chens Usnea antarctica and U snea aurantia co-atra from the Maritime Antarctic

Polarforschung58 (2/3):171-179, 1988

2.7 Net C02 Exchange in Relation to Thallus Moisture and Temperature in two Fruticose Li-

chens Usnea antarctica and U snea aurantia co-atra from the Maritime Antarctic

ByP,M, Harrisson and P, Rothery'

Summary:Spccics01'Usnca (Ncnropogoniarc a major component01'considerablc arensofthcfellficktvcgetation Ixland.SOHlhOrkncvs Islands. Althoughmorphologically similar thc rcspiratory and photosynthctic responses01'thc\WO 10thc samce;:~;~~I;ll~~I','~~:scconditionswcrc totally different. Dcspitc similar growthformsthctwospccies requircd diffcrcnt thallus water fürmaximum pl ItisSlH!!!CSICd thatthismay bc auributahlc10changcs numbcr01'diffusion wilh possiblcdiffcrcnces thcirlightrcsl)onscs, und thc possessinn01'different structurcs conuucnsuratc

Zusammenfassung:l/.\7/ca-Ancn bilden dieHauptkomponentein derRohbodenvegetationvon Signylsland. South Orkncv Islands.Troll. morpho- logischerAhnliehkeltsind Atmunglind beider Arten sehr unterschiedlich. FÜr maximaleNcnophorosynthcscnuen benötigensie unterschiedliche Wassennennen.Diese möacndarinbcuründcr daß die Diffusionswege im Thallus beiwechselndem Wasserachalt verschieden daß dieFI~chtenunterschiedlichauf ÜChl reagieren und daß unterschiedliche Al1 sich zureproduzieren einen Einfluß<lld"die

Lebensweise haben.

LINTRODUCTION

Of the wiele variety of terrestrial ecosysterns those in Antarctie regions are amongst the mosr extreme, They are restricteel to aseries of isolateel snow-free areas which cornprise approximarely 1% of tbe Antarctic,anel temperature and water availability in these ecosystems are subjeet to large and not infrequent changc.

Due to their poikilohydrous nature liehen metabolie processes, particularly gaseous exchange, can vary greatly with changcs in thallus molsture content (LANGE 1980), PERKINS (1945) elrew auennon to the possible effect ofwater availability on the local elistribution of lichens in Antarcticecosysterns.Globally the importance ofthallus moisture content to the survival and growth of lichens has since been extensively stuelieel in a range of environments and sumariseelbyKERHSAW (1985), Gaseous cxchange studies using Antarctic liebens have been summarisedbyKAPPEN (1983, 1985) anel INO (1985),

Neuropogen is the only subgenus of frutieose lichens to be prominent in both rhe eontinental anel pcri-Antarctic flora (KAPPEN 1985 and LAMB 1964), Usnea is the dominant fruticose genus in the flora of Signy lslanel (60"43 'S, 45"38'W), South Orkney Islands (SMITH 1972, 1984), HOOKER (1980) has recently stuelieel the growth raresofUsnea [asciata(= U, aurantiaco-atraWalker 1985)andUsneaantarcticaon Signy Islanel- maritime Antarctic - anel KAPPEN (1983, 1985) has recently investigateel aspects of the gascous-exchange anel water-relations ofUsneafasciata (=U, aurantiaco-atroWalker 1985)and Usnea sulphurea(= U, sphacclata Walker 1985) inthe maritime anel eontinental Antarctic.

The habitat andelistribution of U,antarctica andU,aurantiaco-atraonSigny Islandintheperi-Antaretichas been described by HOOKER (1980), HUNECK et al, (1984), LONGTON (1985), SMITH (1972, 1984) anel WALTON (1984), The environmental factors, temperature anel water availability exert a significant influence on the survial ofterrestrial organisms in Antarctica, along with substrate availability - possibly the dominant factor - anel wind, They are inextricably linked in their action on biological sysrerns, and may largely eletermine the survival not only of inelivieluals, but alsooftheirpopulations, communities, anel species polarecosystems (BLOCK 1985), The aim of this paper is to eompare the role of tbese two variables on the net C02 exchange ofUSllca aural1tiaco-atra anel USl1ca al1tarctica.

