Carbon Dioxide Effects on Fruit Respiration '.II. Response of Avocados, Bananas, & LemonsRoy E. Young, Roger J. Romani, & Jacob B. Biale

Department of Plant Biochemistry, University of California, Los Angeles

,Most studies concerning the effects of the gas

composition of the atmosphere surrounding fruit havebeen directed toward the response of the combinedchanges of oxygen an(d carbon dioxide. This was a

result of the economic need for methods of prolongingthe storage life by the use of modified atmospheres attemperatures above those which cause chilling injury.Several studies have been concerned with the effectsof decreased oxygen alone. Parija (15) obtainedminimum carbon dioxide prodluction by apples at 5 %oxygen. Kidd and WVest (12) found an accelerationof the onset of the climacteric in apples by oxygen

tensions higher than in air. Singh et al. (19) re-

ported for mangos a critical oxygen level of 9.2 %below and above which carbon dioxide evolution in-creased. The studies of Claypool and Allen (6, 7)with apricots, plums, peaches, and pears indicated a

decreased rate of CO., output at oxygen tensions be-low air. For a more comprehensive treatment of thissubject the recent review by Biale (3) is suggested.

For the avocado, a fruit which exhibits a climac-teric pattern of respiration, it was shown by Biale(2) that low oxygen reduced the respiratory activityduring the preclimacteric period andl the (luration ofthe preclimacteric period was prolonged. Within therange of 2.5 to 21 % oxygen the time required toreach the climacteric peak was extended in proportionto the decrease in oxygen tension and the intensity ofrespiration at the peak was reduced. No significantstimulation of respiration wvas observed by concentra-tions of oxygen above 35 %. However, the cumula-tive carbon dioxide production from the time of pick-ing until the climacteric peak was not changed bythe treatment at any level of oxygen. In the case

of the banana, another fruit with a climacteric pat-tern, it was found by Kidd and West ( 11 ) that storagein 2.5 and 5.0 % oxygen did not materially decreasethe rate of ripening. On the other hand, Leonard(13) observed a reduction in CO, liberation by fruitstored in oxygen concentrations lower than air andno effect in concentrations higher than air.

Lemons did not exhibit the climacteric pattern ofrespiration after picking when stored in air or atoxygen tensions below that of air, as shown in thesttudies of Biale and Young (5). The respiratory

1 Received revised manuscript Dec. 20, 1961.

activity under air decreased slightly during storage.Reduction of oxygen in the atmosphere surroundingthe fuit reduced the rate of respiration in proportionto the oxygen concentration in the range of 21 to5 % oxygen. Carbon dioxide evolution increased atoxygen levels below 5 %, indicating the similarity ofthe behavior of lemons with other fruits characterizedby a critical oxygen concentration. The storage lifewas extended and the (lecomposition of chlorophyllin mature green lemons was delayed by loweredoxygen.

The effect of added carbon dioxide on respiratorvactivity of fruits at a particular oxygen concentrationhas been studied little, largely due to technical diffi-culties. Limited data are available on the banana.By the use of the katharometer method Gane (9) ob-served a suppression of the climacteric and reductionof respiratory activity in an atmosphere of 10 %carbon dioxide and 10 % oxygen. Wardlaw (20)subjected unripe bananas to clifferent combinations ofoxygen and carbon dioxide. On the basis of gasanalysis he concluded that there was a 50 X redluctionin rate of respiration over a wide range of 02 andCO2 concentrations as compared with air. His (le-terminations were limited to green fruit andiweredone in a closed system in which the gaseous composi-tion could not be kept constant. The use of analyticalmethods described in the first paper (21) of this serieshas enabled us to study the effect of carbon dioxideat several oxygen levels on the respiratory activityand storage behavior. The responses to CO., ofavocados, lemlons and bananas are reported at thistime.

Materials & MethodsAvocados of the Fuerte variety were picke(d in the

orchard of the Plant Biochemistry department at theUniversity of California, Los Angeles, or were ob-tained from commercial orchards in southern Californ-ia. Fruit was always picked in the morning and putunder treatment on the same day. Stems were cutclose to the fruit and those with detached stemls werenot used. Fruit was selected for uniformity of sizeand 10 to 25 specimens were used per treatment.

