plant development – lessons from animals

copy

New Phytologist

(2003)

157

1ndash7

wwwnewphytologistcom

1

Forum

Blackwell Science LtdOxford UKNPHNew Phytologist0028-646XTrustees of New Phytologist 2003 157

Commentary

Rainforest trees and temperature change

What happens to rainforest trees when the temperaturechanges In this issue of

New Phytologist

(pp 55ndash64)Cunningham amp Read report on an investigation of theacclimation of photosynthesis to temperature in seedlings ofupper canopy rainforest trees The distribution of the eightspecies selected is spread over a wide range ndash 25 degrees oflatitude from cool-temperate Tasmania (41

deg

S) to tropicalQueensland (16

deg

S) For comparison equivalent northernlatitudes would reach from Lisbon to Dakar in Senegalor from Northern California to Southern Mexico Withdecreasing latitude mean annual temperatures range from 9to 23

deg

C and the corresponding annual range of temperaturemaxima ranges from 12 to 7

deg

C

lsquoGlobal warming could potentially result in higher

temperature optima and reduced spans of temperature

for maximum photosynthesisrsquo

Responses to constant and fluctuating temperature regimes

Among the sites used for the study those from the coolestclimates and highest latitudes tend to have the greatestannual range of temperatures whereas the warmest climatesat the lowest latitudes have smallest annual range oftemperatures Diurnal ranges of temperature tend to begreatest at intermediate latitudes (26ndash37

deg

S) and least at thehighest and lowest latitudes Against this backdrop theauthors experimented on seedlings that had been exposedfor 8 wk to either a lsquoconstantrsquo temperature regime (2214

deg

Con a 168 h day-night cycle) or a randomly applied series offluctuating temperature regimes that varied from 1711

deg

Cto 2719

deg

C but which gave an overall mean temperaturefluctuation of 2214

deg

C Instantaneous measurements ofphotosynthesis at various temperatures were made on thenew leaves that had formed during the previous 8 wk

Intriguingly temperature acclimation was similar in plantsthat had been previously exposed to either the constant orthe fluctuating regime Optimum temperatures for maximumrates of photosynthesis were generally higher in tropicalspecies but the temperature span encompassing 80 ofmaximum photosynthesis was greater in temperate than intropical species The authors give two alternative explanationsfor the similar responses of the species to the constant andfluctuating temperature regimesbull The species acclimate to mean temperatures and are notinfluenced by a fluctuating regimebull The acclimation is not relevant for either temperatespecies (which already have a broad photosynthetic responseto temperature) or for tropical species (which have evolvedin a climate with lesser seasonal and diurnal fluctuations intemperature)

The only species that showed increased photosynthesisafter being exposed to the fluctuating regime was

Eucryphialucida

a cool-temperate species Two tropical species respondedsignificantly to the fluctuating regime

Castanospermum australe

showed a slightly decreased optimum acclimation temper-ature and

Alstonia scholaris

showed an increased temperaturespan for 80 of maximum photosynthesis

Contrast and clines ndash beyond the temperatetropical divide

The authors contrasted the temperate and tropical speciesand concluded that the higher acclimation potential shownby the temperate species was consistent with the largerseasonal and diurnal fluctuations in temperature that occurin the temperate environments But multiple regression ofspecies means from Tables 2 3 and 5 of Cunningham ampRead (2003) shows that both the instantaneous temperaturefor maximum net photosynthesis (AT

opt

) and the temper-ature span encompassing 80 of maximum net photo-synthesis (AT

span

) are strongly related to latitude and diurnaltemperature range and account for substantially more ofthe variation in AT

opt

and AT

span

than the simple contrastbetween means for temperate and tropical species Thisindicates that there is a significant latitudinal trend (Fig 1)Latitude and diurnal temperature range account for moreof the variation in AT

opt

and AT

span

of plants subjected tothe constant temperature regime than that of plants fromfluctuating temperature regime

