The 'inch flora': Some observations on the morphology and seed biology of annual plant species common in semi-arid woodlands of western Victoria

2012·1.7 MB·English

by  John W, O'brien Morgan

Research Reports
The ‘inch flora’: some observations on the morphologyand
seed biology ofannual plant species common in
semi-arid woodlands ofwestern Victoria
W
John Morgan andTeri P O’Brien
DepartmentofBotany,LaTrobeUniversity,Bundoora,Victoria3086
Email:[email protected]
Abstract
Annualplantsarean under-studiedcomponentoftheVictorian flora.Wedescribethemorphologyandseed
biologyoftenspecies commontosemi-aridwoodlands infarwestern Victoria:Actinoboleuliginosum, Blen-
nospora drummondii, Calandriniagranulifera, Centrolepis strigosa, Millotia tenuifolia, Podothecaangustifoliay
Pogonolepismuelleriana, Qui—netiaurvillei, Rhodanthepygmaea, Triptilodiscuspygmaeus.Twoformsofgrowth
morphologywereobserved erect’ and‘branched’. Threespecieswerefoundto havemyxospermous seeds.
Here,theseedcoatcontainsamassofdrymucilagewhichhydratesandswellsrapidlyoncontactwithwater.
Eightspeciesappeartohavedormantseedatthetimeofdispersal,presumablyastrategytoavoidgermination
insummer.Thisdormancywasovercomewhenseedwasheatedto35°Cfor4weeks.Ourpreliminarystudies
confirm that annuals are a fascinating group ofplants which warrant much better ecological understand-
ing. Further quantitative investigation ofthe seedbiology(particularlythe extentand roleofmyxospermy),
seedgerminationrequirements(includingdormancy-breakingmechanisms) anddevelopmentalanatomyare
clearlynecessary. {The VictorianNaturalist129(1)2012,4-9).
Keywords:germination,growthform,myxospermy,seeddormancy, annual flora
Introduction
Annual plants of the worlds arid ecosystems cies (Quinetia urvillei) is classified as rare in
can be divided into those that flower and seed Victoria. The maximum heightofall species is
in the summer and those that carry out those <100mm,withseveralspeciesstrictly<30mm
activitiesinwinter-spring.Annualspeciesfrom tall at maturity. All species we have observed
low rainfall zones in western Victoria fit com- demonstrate one oftwo characteristic growth
fortably into the winter-spring active group, morphologies (Table 1). Species eitherdevelop
using the winter-dominant rainfallthat occurs (i) asingleshootwhich bearsjustoneterminal
in this region. While annuals are a common inflorescence(‘erect’habit;Fig. 1),or (ii)oneor
componentofsemi-aridwoodlandsandmallee moreadditional shoots are producedfrom lat-
shrublandsinsouthern Australia (e.g. Rice and eralbudsontheprimaryaxis(‘branched’habit;
Westoby 1983; van der Moezel and Bell 1989; Fig.2).Theselateralshootsappeartogrowdia-
Morgan et al 2011), few species have received geotropically and then curve upwards, bearing
detailed ecological study. Indeed, of the spe- either inflorescences or further shoots that, in
cies we studied, only the seed dormancy and turn,bearinflorescencesin amoreorlesserect
germination ofActinobole uliginosumhas been position. Six species have ‘branched’ habit but
previouslyreported (Hoyle etal 2008). Hence, there is much variation. Blennosperma drum-
in this contribution, we introduce aspects of mondti, for instance, omitsthe developmentof
the morphology and seed biology ofa suite of the lateral shoot stage and has either just one
annual species common to semi-arid eucalypt single shoot with a terminal inflorescence, or
woodlands in Victoria based on our observa- two or more laterals that grow atan angle that
tionsofthese much-overlookedplants. is steeply inclined to the vertical, each termi-
natedbyasinglesmallerinflorescence.InPogo-
MWoerpshtuodlioegdy1a0ndspesceieedsbtiyopliocgalyly found in the nolepis muelleriana^ it is very clear that when
additionalbranchesarisefromthediageotropic
‘inch flora' ofsemi-arid woodlands in western laterals,the sizeofthe inflorescencesproduced
andnorth-western Victoria(Table 1).Onespe- are smaller than those produced terminallyby
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Table1.Height,growthformmorphologyandevidenceofmyxospermyfortenannualplantspeciescommon
tosemi-aridwoodlandsinwesternVictoria. *Height<30mm; Comprisingasingleshootwhichmaybear
justoneterminalinflorescence(erect),oroneormoreadditionalshootsareproducedfromlateralbudsonthe
primaryaxis(branched); Seedsrichinmucilagewhichhydratesrapidlyonwetting;****Seedssurrounded
byhairsthatturnmucilaginousonwetting.