Bananas of the Gros Michel variety grown inCentral America were obtained with the cooperationof the Consolidated Fruit Co. Fruit was picked inthe three-quarter full stage, shipped to Los Angelesby boat, and were still dark green when received.

416

YOUNG ET AL.-C04 & FRUIT RESPIRATION

The transit period was normally 12 days, duringwhich the fruit was subjected to a temperature ofabout 15 C and a relative humidity of about 90 %.Single bananas were separated from the hand and15 to 20 bananas were tested per treatment. Theywere treated within one day after arrival of the boat.

Lemons were obtained from commercial packinghouses in Los Angeles County. They were pickedon the basis of size rather than color, and dark greenfruit was selected for all experiments. Ordinarilyone to two days elapsed between picking and the startof treatment. Fruit was washed, waxed, and sizedat the packing house. In one experiment fruit waxedwith water wax containing 2,4-D was used. To checkthe effect of the 2,4-D treatment a sample of the samepick as used in the experiment was removed from thebelt ahead of the waxer. Respiratory activity andcolor changes were unaffected by the 2,4-D wax treat-ment. Button retention was somewhat better after2,4-D treatment. Composite samples of 30 to 50fruit, uniform as to size and color, were selected foreach treatment.

Fruit was put in respiration jars described byBiale and Shepherd (4), and gas mixtures passedcontinuously as described in paper I of this series(21). Respiratory activity was measured as ml oxy-gen absorbed per kg fresh weight per hour. Oxygenuptake was measured on the Model G-2 oxygen ana-lyzer as described previously (21). Oxygen uptakemeasurements were made every 2 to 8 hours duringthe experimental period, depending upon the numberof experimental samples attached to the automaticsampling system. Where daily values are reported,the average of not less than four separate measure-ments is given.

For the chemical analysis of the lemons, threerandom samples of seven fruit each were taken fromeach treatment. Juice of seven was pooled for analy-sis and the average of the three aliquots is reported.Ascorbic acid was determined by the method ofRamsey and Colichman (17).

Results- Fuerte Avocado. Fruit used for the experimental

data given in figures 1 and 2 was picked on May 20and kept at 15 C throughout the experiment. Figure1 shows a typical climacteric curve of respiration ofavocados under air. The fruit remained hard duringthe preclimacteric period, which was 4 days for thisexperiment, started to soften as the respiratory activi-ty increased, and became soft and edible about twodays after the peak of respiratory activity. The ef-fect on respiration of adding 5 and 10 % carbondioxide to air is also shown in figure 1. As theaddition of carbon dioxide reduces the oxygen con-centration by dilution, sufficient oxygen was addedto these mixtures to maintain the oxygen level at21 % in all cases. When 21 % oxygen with 5 %carbon dioxide was used, the preclimacteric rate ofoxygen uptake was affected only very slightly andthe start of the climacteric rise was delayed by only

about three days. Thereafter there was a gradualrise to a peak of respiration which was 40 % belowthe air control and was reached after 21 days com-pared to 8 days for the air treatment. The peak wasfollowed by a decline and on the 23rd day the fruitwere transferred to air. There was no definite in-crease in oxygen uptake after the transfer, indicatingthat the climacteric had really occurred and thesefruit softened in the normal manner. When fruit wastreated with a gas mixture containing 10 % carbondioxide and 21 % oxygen, there was essentially nochange in respiratory activity for 23 days and therate was the same as that of the initial value of thecontrol. Upon transfer to air there was an increasein respiration, indicating that the climacteric had notoccurred under the carbon dioxide treatment but hadoccurred after transfer to air when the fruit softenednormally.

