Diurnal Variation in Acetylene Reduction and Net Hydrogen Evolution in Five Tropical and Subtropical Nitrogen-Fixing Tree Symbioses

Diurnal variations in acetylene reduction and net hydrogen evolutionwere shown in five tropical and subtropical nitrogen-fixingtree symbioses. The symbioses studied in a growth chamber were:Acacia albida x TAL 1457, Leucaena leucocephala x TAL 1145,Prosopis chilensis x TAL 600, Casuarina glauca x HFP Cc13 andC. obesa x HFP Cc13. Acetylene reduction was highest at the end of the light periodin all symbioses studied. In the A. albida x TAL 1457 symbiosis,the diurnal variations in acetylene reduction and net hydrogenevolution showed a minor synchrony, while in the other symbiosesthe diurnal pattern of acetylene reduction and net hydrogenevolution clearly differed. Also, a diurnal variation in relativeefficiency of nitrogenase was shown in the A. albida x TAL 1457symbiosis. A hydrogen uptake enzyme was detected at a low substrate concentration(24.5 mmol m^–3 H₂) for L. leucocephala x TAL 1145, C.obesa x HFP Cc13 and has earlier been found for C. glauca xHFP CcI3. A hydrogen uptake system was also found for P. chilensisx TAL 600 and A. albida x TAL 1457 at a 17-fold higher substrateconcentration. The results show that a diurnal variation in C₂H₂ reductionand H₂ evolution occurs, and that diurnal variation in the conversionfactor between C₂H₂ reduction and N₂ fixation could occur. Thisfact raises criticisms regarding the use of a point estimateof this factor. Key words: Acetylene reduction, hydrogen evolution, uptake hydrogenase, nitrogen-fixing tree symbioses     

Nitrogen Fixation by Subterranean Clover at Varying Stages of Nodule Dehydration: II. EFFICIENCY OF NITROGENASE FUNCTIONING

Changes in nitrogenase activity (C₂H₂ reduction and H₂ production),nodulated root respiration and the efficiency of nitrogenasefunctioning were measured in response to progressive dehydrationof nodules on intact well-watered plants of subterranean clover(Trifolium subterraneum L.) cv. Seaton Park. The nodulated rootsof vegetative plants grown to the 14-leaf stage were incubatedin a gas exchange system through which a continuous dry airstreamwas passed over an 8 d period. The root tips were immersed inan N-free nutrient solution during this time so that water andion uptake was unimpeded. The decline in nodulated root respirationresulting from nodule drying was associated with a continualreduction in respiration coupled to nitrogenase activity. Asnodule water potential (_nod) decreased, the proportion of totalnodulated root respiration which was nitrogenase-linked declinedfrom 50% (day 1) to 33% (day 8). This was accompanied by a 79%reduction in specific nitrogenase activity (from 3.79 to 0.81umol C₂H₄ g^–1 nodule dry weight min^–1). Nodule dehydrationalso induced a decline in hydrogen (H₂) production in air. Therelative decline in hydrogen production exceeded that of acetylenereduction activity and this resulted in an increase in the relativeefficiency of nitrogenase functioning. However, the carbon costof nitrogenase activity progressively increased above 2.0 molCO₂ respired per mol C₂H₄ reduced as rood decreased below –0.4to –0.5 MPa. Consecutive measurements of the rates ofhydrogen evolution, ¹⁵N₂ fixation and acetylene reduction activityon intact unstressed plants resulted in a C₂H₄/N₂ conversionfactor of 4.08 and an electron balance of 1.08. These resultsindicated that the pre-decline rate of acetylene reduction activitymeasured in a flow-through system provided a valid measure ofthe total electron flux through nitrogenase. Key words: Subterranean clover, dehydration, efficiency, nitrogenase activity     

The Influence of Carbohydrate Reserves on the Response of Nodulated White Clover to Defoliation

