Further Evidence Diffusion the ... - Plant Physiology · PLANT PHYSIOLOGY 2.0 mim and 3.0 mm disks. Each point represents the mean of 4 experiments. \Vith both thicknesses, increased

$Page 1: Further Evidence Diffusion the ... - Plant Physiology · PLANT PHYSIOLOGY 2.0 mim and 3.0 mm disks. Each point represents the mean of 4 experiments. \Vith both thicknesses, increased$
Plant Physiol. (1968), 43, 274-28()

Further Evidence of Oxygen Diffusion as the Determining Factor in theRelation between Disk Thickness and Respiration of Potato Tissue

Ian R. MacDonaldDepartment of Plant Physiology, Macaulay Institute for Soil Research, Aberdeen, Scotland

Received October 23, 1967.

Abstract. The effect of oxygen tension above atmospheric pO. on the development ofrespiratory capacity in potato disks has been examined. Raising the oxygen tension of theaqueous environment to 40 t% during the aging of 2.0 mm or 3.0 mm thick disks at 25progressively increased the respiration rate of the tissue as shown by subsequent assay in100 % oxygen. Disks 3.0 mm thick showed a greater response to increased pO2 than did2.0 mm disks. A comparison of center 1.0 mm sections excised from 3.0 mm diisks afteraging, showed that the respiration rate of internal tissue from disks aged in high p0. was

approximately 40 1% greater than such tissue aged with atmospheric pO,. The characteristicinverse relationship between respiration rate and thickness in aged disks can be modifiedfrom a concave-downwards curve to a convex-downwards curve by pretreating the tissue withincreased p02, thus indicating that raising the P02 during aging can increase the thicknessthreshold at which the transition from tissue manifesting the respiratory charaoteristics ofthin disks to that manifesting the characteristics of thick disks, occurs. Similarly increasedpOG during aging can modify the hyperbolic relationship obtaining between pretreatmenttemperature in the range 100 to 250 and respiratory capacity of aged 3.0 min disks, toapproximate to the linear relationship observed with 0.75 mm disks. It is concluded that thedevelopment of respiratory capacity in disks between 0.75 mm and 3.0 mm thick is restrictedby oxygen deficiency and that the characteristic inverse relationship between respiration rateand thickness in aged disks is largely attributable to this factor, the influence of which isdiscernible both on the development of respiratory capacity and on its subsequent assay.

When storage tissue is cut up into disks, devel-opmenta,l processes are in!iitiated at the ribosomallevel (5, 9). In stuitable incubation conditionsthese bring aboutit ftundamental chaniges in metabolicactivity. A rapid time-dependent, temperature-de-termined increase in respiration (13) commencessoon after excision, and finalily reaches a levelsevera,l times greater than that obtaining in freshlycutt disks. This induced respiration of aged disksis inversely related to the thiickness of the sl,ice,the th'icker the disk the lower the respirati,on peruinit weight o,f tisstue, at least uip to 3.0 mm. Thisrate/thickness effect was one of the earliest obser-vations made in tisstue disk respiration studies (19).Subsequlently, Laties (8) stressed the need to (lis-tingui,sh the effect of thickness on the respiratoryactivity manifested in aged disks fro,m its effect onthe development of the respiratory increment duiringaging. MacDonald (11) recenltly demonstratedthat the respiration of aged thjick disks is rate-limited by oxygen deficiency and that the effect ofthickness on the respiratory activity manifested inaged disks is attributable in large measture to thisfactor. We now report an investigation of theeffe,ct of partial presstures o,f oxygen above atmos-pheric on the development of respiratory capacityin potato (lisks from xvhicli we conc4luide that juistas pO., exercises a controlling influence on therespiratory activity of aged disks so also PO2 is

274

the maini controillfing influence on the developmentof respiratory capacity, as a function of disk thick-ness, during aging. The effects of disk thicknesson respiration in the range 0.75 mm to 3.0 mm maytherefore be ascribed to oxygen tension.

Materials and Methods

Stored potato (Solanum tutberosumn L.) tulbersof the varieties Sharpe's Express, Golden Wonderand Kerr's Pink were u,sed; Ino varietal differenceswere discernible in this s<ttudy. T'ihe tuber wasthickly peeled and slic;e(d with a hand microto,me(American Optical Company, Buffalo, New York).The slices, ranging in thickness from 0.75 mm to3.0 mm, were puinched wi,th a cork borer to give10 mm diameter disks which were then aged for22 hou,rs at 250 in an aerated washing dev(ice (14).The modifications required to vary the pO2 duringpretreatment w,ill be described in the experimentalsection.