The effect of increased carbon dioxide at the 10 %oxygen level on the respiratory trend of avocados isillustrated in figure 2. Fruit for this experiment wasfrom the same lot as that used for the experiment infigure 1. Treatment of 10 % oxygen alone reducedthe climacteric peak value to about 65 ml/kg/hr ascompared to 85 ml/kg/hr for air, but delayed the peakonly very slightly. When 5 % carbon dioxide with10 % oxygen was used, there was again a very grad-ual rise in respiratory activity to a peak value of 48ml/kg/hr on the 21st day, which was the same as thepeak value for 21 % 02 with 5 % CO2. The rise aftertransfer to air was probably due to the fact that onefruit was infected with a fungal rot. It should benoted that the rise in respiration started only abouttwo days after the 10 % oxygen control, but the peakwas delayed from 9 days for the control to 21 daysfor the carbon dioxide treatment. Again 10 % car-bon dioxide in 10 % oxygen produced no change fromthe preclimacteric minimum of the control and showedno change in respiration up to the 23rd day. Ripen-ing was normal after transfer to air. Fruit under alltreatments was of good flavor, according to informaltaste panels at the end of the storage period. Therewas some decay, particularly at the stem end of fruitsstored under carbon dioxide. We believe this to bea function of the length of storage rather than dueto the carbon dioxide treatment itself.

In figure 3 the maximum values for all treatmentsare compared. While the 10 % oxygen treatmentreduced oxygen uptake at the climacteric compared tothe air control, the peak values for 21 % oxygen with5 % carbon dioxide and 10 % oxygen with 5 %carbon dioxide were about the same. Values at the10 % carbon dioxide level do not represent climactericpeak values, and it is not known whether a climactericwould occur or not if more time had been allowed.It would appear, though, that when 5 % carbondioxide was used it was the carbon dioxide whichlimited the reaction rate.

In other experiments, 5, 10, and 15 % carbondioxide have been used with 5 % oxygen, and satis-factory ripening was obtained after 45 days' treatment

417

PLANT PHYSIOLOGY

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YOUNG ET AL.-C02 & FRUIT RESPIRATION

at 15 C when early season fruit were used. Fifteenper cent carbon dioxide appeared to produce someinjury to the fruit.> Bananas. The banana is another fruit which ex-hibits the climacteric pattern of respiration associatedwith ripening. The results of respiratory measure-ments of fruit stored in air, 21 % oxygen with 5 %carbon dioxide, and 21 % oxygen with 10 % carbondioxide at 15 C are shown in figure 4. Here wesee that under all three conditions there was a definteclimacteric, that the peak values of oxygen uptakewere essentially the same, and that the shapes of thecurves of oxygen uptake are similar. While thefruit under 21 % oxygen with 10 % carbon dioxideactually showed a slightly lower peak, the slope ofthe increase was lower and more irregular. We be-lieve this to be an indication that the fruit ripenedless uniformly. If only a few fruit start to increasetheir respiration on particular days, it would beexpected that the curve would be broader and moreirregular. The change in color from green to yellowwhich is coincident with the climacteric was less uni-form for this treatment. The important point to noteis that the only essential difference between the threetreatments is the time of onset of the climacteric andthe day that the climacteric peak occurred. The aircontrol treatment reached the peak on the 14th day,21 % oxygen with 5 % carbon dioxide on the 19thday, and 21 % oxygen with 10 % carbon dioxide after27 days.

In figure 5 is shown the results of treatment with5 and 10 % carbon dioxide added at the 10 % oxygenlevel. Here again peak values at 10 % oxygen andat 10 % oxygen with 5 % carbon dioxide are essen-tially the same as the air treatment, but the 10 %oxygen treatment required 30 days to reach the peakwhile 10 % oxygen with 5 % carbon dioxide required47 days. Fruit treated with 10 % oxygen with 10 %carbon dioxide showed no increase for 50 days andwere still dark green. Upon transfer to air, thesefruits ripened normally and were of good flavor andquality. We have not established whether bananasmaintained under this treatment for more than 50days would undergo a climacteric, but there appearedto be no injury at least for this period. These datashow that avocados and bananas with typical climac-teric patterns of respiration responded to modifiedatmospheres in different ways. For bananas, bothdecreased oxygen and increased carbon dioxidle caused

a delay in the time of onset of the climacteric, butonce the climacteric was induced the treatment seemedto have no effect on the rate of respiration. On theother hand, the rate of respiration of avocados duringthe climacteric was affected by the same treatments.

