Effect of Adenine Nucleotides on the Respiration of Carrot Root Slices

Sodium pyruvate and dinitrophenol stimulated O(2) uptake of freshly cut phloem parenchyma from carrot roots by 63 and 120% at optimal concentrations, indicating that production of pyruvate by glycolysis regulates over-all respiratory rate. Adding 0.5 to 6.7 mm Na(3)ADP and Na(3)ATP to slices rapidly stimulates respiration rate by 20 to 85%. The effect is greater at the lower end of this concentration range and is not due to change in pH or active cation uptake. It is suggested that treating tissue with both nucleotides stimulates pyruvate kinase, the rate-limiting step in respiration of freshly cut slices, by increasing the concentration of endogenous ADP. Adenosine diphosphate continued to stimulate O(2) uptake until the peak of induced respiration, but ATP inhibited respiration during development and decline of this peak. Absence of respiratory stimulation by NaH(2)PO(4) and of respiratory inhibition by added nucleosides confirms that inorganic phosphate is not a limiting factor of respiration in freshly cut slices. The stimulation of respiration rate of these slices by dinitrophenol is consistent with results from experiments in which ADP and ATP were applied to the tissue.     

Sulfide-Resistant Respiration in Leaves of Elodea canadensis Michx: Comparison with Cyanide-Resistant Respiration

The rate of dark O₂ uptake of Elodea canadensis leaves was titrated with either cyanide or sulfide in the presence and in the absence of 5 millimolar salicylhydroxamic acid (SHAM), an inhibitor of the alternative oxidase. The inhibition of O₂ uptake by SHAM alone was very small (3-6%), suggesting that actual respiration mainly occurred through the cytochrome pathway. O₂ uptake was slightly stimulated by cyanide at concentrations of 50 micromolar or higher, but in the presence of SHAM respiration was strongly suppressed. The effects of sulfide on O₂ uptake were similar to those of cyanide, except that the percent stimulation of O₂ uptake by sulfide alone was somewhat higher than that of cyanide. However, the estimates of the capacity of the alternative pathway were similar with both inhibitors. Another difference is that maximal inhibition of respiration in the presence of SHAM was observed with lower concentrations of sulfide (50 micromolar) than cyanide (250 micromolar). The results suggest that sulfide can be used as a suitable inhibitor of cytochrome c oxidase in studies with intact plant tissues, and that sulfide does not apparently inhibit the alternative oxidase.     

Leaf respiration: Possible cause of stimulation after low temperature treatment

Uncoupling of oxidative phosphorylation by 2,4 dinitrophenol stimulated leaf respiration rate measured as CO₂ production or O₂ uptake after low temperature treatment.     

Effects of KCN and Salicylhydroxamic Acid on Respiration of Soybean Leaves at Different Ages

Measurements of respiration were made on leaf discs from glasshouse-grown soybean (Glycine max [L.] Merr. cv `Corsoy') plants in the presence and absence of cyanide (KCN) and salicylhydroxamic acid (SHAM). O₂ uptake by mature leaves measured at 25°C was stimulated by 1 millimolar KCN (63%) and also by 5 millimolar azide (79%). SHAM, an inhibitor of the alternative oxidase and a selection of other enzymes, also stimulated O₂ uptake by itself at concentration of 10 millimolar. However, in combination, KCN and SHAM were inhibitory. The rate of O₂ uptake declined consistently with leaf age. The stimulation of O₂ uptake by KCN and by SHAM occurred only after a certain stage of leaf development had been reached and was more pronounced in fully expanded leaves. In young leaves, O₂ uptake was inhibited by both KCN and SHAM individually. The uncoupler, p-trifluoromethoxy carbonylcyanide phenylhydrazone, stimulated leaf respiration at all ages studied, the stimulation being more pronounced in fully expanded leaves. The uncoupled rate was inhibited by KCN and SHAM individually. The capacity of the cytochrome path declined with leaf age, paralleling the decline in total respiration. However, the capacity of the alternative path peaked at about full leaf expansion, exceeding the cytochrome capacity and remaining relatively constant. These results are consistent with the presence in soybean leaves of an alternative path capacity that seems to increase with age, and they suggest that the stimulation of O₂ uptake by KCN and NaN₃ in mature leaves was mainly by the SHAM-sensitive alternative path. The stimulation of O₂ uptake by SHAM was not expected, and the reason for it is not clear.     