Previous studies of Australian rainforest trees (Hill

et al

1988 Read amp Busby 1990 Read 1990 Read amp Hope1996) have studied the effects of ambient and controlled

Commentary


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New Phytologist

(2003)

157

1ndash7

Forum2

environments on the acclimation potential of the examinedspecies but what makes the paper by Cunningham amp Readspecial is that it is the first attempt to separate the effectsof constant and fluctuating temperature regimes onacclimation potential of rainforest tree species originatingfrom different latitudes Mean temperatures and annual anddiurnal temperature ranges vary along latitudinal gradientsbut temperature fluctuations may also vary seasonally inthe same site In deserts seasonal and diurnal temperatureranges are greater in winter than in summer and winter desertannuals have been shown to have a greater capacity forthermal acclimation than summer annuals (Downton

et al

1984)

Acclimation to light regimes

Are there other examples where tree species from fluctuatingenvironments show greater potential for acclimation thansimilar species from more constant environments Differencesin light regime provide such an example Species that growin high light environments are subjected to greaterfluctuations in light intensity than those that grow in shadeor climax communities where the fluctuations in lightintensity are less Trees from open environments are typicallyprimary colonisers and early successional species whereastrees from shaded environments are usually late successionalspecies from climax vegetation

Do early successional species show greater plasticity inresponse than late successional or shade-adapted species Thisquestion has been addressed by investigations of Australianrainforest species (Read amp Hill 1985)

Ceratopetalum apetalum

and

Doryphora sassafras

are subsidiary species that co-occur

in temperate rainforests in south-eastern Australia whichmay be dominated by either

Nothofagus moorei

or

Eucryphiamoorei

These canopy dominants show acclimation ofphotosynthesis (on an area basis) to high and low light regimeswhereas the photosynthesis of the subsidiary species showslittle acclimation to differing light regimes Similar differencesin acclimation are shown in Chilean

Nothofagus

species theshade tolerant

N alpina

shows little acclimation to highlight regimes while the canopy dominants

N dombeyi

and

N obliqua

show greater acclimation to changed light regimesSimilar observations have been made in North Americawhere late successional shade-adapted tree species such as

Tilia americana

and

Acer saccharum

showed little alterationof photosynthesis rates when grown in contrasting lightregimes and even showed enhanced photosynthesis aftershade treatment By contrast mid-successional tree speciessuch as

Quercus imbricaria

and

Platanus occidentalis

showedlarge differences in photosynthesis rates after being grown insun and shade (Bazzaz amp Carlson 1982)

Acclimation to freezing temperatures

A final example is concerned with frost resistance of treespecies along a latitudinal gradient It is well known thatspeciesrsquo frost resistance is well correlated with latitude speciesfrom high latitudes have high frost resistance those fromlow latitudes have low frost resistance (Sakai amp Larcher1987) As mentioned previously high latitude temperateenvironments are characterised by lower mean temperaturesbut greater seasonal and diurnal variation than low latitudetropical environments If fluctuating environments leadto the selection of species with greater acclimationpotential then species from lower latitudes would be expectedto show lesser acclimation potential than those from higherlatitudes

Read amp Hope (1989) provide an appropriate Australasianexample They measured the frost resistance of species of

Nothofagus

from Tasmania south-eastern Australia and PapuaNew Guinea These were artificially hardened in growthchambers with a short (8 h) photoperiod at 15

deg

C and a 16-hdark period at 2

deg

C The maximum hardiness obtained inthe growth cabinets was similar to that in plants that hadbeen hardened naturally outside Frost resistance increasedwith latitude and decreased with both accumulated temper-ature (day-degrees) and the estimated temperature at the sitesof origin of the species and was also associated with photo-synthetic parameters Both the temperature span for max-imum photosynthesis of the species and the degree of frosthardening increase with latitude (Fig 2) indicating a similarresponse to environmental temperatures