In every case tissue respiration was determinedin aqueotus soltition by Warbburg manometry at 300(11). Initially, and following gassing or tempera-ture chiange, a 30 minute equ,ilibratio,n period wasinvariably allowed before zero readlings were taken,and the values reported are those observed overthe initial hour. When measurement,s were con-tinued over a second hiour they were either similar

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MAC DONALD-POTATO DISK RESPIRATION RATE-LIMITED BY OXYGEN

to, or only slightly higher than, the first hour.Vessels were gassed as appropriate by displacementwith oxygen or commercial oxygen/nitrogen mix-tures. The oxygen content of the cylinders waschecked and oxygen tengions during pretreatmentmonitored with a Beckman 39065 oxygen electrodeused in conjunction with a Beckman Potentiometricstrip-chart recorder.

Except for disks aged in flasks (,see below),aseptic procedures for slicing and aging the tissuewere dispensed with since bacterial contaminationis no,t a factor in the respiratory rise occurring inthe 24 hours after excision (15).

Results

The Effect of pO2 on the Oxygen Uptake ofAged 2.0 mzm Disks. MacDonald (11) showed that100 % O, in the Warburg flasks during a respira-tory assay significantly increased the respirationof disks above 1.0 mm thick. The effects ofoxygen tension ranging from 30 % to 100 % wastherefore tested on 2.0 mm thlick disks to determinethe optimuim pO. for respiratory assay. Disksaged 22 houtrs at 250 were transferred to Warburgflasks and their respiration assayed over 1 hourwith air as the gas phase. Differing O/N2 mix-tures were then introduced into dutplicate vesselsand respiration assayed over a second hour. Theresulting percentage increase in oxygen uptake isshown in figure 1, each point representing the mean

a40a

c 30._

0()

a 20L

i-11c-W 10

24P),0er

~~ ~X5.-x-~ x -xX

/x

I

x40 60 80.40 60 80 100

cent o° In gas phaseFIG. 1. The effect of increased oxygen tension dur-

ing assay on the respiration rate of 2.0 mm thick disksassayed at 300. The disks were aged at 250 for 22hours. Respiration was assayed first with air as thegas phase and the percentage increase induced by sub-sequently gassing the vessels with oxygen of differenttensions is plotted against concentration.

175 _

L:

0~~'10 0

o 0~~~~~0 0

75X25 50 75 100

per cent o, in pretreatmentFir.. 2. The effect of pretreatment with increased

oxygen tension on the development of respiratory ca-pacity of 2.0 mm and 3.0 mm disks aged for 22 hoursat 250 in water in equilibrium with varying pO2.Respiration was assayed at 300 first with air as thegas phase and then with 100 % 02.

of 6 experiments. Disks flushed with air showeda 5 % stimulation; this increased as the pO wasraised to 50 % but showed no fall at 100 %. Allsubsequent assays with above atmospheric PO2 weretherefore carried out using 100 % 0Q. Disks 3.0mm thick showed a similar pattern of response.

The Effect of PO2 on the Development ofRespiratory Capacity. Six 500 ml capacity glasscylinders made from separating funnels were im-mersed in a constant temperature bath at 250 anda different 02/N2 mixtture ranging from 20 % to100% oxygen was introduced inito each cylindervia the stem. The gas stream circulalted the disksand maintained a constant oxygen tension in thewater. This modificaition reprodtuced the featuresof the aerated washer (14) except that there wasno continuous changing of the medium. Howeverthe water in the cylinders was changed 3 timesduring the 22 hour pretreatment. The oxygen con-tent of the gas mixtures entering the cylinders was21, 32, 41, 52, 69, and 100 % respectively. Afteraging, the respiration of duplicate samples of eachtreatment was determined using air as the gasphase. The Warburg vessels were then flushedwith 100 % O. and the respiration again deter-m'ined. Figure 2 shows the results &btabined with