Regulation of glycolysis in heterotrophic cell suspension cultures of Chenopodium rubrum in response to proton fluxes at the plasmalemma

The present experiments were carried out to investigate the effect of increased fluxes of H⁺ across the plasmalemma on glycolysis in heterotrophic cell suspension cultures of Chenopodium rubrum L. (1) Increased H⁺ influx was produced by adding glucose, 6-deoxyglucose, 2-deoxyglucose, or sodium fluoride. The net influx decreased to zero after 3 min. This recovery was accompanied by an increase in the rate of O₂ uptake, but not of dark CO₂ fixation. When glucose or fluoride were added, the increase of O₂ uptake occurred without a decrease in the ATP/ADP ratio, and was large enough to provide the ATP that would be needed for compensatory H⁺ extrusion via the plasmalemma H⁺-ATPase. When 2-deoxyglucose was added, the rise of respiration was restricted by sequestration of phosphate and depletion of phosphorylated metabolites, the ATP/ADP ratio declined, and a slow net H⁺ influx started again after 4 min. (2) Alkalinisation of the medium to induce an H⁺ efflux resulted in rapid activation of dark CO₂ fixation, but not of O²-uptake. (3) A stimulation of respiration or dark CO₂ fixation was always accompanied by a decrease of phosphoenolpyruvate. This shows that the primary sites for regulation of glycolysis are pyruvate kinase and phosphoenolpyruvate carboxylase, respectively. (4) There was no consistent relation between glycolytic flux and triose-phosphates or hexose-phosphates. This shows that the reactions involved in carbohydrate mobilisation and the conversion of hexose-phosphates to triose-phosphates only have a secondary role in stimulation of glycolysis. (5) Phosphofructokinase will be stimulated as a consequence of the decrease in phosphoenolpyruvate. (6) The increase in glycolytic flux occurred independently of (in the case of 2-deoxyglucose and fluoride), or before (in the case of glucose), any increase of fructose-2,6-bisphosphate. When fructose-2,6-bisphosphate did increase (after supplying glucose), this was accompanied by an increase of triose-phosphate and fructose-1,6-bisphosphate, which otherwise remained very low. It is argued that fructose-2,6-bisphosphate increases as a consequence of the decrease of glycerate-3-phosphate, a known inhibitor of the synthesis of this regulator metabolite. However, activation of pyrophosphate fructose-6-phosphate phosphotransferase by fructose-2,6-bisphosphate does not play an obligatory role in the stimulation of glycolysis.     

Involvement of respiratory processes in the transient knockout of net CO2 uptake in Mimosa pudica upon heat stimulation

Leaf photosynthesis of the sensitive plant Mimosa pudica displays a transient knockout in response to electrical signals induced by heat stimulation. This study aims at clarifying the underlying mechanisms, in particular, the involvement of respiration. To this end, leaf gas exchange and light reactions of photosynthesis were assessed under atmospheric conditions largely eliminating photorespiration by either elevated atmospheric CO₂ or lowered O₂ concentration (i.e. 2000 μmol mol^?1 or 1%, respectively). In addition, leaf gas exchange was studied in the absence of light. Under darkness, heat stimulation caused a transient increase of respiratory CO₂ release simultaneously with stomatal opening, hence reflecting direct involvement of respiratory stimulation in the drop of the net CO₂ uptake rate. However, persistence of the transient decline in net CO₂ uptake rate under illumination and elevated CO₂ or 1% O₂ makes it unlikely that photorespiration is the metabolic origin of the respiratory CO₂ release. In conclusion, the transient knockout of net CO₂ uptake is at least partially attributed to an increased CO₂ release through mitochondrial respiration as stimulated by electrical signals. Putative CO₂ limitation of Rubisco due to decreased activity of carbonic anhydrase was ruled out as the photosynthesis effect was not prevented by elevated CO₂.     