P.lvI. H31Tisson allelP. Rothery. Brilish Antarctic Survcy, Natural Environment Research CounciL High Cross. rvladinglcyRoad. Cambridge eS3 OET. Uniled Kingdom.

171

2. MATERIALS AND METHODS 2.1 General

C02exchange anel thallus water content measuremcnts were made in the light and dark using the methodology described in HARRISSON et al, (1986). Experiments were conducted between 22 May and 3 December 1982.

Single thallus size classes of mean dry weights 0.81 g (SE

=

0.05. n

=

26) anel 2.26 g (SE

=

0.1-3. n

=

26) for Usnea antarcticaanelU.aurantiaco-atrarespectively were collected front quartz mica schist clitf faces. At the start of each experiment the saturated rhallus water content (SAT gg") was estimated by fully hydraring the thallus in double-distilled water for 15 min followed by light shaking to remove excess surface water. Four replicates were used in the light experiment and three replicates for dark experiments for eaeh species. Thallus temperatures of -5.O.5. 10. 15.20 anel 2Y C were used in the light and dark with two exceptions. 2Y C was unattainable for both specics in the dark anel only 19' C was used instead of20' C forUsnea aurantiaco-atra,An irradiance level in the 400--700 nm waveband- which was thought to be typical of field conditions on Signy Island - of200 umol m-\-I was used in all the net photosynthcsis measuremenrs. Unfortunately, no experiments could be perforrned to investigate the pattern of response of net photosynthesis to incxreasing irradiance levels - at individual thallus watcr contents.

2.2 Parameter Idcntification

Thc following photosynthctic anel respiratory parameters werc idcntificd and analysed:

DRMAX and NPMAX (mg C02 i1h-^l) are the maximum rares of dark respiration and net photosynthesis respcctivcly. At each temperature - for both species - for eaeh individual thallus, maximum rares of dark respiration (DRMAX) occurrecl at the highest rccordable thallus watcr contents. Whereas NPMAX oeeurs at sub-saturatcd thalluswatercontcnts.These sub-saturated water contents form the third parameter to be analysed - WCNPMAX (gg ..I). The thallus water eontent at which the maximum rate of net photosynthesis (NPMAX) occurs is both easy to measure and c1early shows the inextrieable link between the two important enviromental variables temperature anel water in the liehen thallus. The fourth parameter SAT (gg-I) is the saturated thallus water content. This parameter may give information of the relative water storage capacity ofthe two liehen thalli, However, this parameter does not give any information on the location01'nature of the water storage sites. The final parameter %DEP (%) is the pcrcentagc depression in net photosythesis oecurring at full thallus hydration (SAT). It is given by the equation:

% DEP= ((NPMAX - NPSAT)/NPMAX) x 100

where NPSAT is the rate of net photosynthesis at full thallus hydration. Once more this is an easy parameter to measure. As the thallus water eontent increases the rate of net photosynthesis (NP) increases anel reaches a maximum value (NPMAX) at a sub-saturated water content (WCNPMAX) a further increase in thallus water eontent results in a decline or depression in NP. By studying the depcndence of this depression in NP on temperatureitmay be possible to elucidate the meehanism(s) causing the depression.

2.3 Analysis of Parameter Dcpendence on Temperaturc

Tbe responses of the two species were comparec1 using a two-way analysis of variance. Differences in mean pattern response were examined by testing for a starisrically significant interaction. Where no interaction was derecred, i.e, indienring parallel responses wirh temperature, we tested for a eonsistent speeies c1ifferenee across the range of temperatures used. In so me cases, the variables were first transformec1 by ea1eulating the loge of the variable before analysis to allow for inereased variation amongst relieates as the response inereased. In these analyses speeies c1itferenees are measured as proportionate temperature etfeets, and species differences are proportionate change ditferenees as opposed to absolute differenees. AlsoQIOvalues (SCHMIDT-NIELSEN 1975) were ca1eulated to study the depenc1ence of the rates of change of the maximum rates of dark respiration and net photosynthesis on temperature.