- Lemons. It has been shown by Biale and Young(5) that lemons did not exhibit a climacteric patternof respiration, at least at the oxygen tension of airor below. All other citrus fruits which have beenexamined behaved in a similar manner. The respira-tory activity declined steadily from picking until theonset of decay by fungi. The respiratory quotient(RQ) of lemons is just a little less than one andchanges only slightly with oxygen tension between2.5 and 100 % oxygen (5), so that either oxygenuptake or carbon dioxide production may be used tomeasure respiratory activity.

Earlier experiments (5) had shown that 5 %oxygen was a desirable oxygen concentration for thestorage of lemons. In our first experiments we used5 % oxygen with 0, 5, and 10 % carbon dioxide at20 C. Respiratory activity of these samples was es-sentially constant after the first 2 weeks of storage.Average values for oxygen uptake of these treatmentsare shown in table I. While 5 % oxygen did bringabout a decrease in respiratory activity, the additionof 5 % carbon dioxide caused little further decrease,while at the 10 % CO, level a slight increase inrespiratory activity was noticed.

Similar experiments at the 10 % oxygen levelwere carried out at 20 C. One air treatment wasalways used as a check on the reproducibility of be-havior of the fruit. Curves of the oxygen uptake are

Table IRespiration of Lemons in Relation to

Carbon Dioxide Tension

ml 02 per kg-hour

5% O., 5% O2Week Air 5 % O., + +

5% CO2 10 % CO2First 4.7 1.8 1.6 2.0Second 4.5 1.7 1.5 2.0Fourth 4.7 1.7 1.6 2.1Sixth 4.8 2.5 2.0 2.4Eighth 4.4 2.9 2.2 3.0

Fig. 1. Oxygen uptake by Fuerte avocados in air and in 21 % 02 with 5 and 10 % CO2 at 15 C. Arrow indicatestransfer from CO2 treatment to air.

Fig. 2. Oxygen uptake by Fuerte avocados at 15 C in 10 % 09 and in 10 % O., with 5 and 10 % CO, added.Arrow indicates transfer from CO2 treatment to air.

Fig. 3. Maximal respiratory rates of Fuerte avocados in relation to 02 and CO, concentration.Fig. 4. Oxygen uptake by bananas at 15 C in air and in 21 % 02 with 5 and 10 % CO.Fig. 5. Oxygen uptake by bananas at 15 C in air, 10 % O2, and in 10 % 02 with 5 and 10 % CO,Fig. 6. Oxygen uptake by lemons at 20 C in relation to 02 and CO2 concentration.Fig. 7. Oxygen uptake of lemons at 20 C under air anl 21 % 02 with 5 and 10 % CO,.Fig. 8. Oxygen uptake by lemons at 20 C in relation to CO2 and ethylene treatment.

419

PLANT PHYSIOLOGY

shown in figure 6. During the first 6 weeks of theexperiment the respiratory activity under both treat-ments was markedly lower than in the air control.After 3 weeks, the rate of respiration in the 10 %oxygen treatment leveled off while the activity of thefruit under 10 % 0, with 10 % CO, actually in-creased slightly. At 8 weeks, the respiration of theCO2 treated lot exceeded that of the control fruitand remained higher for the final seven weeks of theexperiment.

Treatment in atmospheres containing 21 % oxy-

gen with 5 % carbon dioxide and 21 % oxygen with10 % carbon dioxide showed much more strikingstimulation of respiration (fig 7). For the first 3weeks oxygen uptake of all samples decreased. Thenwhile the rate of respiration in air continued to de-cline slowly for the next 21 weeks, both the 5 and10 % carbon dioxide treatments brought about an

increase in respiratory activity, reaching peak valuesat about nine weeks and declining thereafter, but al-ways remaining above the air control. At the 10thweek, the oxygen uptake in the 5 % carbon dioxidetreatment was about 31 % above the air control andthe 10% carbon dioxide treatment was 50 % abovethe air control.