Is the mean temperature or the annual or diurnal temperat-ure range the controlling factor At low altitudes temperat-ures decrease and diurnal and seasonal ranges of temperatureincrease with latitude ndash consequently tropical latitudes are

Fig 1 Acclimated temperature span (AT span degC) for 80 of maximum photosynthesis in relation to latitude of origin of the species Filled circles indicate acclimation to the lsquoconstantrsquo temperature regime and open circles represent acclimation to the fluctuating regime Data from Tables 2 and 5 (Cunningham amp Read pp 55ndash64)

Commentary

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New Phytologist

(2003)

157

1ndash7


Forum 3

characterised by high temperatures with little seasonal anddiurnal fluctuation Diurnal ranges of temperature howeverincrease dramatically with altitude at tropical latitudes sothat at high altitudes seasonal variation in temperature isslight but diurnal variation in temperature becomes extremeThese diurnal fluctuations do not result in increasedacclimation (hardening) of tropical alpine plants to lowtemperatures but rather to avoidance of the effects of lowtemperature by insulation supercooling leaf movement andsheer bulk (Beck

et al

1982 Goldstein

et al

1985 Squeo

et al

1991) The plantsrsquo responses to diurnal changes intemperature may not be rapid enough (or perhaps tooenergetically expensive) to accommodate acclimation Rapidacclimation to high temperatures does occur for exampleMacaronesian succulents show marked diurnal changes inheat tolerance (Loumlsch amp Kappen 1984)

Plant strategies and acclimation

The trees investigated by Cunningham amp Read are mostlylong-lived late-successional canopy species that belong toGrimersquos stress-tolerant category (Grime 2001) They areconsidered to show slow morphogenetic responses but rapidacclimation of photosynthesis and other physiological func-tions (Grime 2001) In the studies mentioned previouslyacclimation of photosynthesis and frost resistance increasewith latitude indicating that greater acclimation occurs inharsher environments with colder temperatures and greatertemperature extremes than in tropical environments Bothphysiological acclimation of photosynthesis and morph-ological acclimation of leaves are typically less in shade-tolerant trees than in emergent trees from open canopiesThis appears to be a general phenomenon (Grime 2001)

that is to some extent at variance with the characteristics ofstress-tolerant species (Grime 2001) as both morphologicaland physiological plasticity are low However species adaptedto shade exist in a more constant light environment and theirlesser plasticity reflects this

Perspectives

Cunningham and Read have produced a carefully designedexperiment that was designed to detect any differences inphotosynthetic acclimation to temperature after exposure toa constant thermal regime or a fluctuating regime Thesecontrasting regimes made minimal differences to the degreeof thermal acclimation of photosynthesis The variousmeasures of temperature at sites of origin of the species arehighly intercorrelated but annual mean temperaturesalone account for more of the variation in photosyntheticacclimation than any other combination of temperaturesThis may indicate that mean temperatures are the drivingforce and annual and diurnal ranges of temperature haveonly a small modifying effect If photosynthetic acclimationis a function of mean temperature at sites of origin of thespecies then global warming could potentially result inhigher temperature optima and reduced spans of temperat-ure for maximum photosynthesis Time further experimentand the species will tell

Peter Bannister

Department of Botany University of Otago PO Box 56Dunedin New Zealand

(email peterplantaotagoacnz)

References

Bazzaz FA Carlson RW 1982

Photosynthetic acclimation to variability in the light environment of early and late successional plants

Oecologia

54

313ndash316

Beck E Scheibe R Steiger D Pongratz P 1982

Frost avoidance and freezing tolerance in Afro-alpine lsquogiant rosette plantsrsquo

Plant Cell amp Environment

5

215ndash222

Cunningham SC Read J 2003

Do temperate rainforest trees have a greater ability to acclimate to changing temperatures than tropical rainforest trees

New Phytologist

157

55ndash64

Downton WJS Berry JA Seemann JR 1984

Tolerance of photosynthesis to high temperature in desert plants

Plant Physiology

74

786ndash790

Goldstein G Rada F Azocar A 1985

Cold hardiness and supercooling along and altitudinal gradient in Andean giant caulescent rosette species