3, RESULTS

Figures la and 1b show the tcmperature and moisture depcndencies ofnet photosyntbesis (NP) very clearly. Under the light regime used maximum rares of net photosynthesis (NPMAX) occur at 0' C forUSI1C(ltrurantiaco-atro ancl at 5' C for U. (l11/(lJ'ClicaThis would be an cxpceted result ifaurantiaco-atra is not light saturated, Unfortunately, no experiments eould be performed to ascertain light saturation levels for cither species. The responses of individual replicates at these temperaturcs are shown in more detail in Figures le ancl l d. A degree of variability in the pattcrn and absolute rares of response is apparent. The variability in thc absolute rares of net photosynthesis of the individual rcplieates could bc due to between rcplieate diffcrcnees in the total thallus chlorophyll eontent and or algal cell number, The differcnces in the pattcrns of responsc eould be caused by between replieatc diffcrences in the watcr storage capacity of the thalli, There is also a difference between speeies in the depression in net photosyuthesis at full thallus hydranon. The depression in net photosynthesis is greater inU. aurantiaro-atraas is also clearly shown in Table I,Itshould be noted that at -5' C Figure la shows virtually no depression in net photosynthesis (NP) for U,aurantiaca-atra,and Figure I b shows no depression in NP for U,antarctica. However, Table 1 shows small depressions in NP for the two species. This is because the saturated thallus water content (SAT) is diffieult to measurc, which results in a wiele variation of values for this parameter.

This is visible in Figures le ancl Id where the inidividual replicate data is plotted. Figures la and Ib show the mean rates of NP, where n=4 for each ternperature. This restriets the water content range of the plottcd data because, as water eontent incrcases, sornc thalli are recorded as being saturated before others, ancl as soon as n=3

01'less no more data ean be plotred. Thcrefore the effectivc lack of a depression in NP at -5' C in Figures l a and Ib is an unavoidable artefact of data prcscruation. At a glancc Figure I shows the rates ofnet photosynthesis for U.antorcticaare greater than forU.aurantiaco-atra.

Figures 2a and 2b show the tcmperaturc and moisture dependeneies of dark respiration (DR) very clearly. In both speeies DR inereases with temperature ancl is greatest at the highest experimental temperarures. At a glanee Figures 2a. 2b, 2e ancl 2d show that the dark respiration rares of U, antarcticaare greater than that of U.

aurantiaco-atraat similar thallus tcmperatures ancl water content. The darkrcspiraiion (DR) rates show different moisture dependent responses in both spccies. These responses are approximately linear forU. antarrticaand curvilinear for U. aurantiaco-atra. Rates of DR increase with increasing thallus water eontent. At eaeh tcmpcrature - for both species - for eaeh individual thallus maximum rares of DR (DRMAX) occurred at the highest recordable thallus watcr contents (SAn, Figurcs 2c and 2d inciicate the degree of variability in the DR rares.This saturation water content is speeies dependent - U, ant arctica(Uia.)mean SAT = 2.13

ss'

(SE = 0,04, n = 46) andU,aurantiaro-atra(Ll.a-a.) mean SAT= 1,78 gg-l (SE = 0,03, n = 46) where theSAT ratio, U.a.Zl.l.a-a.

= 1.20 - indieating thatU.antarcticahas the higher water storage capacity, However. this data sheds no light on the nature01'location of the wafer storage sites in the two speeies,

The statistical analysis of the selectcd photosynthetic and respiratory parameters ean be summarised as folIows:

%DEP: (%) for both species a curvilincar increase with temperatllre is apparent (Tab, 2), However, it is possible that the formlIla lIseci to calclliate %DEP cOllld give extreme vallIes when NPMAX is very low at high temperatures. Whether this shou1d be treatecl as an artefact01'areal effect can be c1ebatecl, The eurvilinearity of the response may in reality be lcss marked than the analysis sllggests, Differences between species varied with temperature (F6.42 = 3,28; p< 0.01), The speeies effect is, however, probably best measured as a ratio ofU, oillarclicato U.allrailliaco-alm,These ratios are given in Table I, Thus, apart from the high value - around unity - at 25' C, wben the observed responses are very similar, the ratios are relatively stablc, The above finclings and thc observation that between replieate variation increases with the response sllggested repeating the two-way analysis of variance lIsing loge transformed data, Even after loge transformation there is a statistically significant interaction(F6,42= 4,16: p< 0.01) inclieating some temperature dependence of the ratios, presllmably most of the effeet is due to the observation at 25"C.