Species Family InchPlant* Morphology** Myxospermous***
Actinoboleuliginosum Asteraceae Strict Branched Yes
Blennosporadrummondii Asteraceae Strict Branched Yes
Calandriniagranulifera Portulacaceae Strict Branched No
Centrolepisstrigosa Centrolepidaceae Strict Branched No
Millotiatenuifolia Astercaeae No Erect No
Podothecaangustifolia Asteraceae Variable Branched No
Pogonolepismuelleriana Asteraceae Strict Branched Yes
Quinetiaurvillei Asteraceae No Erect No
Rhodanthepygmaea Asteraceae Variable Erect No****
Triptilodiscuspygmaeus Asteraceae Variable Erect No
the original diageotropic shoot. This is equally little as a few minutes (Fig. 3). In Rhodanthe
truewith Blennospermadrummondii. pygmaea, seeds are surrounded by hairs that
Three species {Actinobole uliginosum, Blen- turn mucilaginous on wetting. Myxospermy
nospora drummondii, Pogonolepis muelleriana)
have myxospermous seeds. Here, theseed coat
containsamassofdrymucilagewhentheseeds
areshed.Themucilagehydratesandswellsrap-
idly on contactwith water, in some cases in as
Fig. 1. Examples ofannualplantswith erect' growth morphologywhereasingleshootbears aterminal in-
florescence. (a) Millotia tenuifolia, (b) Siloxerus multiflorus and (c) Gnephosis drummondii. Photos by Pete
Green.
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Fig.2.Actinoboleuliginosumisanexampleofaspecieswith‘branched’growthmorphologywhereoneormore
shootsareproducedfromlateralbudsontheprimaryaxis.PhotobyPeteGreen.
has been studied in many species of desert Observationsondevelopmentalanatomy
plants (reviewed by Fahn and Cutler 1992), A feature of annual plants is their ability to
and been found to be common in plant fami- growrapidlyaftergermination,establishoneor
lies such as PlantaginaceaeandAsteraceae. In- more shootsthat bear inflorescences, andthen
terestingly, myxospermy is common in winter to flower and produce seed rapidly. One can
annualsofIsraelandSouthAfrica (van Rheede speculate that achieving this end has sparked
van Oudtshoorn and van Rooyen 1998); how- the creation ofthe 'branched’ morphology we
ever, it does not appear to have been investi- observed. First, photosynthetic tissue needs to
gated much in Australian annual species. A be close to the sites at which photosynthates
number oftheories have been suggested about are consumed,becauseit isalmostan axiomof
its significance and mxyospermymightbe im- phloem function that the shorter the distance
portant when seeds are both wet and dry. We betweensourceandsink,themorerapidlypho-
areinclinedtotheideathatitassiststheseedto tosynthetic products are moved to their desti-
attach to the soil, giving the radicle something nation.Inplantswiththis ‘branched’morphol-
topushagainstasitstartstoelongate,minimis- ogy,suchmorphologyguaranteesthatthesinks
ingthe riskofpushingthe roothairzoneaway and the source of photosynthates are never
fromthesoilsurface.Thismightbeparticularly more thanafewcentimetresapart.
important where the substrate is covered by a The water and mineral nutrition for these
biological soilcrustas is commonin semi-arid axes will be drawn from soil and the root sys-
areas. Clearly this is an area in need ofmuch tems,andsomewaterwillnodoubtbetakenup
greaterstudy.
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Research Reports
Fig. 3. EvidenceofmyxospermyinBlennosperma drummondii (foreground) compared lo aspecies which is
not myxospermous {Podotheca ungustifolia, background). In this example, the mucilage had hydrated and
swelledrapidlyoncontactwithwaterapplied 15minutespreviously.PhotobyJohnMorgan.
byabsorption throughgrowingleafsurfaces,in ledons have barely finished unfolding. In soil,
somecasesas dew; however,thereareno stud- this primary root develops lateral branches at
ies on the developmental anatomy and histol- regularintervals (5-10mm),lateralswhichare
ogyofanyofthese specieswith theseissues in also strongly diageotropic from the moment
mind. Theuptakeofwater, photosynthates and they emerge from the stele ofthe parent root.
minerals into the developing flower heads and These laterals then produce vertical roots with
seedsofthesespecies, w'iththeurgencycreated numerousroothairswhichcanadherestrongly
by the relatively short growing season, and its to sandgrainsin thesoil. A detailed analysis of
timing with respect to vascular development, therootmorphology,includingdevelopmentof
will likely be of considerable interest to eco- themycorrhizasknowntobeafeatureofplants
physiologists. growing in these habitats (Warcup and McGee
Our preliminary observations suggest that 1983),and ofthesoil sheathsidentifiedbyMc-
the root system engages in asimilarpattern of Cully(1987), whichhavebeenshowntopermit
development to that ofthe shoots. A primary non-rhizobialN2fixation,awaitdetailedstudy.