Oecologia

68

147ndash152

Grime JP 2001

Plant strategies vegetation processes and ecosystem properties

2nd edn

Chichester UK John Wiley amp Sons Ltd

Hill R

S

Rea

d

J Busby JR 1988

The temperature-dependence of photosynthesis of some Australian temperature rainforest trees and its biogeographical significance

Journal of Biogeography

15

453ndash465

Fig 2 Increase of frost resistance (degC) in acclimated Nothofagus spp in relation to their temperature span for 80 of maximum photosynthesis Data from Read amp Hope (1989 Table 2) and Read (1990 Table 4)

Meetings


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New Phytologist

(2003)

157

1ndash7

Forum4

Loumlsch R Kappen L 1984

Diurnal patterns of heat tolerance in relation to CAM

Zeitschrift fuumlr Pflanzenphysiologie

114

185ndash112

Read J 1990

Some effects of acclimation temperature on net photosynthesis of some tropical and extra-tropical Australasian

Nothofagus

species

Journal of Ecology

78

100ndash112

Read J Busby JR 1990

Comparative responses to temperature of the major species of Tasmanian cool temperate rainforest and their ecological significance II Net photosynthesis and climate analysis

Australian Journal of Botany

38

185ndash205

Read J Hill RS 1985

Photosynthetic responses to light of Australian and Chilean species of

Nothofagus

and their relevance to rainforest dynamics

New Phytologist

101

731ndash742

Read J Hope GS 1989

Foliar frost-resistance of some evergreen

tropical and extratropical Australasian

Nothofagus

species


37

361ndash373

Read J Hope GS 1996

Ecology of

Nothofagus

forests of New Guinea and New Caledonia In Veblen TT Hill RS Read J eds

The ecology and biogeography of Nothofagus forests

London UK Yale University Press 200ndash256

Sakai A Larcher W 1987

Frost survival of plants Responses and adaptations to freezing stress Berlin Germany Springer

Squeo FA Rada F Azocar A Goldstein G 1991

Freezing tolerance and avoidance in high tropical Andean plants Is it equally represented in species with different plant height

Oecologia

86

378ndash382

Key words

acclimation climate change photosynthesis frost tolerance rain forest trees

Meetings

Plant development ndash lessons from animals

EuroConference on Tissue Specification and Patterning during Development Granada Spain 11ndash17 May 2002

What has allowed the vigorous progress that has characterizedresearch in plant development during recent years A criticalfactor beyond the obvious widespread adoption of

Arabidopsis

as a model organism is the availability of detailed knowledgegathered through the dissection of development in othermodel organisms Highlights from the lsquoEuroConference onTissue Specification and Patterning during Developmentrsquoindicate just how rich this mine of information remains

Past progress

Much of the progress in understanding the regulation ofthe cell cycle in plants (Stals amp Inzeacute 2001) has been madepossible thorough the advantages of having a firm animalmodel (Nurse 1990 Murray 1992) providing a very usefulconceptual framework for the interpretation of experimentaldata Also consider flower organ specification Even thoughthe homeotic genes involved in both kingdoms turned outto belong to different classes (homeobox genes in animalsand MADS box genes in plants) the general scheme fororgan specification previously depicted for Drosophila heldroughly true for Arabidopsis (Meyerowitz 2002) Fields suchas cell fate control via chromatin remodelling in plants are

now experiencing fast growth thanks to the application ofknowledge and models developed in animals Here there isnot just an analogy in the general strategy comparing plantswith animals but also the involvement of very similar genesndash homologs of the Drosophila Polycomb group of proteinsseem to cover equivalent functions in plants (Goodrich ampTweedie 2002)

Morphogens

Morphogens are signalling molecules that satisfy severalcriteria (Gurdon amp Bourillot 2001)bull Release from a localized source forming a concentrationgradient over a group of cellsbull Ability to induce at least two cell responses in a concentration-dependent waybull Direct action