Denny (8) demonstrate(d that ethylene increasedthe respiratory activity as well as hastened the de-composition of chlorophyll in the rind of maturegreen lemons and accelerated the onset of decay. Infigure 8 is illustrated the effect of adding 10 ppm ofethylene to air and to 21 % oxygen with 5 % carbondioxide. The addition of ethylene to the air treat-ment nearly doubled the respiratory activity, but addi-tion of ethylene to the 21 % oxygen with 5 % carbondioxide treatment caused an additional small but sig-nificant stimulation. It is not possible to pick com-

parable time periods to calculate the exact percentagestimulation. The stimulation by both ethylene andCO2 is greater than that caused by either carbondioxide or ethylene separately. It would appear thatthe stimulation of respiration by ethylene is broughtabout by a different mechanism than the stimulationdue to carbon dioxide.

On the basis of the relationship establishedl be-tween respiratory activity and storage life as meas-

ured by the occurrence of physiological breakdown,invasion of fungal decay, color changes, and abscissionof the calyx (5), one would expect that the carbondioxide enhanced respiration would shorten the stor-age life, hasten chlorophyll decomposition in the rindof mature green lemons, and accelerate abscission ofthe calyx. Actually, however, there was a markeddelay rather than acceleration of decomposition ofchlorophyll, as is shown in table II, and of abscissionof the calyx. Observations recorded in this tablewere made after 10 weeks of storage when the bestcolor contrasts occurred. In all three experimentsthere was a definite delay in chlorophyll decomposi-tion due to the CO2 treatment. It was true, however,that there was more variability under the carbondioxide treatment than under air or 10 % 02. This

Table IIColor Transitions of Lemons in Relation to

Carbon Dioxide Treatment

Gas mixture % in Color class

Exp. 0,/CO2 Yellow Silver Light Darkgreen green

A 21/0 90 10 0 05/0 0 40 37 235/5 0 3 87 105/10 0 0 79 21

B 21/0 100 0 0 010/0 0 100 0 010/10 35 28 33 4

C 21/0 26 70 4 021/5 0 90 10 021/10 0 76 20 3

was particularly noticeable in experiment C for 10 %carbon dioxide with 10 % oxygen, in which case 35 %of the fruit were yellow and 4 % were still darkgreen, while under 10 % oxygen with no carbondioxide all fruit were silver.

Abscission or drying of the calyx or button oflemons is used by the citrus industry as an indicationof the keeping quality of a sample of fruit. In theexperiments listed in table II the buttons were greenand normal after 10 weeks storage for all treatments.However, fruit under the same oxygen and CO2 treat-ments but in the presence of 10 ppm ethylene wasyellow after 10 weeks storage. The buttons of thefruit kept in air with ethylene were all dry or hadabscised. Approximately 50 % of the buttons wiltedor abscised after 10 weeks on fruit treated with ethyl-ene in the presence of 10 % oxygen or in 21 % 02combined with 5 % CO2. When ethylene was addedto 21 or 10 % oxygen with 10 % CO, buttons of es-sentially all fruit remained healthy for the 10 weekperiod.

Lemons used in experiment C were analyzed fortotal soluble solids (sugars), total titratable acidity(as citric acid), and for ascorbic acid at the time the

Table IIIChemical Analysis of Lemons Before & After Storage

Initial Final

%O2/% CO2 2110 21/5 21/10 10/5 10/10Brix 9.6 7.7 7.0 6.8 7.2 6.7Acid 5.58 5.38 4.63 4.42 4.15 3.39Ascorbic acid 67.1 45.6 40.8 34.4 34.5 30.2

Brix refers to total soluble solids on Brix hydrometerscale.

Acid expressed as mg citric acid per 100 ml juice.Ascorbic acid as mg per 100 ml juice.21 % oxygen levels stored for 28 weeks.10 % oxygen levels stored for 41 weeks.