radicle emerges rapidly from below the hy-
Preliminarystudiesofgermination
pofocohtyyplocwohtiylchhamiarsy.oDrumrianygnothtishasvteagae,comraonnya We collected ripe seeds from all species in
October-November 2009 from a number of
seedlings have theircotyledons still encased in
state forests between Horsham, Dimboola and
a seed coat and, in the case ofmyxospermous
GorokeinwesternVictoriatoexaminethecues
species, the seed coats are firmly stuck to the
for germination in annuals. In particular, we
substrate. This radiclegrowsverticallyand rap-
idly,reachingadepthof70 mmwhen thecoty- were interested in the temperatures that pro-
mote germination and the environmental cues
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Research Reports
Toanbflree2s.hSsueemdm.aAryfooufrtthheggeerrmmiinnaattiioonntrreesaptomnesnetoofntefnreasnhnsueaeldpwlaanstpsrpeeccieedsedtobmyovheea-tailnognsge(edMsAaIt-335)°tCrefaotrmefnotusr
weekspriortosowing.
ExperimentalTreatment
Species MA1;30“C MA2;20°C MA3:GA3@20°C Seedheating@
(12hrslight/dark) (12hrslight/dark) (12hrslight/dark) 35°Cthen20°C
(12hrslight/dark)
Actinoboleuliginosum No No No Yes
Blennosporadrummondii Yes-substantial Nottested Nottested Yes
Calandriniagranulifera No Yes-substantial Nottested Yes
Centrolepisstrigosa No No Yes-minimal No
Millotiatenuifolia No No No Yes
Podothecaangustifolia No No No Yes
Pogonolepismuelleriana No No Yes Yes
Quinetiaurvillei No No No Yes
Rhodanthepygmaea No No No Yes
Triptilodiscuspygmaeus No No Yes Yes
thatmightbenecessarytoovercomedormancy. Afteranother3weeks,westartedGermination
Wedonotreportgerminationexperimentsper Trial3 in an attemptto overcome dormancyof
se; rather, we make observations oflikely ger- seeds. Gibberellic acid (GAft, known from the
minationstrategiesinthesespeciesasaprecur- literaturetobecapableofbreakingdormancyin
sortomore formal study. some desertannuals (Plummerand Bell 1995),
Within two months of seed collection, we was added to all Petri dishes where no germi-
started Germination Trial 1. Weplaced 70-100 nation had previously been recorded (i.e. 8 of
tilled seeds ofeach species on Whatman No. 1 the 10 species). WetreatedeachPetridishwith
mm
filter paper in individual 90 Petri dishes 7.5 mlofGA3 (at 200 mg/1), andthen returned
andplaced them at30°C constanttemperature dishes to the growth cabinet at 20 °C (12 hrs
inaThermolinegrowthcabinet(12hrslight/12 light/12hrsdark).Thisexperimentreleasedthe
hrs dark). Wewere interested in whetherfresh dormancyofsomespeciesincludingCentrolepis
seed would resist germination upon wetting strigosa, Pogonolepis muelleriana and Triptilo-
at high temperature. Within one week, Blen- discuspygmaeus; however, for Actinobole uligi-
nosperma drummondii had produced a few nosum, Millotia tenuifolia, Podotheca angustifo-
germinants; all other species showed no signs lia, Quinetia urvillei and Rhodanthe pygmaea,
of activity. After 3 weeks, Blennosperma had nogerminationwas observed.
achieved agermination ofapproximately 70%, Finally, in Germination Trial 4, we exposed
butnootherspeciesgerminatedatthistemper- filled seed ofall species to 28 days ofheating
ature.Atthistime,wemoved all Petridishesto at 35 °C. This is considered a form ofdry af-
20°C constanttemperature (12 hrslight/12hrs ter-ripening. Heating was achieved by placing
dark) (Germination Trial 2) in a ‘move-along’ seeds in paper bags inside a thermostatically
experiment (i.e. seedexposedto one treatment controlled oven. After heating, 70-100 seedsof
thatdoesnotinducegerminationaremovedto eachspecieswereplacedonWhatmanNo. 1 fil-
asecondtreatmentinan attempttodoso). terpaper in 90 mm Petri dishes, asbefore,and
Calandrinia granulifera germinated rapidly placed at 20 °C constant temperature, with 12
whenseeds were moved to 20°C; >80% germi- hrslight/ 12 hrs dark. All species, except Cen-
nationwasobserved withinthree daysofstart- trolepisstrigosUygerminatedinthe3weeksafter
ing this trial. No germination, however, was westartedthistrial.Germinationwashigh;60-
recordedfor anyother species at thistempera- 80% germination was observed in all species.
ture suggesting that seeds have aphysiological WesummarisetheseoutcomesinTable2.
dormancyuponbeingshed.