Fluoride Inhibition of Respiration and Fermentation in Cultured Cells of Acer pseudoplatanus

Low concentrations of sodium fluoride severely inhibit anaerobic CO₂ evolution in Acer pseadoplatanus L. cells but have relatively little effect on aerobic respiration. The insensitivity of respiratory O₂ uptake to fluoride is due in part to the fact that fluoride reduces the intracellular pyruvate concentration to only a relatively minor extent under aerobic conditions, although it prevents the several fold increase in endogenous private which is normally brought about by anoxia. The respiratory insensitivity is also ascribable to the existence of a respiratory component which is unaffected by the decrease in endogenous private resulting from fluoride treatment. The extremely severe respiratory inhibition brought about by fluoride plus dinitrophenol is not appreciably relieved by exogenous private, indicating that this inhibition is the result of interference with the aerobic oxidation process per se and is not solely a consequence of glycolytic inhibition.     

The tricarboxylic acid cycle and glycolysis in relation to ion transport by the ciliary body

1. The respiration and aerobic glycolysis of pig ciliary processes in oxygenated phosphate and bicarbonate buffers have been investigated. 2. Significant amounts of lactic acid are produced only in the presence of added glucose, but this does not change the endogenous respiration rate. 3. Succinate and citrate increase the oxygen uptake considerably, but pyruvate has almost no effect; oxaloacetate and fumarate stimulate slightly in the presence of glucose. Aspartate and fumarate together stimulate pyruvate utilization and are oxidized as fast as citrate. 4. Ouabain inhibits the oxidation of glucose and other substrates by limiting the ADP supply from the sodium transport system. Cyanide and azide inhibit respiration and stimulate glycolysis. 5. The transport mechanism depends largely on ATP from oxidative phosphorylation and regulates the rate of respiration and glycolysis by controlling ADP production from the Na(+)-K(+)-activated adenosine triphosphatase.     

Kinetics of the Oxyhydrogen Reaction in the Presence and Absence of Carbon Dioxide in Scenedesmus obliquus

The oxyhydrogen reaction in the presence and absence of CO₂ was studied in H₂-adapted Scenedesmus obliquus by monitoring the initial rates of H₂, O₂, and ¹⁴CO₂ uptake and the effect of inhibitors on these rates with gas-sensing electrodes and isotopic techniques. In the presence of 0.02 atmosphere O₂, the pH₂ was varied from 0 to 1 atmosphere. Whereas the rate of O₂ uptake increased by only 30%, the rate of H₂ uptake increased severalfold over the range of pH₂ values. At 0.1 atmosphere H₂ and 0.02 atmosphere O₂, rates for H₂ and O₂ uptake were between 15 and 25 micromoles per milligram chlorophyll per hour. As the pH₂ was changed from 0 to 1 atmosphere, the quotient H₂:O₂ changed from 0 to roughly 2. This change may reflect the competition between H₂ and the endogenous respiratory electron donors. Respiration in the presence of glucose and acetate was also competitive with H₂ uptake. KCN inhibited equally respiration (O₂ uptake in the absence of H₂) and the oxyhydrogen reaction in the presence and absence of CO₂. The uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone accelerated the rate of respiration and the oxyhydrogen reaction to a similar extent. It was concluded that the oxyhydrogen reaction both in the presence and absence of CO₂ has properties in common with components of respiration and photosynthesis. Participation of these two processes in the oxyhydrogen reaction would require a closely linked shuttle between mitochondrion and chloroplast.     