275

1[



PLANT PHYSIOLOGY

2.0 mim and 3.0 mm disks. Each point representsthe mean of 4 experiments. \Vith both thicknesses,increased pO, during pretreatment tended t,o de-crease the oxygen uptake of the tissuie assayed ine,(luilil)rilim with air and an inhibitory effect wasparticuilarly evident with 100 % O. (cf. 8, tableIN'). However N-hen respiration is assayed in100 % 0O pretreatment in high PO2 is seen to haveI)rodlicedl a substantial increase in the respiratorycapacity of the tissuie. Assay in 100 % 0° with2.0 mm disks cauises a stimulation of °2 tuptakeranging from julSt over 40 % with disks aged inequilibriumwnith air to 75% with disks aged in100 % O., and with 3.0 mm disks the percentagestimulation ranged frolm 50 % to 100 %. Since inhoth ca.ses the highest respiration rate wvas recordedfrom tissue pre,treated with 40 to 50 % O., thistenslion was considered optimuim and uised stub-se-(Illentlv.

The Effect of -o % 0., Pretreatment o0t theDevelopinent of Respiratonr Capacity in Disks of

L

>"'175

150cm

V

-L1250eoon

c5s100

N.Of1 0 20 3-0

Disk Thickness (mm.)FIG. 3. The ef fect of pretreatment witli 40 %

oxygen otn the (levelopment of respiration in disks ofx-arying thickness from 0.75 mm to 3.0 mm. Diskswere aged for 22 hours at 250. X) Air as the gas

phase during pretreatment and assay. 0) Air as thegas phase during pretreatment. Assay in 100% 0.

0) 40 % O, as the gas phase during pretreatment.Assay in 100 % 0. The 1.0 mm points for each treat-ment are all superimposable.

Varying Thicklness. Disks of thickness 0.75, 1.0,2.0, and 3.0 mm were prepared from 1 potato an(Idivided into 2 lots 1 of which was aged in theusual way w-ith air blown throuigh the stem olf thewrasher (14). The other grouip were aged in anidentical washer, htut 1 which, in addition to theair introduced via the stemr also had 100 % 0.buihbled into the water bathing the disks. Theratio of oxygen to air was adcjusted stuch that theoxygen tension of the water wTas 40 % as indicatedll)y the electrode which contilnuously monitored themeditum. After aging, the respiration of bothgrouips of (lisks w-as assayed in 100 % 0O and therespiration of the tissuie pretreated withouit in-creasedl oxygen, was also assayed with air as thegas phase. Pigure 3 illustrates the resuilts obtainedlfrom 1 experiment, the points be-ing the means of3 replicates. The experiment was repeated severaltimes with similar restulits. Clearly the thicker thedisk the greater is the response to pretreatmentwith high p0o.

The Effect of 40 % 02 Pretreatinenit on1 theDevelopmnent o f Respiratory Capacity in ThickDisks (Is Demionstrated by Subsequent Division.The experiments reported above indicate that pre-treatment of thick disks with increased pO., stimu-lates the development of respiratory capacity butthait increased oxygen tenslion dtriing the assay isnecessar\- to reveatl this increase. Prestumably thisis dlue to limitatiions in the rate of oxygen diffusionto the center tissuie. If, however, pretreatment inelevated pO, had led to an increased synthesis ofres'piraitory capacity in the center of a thick disk,tissiue taken fronm the center slhould show a higherres)iration rate when assayed in air.

Disks 1.0, 2.0, and 3.0 mm thick were preparedfrom I potato alld aged in water in ecquiilibriutmwith air or 40 % oxygen as described in theprevious section. Tissuie from both pretreatmentswvas assayed first with air as the gas phase andthen with 100 % oxygen. The restults shown intable I confirm those reported above, addition ofoxygen to the assay conditions induicing a 35 %stimullation in 3.0 mm disks aged with 21 % 02and a 74 % stimulation in those aged with 40 % 02.These 3.0 mm disks were then each sectioned into3 disks, approximately 1 mm thick (see 11) andthe respi,ration of the ouiter and inner sectionsassayed separately using air as the gas phase. Thevalues obtained (table I) show that when theinner tissue is excised from a thick disk afterpretreatment with increased pO, its respiration rateis approximately 40 % greater than that of innertissuie from control disks and additional 09 in theassay sxystem is not necessary- to demonstrate this.