8 TheVictorianNaturalist
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Research Reports
We conclude from these observations that Acknowledgements
the common seasonal pattern of high sum- Technical assistance was provided by Max Bartley
mer temperatures and a variable period oflow and Pete Green kindly allowed us to use his photo-
graphs. Professor Roger Parish gave on-going en-
rainfall combine to render the seed of many couragementandsupport.
annual species initially dormant upon disper-
sal. Blennospora drurnmondii and Calandrinia References
graniiliferawouldappeartobeexceptions;they FathrnaegAera:nBderCluitnl)erDF(1992)Xerophytes.(GebruderBorn-
await the arrival of appropriate temperatures Hoyle GL, Daws MI, Steadman K) and AdkinsSW (2008)
and moisture to germinate. As the summer mPraen-cayndalploesvti-athiaornvesotf ainnflAuuesntcreasliaonn Apshtyesriaocleoageicaslpecdioers-;
progresses, the high temperatures that seeds Actinobole uliginosum (A. Gray) H. Eichler. SeedScience
experience at (or near) the soil surface ap- McRCeuslelarychM1E8,(119918-71)99S.elected aspects ofthe structure and
pear likely to slowly reduce the dormancy of development offield-grown roots with special reference
the embryo, which eventually responds to the tomaize.InRootdevelopmentandfunction,pp53-70.Eds
combinationofcoolerconditionsandmoisture PPrJesGsr:egCoarmyb,riJdVgLea)keandDARose.(CambridgeUniversity
by rapidly germinating. Actinobole uliginosum, Morgan [W. Wong NK and Cutler SC (2011) Life-form
Millotia tenuifolia, Podothecaangustifolia, Qui- wspoeocdilesa-nadrecaommruelnaittiyo.nsPhlipasntEicnoloagy2t1e2m,pe1r0a4t7e-105eu5c.alypt
netia urvillei and Rhodanthe pygmaea all ap- PlummerJAandBellDT(1995)Theeffectsoftemperature,
pearto respondinthismanner. lightandgibberellicadd(GA?)onthegerminationofAus-
tralianeverlastingdaisies(Asteraceae,TribeInuleae).Aus-
Conclusions RictrealBiaannJdouWrenasltoobfyBoMtan(y194833,)93P-l1an0t2.speciesrichnessatthe
The ecology ofannual plants has largely been 0.1 hectare scale in Australian vegetation compared to
overlooked in southern Australia, despite the vaonthdeerrcMoonteizneelntPs.GVaegnedtaBteilol5D2,T12(91-91894)0.Plantspecies rich-
important contribution they make to diversity nessinthemalleeregionofWesternAustralia.Australian
insemi-aridwoodlandsandmalleeshrublands. vaJnouRrhtieaeldoefEvcaonlOoguydt1s4,ho2o2r1n-2K26a.ndvanRooyenMW(1998)
Hence, our observations were undertaken in Dispersalbiologyofdesertplants.(Springer:NewYork)
the almost complete absence ofpublished lit- WarcupJHandMcGee PAU983)Themycorrhizalassocia-
erature. By describing the variation that exists 6ti6o7n-s67o2f.someAustralian Asteraceae.NewPhytologist95,
in morphology and seed biology ofthis group,
we have demonstrated that annual plant spe-
cies exhibit a fascinating array ofstrategies to
cope with their environment. We hope that
thisstimulates furtherquantitative studyofthe
‘inch flora. Received11August2011;accepted24November201
Onehundredandtwenty-eightyearsago
MALLEEHENSANDTHEIREGGMOUNDS
ByA.J.Campbell
(ReadbeforetheFieldNaturalists'ClubofVictoria,Dec.8, 1884)
...WhenanythingismentionedabouttheMalleecountry,thereisinstantlyconveyedtothemindofmany
peopleavividpictureofdesertland, theabodeonlyofwilddogsandlatterlyofrabbits.Neverwastherea
greatererror. ItispredictedthatinthenearfuturetheMalleecountrywillbeamongthemostfruitfuland
productivepartsoftheColonyofVictoria.TheMalleescrubisaspeciesofdwarfgum-treeorEucalyptus,
and 10to 12 small treesspringfrom the oneroot. The foliage spreads over head 10 to 20 feet abovethe
ground. Thewoodishardanddurable,thebarkasarulesmoothandthin.
Thisscrubgenerallygrows inalooseyellowish sandysoil, andin longbeltsvaryingfromhalfamileto
milesinbreadth. Betweenthebeltstherearecorrespondinggoodtractsofcountrymoreopenlytimbered
withbull-oak,andstillmorethicklyinpartswheretheyareinterspersedwithoneormorevarietiesofgum
trees,Murraypine,&c.
From The VictorianNaturalistI,p. 124,December 1884
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