A growth-chamber study was carried out to determine whetherthe response of apparent nitrogenase activity (C₂ H₂ reduction)to complete defoliation is influenced by the availability ofcarbohydrate reserves Reserve carbohydrate (TNC) concentrationsof 6-week-old white clover (Trifoliun repens L) plants weremodified by CO₂ pretreatments There was no difference in theresponse of apparent nitrogenase activity to defoliation betweenplants with different TNC concentrations C₂H₂ reduction activitydeclined sharply after defoliation and then recovered similarlyin both high- and low-TNC plants Further experiments were conductedto explain the lack of response of apparent nitrogenase activityto TNC levels Bacteroid degradation was ruled out because invitro nitrogenase activity of crude nodule extracts was stillintact 24 h after defoliation Sufficient carbohydrates appearedto be available to the nodules of defoliated plants becauseadding [¹⁴C]glucose to the nutrient solution did not preventthe decline in apparent nitrogenase activity These conclusionswere supported by the finding that an increase in pO₂ aroundthe nodules of defoliated plants completely restored their C₂H₂reduction activity The comparison of the effects of defoliationand darkness suggested that the decrease in apparent nitrogenaseactivity was not related directly to the interruption of photosynthesisIt appears that lack of photosynthates is not the immediatecause of the decline of nitrogen-fixing activity after defoliation White clover, Trifolium repens L, defoliation, nitrogen fixation, regrowth, reserves, carbohydrates, acetylene reduction, nodule extract     

Acetylene-induced Changes in the Oxygen Diffusion Resistance and Nitrogenase Activity of Legume Root Nodules

Legume symbioses such as pea, lucerne and clover exhibit a substantialdecline in nodular respiration and nitrogenase activity whensubjected to gas streams containing C₂H₂. Assuming a constantrate of O₂ diffusion into the nodule this decrease in respiratoryO₂ consumption would lead to an increase in internal O₂ concentrationwhich would inactivate nitrogenase. No such inactivation hasbeen observed indicating that a change in diffusion resistanceis involved in the C₂H₂ response. Root nodules of C₂H₂ responsivesymbioses are distinguished by their tolerance to high (80 percent) O₂ levels. The nitrogenase of soya-bean and sainfoin,which do not respond to added C₂H₂, is denatured at O₂ levelsabove 40 per cent. Even in O₂-tolerant systems (e.g. pea) theenzyme is damaged if the increase in O₂ levels is rapid (ca.45 s) indicating that an induced change rather than a ‘permanent’feature is involved. The O₂-tolerant, C₂H₂-responsive behaviourpattern of white clover nodules was reversed by pulse feedingwith nitrate-N indicating that the overall C: N ratio in thenodules is involved in determining the response.     

Influence of Cadmium on Growth and Nitrogen Metabolism of Anabaena cylindrica Lemm

A study was made of the effects of cadmium on the Cyanobacterium(blue-green alga) Anabaena cylindrica Lemm. as part of the paddy-fieldecosystem. A simple culture vessel has been designed, which allows periodicalmeasurement of growth (optical density) and nitrogenase activity(C₂H₂-C₂H₄ method). The influence of medium renewal was checked:the renewal of the medium maintained a higher growth rate andhigher nitrogen fixation ability. The cadmium effects were studied using six concentration levelsranging from 0 (control) to 2 parts 10^–6 with renewedmedia (10% every day). No significant differences could be seen up to 1 part 10^–6for nitrogenase activity and relative percentage of heterocysts(decreasing as a function of time from ±4% to ±1.5%). Inhibition of growth (OD and dry weight) was weak at 1 part10^–6 but important at 2 parts 10^–6; at this concentrationcadmium induced morphological and physiological effects: chlorosis,cellular malformations and destruction, and increase in heterocystfrequency (up to 7.72% ±0.19). The cadmium concentration factors were much lower than thosereported for other plants like Chlorella and water pests     