The Effect of Intcreased pO, on Disks of Vary-ing Thickness Aged Aseptically in Flasks. Manyinvestigators uising poltato slices commonly age theirdisks in closed flasks. Disks 1.0, 2.0, and 3.0 mmthick were therefore prepared aseptically (12) andsome of each thickness transferred to each of 4

.

x-~~~~~~~~~~~~~~~

2/76

R(. I I



MAC DONALD-POTATO DISK RESPIRATION RATE-LI MITED BY OXYGEN

Table I. The Effect of Pretreatmient withthe Developmient of Respir-atory Cal

1.0 mm, 2.0 mm, and 3.0 mm disks wehours at 250 in water in equilibrium with aoxygen. Following pretreatment, respiraticsayed at 300 initially with air as the gassubsequently with 100 % oxygen. The 3.0were then subdivided into outer and innand the respiration of these separately deterair as the gas phase. Fresh weights wereat the time of cutting.

Gas phase dturing pretreatment AirGas phase dturinig assay Air 0.,

1.0 nmmi Thick disks2.0 nmmi Thick disks3.0 mm Thick disks3.0 mim Disks subdivided

Outer sectionsInner Sections

17411087

ul O,2abs((qf fr wf,t) -

183132118

163104

sterile liter- filter flasks which were gaair, 30%, 60%, and 100% oxygenl re'T'he flasks were sea'led and olaced olBrunswick Gvroftorv Shaker. After aging, theres!pirat;ion1 of tisstue from each treatment wasassayed iiitially wilth air as the gas phase anid theniwi^th 100 % 0.- As noted above, the effect ofpretreatment only becomes apparent on the a(lditiouiof oxxvgen to the \Varburg flasks. The percentagestimulation of respiration with added oxygen isshowNn in table II. The thicker disks show asimilar pattern of response to those aged in aniopen system. The low%er valtues obtainedl in thi-experimenit, especia'lly for disks aged with air asthe gas phase, may be duie to the different circuim-stances of aging or possibly to the fact that thepotatoes uised had been stored for 12 molnths.

The Effect of 40 % Oxyge7i Ott tltc Develop-mlent of Respiratory Caparity in. ,.o mmt)7t Disks as

Table IT. The Effect of pO0 on Disks of VaryinigThickness Aged in Closed Flasks

Disks 1.0 mm, 2.0 mm. and 3.0 mmn thick were aged22 hours at 250 in flasks containing different oxygentensions. Following pretreatment respiration was as-sayed at 300 initially with air as the gas phase andsubsequently with 100 % 0,. The percentage increasein respiration evoked by 100 % 02 in the gas phasereflects the increase in respiratorx capacity induced bythe pretreatment.

Gas phase duringpretreatment

Air

30% O..60% O.,100%

Oxygen Oil a Function of Temperature. The rate at whichOacity respiratory capacity increases (Iiring aging isre aged 22 tem(perature determined (13). When thin slicesir or 40 % are aged for 22 hours at different temperatures andrn was as- their respiration then assayed at 30°, the respirationphasedankd increases linearly a,s a function of pretreatment

er sections temperature between 100 and 250 (fig 4). Inmined witlh contrast to this, 3.0 mm disks respond only mar-determined ginally to incubation temperattures above 150 (fig

4). If in this case oxygen is a rate-limiting factorit should be possible to induce a linear response to

40 % 02 temperatuire by raising the oxygen tensi,on dturingAir 0.2 pretreatment. Figture 4 shows that when the)) bcd oxygen tension is maintained at 40 % dutiring pre-(hr)-' treatment at different temperatuires thick disks171 203 respond to temperature in a manner similar to thin124 167 disks. The points in figure 4 are the means of94 164 2 experiments in each of which the assay was

carriied otut in triplicate. In this series of experi-172 ments, the respiration of 3.0 mm di,sks aged and

assayed in oxygen was consoistently higher thanithat of 0.75 mm disks aged and assayed with air as

tssecd w-ith the gas phase. Addition of oxygen to 0.75 mmbspectivel . disks during the assay would have increased theiralNlyewXK respiration slightly.

L

-"'1503:

c, 125-cm

n00ioo0naOm75:1

10 15 20 25Pretreatment Temperature

FIG. 4. The development of respiratory capacit-in 0.75 mm and 3.0 mm thick disks as a function ofpretreatment temperature. Disks were aged for 22

hours in a washing device at the temperature shownwith either air or 40 % oxygen as the gas phase. Oxy-gen uptake was assayed at 300 with air as the gasphase for 0.75 mm and 3.0 mm disks pretreated withair, and with 100 % oxygen as the gas phase for 3.0mm disks pretreated with oxygen.