420

YOUNG ET AL.-C02 & FRUIT RESPIRATION

experiment was started, and again as the fruit was

approaching the end of their storage life. Data forthese analyses are shown in table III. Samples oflemons held under 21 % oxygen and under 21 %oxygen combined with 0, 5, and 10 % carbon dioxidewere analyzed after 28 weeks of storage. Sugars,acids, and ascorbic acid all decreased as the carbondioxide concentration was increased. Data for 10 %oxygen treatment were not available for this experi-ment, but these same three analyses yielded lowervalues for 10 % oxygen with 10 % carbon dioxidethan for the 5 % carbon dioxide treatment at 10 %oxygen when analyzed after 41 weeks. It appears,

therefore, that carbon dioxide-stimulated respirationdoes bring about significant decreases in sugar, acid,and ascorbic acid in the juice of lemons.

Determinations were also made of the volume ofextractable juice, fresh and dry weight of the rind,thickness of the rind, and change in total weight ofthe fruit. There was no correlation between thesemeasurements and the treatments.

Discussion

While most fruits respond to reduced oxygen inthe atmosphere by reduced respiration and a delay ininduction of the climacteric, these data show thatavocados, bananas, and lemons each reacted in differ-ent ways to the addition of carbon dioxide to theatmosphere surrounding the fruit. Carbon dioxidedepressed the rate of respiration in avocados, delayedthe induction of the climacteric without affecting therate of respiration in bananas, and stimulated therespiratory rate of lemons.

Carbon dioxide appeared to have no effect on therate of respiration of either avocados or bananas priorto the induction of the climacteric rise. Avocadostreated with 5 % carbon dioxide at either 21 or 10 %oxygen showed a delay in the onset of the respiratoryrise of not more than 2 or 3 days, but the peak activityoccurred 12 days after the controls. Five per centcarbon dioxide with 21 % oxygen reduced the peakactivity by 40 %, and at the 10 % oxygen level 5 %carbon dioxide reduced the respiratory rate at thepeak by 25 %. The actual value of oxygen uptake atthe climacteric peak was the same under both treat-ments.

These data are consistent with the idea that when10 % oxygen is used with no carbon dioxide added,the rate of respiration is limited by availability ofoxygen as an electron acceptor. Experiments dealingwith oxygen effects alone reported earlier (2) showedthat the rate of respiratory activity of avocados was

not changed materially by increasing the concentra-tion of 02 from that in air to 100 %. It is reasonable,therefore, to suggest that when CO2 is added to 21 %oxygen the rate of oxidation is not limited by avail-ability of oxygen as an electron acceptor. Conceiv-ably during the climacteric and not prior to its onset,a different metabolic pathway is superimposed on thebasic pathway. This might be accounted for by a

CO2 induced inhibitor of the new pathway such as

malonate, the inhibitor of the citric acid cycle. Alter-natively, the partial reversal of a decarboxylation re-action necessary to permit further dehydrogenaseactivity could limit the availability of electrons to theelectron transport system. In avocados and bananasunder 10 % CO2 there was no evidence for a climac-teric and we presume that here the new pathway iscompletely suppressed. If avocados are held muchlonger than 23 days at 15 C under 21 % oxygen and10 % CO2, physiological breakdown occurs whichsuggests that some inhibitor or product has accumu-lated to the point that normal metabolism can nolonger take place.

The rate of respiration of bananas is not affectedby the addition of 5 or 10 % carbon dioxide eitherbefore or after the climacteric has been induced, butonly the time of induction is affected. Low oxygenalso appears to delay the induction of the climacteric,but in addition it reduces the pre-climacteric oxygenuptake. The effect of low oxygen and increased car-bon dioxide appears to be synergistic in delaying theinduction of the climacteric. Five per cent carbondioxide in air delays the climacteric peak by 5 days.Ten per cent oxygen causes a delay of 16 days. Thecombination of 10 % oxygen and 5 % carbon dioxidebrings about a climacteric peak after 34 days, or 13days later than the sum of the delays caused by eitherreduced oxygen alone or increased carbon dioxidealone.