One last point eonceming the dcpcndcnee of %DEP on temperature is that some curvilinearity is still present in the relationship between loge (%DEP) and temperaturc (Tab, 2),

WCNPMAX (gg-l): for both species there is some dec1ine with temperature. ForU.antarclica,some curvilinearity in the relationship is apparent (Table 2), Two-way analysis ofvariance shows a statistically significant interaction 173

0.1I I

ooo~

I

5"C

n=4

25 o S"C V' 10"C .6. o~c

o ^-S°C o 15"C ... 20"C

+ 25"C

2.0 1.0 1.5

Thallus water content (gg-l)

"

b

0.1

0 0 0 0 0 0 0 0CIccö0 0 0

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Usnea antarctica 0.2

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Usnea aurantiaco - atra

Thallus water content (gg-1)

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--...---,----~--_r--_.--_,__----,r--_rl'

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(F6.42=4.19: p< 0.0I).Differcnces betweenU.antarcticoandU.anrantiaco-otro(frornTab. I)over the range

Tcrnpcraturcf'C) - )

10 15 20 25

(,;~DEP((,:(l.

Spccics u.,

:'i.ni 5.0 R..1± l.R n.7± 6.6 9.2+ 2.4 J5.0± 10.7 187.9±I06.2 1737.7±S37.6

Parameters qOEPraue

. <I--a. U.a./U.a.-a.

31.7± 4.1 0.16

ni.Xt 6.4 0.12

)5.4± 17.1 OA]

51.3± 4.8 0.18

171.2± 46.1 0.10

I 177.5±205.4 0.16

1560.0±502.1 1.11

WCNPi'vIAX (gg I) Speeres U.a .

1.84±0.c1'J 2.14±O.20 1.61±O.16 2.0B±O.22 1.50tO.27 1.62±O.25 O.20±{l.02

Uia-a.

1.34±0.16 O.92tO.OS O.X9±O,03 O.69±O.OS O.66±O.IS 0.16±0.0.1 11.I1±1I.1I4 Tab.I: Mcan cstimatcs SE) for (,tDEP und \vCNPl.,,!AXwithappropnatc rnüos at sevcn tcmpcraturcs. U.a.=USI/('(J (/l/lon11("(1and U.a.-a.l/snca aurantiaco-otra

Usncaaurantiaco-astra

Specics

Usnca antarcnca Parameters

DRMAX

47.2'"

O.12?\' 0.114 11.009"

O.12?\

--0.002--

QE

NS NS

l.F NS

NS NS

:)6.4;"·~

0.176'"

(J.O)'·

II.OIR'"

O.Ln

~-O.OO)"

I:F

Tah.2:

'. ··pO.OOI:

on tcmpcraturc.Levels of .... ignificancc arc indicatcdrvp0.05: P 0.01:

::;; quadraticcffcct:LF::;; lest.Rcfcr to tablcs I and Sfor uuits.

-5to25"C are as folIows: 0.4 1. 1.22.0.72. 1.39.0.84. 1.46. ancl 0.09 gg -I (SE (dif)=0.21). Thus,U. antarctica emerges as higher over the range-5to 20' C with the differcnce being most pronounced in the range 0 to 20" C.

DRMAX (mgC02

g'

h-\ both species show a significant curvilinear increase with temperature (Tab. 2).Over the range-5to 2et CU.aurantiaco-rttrachanges 24-folcl anclU.antarctica32-fold. As judged by the analysis of variance on the raw data, there is a highly statistically significant specics by temperature interaction(F5.24= 31.1: Jl< 0.0(1). However. similar consiclerations to those for %DEP show that the analysis of ratios is morc relevant to the species comparison. The U. antarctica:U. aurantiaco-otraratios over-5to 2fr C are given in Table 3. Again, these appear to be relatively stable over the temperature range usecl. This is supportedbythe observecl lack of statistically significant interaction(F).24=2.49: p< 0.(5). Thus, valucs of DRMAX forU.

antarcticaare approximately twice thosc forU. aurantiaco-atraover the observed temperature range. Using log;

transformed data still shows some curvilinearity in the relationship with tcmperature for both species (Tab. 2).