Increase in respirationThickncss 1.0 mm 2.0 mm 3.0 mm

-1.5-1.0

1.90.7

11.316.942.243.8

22.732.758.662.5

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277



278

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2

~0.0~~

0

.0 A~~x0o 01~~~_5

0T75 1-5 3 0Disk Thickness (mm.)

FIG. 5. The effect of assay temiiperature oni therespirationi rate of disks of varying thickness. Diskswere aged for 22 hours at 250 and their respirationrate assayed as a functioni of temperatuire with air asthe gas phalse.

The Effect of A4ssay Temtperatture ont the In-verse Relation Between Respiration Rote and DiskThickness. Disks 0.75, 1.5, and 3.0 mm thick wereaged for 22 hoturs at 25° in equiilibriuim with airanid the re,spiraftion rate of 4 replicaites of eachthickness was determined o-ver 1 houir at 300. Thebath temperature was theni lowered prog-ressively(11) and the respiration raite assayed at each tem-peraiture initerviall down to 5°. The results areshown in figure .5. A similar pattern is obtainedwhen the temperature sequence is reversed.

Discussion

'I'he stimulatory effect of oxygeni on the res-P)iratorv (levelo'pnent of thick (lisks (fig 2) can beexplained in terms of an increased synithesis of res-piratory cap)acity. Incuibationi of sliced storage tis-suie induces metabolic changes which can be broadlygroupe(d either as t,he quiickening of existing meta-bo.lic processes, e.g. respiration, or as the initiationof processes previoulsly absent or totally stuppressed,e.g., enzyme induction ani{d ion absotrpbion. Allthese changes are dependent on aerobic metabolism.Some a,t least, and possibly all, depend on the

P H: YS IOLOGY

synthesis of new comiponents. It has been claimed(10) that the respiratory increase in potato disksis due to an increase in the number of mitochondriarather than to a quickening of the exi,sting mito-chondria. Hackett et al. (6) examined the oxida-tive ani(l phosphorylative activities o,f mitochondriafrom fresh and( aged poltalto disks and were uinablcto (lemonstrate any increased activity with aging.A variety of evidence from inhibitor and lowtemperature experi;menlts points to the dependenceof the respiratory rise on synlthe-tic processes (4).That these sy-nthetic processes are considerabl)represse(l in disks from 1.0 mm to 3.0 mm thickbecause of oxvgei (leficiency is bornle oult by theevildence owl)presented. The overaill respirationof 2.0 mm and 3.0 mm disks cani be increasedsuibstanitially by oxygeni pretreatment ( fig 2 and3) anid especially pertinent is the fin,ding (taible I)that the respiration rate of ininer sectionis of3.0 mim disks pretreated with oxygen is almost.50 % greater tihan comparable seotions from dcisk-saget in equilibriu-m with atmospheric pO.,.

With,in the range of thickness investigated here,it would seem that the original explianation (19) ofthe inverse relationship betw-een respiration rateand disk thickness, namely thait it is due to anioxygeni gradient across the disk, is correct. Laties(8) wals the first to po,int oult that the respiratory!limitationl in thick diskis included ani inhihiltion oftihe development of respiration in the ceniter of thedisk, earlier wvorkers having incorrectly attributedall the effect of thickness to an effect oni imeasiiredrespiratory activKty. He concluded that it is thedevelo.pment of the respiratory increment wNhich iscontrolled by tissuie thbickness and not respiratorylact.:vity per se, buit that in neither case wais thecontrol effected by oxygeni limitationi. Siubsee-quently, however, it was sho-wnl (11) thait there.spiration rate of thick tissule may be increasedafter aging by rais-ing the external 1)0p, or bysubdividing the tisstue into thinner sectiolls toshorteni the diffusion path of oxygeni, indicatingthat respiratory activiity per se i,s influenced bydisk th,ickness. Tlit is signiificant in this connectionthat the inverse relationship between thickness andraite is not maniifested when respiration rate isassayed at lower temperatures (fig 5).