Hydrogen evolution by a thermophilic blue-green alga Mastigocladus laminosus

The thermophilic blue-green alga (cyanobacterium), Mastigocladuslaminosus isolated from a hot spring, evolved hydrogen gas undernitrogen-starved conditions in light when algal cells were grownin a nitrate-free medium. Cells grown in a nitrate-medium evolvedno detectable hydrogen gas in light. The optimal temperatureand pH for hydrogen evolution were 44–49?C and 7.0–7.5.High activity of hydrogen evolution. 1.6 ml H₂/mg chl.hr, wasinduced when algal cells grown in the nitrate medium were activelyforming heterocysts in the nitrate-free medium in air. Hydrogenevolution in light was depressed by nitrogen gas and inhibitedby salicylaldoxime or DNP. This hydrogen evolution by M. laminosusis attributed to the action of nitrogenase. (Received June 20, 1979; )     

Oxygen Supply Limits Nitrogenase Activity of Clover Nodules After Defoliation

The aim of this study was to test the effect of oxygen partialpressure as a possible limiting factor of nitrogen fixationfollowing defoliation. The response of nitrogenase activity(C₂H₂-reduction) of defoliated and undefoliated white and redclover plants (Trifolium repens L. and Trifolium pratense L.)to either 19 kPa oxygen or 55 kPa oxygen was investigated. Priorto defoliation, white clover plants were grown for five weeksin a growth chamber, and red clover plants for 7 or 11 weeksin a glasshouse. The results included measurements of ¹⁶N₂-uptake. Increasing oxygen partial pressure from 19 to 55 kPa severelyrestricted nitrogenase activity of undefoliated white cloverplants; however, 2 h after complete defoliation, the same treatmentcaused a significant increase. A fivefold increase in nitrogenaseactivity upon exposure to the elevated oxygen partial pressurewas found at the end of a 24 h period. This beneficial effectdecreased gradually from 1 to 5 d after defoliation. The responseof recently defoliated red clover plants to 55 kPa oxygen partialpressure was similar to that of white clover, independentlyof plant age. The gradual recovery of nitrogenase activity duringthree weeks of regrowth was associated with a simultaneous changein the response to increased oxygen partial pressure, leadingagain to the response of undefoliated plants. These data suggested that lack of oxygen at the site of nitrogenfixation, resulting from a dramatic increase in oxygen-diffusionresistance, is the main factor limiting nitrogenase activityfollowing defoliation. Trifolium repens L., Trifolium pratense L., white clover, red clover, defoliation, regrowth, nodules, nitrogen fixation, nitrogenase activity, oxygen limitation     

Changes in Nitrogenase Activity and Nodule Diffusion Resistance of Subterranean Clover in Response to pO2

Diffusion resistance to oxygen within nodules was calculatedusing the respiratory quotient (RQ) of nodules from intact plantsof subterranean clover (Trifolium subterraneum L.) cv. SeatonPark nodulated by Rhizobiun trifolii WU95. From 21 to 52% O₂,the RQ remained between 0.94 and 1.04, whereas at 10% O₂, theRQ was 1.65. When nodulated roots of intact plants were exposedto sub-ambient pO₂ in a continuous flow-through system, respirationdeclined immediately, followed by a partial recovery within30 min. The magnitude of the final respiration rate was dependentupon the pO₂ in the gas stream. Initial rates of respirationwere re-established after 24 h at sub-ambient pO₂ as a resultof changes in the resistance of the variable barrier to oxygendiffusion within the nodules. Nitrogenase activity also decreasedlinearly with decreasing pO₂ in the gas stream, but partialrecovery occurred after 24 h incubation at sub-ambient pO₂.Maximum rates of nitrogenase activity occurred at rhizosphereoxygen concentrations between 21% and 36% O₂. Resistance tothe diffusion of oxygen within the nodules increased at supra-ambientpO₂ and at oxygen concentrations above 36% O₂, resulted in adecrease in both nitrogenase activity and nodulated root respiration.The diffusion resistance of nodules to oxygen increased rapidlyin the presence of either supra-ambient pO₂ or saturating pC₂H₂.Reductions in nodule diffusion resistance either during recoveryfrom exposure to 10% acetylene or to sub-ambient pO₂ occurredmore slowly. It is concluded that subterranean clover is welladapted for maximum nitrogen fixation at ambient pO₂. Key words: Nitrogenase activity, oxygen, subterranean clover, diffusion resistance     