NPMAX (mgC02 g" h~I):U. aurantiaco-atrashows a statistically significant decrcase with temperature, where as forU. tmtarctirathere is a pronounced quadratic effect with NPMAX increasing to a peak between5to 10' C and then falling off again (Tab. 2). Two-way analysis of varianeo ShOWR a highly statistically significant

NPi\iI;\X(mgCO~gtr':

Spccics Tempcraturc (Cl

-5

111 15 approx

20 25

u.«

0.0I)±O.OO.1 0.0))±O.004 0.IIR±O.OI4 0.2R)±0.027 O.276±0.c109 OAR7±O.OOR

U.a.-a.

0.009±O.002 0.029±O.00.1 0.(1)I±0.004 II.IIRS±O.O IS 0.167±0.OU O.220±O.023

Parameters DRivl;-\X ratic

li.a./U.'-L-a.

1.7 1.9 2.3 3.4 1.7 2.2

u.».

0.107±0.1I1.1 0.1 R6±O.020 Ü.224±O.(121 O.216±O.023 O.OR6±O.OOR O.05()±0.025 0.01.1±0.006

Ll.a..a.

0.060±0.004 O.OR 1±0.01l4 O.063±O.(103 0.04S±0.00S OJm±II.009 1I.IIOS±0.OU2 0.004±0.0I11

NPiViAXratio U.a./U.a.-a.

I.R 2.3 3.6 4.R 2.7 6.3 3.3 Tab. 3:Mcan estimated (±SE) forDRMAXandNPivlAXwithappropriatcrattos ateight tcmperaturcs. U.n. ::;;USlICaantarctica and U.a.-a.::;;Usnca aurantiaco-atro.DRl'vlAX:NP;'vIAX rattosbctwcen-.:'i10approximatcly2(YCfor1).<1, undti.a.-a. respcctively are:Ll.a.:0.14.0.30. OS'). 1.32.3.21.

9.74:U.a-.a.:0.15. 0.36.0.81.1.89.5.12.27.50

175

internenon effect (F6.42= 14.1: p< 0.00 I) confinning the obvious that thc elifference betwccnU. antnrctiraanel U.aurcmtuno-atrnvaries with tcmperature. Ovcr the range -5 to 25" C the elifferences betweenU.antarctica andU.ournntincoa-otra(from Tab.3)arc as follows: 0.047. 0.105. 0.161. (J.l71. 0.054. 0.042. and 0.009 mu C02 g-t

n'

(SE (dif) = 0.(19). Thus. U. antarctira has a rate which is consistently lugher than for

U.

anmnriaco-atmand most marked in the range 0 to 10"C.

The variation in QIO values 1'01' DRMAX and NPMAX across the experimental temperature range is presenteel in Table 4. Thc DRMAX:NPMAX QIO value ratio rernains rclatively stable between-5 anel 10· C inU.aurantia-

CO-Oll"({and then increases between 10 and 20" C. The cquivalcnt ratios arc rnore variable inU. antarcticabut show^anoverall increasingtrend acrossthe^samercmperarures^range.

Tcmpcr.uurcintcrvalI"Cl

0,(,valucs Speeres -5-0 0- 5·-10 ^10-1) 15~20or20 20-25

for parametcrs

DRM;IX ^Li.a."'I. IO.3~±5.84 3.09±1.69 2.7X±2.3S 3.X6±2.80 1.99±2.09

NI'MilX u.«.a. 1.82±0.83 O.60±O.39 n.:'iI±O.64 0.51±1.59 IL06±O.70 1.25±5.39

U.a.-'L 5.7 5.2 5.5 7.6 33.2

li,a. 13....1-4±5.77 4.60±2.2! 5.1)3±2.71 O.94±O.72 3.1 I±O.48

NPJvlAX LI.n. ?dJ2±2.0X 1.61:t1.33 O.97±l.O I ^O.14±n.38 (J.:\4±2.18 O.05±l.JO

U.n. 4.5 2.9 6.0 6.7 9.1

Tab. 4: Q:nvalucx.andnllios.forthc mcnn valucs01'DIVvJi\XrtromTnb!c2)undNPivJAX(fromTablc2)with approximatc~':SI .U,n.=Usnca ollfaFcri("{/and U.a.---a.={/SIIC{/aurantinca-cma