The effect of pO2 onl respiraltory activrit suig-gested that pO could equally well be the agency1w which the controlling influence of thickness O01respiratory development is exerted. TIn tbhiis event,increase(l pO. shoiuld inlduice in thick disks a levelof resl)irator\- activity comlparal)le \\ith that mauii-feste(d b- thin disks. This has iio\\ been estab-lished in 2 (lifferent ways. TIn the first place.wlhei dlisks are pretreated and assayed in elevated1)02, the rate/thickness relationship is altere(d fromthe characteristic concave-downwards curve to aconvex-downw%vards culrve (fig 3) and, interestinglyenoutgh. now, assumes the appearance of the ideal-.zed relationship T-hchIaties (8. fig .5. Cuirve A)

,30

-J6~~ ~



.MIAC DONALDP)-1'OTATrO DISK RE'SPIRATIro.N RATEll-f.i-\lTE D BY OXYGEN

theorized should obtain where respiration is propor-

tional to pO.. In the second place when thickdisks are aged and assayed in the presence of highpO, they respond to temiperatuire in the range 10(to 250 in a manner identical to that of thin dcisks

ith respect to respiratory development (fig 4).In oither words, the limitation imposed on therespiratory development of thick disks duiringaging can be removed by suipplying oxygen at ailapprioprmate tension.

Bulrton (3) has shown that the oxygen tensionthroutghout an intact potato tuber in equtilibriumwilth atmospheric pO. is more than enotugh tosupport respiration, even the high rate character-istic of young developing tubers. Why then shouildoxygen penetrability become a limiting factor intissute diisks a mere 2 mm and 3 mm thick? Laties(8) poiinted ouit that this situation could be brouightabout by (i) a decrease in the oxygen affinity ofthe terminal oxidase, (ii) by an increased respira-tion rate ra,i,sing the oxygen requirement, or (ii,i)by a decrease in the diffusion coefficient of oxygen

owing to wvater injection of intercellular spaces.

As regards (i), opinions differ and the evidenceis equivocal. Schade et al. (18) reported an in-creased sensitivity to pO2 during aging and con-

cluded that this indicated an increased participationof a pO., sensiitive terminal oxidase system. Thi-mann et al. (20) recognized that an increaseddependence on pO.) coulid be dule simply to limita-tions in oxygen diffulsion and having taken stepsto redulce this possibility they concluded that theoxidases of fresh and aged disks had a similaraffinity. On the other hand, Mapson and Burton(16) obtained evidence of the participation of 2termlinal oxisdases, 1 wiith a low oxygen affinity,in the respiration of whole tubers.

Regarding (ii), the question raised is not

whether respiration rises buit whether it risesenough to bring about the observed dependence onlpO.2 Laties (8) argued on the basis of Warbuirg'sequation and the use of a diffusion coefficient foroxygen calculated for the intact tuber (2) that theobserved respiratory rise is insufficient to imposea limitation on 1.0 mm aged disks.

There thus remains the third possibility that thediffusion coefficient of oxygen is decreased b)slicing. Since the rate of oxygen diffusion in airis approximately 105 times faster than throughwarter (7), a film of moisture surrounding planttissue can reduice oxygen diffusion by several ordersof magniltude. Ohmura and Howell (17) haveshown that adding water to a variety of plainttissues leads to a marked decrease in oxygen uptake.WVith slices of corn scutellum, the inhibition in-creased with the thickness of the slice and thevconcluded that this was duie to a decreased rate ofoxygen diffusion through the tissue. Similarly,Berry and Norris (1) concluded from a study ofthe respiration of onion root segments at differenttemperatures that oxygen diffusion is the limiting

factor at oxygen tensiolns below the critical pres-sture wh,ich, in the case of apical segmelnts at 300,was 45 %. And most pertinent olf all WVoollev (21)esqtablished, both by experiment and observation,that the suirface of freshly cutt potato tissuie dif-fered from the bu-lk tissuie in that most of theintercelltu,lar spaces w-ere fille(d with water at thesurface, although internally they were air filledIand interconnecting.

It woulld seem therefore, that oxygen diffusionis nlot a limiting factor in the whole tulber becauisethe intercelluflar space system can supply oxygenat a rate greater thaii that riequiired to maintainrespirationl. However, slicing the tissuie reduicesthe free flow of oxygen by occltuding most of theexposedl cuit ends of the interconnecting air spaces.Diffulsion then becomes a limiting factor probablyevren in freshly cut disks above 2.5 mm thick, therespiration of which decreases with fturther in-crease in th;ickness (8, 11, 21). As the respirationrate increases with aging the oxygen limitationwill take effect in thinner disks and the rate/thick-ness effect is manifested uinless the temperatuire ofrespiratory assay is lowered to the point where,becau1se of the rediuced respiratorv demand, oxygensatutration is achieved at lower partial pressulres(fig 5). Oxygen is ulot the cauisative factorinitiating the metabolic processes occasioned byslicing, blut it is necessary in increasing qulantityfor the fuill development of these processes and itrapidly becomes the limitillg factor controlliing atleast the quantitative aspects of resl)irationll bothas regards development aili( activity.

