利于防御低氧伤害.     

Anoxia tolerance in rice roots acclimated by several different periods of hypoxia

Rice (Oryza sativa L.) seedlings were subjected to hypoxic pretreatment (H-PT; incubated in 5% O2 atmosphere) for various lengths of time followed by an anoxic stress. Anoxia tolerance of rice roots was improved with increasing duration of H-PT, but longer H-PT than 12 h gave no additional improvement. Concentrations of ATP and ethanol, and activities of pyruvate decarboxylase (EC 4.1.1.1) and alcohol dehydrogenase (EC 1.1.1.1) in the roots were increased by H-PT, and the times and patterns of increasing in these concentrations and activities were similar to those of increasing in the anoxia tolerance. These results suggest that the H-PT may increase anoxia tolerance due to maintenance of ATP levels with rapid induction of ethanolic fermentation, and hypoxic acclimation may occur within 12 h.     

Metabolic Control of Anaerobic Glycolysis (Overexpression of Lactate Dehydrogenase in Transgenic Tomato Roots Supports the Davies-Roberts Hypothesis and Points to a Critical Role for Lactate Secretion

Roots of all plants examined so far have the potential for both ethanol and lactate fermentation. A short burst of lactate fermentation usually occurs when plant tissues are transferred from normoxic to anoxic conditions. According to the Davies-Roberts hypothesis, the consequent pH drop both initiates ethanol fermentation and blocks further production of lactate by inhibiting lactate dehydrogenase (LDH). However, the role of LDH in this pH control mechanism is still a matter of debate. To perturb the control system in a defined way, a barley LDH cDNA under the control of the cauliflower mosaic virus 35S promoter was introduced into tomato (Lycopersicon esculentum Mill. cv VFMT) using Agrobacterium rhizogenes. The transgenic root clones expressed up to 50 times the LDH activity of controls. The fermentative metabolism of these clones was compared using roots grown previously in normoxic conditions or roots given a 3-d hypoxic pretreatment. During the transition from normoxia to anoxia, lactate accumulation was no faster and no more extensive in transgenic roots than in controls. Similarly, during prolonged anoxia the flux of 14C from [U-14C] glucose to lactate and ethanol was not modified by the expression of the transgene. However, in both transgenic and control roots, hypoxic pretreatment increased the flux to lactate and promoted lactate export to the medium. These results show that LDH has a very low flux control coefficient for lactate fermentation, consistent with the Davies-Roberts hypothesis. Moreover, they suggest that lactate secretion exerts major control over long-term lactate glycolysis in vivo.     

根际低氧逆境对网纹甜瓜幼苗生长及根系呼吸代谢途径的影响

以耐低氧性具有明显差异的两个网纹甜瓜(Cucumis melo var. raticulalus)品种为试材,研究了根际低氧胁迫下幼苗生长、根系活力及根系呼吸关键酶活性的变化。结果表明,根际低氧胁迫下,两品种幼苗生长均受到明显抑制,而根系活力升高;根系PDC活性两品种均显著提高,品种间无显著差异; MDH活性两品种均显著降低,且耐低氧性弱的‘西域一号’下降幅度较大;根系ADH和LDH活性两品种均显著提高,耐低氧性强的‘东方星光’ADH活性增加的幅度显著高于耐低氧性弱的‘西域一号’,而‘西域一号’LDH活性增加幅度显著高于‘东方星光’。说明‘东方星光’在低氧胁迫下能保持较高的有氧呼吸水平,无氧呼吸的主要途径为乙醇发酵,而‘西域一号’在低氧胁迫下无氧呼吸的主要途径为乳酸发酵。     

Fermentation and malate metabolism in response to elevated CO2 concentrations in two strawberry cultivars

Concentrations of acetaldehyde, ethanol, ethyl acetate (EA), organic acids and activities and gene expression of alcohol dehydrogenase (ADH; EC 1.1.1.1), pyruvate decarboxylase (PDC; EC 4.1.1.1), alcohol acyltransferase (AAT; EC 1.4.1.14), malate dehydrogenase (MDH; EC 1.1.1.37), malic enzyme (ME; EC 1.1.1.40) and glutamate dehydrogenase (EC 1.4.1.14) were investigated in two strawberry ( Fragaria × ananassa Duch) cultivars with different responses to CO₂ during storage. 'Jewel' fruit treated with CO₂ accumulated acetaldehyde and ethanol but little EA, while 'Cavendish' accumulated little acetaldehyde or ethanol but accumulated EA. In CO₂-treated fruit, PDC activity was positively correlated with EA accumulation in 'Jewel' but not in 'Cavendish', while no differential effect of atmosphere was observed on its gene expression. ADH activity and gene expression show a correlation with ethanol accumulation in 'Cavendish'. In 'Jewel', there was a positive correlation between ADH gene expression and enzyme activity; however, this correlation does not explain ethanol accumulation in this cultivar. EA accumulation did not show any correlation with AAT activity and gene expression in any of the cultivars. Succinate concentrations were highest and those of malate lowest in CO₂-treated fruit of both cultivars, but MDH and ME activities were not affected by CO₂. Gene expression of MDH and ME were not affected by atmosphere in 'Cavendish', although in 'Jewel' the MDH expression was slightly lower in CO₂- than air-treated fruit. The results of this study show that differences in fermentation products and malate accumulation in CO₂-treated strawberry fruit are not consistently correlated with enzyme activities and gene expression.     

Alcohol Dehydrogenase and Pyruvate Decarboxylase Activity in Leaves and Roots of Eastern Cottonwood (Populus deltoides Bartr.) and Soybean (Glycine max L.)

Pyruvate decarboxylase (PDC, EC 4.1.1.1) and alcohol dehydrogenase (ADH, EC 1.1.1.1) are responsible for the anaerobic production of acetaldehyde and ethanol in higher plants. In developing soybean embryos, ADH activity increased upon imbibition and then declined exponentially with development, and was undetectable in leaves by 30 days after imbibition. PDC was not detectable in soybean leaves. In contrast, ADH activity remained high in developing cottonwood seedlings, with no decline in activity during development. ADH activity in the first fully expanded leaf of cottonwood was 230 micromoles NADH oxidized per minute per gram dry weight, and increased with leaf age. Maximal PDC activity of cottonwood leaves was 10 micromoles NADH oxidized per minute per gram dry weight. ADH activity in cottonwood roots was induced by anaerobic stress, increasing from 58 to 205 micromoles NADH oxidized per minute per gram dry weight in intact plants in 48 hours, and from 38 to 246 micromoles NADH oxidized per minute per gram dry weight in detached roots in 48 hours. Leaf ADH activity increased by 10 to 20% on exposure to anaerobic conditions. Crude leaf enzyme extracts with high ADH activity reduced little or no NADH when other aldehydes, such as trans-2-hexenal, were provided as substrate. ADH and PDC are constitutive enzyme in cottonwood leaves, but their metabolic role is not known.