Evidence of Hulme (10), Pearson and Robertson(16), and of Rowan et al. (18) indicate that in threedifferent fruits, the apple, tomato, and avocado, theclimacteric is accompanied by formation of more pro-tein, which suggests the possibility of synthesis ofnew enzymes and the introduction of new metabolicpathways. Some support for this contention is sup-plied in the report by Allentoff (1) in which C1402fixation into amino acid residues of protein of de-tached apples was demonstrated. Specific activityof four amino acids in the bound and free states wasmeasured. Two had much higher and the other two,lower specific activity in the protein than in the freeform. If this is true for the banana, reduced oxygenmight be expected to delay the induction of the cli-macteric by decreasing the available ATP for syn-thesis, and increased carbon dioxide might delay theformation of a specific amino acid necessary for syn-thesis of a specific enzyme or delay decomposition ofan inhibitor of either new enzyme synthesis or anotherpathway. Although the above mechanisms are com-pletely speculative, the divergent responses to CO2suggest that the climacteric in the banana and avocadorepresent different biochemical phenomena in contrastto the general assumption that the climacteric has as

its basis a single specific reaction common to allfruits which exhibit this phenomenon.

Neal and Hulme (14) have shown that apple skinslices produce carbon dioxide when fed malic acidwithout equivalent oxygen uptake. He suggests thatthe climacteric in apples may be due to this malic de-carboxylase. If this malate decarboxylase is respon-sible for the climacteric in apples as shown by carbon

421

PLANT PHYSIOLOGY

dioxide prodluction, no climacteric should appear byoxygen uptake measuremiients and this would repre-sent a third type of climacteric.

Lemons do not exhibit a climacteric at the oxygentension of air or below. WVhen subjected to addedcarbon dioxide, the curve of respiratory activity ap-pears somewhat like a cliniiacteric but actually is quitedifferent from the rise which normally occurs in avo-cados and bananas. All fruits whiclh exhibit a cli-macteric can be induced to a climacteric by treatmentwith low concentrations of ethylene, but once theclimacteric is induced ethylene has no additional ef-fect. Lemons, on the other hand, which are inducedto a stimulated respiration by carbon (lioxide arefurther stimulated by treatment with ethylene, and theethylene stimulated respiration persists for the dura-tion of the treatment. It would appear that these twostimuli act at different sites of the metabolic pathway.It will be shown in paper three of this series thatcarbon dlioxide probably acts in the lemon by ac-celerating the turnover of a rate limiting step in themetabolic pathway rather than in(luction of a newpathway.

Summary

Avocados, bananas, and lemons were subjected to0, 5, and 10 % carbon (lioxide at 5, 10, and 21 %oxygen.

Carbon dioxide delayedl the onset of the respira-tory rise in the avocado, reduced the rate of oxygenuptake at the climacteric peak and prolonged storagelife.

In the banana the induction of the cliniacteric waspostponed but the rate of respiration at the peak wasunaffected by CO., treatment xvhenever the peak oc-curred; the suppression of the climacteric pattern andthe extension of storage life was particularly pro-nounced in an atmosphere of 10 % carbon dioxidecombined with 10 % oxygen.

Carbon dioxide cause(l an unprecedeinted stimula-tion of respiration in lemlons. This phenomenon wasmore striking with 10 than with 5 % CO2 and morein combinations of carbon dioxide with air than withoxygen concentrations below air. There appearedto be an increased utilization of sugars and acids asa result of carbon dioxidle treatment. In general, thelife of lemons was prolonged by carbon dioxide, espe-cially at the lower levels of oxygen.

Literature Cited1. ALLENTOFF, N., W. R. PHILLIPS, & F. B. JOHNSTON.

1954. A C; study of carbon dioxide fixation inthe apple. I. The distribution of incorporated C14in the (letache(l MIcIntosh apple. J. Sci. FoodAgric. 5: 231-234.

2. BIALE, J. B. 1946. Effect of oxygen concentrationon respiration of the fuerte avocado fruit. Am.J. Botany 33: 363-373.

3. BIALE, J. B. 1960. Respiration of fruits. Hand-huch der Pflanzenphysiologie 12, Part II: 536-592.Julius Springer, Heiclelberg.

4. BIALE, J. B. & A. D. SHEPHERD. 1941. Respirationof citrus fruits in relation to the metabolism offungi. I. Effects of emanations of Penicillium digi-tatut;t, Sacc. on lemons. Am. J. Botany 28: 263-270.

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