Acknowledgment

The techiiical assistanice gi- uen by Miss Normla Sad-dler is gratefully ackiuowk0dged.

Literature Cited

1. BERRY, L. J. AND WV. E. NORRIS. 1949. Studiesof onion root respiration. I. Velocity of oxygenconsumption in different segments of root atdifferent temperatures as a function of partialpressure of oxygen. Biochim. Biophys. Acta 3:593-606.

2. BURTON, MV. G. 1950. Studies on the dormancyand sprouting of potatoes. 1. The oxygen con-tent of the potato tuber. New Phvtologist 49:121-34.

3. BURTON, NV. G. 1965. The permeability to oxy--gen of the periderm of the potato tuber. J. Exptl.Botany 16: 16-23.

4. CLICK, R. E. AN-D D. P. HACKETT. 1963. Therole of protein and iucleic acid synthesis in thedevelopment of respiration in potato tuber slices.Proc. Natl. Acad. Sci. U.S. 50: 243-50.

5. ELLIsJ R. J. AND I. R. MACDONALD. 1967. Activa-tion of protein synthesis by microsomes fromaging beet disks. Plant Physiol. 42: 1294-1302.

6. HACKETT, D. P., D. XW. HAAS, S. K. GRIFFITHS,AND D. J. NIEDERPRUEM. 1960. Studies on de-

27()



80PLANT PHYSIOLOGY

VelolIinent of cy anide-resistant respiration InIIpo-tato ttlb r slices. Plant Physiol. 35: 8-19.,-

7. JAMES, \\. 0. 1953. Plant respiratioin. ClareindoiiPress, Oxford.

S. l ATIES, (G. ( i. 1962. Colitrollitg inifnltienc 01thickicss on dcv clopmnt aild ty-pc of respiratorvactivitv ini potato slices. Plaint Phvsiol. 37: 679-90.

9. LEAVER. C. J. AND J. L. K-EY. 1967. PoIxyribosomeforimiation :md RNA svnthesis during aging ofcarrot- oot tissnie. Proc. Natl. Acad. Sci. U.S.57: 1338-44.

10. LEE, S. G. AND) R. Ml. CHASSON. 1966. Agingand mn itochlon drial (levelopment in potato twbertissue. Phvsiol. Plantarum 19: 199-206.

11. MACDONALD. 1. R. 1967. Oxygen tension a de-terminiiin- facto- in the respirationi of plotato disksoftarying thilckiness. Plant Phvsiol. 42: 227-32.

12. \tACDON.AL), 1. R 1967. B<acterial infection andioll absorption capacitv in beet disks. Ann. Botanv311: 163-72.

13. \IACDONAn.1T. R. AND P. C. DEKOCK. 1958. Tern1-peratnre control and metabolic drifts in aging(lisks of stora-e tissu1e. \in. Botatnv N.S. 122429-48.

14. -MACDONALD., I. R. AND A. H. KNIGHT. 1958. .\niapparatns f )r maintalining tisk(s of ti -;sue iniuiniforimi menvirInineut. .\nn1. l 1talax- N.S. 22:423 27.

15. -MACDONALD, 1. R., 1). Mr. WEBLEY, AND J. S. D.TBACON. 1963. VWashed disks of storage tissuefiree from bacterial contaminationi. Biochem.J. 89: 104 p.

16. \IAPSON, L. XW. AND XX. G. BURTON. 1962. Theterminal oxidases of the potato tuber. Biochemil.J. 82: 19-25.

17. O0IxtuRA, T. AND R. XW. HOWELL. 1960. Inluii-tory effect of water- on oxygen consumilptioni byplant miaterials. Plant Phv-siol. 35: 184-88.

18. SCHAI)E, A. L., H. LFvy. L. 1BERG-MANN, AND S.HARRIS. 1949. Studies on the respiration ofthe white potato. III. Chaniges in the termiinaloXidase pattern of potato tissuc associated wVitltimiie of suispension in -water. Arch. Biochemii. 20:211-19.

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Further Evidence Diffusion the ... - Plant Physiology · PLANT PHYSIOLOGY 2.0 mim and 3.0 mm disks. Each point represents the mean of 4 experiments. \Vith both thicknesses, increased