Antagonism Between Malate and Ethylene in the Regulation of Avena sativa Coleoptile Elongation

Etiolated Avena sativa L. coleoptile sections were used to determinethe influence of C₂H₄ on in vivo and in vitro rates of CO₂ fixation,and to measure the influence of various permutations of C₂H₄,CO₂, and malate on growth. Whereas 1 mM malate or 320 µI^-1 CO₂ stimulated growth by approximately 100 per cent, inhibitionof growth by 10-8 µ I_-1 C₂H₄ was substantial only in thepresence of malate or CO₂ The increase in growth rate in responseto these two agents was eliminated by the simultaneous applicationof C₂H₄. The in vivo rate of dark [¹⁴C]bicarbonate fixationand in vitro enzymic assays of fixation were not measurablyinhibited by C₂H₄. These results are discussed in the lightof evidence which indicates that CO₂-stimulated growth is mediatedby dark fixation. The data do not support the view that C₂H₄inhibition of growth results from an inhibition of fixation,but suggests that C₂H₄ may inhibit some step in the processby which malate stimulates growth.     

Nitrogenase Activity in Response to Darkening and Defoliation of Alnus incana

Huss-Danell, K. and Sellstedt, A. 1985. Nitrogenase activityin response to darkening and defoliation of Alnus incana. —J.exp. Bot. 36: 1352–1358 In the Alnus-Frankia symbiosis the nitrogen-fixing root nodulesare one of the sinks for carbon compounds newly formed in photosynthesisand exported from the leaves (source). The source-sink ratioof cloned plants of Alnus incana was reduced by darkening orby total or partial defoliation and the resulting nitrogenaseactivity (C₂H₂-reduction) was measured. Nitrogenase activityhad nearly ceased 5 h after total defoliation but not untilca. 5 d after total darkening. Most of the activity was lostduring the initial hours and days, respectively. When leaf areawas reduced approximately by half nitrogenase activity decreasedslightly less than by half. Removal of upper leaves seemed lessharmful than removal of lower leaves one day after defoliation.On the following 2 d the treatments appeared to be similar.Thus, nitrogenase activity was largely dependent on newly formedassimilates but could also depend on stored reserves that weremobilized. Measurements of in vitro nitrogenase activity inroot nodule homogenates from darkened plants indicated thatnitrogenase gradually became inactivated and/or depleted after1 and 2 d in darkness Key words: Carbon supply, Frankia, nitrogen fixation     

Levels of endogenous ethylene, carbon dioxide, and soluble pectin, and activities of pectin methylesterase and polygalacturonase in ripening tomato fruits

In 4 cultivars of tomato (Lycopersicon esculentum Mill.), theearly detachment of fruits advanced ripening and considerablyreduced the threshold value of endogenous C₂H₄. This indicatesa supply from the vegetative parts of (a) labile ripening-inhibitingsubstance(s) antagonizing the action of C₂H₄. The endogenous level of CO₂ increased shortly after the risein C₂H₄, and maximum levels of C₂H₄ and CO₂ occurred almostsimultaneously. The activity of PE showed no connection with ripening, but PGactivity did not occur until the onset of ripening. However,this activity increased at considerably higher C₂H₄ concentrationsthan the rise in WSP, and was independent of the possible presenceof ripening inhibitor(s). Hence PG is considered not to be involvedin the primary events leading to fruit ripening. Exposure of fruits to different C₂H₄ concentrations in the ambientatmosphere also showed PG activity to increase only after therise in WSP had started. Other pectin degrading or synthesizingenzymes may be involved. In the non-ripening Rin mutant of cv. Rutgers, no rise occurredin C₂H₄, CO₂, WSP, and PG activity. ¹ Present address: Department of Agricultural Chemistry, Facultyof Agriculture, Kochi University, Otsu 200 Monobe, Nangoku City,Kochi Prefecture 783, Japan. (Received February 16, 1978; )     