4. D1SCUSSION

Thc depression in net photosynthesis at full thallus hydration is grcarer inU.anrantiaco-atrathanU.antarctica at all tcmperaturcs studiert except 25' C (Tab. I). This could possiblybeintcrpreteclbyLANGE (IY80) as being due. in part, to an increase in the resistance 01'C02 diffusion through water in the liquid phase, as opposed to through the air. However, diffusionneeclnot be through water to increase the resisrancc. there could be fewer air diffusion pathways. Howcver, theU.antarcticathallus contains more watcr at full hydranon thanU.aurantia- co-atra.11' the higher wateI'content 01'U. antarcticaresults in a rcduction 01'air diffusion pathways as eompareel toU.anrantiaco-atra.rhis mal' result in a greater resistance to C02 diffusion in theU.antarcticathallus at ful1 hydration, ancl thus lead to a greater elepression in net photosynthesis in this species when compareel with U.

atnantiaco-atra,With the cxceprion at 25' C the reverse is observed. Higher rares OfC02 fixanon inU. antarctica could offset sorne 01'the effects a higher fully hydrared water eontent would have in decreasing the rate 01'C02 diffusion from the air to the algal cells. However. we would expect rhat this gain would be offsct by thc mucb grcatcr rates 01'dark respiration inU.antarctica,therefore, another reason must be sought.

The increased depression in net photosynrhcsis values inU. aurantioco-atmcomparecl withU. antarctiraeouIeI be due to a greater ratio 01'net respiration to net photosynthesis in the forrner speeies. This possibility is suggested by a comparison oIthe DRMAX:NPMAX ratios in Table 3 wh ich are always greater inU. auwilliaco-alm.This is also paralleled in the ratio oltbe DRMAX:NPMAX QIO values in Table 4. where overall they are greater inU.

a[frailliaco-alra.

HARRISSON et al. (1986) analysed the shape 01'the moisture depenelent dark respiration eurves for the foliose lichenUmbilicaria antarctica.They conclucled that the diphasie moisture depcnclent relationsbip indicated that water was being stored in the intereel1ular air sp3ces in the medul1a 01'the thallus. 11' this proposition is true then the moisture elepenelent forms ofthe dark respiration responses in Figure 2 woulel inelieate thatU. auwntiaco-alm - with its moderately elipbasic mai sture elependent response - has a mocleratcly high intercel1uIar water storage capaeity - assumecl to be primarily Ioeatecl in the medul1a - similar to that 01'Umbilicaria anlaretica.However.

USllea aillarclica with its virtual1y linear moisture depenclent response would appeal' to have a very 10w intereel1ular water storage eapaeity. wh ich is also assumed to be primarily locatecl in tbe meelul1a.

HARRISSON et al. (1986) usecl Q]() values to suggest the presence 01' ice within the thal1us 01'Umbilicaria

Usnea aurantiaco - atra Usnea antarctica

. . . . . . . . . .

. . . . . .

. .

.4.=20"G

0 =15°G

'17=10^{0 G} 0= SOG 6= OOG o=-S°G

8".8 8 38 8 83 3 000~~'S

0°° 9

° ^V

0°°° vvv v

... 0 0 0 vvv^v 00

0 0 0

. ' 0 VV 0 0 0 0 0 0 0 0 0 0 0

•

_nCQe~B~~OOOOOOo

AAAA6.6.66.A6.6666.666.66.6666.

~~ A,~000090iS8$800ooo~g8f

1.5 2.0

b

02

f-

0.1 0.3 04

2.0 .4.=19°G 0= 15"G '17=10"G 0= 5"G Ll.= O"G

<)=-soG

...

...

..

^'

...

.

o^{o0 0 0}

0 °

0 0 0 0 0

00 0

0 0

0 0 ° o o o

. . .