Re-interpreting anaerobic metabolism: an argument for the application of both anaerobic glycolysis and excess post-exercise oxygen consumption (EPOC) as independent sources of energy expenditure

Due to current technical difficulties and changing cellular conditions, the measurement of anaerobic and recovery energy expenditure remains elusive. During rest and low-intensity steady-state exercise, indirect calorimetric measurements successfully represent energy expenditure. The same steady-state O₂ uptake methods are often used to describe the O₂ deficit and excess post-oxygen consumption (EPOC): 1 l O₂ = 5 kcal = 20.9 kJ. However, an O₂ deficit plus exercise O₂ uptake measurement ignores energy expenditure during recovery, and an exercise O₂ uptake plus EPOC measurement misrepresents anaerobic energy expenditure. An alternative solution has not yet been proposed. Anaerobic glycolysis and mitochondrial respiration are construed here as a symbiotic union of metabolic pathways, each contributing independently to energy expenditure and heat production. Care must be taken when using O₂ uptake alone to quantify energy expenditure because various high-intensity exercise models reveal that O₂ uptake can lag behind estimated energy demands or exceed them. The independent bioenergetics behind anaerobic glycolysis and mitochondrial respiration can acknowledge these discrepancies. Anaerobic glycolysis is an additive component to an exercise O₂ uptake measurement. Moreover, it is the assumptions behind steady-state O₂ uptake that do not permit proper interpretation of energy expenditure during EPOC; 1 l O₂≠ 20.9 kJ. Using both the O₂ deficit and a modified EPOC for interpretation, rather than one or the other, leads to a better method of quantifying energy expenditure for higher intensity exercise and recovery. Accepted: 23 September 1997     

Comparative Physiology of Respiration in Aquatic Fungi II. The Saprolegniales, Especially Aphanomyces astaci

The endogenous respiration of six members of Saprolegniaceae (Oomycetes), Saprotegnia sp., Thraustotheca sp., Achlya sp., Dichtyuchus sp., Aphanomyces euteiches, and A. astaci were studied in the presence and in the absence of exogenous substrate using conventional manometric techniques. Glucose stimulated the rate of oxygen uptake of unstarved mycelia to some extent in all the fungi. Attempts to increase the weak stimulation of respiration by glucose in A. astaci were not successful. The respiratory quotients of the fungi tested were usually in the range of 0.7 to 0.9 during endogenous respiration, and addition of glucose increased these values more than expected. L-leucine and L-glutamic acid stimulated respiration of A. astaci only when the fugus was starved, and acetic acid and butyric acid were inhibitory. Fructose and acetic acid increased respiration in starved mycelium of A. euteiches while L-leucine and L-glutamic acid had little effect. Antimycin A, HOQNO, HCN, and fluoroacetate strongly inhibited endogenous oxygen uptake by A. astaci. Amytal and azide were also markedly inhibitory while rotenone and CO had little effect. DNP and diphenylamine inhibited respiration at a high concentration but at a lower concentration DNP was stimulatory. In contrast the respiration of Saprolegnia sp. was resistant to cyanide, antimycin A, and HOQNO. Spectrophotometric observations on homogenized mycelia of Saprolegnia sp. and of A. astaci indicated the presence of cytochrome c (551 nm), two b-type cytochromes (557 and 564 nm) and cytochrome a-a₃ (605 nm) all in approximately equimolar concentrations. In both strains CO combined with cytochrome a₃ and an unidentified pigment. A remarkable similarity in the cytochrome system seems to exist between these two strains and some members of the Leptomitales.     