Hydrogen Effect on Nitrogenase (C2H2 Reduction) Response to Oxygen in Bradyrhizobium japonicum-Phaseoleae Symbioses

The influence of hydrogenase in Bradyrizobum-Phaseoleae symbioseswas studied ex-planta and in-planra in soybean (Glycine max)and cowpea (Vigna unguiculata). The hydrogenase was activatedby the addition of hydrogen in the incubation gas phase whichmodified the response of nitrogenase activity of Hup⁺ (hydrogenuptake positive) symbiosis to the external oxygen partial pressure.For bacteroids the hydrogenase expression increased nitrogenaseactivity at supraoptimal pO₂, acting possibly as a respiratoryprotection of nitrogenase. However, at suboptimal pO₂, nitrogenaseactivity of Hup⁺ bacteroids decreased with hydrogen, a phenomenonattributed to the lower efficiency of ATP synthesis from hydrogenthan from carbon substrates oxidation. For undisturbed nodules,the hydrogenase expression in soybean increased the optimalpO₂ for ARA (COP), from 35.3 to 40.3 kPa O₂, and the ARA atsupraoptimal pO₂; at suboptimal PO₂ there was a negative effectof hydrogenase on ARA, although this inhibition was less thanon bacteroids and was not detected if plants were grown at 15°C rather than 20 °C root temperature. No H₂ effectwas detected on cowpea nodules. The results on soybean nodulesare consistent with the concept that symbiotic nitrogen fixationis oxygen-limited and that hydrogenase activity has no beneficialeffect on nitrogen fixation in O₂ limitation. Key words: Glycine max, hydrogenase, nitrogenase, nitrogen fixation, nodules, Vigna unguiculata     

Possible involvement of ethylene-activated {beta}-cyanoalanine synthase in the regulation of cocklebur seed germination

A possible involvement of ß-cyanoalanine synthase(CAS: EC 4.4.1.9 [EC] ) in germination processes of seeds was demonstratedusing pre-soaked upper seeds of cocklebur (Xanthium pennsylvanicumWallr.). Pretreatment in anoxia not only with KCN but also cysteine,as the substrates for CAS, stimulated the subsequent germinationof cocklebur seeds in air. However, the effect of cysteine wasmanifested even in air when applied together with C₂H₄, andits effect was further enhanced in combination with KCN. Thegermination-stimulating effect of KCN was intensified by C₂H₄only when 0₂ was present. In contrast, serine, another substrateof CAS, was effective in air only when combined with C₂H₄ and/orKCN. The addition of cysteine greatly reduced the cyanogenicglycoside content of seeds, but increased HCN evolution. Onthe other hand, glutathione did not have any effect on cockleburseed germination, HCN evolution or bound cyanogen content, suggestingthat cysteine is not acting as a reducing reagent. It is suggestedthat CAS regulates the process of cocklebur seed germinationby the dual action of enlarging the pool of amino acids andsupplying sulphydryl bases, the latter being more determinatelyimportant. Serine is effective only via the former action, whilecysteine would act via both. Key words: Cyanide, cyanogenic glycoside, ß-cyanoalanine synthase, seed germination, Xanthium pennsylvanicum     

Effect of Denitrification on Nitrogen Fixation in Light in a Photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