0.5 1.0 1.5

a

vvvv VV

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antarctica.If their argumcnts arc correet the Q¹⁰values in Table 4 would indicate the presence of icc within the thalli of U.aurantinco-atra and U.antarctica. The QIO values for the temperature interval -) to0' C are exceptionally high for maximum rate of dark respiration - 10.38 and 13.44 for U.auraniiaco-atraand U.

antarctica respeetively. HARRISSONet al. (1986) suggested icc formation wirhin the thallus would lcad to incrcased internal C02 diffusion rcsistances, thus elevating thc^QIOval ucs between -5 und 0'C.The corresponeling QIOvalues for the maximum rate of net photosynthcsis are also slightly elevared Ior the remperature range --5 to 0' C. possibly for the same reasons. However, the effect is negligiblc because at the maximurn rate of ner photosynthesis there is less water in the thallus (NASH et al. 1983). This woulel result in a much reduced volumc of icc forming, proelucing negligible increases in internal C02 eliffusion rcsistances. Howevcr, it must be admitted that at-5'C the intercellular water may not be frozen, anel increases in C02 cliffusion resistanees do not easily explain dccreascs in respiration rares, since the possibility exists that high local CO} conccntrations could be expcctcd, which would easily overwhelm the resistancc effeet. However, it must bc noted that a patina of ice always formed on the surfaee of each rhallus at-5'C. Therefore. even if the inrercellular water is not frozen there still exists an ice barrier to gnseous diffusion. Thc possibility of membrane darnage occuring at-5'C shoulel bc invcsrigared. Such a possibility could provide an alternative explanation for decreased C02 exchange rates - in the light and dark - and thus elcvatedQIOvalues for the temperature interval-5 to 0' C. If fungalmembranes are more susceptible than algalmembranes. this could account for the grcater elevation inQIOvalues for dark rcspiration when compared with net photosynthesis.

LANGE (1980) suggested that interaction between changcs in the C02 diffusion rcsistance through water with tcmperaturc and the mass ofthallus water present mighr be responsiblc for thc shift in the wateI'content yieleling the maximum rate of net photosynthesis (WCNPMAX) to lower water contents at higher tcmperatures (Tab. I).

NASH ct al. (1983) suggested thar water contents yielcling optimal photosynthetic rares occur whcn rhc intercellular spaces arc not fillecl with watcr, and assumed thar CO} does not pass through a water layer as it diffuses inward to the algae. Such conditions would be directly analogous to C02 travell ing though vascular plant stomata allCl substomatal cavities to the mesophyll cells.

In both species there is some decline in the water content yielcling the maximum rate of net photosynthesis with temperature. VaIues forU. allretllliaco-alraare less than for U. alilarelicawhich could suggest that the former species has a larger intercellular water mass than the laller. This conclusion was previously rcached from an analysis of the shape of the moisture elepenclent clark respiration eurvcs.Fmmueh of the temperature range ofU.

anlarcticathc low mass of intercellular water - ifthe suggestion is correct - woulcl result in a relatively high and fairly eonstant water content yielcling the maximum rate of net photosynthesis. Only at lligher temperatures woulel increases in the CO} cliffusion resistance of water be large enough to require a much recluced thallus water content, inU. (J/llarclica,in ordertoyield the maximum rate of net photosynthesis. Alternatively it must be noted that the greater increase in dark respiration with temperature than photosynthesis would also give this result.

To concluele, although morphologieally similar (WALKER 1985) the respiratory ancl photosynthetic responses üfthe two speeies to the same experimental conclitiüns were totally clifferent. The possibility exists that they differ in their light response. but unfortunately no experiments coulcl be perfmmeclto aseertain the pattern of response of net photosynthesis to increasing irracIiance levels - at eaeh thallus water eontent ancl temperature. Also the two speeies used in the experiments clifferecl with regarclto their müdes of reproeluction. Within the subgenus NClIropogon Usnca allranliaco-atret is one of six species which reproduces sexually. by means of abunelant ascospOl'e producing apothiea, lacking any vegetative propagules. 'vVhereas, Usnca anlarcl;ca is one of four asexual speeies whieh are üccasionally fertile (WALKER 1985). The U. allretnliaco-alra thalli used in the experiments bore abunclant apothecia. Whereas, the U. anlarclica thalli possesseel abundant soredia. These vegetative propagules are clefinecI as clusters of funaI hyphae and algal eells without cortex, anel are classified as seconelary in that they are proeluceel on papillae, and unlike species in which smalia develop from the cortex, are not eünfined to apices m seconclary branches (WALKER 1985). The possibiIity has not been eliscounteel that the presence of these wideIy different reproductive structures couId account fm some of the differences in CO}

exchange between the two species, under the same experimental eonditions.

5. ACKNOWLEDGEMENT

We woulcl like to thank Drs. D. W. H. Walton and R.J.Le wis-Smith for their most helpful commcnts. cditorial assistance and supervision of this projcct, and thank June Stokesfortyping the manuscript, and Roger Missing for preparing^the figures.