On the catabolic pathways of sugars in green algae

Respiratory metabolism of the cultures of algaeChlorella pyrenoidosa (82) andScenedesmus obliquus (125) was investigated. One part of algae were cultivated on a mineral nutrient solution, another two parts on the solution with glucose and on the solution with glucose and yeast decoction. Individual steps of respiratory metabolism—endogenous as well as in the presence of exogenous sugars—were estimated according to the response to the following inhibitors: monoiodacetate, NaF, NaN₃, and 2,4-DNP. In the last two cases, fructose, glucose-6-phosphate and fructose-1,6-diphosphate were applied in parallel for comparison. Na-monoiodacetate was found to inhibit the respiration of both endogenous and exogenous (with glucose) substrates, NaF (concentrations up to 2.5.10^?2 m) stimulated the O₂ uptake. The effect of sodium azide and 2,4-DNP depended in both strains on previous cultivation. On the basis of the results obtained, the presence of particular respiration pathways in the dissimilation of glucose is discussed. The following catabolic processes are to be considered: a) direct oxidation (with both autotrophically cultivated strains and with theChlorella cultivated on glucosecontaining medium), b) the process similar to glycolysis, which, however, does not necessarily involve the enolase (it is not inhibited by NaF) c) pentosephosphate cycle (Chlorella), and d) glycolysis, in which both algae can operate when sugars previously phosphorylated are applied.     

Oxygen consumption by purified plasmalemma vesicles from wheat roots: Stimulation by NADH and salicylhydroxamic acid (SHAM)

Plasmalemma vesicles from wheat (Triticum aestivum L.) roots consumed O₂ and the addition of 1 mM NADH increased the rate ~ 3-fold (to 15-30 nmol O₂·mg⁻¹·min⁻¹). The NADH-dependent O₂ uptake was abolished by catalase. In the presence of salicylhydroxamic acid (SHAM), an inhibitor of the alternative oxidase pathway in plant mitochondria, NADH-dependent O₂ consumption was stimulated 10–20-fold (to 200–400 nmol·mg^1̄·min⁻¹). Catalase also abolished this stimulation, which was KCN-sensitive but antimycin A-insensitive, and the production of H₂O₂ during SHAM-stimulated NADH-dependent O₂ uptake was demonstrated. Irrespective of the mechanism, SHAM-stimulated respiration by root plasmalemma makes it difficult to interpret results on root respiration obtained using KCN and SHAM.     

Cyanide-resistant respiration in light- and dark-grown soybean cotyledons

Measurements of respiration were made on intact tissue and mitochondria isolated from soybean (Glycine max [L.] Merr. cv `Corsoy') cotyledons from seedlings of different ages grown in light and darkness. Effects of cyanide (KCN) and salicylhydroxamic acid (SHAM) on O₂ uptake rates were determined. O₂ uptake was faster in light-grown tissue and was inhibited by both KCN and SHAM in all except light-grown tissue older than 9 days. Both inhibitors stimulated O₂ uptake in tissues more than 9 days old. Mitochondria in which O₂ uptake was coupled to ATP synthesis were isolated from all tissues. O₂ uptake by mitochondrial preparations from light- and dark-grown cotyledons was equally sensitive to KCN. Similarly, age did not affect KCN sensitivity, but sensitivity to SHAM declined with age both in the presence and absence of KCN. Estimated capacities of the cytochrome and alternative pathways of the mitochondrial preparations indicated considerably larger cytochrome than alternative pathway capacities. The cytochrome pathway capacities paralleled the state 3 mitochondrial respiration rates, which increased from day 5 to day 7 then declined thereafter. The alternative pathway capacities were not affected by light. The uncoupler, p-trifluoromethoxycarbonylcyanide phenylhydrazone (FCCP), increased the flow of electrons through the cytochrome pathway at the expense of flow through the alternative pathway in isolated mitochondria. However, the combined capacities did not exceed the rate in the presence of FCCP. The results are interpreted to indicate that the stimulation of respiration by KCN and SHAM observed in the 12-day-old green cotyledons and previously observed in older soybean leaves is not explained by characteristics of the mitochondria.     