The effect of denitrification on N₂ fixation was studied ina denitrifying photosynthetic bacterium, Rhodopseudomonas sphaeroidesforma sp. denitrificans. KNO₃ inhibited diazotrophic growthin light, but NC₃–-dependent diazotrophic growth in darknesswas found. NO₃– inhibited C₂H₂ reduction activity in lightin cells grown with NO₃–. NO₃–-dependent C₂H₂ reductionactivity in darkness also was present in cells grown with N₂plus NO₃–, but not in cells grown on glutamate with NO₃–.NO₃– repressed the synthesis of nitrogenase in light.This repression was not overcome by the addition of methioninesulfoximine. The inhibitory and repressive effect of NO₃– was not mediatedby the NO₂– produced from NC₃– nor by the NH₄+ excretedinto the medium. But, as N₂ fixation is controlled by O₂ (redoxcontrol) it seems to be mediated through the denitrificationprocesses. Much of the glutamine synthetase was adenylylatedin cells grown with NO₃– and its adenylylation state closelyparallelled nitrogenase activity in the cells. (Received September 4, 1982; Accepted December 11, 1982)     

Role of uptake hydrogenase in providing reductant for nitrogenase in Rhizobium leguminosarum bacteroids

The role of uptake hydrogenase in providing reducing power to nitrogenase was investigated in Rhizobium leguminosarum bacteroids from nodules of Pisum sativum L. (cv. Homesteader). H₂ increased the rate of C₂H₂ reduction in the absence of added substrates. Malate also increased nitrogenase (C₂H₂) activity while decreasing the effect of H₂. At exogenous malate concentrations above 0.05 mM no effect of H₂ was seen. Malate appeared to be more important as a source of reductant than of ATP. When iodoacetate was used to minimize the contribution of endogenous substrates to nitrogenase activity in an isolate in which H₂ uptake was not coupled to ATP formation, H₂ increased the rate of C₂H₂ reduction by 77%. In the presence of iodoacetate, an ATP-generating system did not enhance C₂H₂ reduction, but when H₂ was also included, the rate of C₂H₂ reduction was increased by 280% over that with the ATP-generating system alone. The data suggest that, under conditions of substrate starvation, the uptake hydrogenase in R. leguminosarum could provide reductant as well as ATP in an isolate in which the H₂ uptake is coupled to ATP formation, to the nitrogenase complex.     

Induction of Ferric Reductase Activity and of Iron Uptake Capacity in Chlorococcum littorale Cells under Extremely High-CO2 and Iron-Deficient Conditions

The marine green alga, Chlorococcum littorale, accumulated ironin its cells and showed high activity of plasma membrane ferricreductase under high-CO₂ and iron-deficient conditions. Theseactivities disappeared upon exposure to ordinary air and byadding excess FeSO₄. The iron uptake had high affinity for theFe(II) form (K_m of 0.13 µM). Carbonylcyanide m-chlorophenylhydrazoneand N,N-dicyclohexylcarbodiimide significantly suppressed theiron uptake, suggesting that the Fe(II) uptake was driven byATPase. These results indicate that high CO₂ and iron deficiencycooperatively induce the Fe(II) uptake and cell-surface ferricreductase activity. (Received October 20, 1997; Accepted January 29, 1998)     

Phosphorus Deficiency Impairs Early Nodule Functioning and Enhances Proton Release in Roots of Medicago truncatula L.