Rcl"crcllccs BIo c k . W.(\9XS):SurvivalOllland. Biologist 32er):!33-138.

Ha r r iss o11 •P. .\V a I!0 11 .D. W. I-I.8.:Rot her Thc cffccts oftcrnpcrnturc nndrnoisturc on dark rcspiration in thc foliosc liehen antarrtu:a. Ncw Phytol 103:

Ho0k er.T. N. Growth and producnon01'USl1e(/antarcnca and UvfascicnoOllSigny bland. South Orkncy lsland. Br.Antarct.Sutv.

Bull . ."i0:

HLI 11cc k . S. Sn ins11urv. lVI. Ri ck ar d .T. lVI. A.8.: S 111i (h. R.I. L. Ecolcgicnl and chcmical invcstigations01'lichcns Irom South Gcorgia and thc maritime Antarcticn. -Journ. Hattori BOI. 56:

1 n o . Y. Comparativc 01'thc cffccts of rcmpcranncOllnct photosvnthcxis nnd rc.spiration in lichcns Front thc Antarctic aud subalpine zoncs Japan. - Thc Magazine Tokyo98: 41--.'13.

Kap p c n.L.(1983): Ecofogv and physiology of thcAntarcticfruticosc liehenLiSI1C(fsulphurca (Koeuig) Th. Fries. --- Polar Biol. I: 249--255.

Kap pcn. L. (1985): Watcr rclurions und not photosynthcsisof[/SI1C{[.A compnrison bctwecnUSI/C({ fasciata(maritime Antarctic) undUSI/ca snlphurca(contmcntal Antnrctic}. - In: Brown. D.l-i.(C(U Lichenphysiclogv and cell hiology: 41--56. Plenum Press. NcwYorkand l.ondon.

Ke r shaw .K.A. (! (85): Physiological ccology01" lichcns ...~.Camlnidgc Univcrsity Press. Cambridpc.

La m b .I.M. <.19(4): Anrarctic Iichcns. I. Thc aencraUsnca.Romalino.1-/iIlWIlIOFIIli(f.AlcC/or!(/ and Corniruknin. --- BI'. Antatet. Surv. Sei. Rcp.

31\: 1-34. ~

La ng c . O.L. !98{)): Meisture contcnt and CO: 01"hchcns. 1. lnflucnccoftcmpcraturc on mciisturc-dcpcnclcntHelphotosynthcsis and dark inRanuüinasnacitonnis . .-- (Bcrl.) 45: 81---87.

L0ng1 0 Il. R. E. (1985): Tcrrcstrinl habitats-vcgctation. -In: Bonne!". W. N. und Walton. D.\V.1-1.(cds.) Kcy cnvironmcnts: 73-105. Pergarnon Press.Oxford.

Nash 111. T H. ;\'10scr. T. 1. Link. S.O.. oss .L.J..Ola1"sen. A. 8:.1''o'latthes. U. (198_l):l.iehenpholosynlhesisin rclntion 10CO~conccnrration. - Occologin 58:.'12-56

Per killS .J. E. (1945): Reports on scicmific rcsults of rhc United Stares Antarctic Service Expedition. -- Proc. Amcr. Plul. Soc. 89 (I): 1--398.

Sc 11111id 1- Ni c I s c n K.(1975):Anima! physiology:Cambndgc Univcrsity Press. Cnmbriclgc.

Smit h . R. l. L. (1971): Vegetation of the South Orkncy lslnndswirhparticularrcfcrenccto Signy lsland. -" Br. Antnrct. Surv. Sei. Rcp. 68:

1----124.

Smith . R. I.L. In: Laws. R. lVI. (cd.j Antarctic ecologyVol.1:61~-162.

Acaclcmic

Wa J k er, F.J.(1985): The liehen gcnusUSl1casubgcnusNC/lropogon. BuH. Br. l\'lus. nal. His\. (Bot.) IJ(1):1-I JO.

\Va 1I0n. D. W. 11.(1984):Theterreslrialenvironment. -'" In: Laws. R. M. (cd.) Anlarctic eeology Vo1. 1:1·"-"60.AcademicPress, London.

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