Wounding Stimulates Cyanide-sensitive Respiration in the Highly Cyanide-resistant Leaves of Bryophyllum tubiflorum Harv

The rate of respiration in sectioned leaves of Bryophyllum tubiflorum Harv. increases with decreasing section thickness. The rates of uninhibited respiration in 2- and 8-millimeter-thick sections are 74 and 46 microliters of O₂ per gram fresh weight of unruptured tissue per hour at 20 C, whereas the rate in the presence of cyanide is 31 microliters of O₂ in each case. The rates are unaffected by salicylhydroxamic acid, but cyanide and salicylhydroxamic acid together completely eliminate O₂ uptake. The capacity of the alternative respiratory pathway is thus initially high (estimated at 84% of the uninhibited respiratory rate in whole leaves) and remains constant but probably unexpressed subsequent to the rapid induction of wound respiration.     

Further studies on the stimulation of glycolysis by previous aerobiosis

1. A short period of incubation in oxygen increases the rate of anaerobic glycolysis in all the normal adult tissues that have been tested, with the exception of erythrocytes. 2. This stimulation does not occur in the six different tumours and in the two embryonic tissues that have been studied. 3. In rat liver and in chicken heart, stimulation is first seen at birth. 4. Stimulation in rat liver slices is decreased in the presence of some inhibitors of oxidative metabolism (cyanide, Amytal, dinitrophenol, malonate), but is not affected if the aerobic preincubation is carried out at 1°. 5. The presence in the medium of some metabolites that are known to be important regulators of the glycolytic rate in living tissues has essentially no effect on stimulation. 6. The pretreatment of animals with inhibitors of oxidative metabolism and with antioxidants does not suppress stimulation; the observed effect of NN′-diphenyl-p-phenylenediamine is probably the consequence of the fall of the glycogen content in the liver. 7. The stimulation of anaerobic glycolysis by previous aerobiosis could not be demonstrated in liver homogenates.     

The stimulus-secretion coupling of glucose-induced insulin release. Effect of exogenous pyruvate on islet function.

1. In isolated pancreatic islets, pyruvate causes a shift to the left of the sigmoidal curve relating the rate of insulin release to the ambient glucose concentration. The magnitude of this effect is related to the concentration of pyruvate (5--90 mM) and, at a 30 mM concentration, is equivalent to that evoked by 2 mM-glucose. Pyruvate also enhances insulin release in the presence of fructose, leucine and 4-methyl-2-oxopentanoate. 2. In the presence of glucose 8 mM), the secretory response to pyruvate is an immediate process, displaying a biphasic pattern. 3. The insulinotropic action of pyruvate coincides with an inhibition of 45Ca efflux and a stimulation of 45Ca net uptake. The relationship between 45Ca uptake and insulin release displays its usual pattern in the presence of pyruvate. 4. Exogenous pyruvate rapidly accumulates in the islets in amounts close to those derived from the metabolism of glucose. The oxidation of [2-14C]pyruvate represents 64% of the rate of [1-14C]pyruvate decarboxylation and, at a 30 mM concentration, is comparable with that of 8 mM-[U-14C]glucose. 5. When corrected for the conversion of pyruvate into lactate, the oxidation of 30 mM-pyruvate corresponds to a net generation of about 314 pmol of reducing equivalents/120 min per islet. 6. Pyruvate does not affect the rate of glycolysis, but inhibits the oxidation of glucose. Glucose does not affect pyruvate oxidation. 7. Pyruvate (30 mM) does not affect the concentration of ATP, ADP and AMP in the islet cells. 8. Pyruvate (30 mM) increases the concentration of reduced nicotinamide nucleotides in the presence but not in the absence of glucose. A close correlation is seen between the concentration of reduced nicotinamide nucleotides and the net uptake of 45Ca. Menadione inhibits the effect of pyruvate on insulin release, without altering its rate of oxidation. 9. Pyruvate, like glucose, modestly stimulates lipogenesis. 10. Pyruvate, in contrast with glucose, markedly inhibits the oxidation of endogenous nutrients. The latter effect accounts for the apparent discrepancy between the rate of pyruvate oxidation and the magnitude of its insulinotropic action. 11. Dichloroacetate fails to affect glucose oxidation and glucose-stimulated insulin release. 12. It is concluded that the effect of pyruvate to stimulate insulin release depends on its ability to increase the concentration of reduced nicotinamide nucleotides in the islet cells.     