The effect of phosphorus supply (1–15 µM) on protonrelease and the role of P in symbiotic nitrogen fixation inmedic (Medicago truncatula L. ‘Jemalong’) was investigated.As P concentration in the nutrient solution increased, shootand root growth increased by 19 and 15%, respectively by day35, with maximal growth at 4 µM P. A P concentration of15 µM appeared to be toxic to plants. Phosphorus supplyhad no influence on nodule formation by day 12 but increasednodule number by day 35. Nitrogenase activity was estimatedby in situ measurement of acetylene reduction activity (ARA)in an open-flow system. During the assay, a C₂H₂-induced declineof ARA was observed under all P concentrations except 4 µM.Specific ARA (per unit nodule weight) doubled when P supplywas increased from 1 to 8 µM. This effect of P was muchgreater than the effects of P on nodulation and host plant growth.Concentrations of excess cations in plants decreased with increasingP concentration in the nutrient solution. Phosphorus deficiencystimulated uptake of excess cations over anions by the plantsand hence enhanced proton release. The results suggest thatP plays a direct role in nodule functioning in medic and thatP deficiency increases acidification which may facilitate Pacquisition. Copyright 2001 Annals of Botany Company Medicago truncatula L. (medic), P deficiency, C₂H₂-ID, nitrogenase activity, proton release, cation-anion balance, open-flow system     

Interrelation between Ethylene and Carbon Dioxide in Relation to Respiration and Adenylate Content in the Pre-Germination Period of Cocklebur Seeds

Effects of C₂H₄ and CO₂ on respiration of pre-soaked upper cocklebur(Xanthium pennsylvanicum Wallr.) seeds during a pre-germinationperiod were examined in relation to effects of the two gaseson germination. At 33?C, cocklebur seed germination was greatlystimulated. This high temperature-stimulated germination wasseverely inhibited by C₂H₄, but not by CO₂, although both gasesstimulated germination at 23?C. C₂H₄ promoted seed respirationat 23?C, but its promotive effect decreases with increasingtemperature and disappeared at about 35?C, while CO₂ stimulatedrespiration regardless of temperature. CO₂ augmented the operationof the CN-sensitive, cytochrome path (CP) regardless of temperature,resulting in an increase in the ratio of the CP flux to a CN-resistant,alternative path (AP) flux. On the other hand, C₂H₄ augmentedthe operation of both paths, particularly of the AP, at 23?C,where it promoted germination. However, at 33?C where germinationis suppressed by C₂H₄, C₂H₄ preferentially stimulated respirationvia the AP, thus leading to an extremely high ratio of AP toCP. The inhibitory effect of C₂H₄ on germination at 33?C disappearedcompletely in enriched O₂, under which conditions CP is knownto be augmented. At 23?C, CO₂ and C₂H₄ acted independently incontrolling seed respiration, but they were antagonistic at33?C. The independent action appeared when the AP flux was verylow relative to the CP flux, while the antagonism appeared whenthe AP flux had risen. This differential action of the two gasesat different temperatures was also observed in the ATP level,adenylate pool size and energy charge of the axial tissues.These results suggest that the germination-controlling actionsof both CO₂ and C₂H₄ are fundamentally manifested through themodification of respiratory systems. However, the germination-inhibitingeffect of C₂H₄ at 33 ?C was not removed by inhibitors of AP,and there was little difference in the adenylate compounds betweenthe C₂H₄-treated and non-treated seeds at 33?C. Therefore, thephysiological action of C₂H₄ can not be explained only in termsof regulation of the respiratory system. (Received January 24, 1986; Accepted November 17, 1986)     

Chlorophyll Fluorescence and Organic Acid Oscillations during Transition from CAM to C3-photosynthesis inClusia minor L. (Clusiaceae)

In species of Clusia, switching from C₃-photosynthesis (C₃-PS)to crassulacean acid metabolism (CAM) may be a means of optimizingwater use, plant carbon balance and photon utilization duringperiods of stress. We ask whether, in perennial species of Clusia,the switch from CAM back to C₃-PS is also of ecophysiologicalsignificance. Our objective was to investigate the performanceof C. minor L. during a short-term shift from CAM to C₃-PS.During the transition from CAM to C₃-PS, nocturnal malate andcitrate accumulation decreased whereas CO₂uptake increased duringthe daytime. However, after 7 d, marked nocturnal accumulationof citrate and 24 h CO₂uptake occurred. In contrast to C₃-likephotosynthesis, a pronounced reduction in the effective quantumyield of photosystem II,