Salicylic Acid induces cyanide-resistant respiration in tobacco cell-suspension cultures

Cyanide-resistant, alternative respiration in Nicotiana tabacum L. cv Xanthi-nc was analyzed in liquid suspension cultures using O₂ uptake and calorimetric measurements. In young cultures (4-8 d after transfer), cyanide inhibited O₂ uptake by up to 40% as compared to controls. Application of 20 μm salicylic acid (SA) to young cells increased cyanide-resistant O₂ uptake within 2 h. Development of KCN resistance did not affect total O₂ uptake, but was accompanied by a 60% increase in the rate of heat evolution from cells as measured by calorimetry. This stimulation of heat evolution by SA was not significantly affected by 1 mm cyanide, but was reduced by 10 mm salicylhydroxamic acid (SHAM), an inhibitor of cyanide-resistant respiration. Treatment of SA-induced or uninduced cells with a combination of cyanide and SHAM blocked most of the O₂ consumption and heat evolution. Fifty percent of the applied SA was taken up within 10 min, with most of the intracellular SA metabolized in 2 h. 2,6-Dihydroxybenzoic and 4-hydroxybenzoic acids also induced cyanide-resistant respiration. These data indicate that in tobacco cell-suspension culture, SA induces the activity and the capacity of cyanide-resistant respiration without affecting the capacity of the cytochrome c respiration pathway.     

Influence of factors affecting germination on respiration of Phacelia tanacetifolia seeds

Summary Germination of the seeds of Phacelia tanacetifolia is inhibited by light. Removal of that part of the covering structures of the seeds which directly covers the radicle allows full germination in light. The rate of O₂ uptake in the seeds increases following imbibition, and reaches the same steady rate in light and in darkness after 3 hr. From the 14 th hour on, dark-imbibed seeds show a linear increase in the rate of respiration. This increase is not observed in dormant seeds incubated in light. In normal dark germination, protrusion of the radicle begins at 12 th hour following soaking, and by the end of 18 th hour approximately 60% of the seeds have germinated. The seeds which have been scarified at the radicle end and germinate readily in light show a steady increase in Q _{O 2}. If scarified seeds are allowed to imbibe 0.3 M mannitol and are then incubated in light, the embryo does not grow and the pattern of O₂ uptake becomes identical with that of intact seeds in light. Mannitol, however, does not inhibit respiration by itself. These observations indicate that the increased O₂ uptake is the result rather than the cause of seed germination, and that light does not cause dormancy by inhibiting O₂ uptake. Measures effective in releasing dormancy (dark incubation, mechanical scarification, gibberellin treatment) do not induce germination by facilitating oxygen entry.This work was supported in part by the Jane Coffin Childs Memorial Fund for Medical Resarch, and by the U. S. Atomic Energy Commission under Contract No. AT (11-1)-1338.     

Oxygen Dependence of Elongation Growth and Oxygen Uptake in Maize Coleoptiles

Auxin-mediated elongation growth of maize coleoptile segments is inhibited by reducing the O₂ concentration in the incubation medium to GT 100 μmol . 1^?1. The half-maximal elongation rate is reached at 40 μmol . 1^?1 O₂, i.e. about two orders of magnitude higher than with mitochondrial respiration. O₂ uptake of the segments measured under similar conditions with an O₂ electrode shows a very similar dependence on O₂ concentration. Auxin increases O₂ uptake by 5–10% when it induces growth. About 40% of the O₂ uptake is insensitive to inhibition by KCN. Auxin has no effect on O₂ uptake in the presence of KCN. The possibility that auxin-mediated elongation growth depends on a KCN-sensitive oxidative process, other than cytochrome c oxidase-catalyzed respiration, is discussed.