John Gurdon (WellcomeCRC Ins Cambridge UK)explained how the existence of a morphogen gradient canbe deduced from its ability to induce gene expression in aconcentration-dependent sequence and that cells interpretmorphogen concentrations independently of their neighboursThis latter point was illustrated by using dissociated cells fromXenopus blastula ndash these cells were exposed to the morphogenactivin and then cultured either as single cells monolayersor lsquoreaggregatesrsquo All these configurations showed similar geneexpression patterns indicating that cell fate is not affectedby cell communication

But how can a cell measure different concentrations of agiven morphogen Gurdon explained how morphogen-receptorbinding experiments conducted in isolated cells from thepigmented animal hemisphere of the Xenopus blastula have

Meetings

copy New Phytologist (2003) 157 1ndash7 wwwnewphytologistcom

Forum 5

shown that activin is only bound by the type II receptorindicating that cells lsquoreadrsquo morphogen concentrations byvarying the activity of a single receptor and not by usingdifferent receptors for the same morphogen The geneexpression response depends on the absolute numbers ofreceptors occupied by activin ndash independently of the numberof unoccupied receptors ndash and this occupancy increases withthe time

Morphogens have also been identified in Drosophilazebrafish and vertebrates (Gurdon amp Bourillot 2001) Formany morphogens we know their molecular nature thesource from which they are released how they are perceivedand the signal transduction pathways from the cell surface tothe nucleus (Gurdon amp Bourillot 2001)

Plant biologists have proposed that morphogen gradientsmight guide trichome positioning (Lloyd et al 1994) orstomatal patterning (Gray et al 2000 Holroyd et al 2002)Also the possibility that auxin could act as a morphogen hasbeen discussed in several contexts (Berleth amp Sachs 2001)But are these plant molecules satisfying the three criteriarequired to be considered as morphogens Perhaps our firststep should be to ask whether auxin ndash or other plant mole-cules ndash satisfies such criteria Then the conceptual frameworkfrom the animal system might guide the characterization ofplant morphogens for every morphogen we should deter-mine the molecular structure its source and sink and itsperceptiontransduction pathway

Cell diversity

Generation of cell diversity also depends on asymmetric celldivisions in which cell fate determinants are asymmetricallydistributed in the mother cell and unequally partitionedbetween daughter cells (Knoblich 2001) The polarizationof the mother cell and the appropriate orientation of themitotic spindle are prerequisites for the unequal segregationof cell fate determinants (Knoblich 2001) Domingos Henriquersquosgroup (University of Lisbon Portugal) has identified severalchick genes that encode proteins with polarized distributionin dividing neural progenitors They have found that thislocalization is intimately correlated with the apical-basalpolarity of the embryonic neuroepithelium These genes arehomologs to the genes that encode the Bazooka (fromDrosophila) and PAR-3 (from C elegans) macromolecularcomplexes which control the apical-basal polarity in neuro-blasts (Knoblich 2001) These results support the viewthat the molecular mechanism that guides asymmetric celldivision in invertebrates is conserved in vertebrates

Are there BazookaPAR-3 homologs in Arabidopsis If sothe relative ease of generating knockouts might reveal thatthis mechanism is conserved in all multicellular organismsincluding plants

There are beautiful examples of asymmetric cell divisionsin plants (Scheres amp Benfey 1999) However none has been

studied in the depth that has been applied to animal systemsMuch of the effort has been put on the molecular-geneticdissection of this process and now we have a number ofmutants that show disruptions in the basic mechanismrequired for generating cell diversity from asymmetric celldivisions The ability to visualize plant cell structures ndash suchas the spindle or the phragmoplast (Cutler amp Ehrhardt2002) ndash and protein dynamics by GFP fusion in livingcells in combination with these very well characterizedmutants will help us to face this task successfully Also know-ledge of the molecular mechanisms that control asymmetriccell divisions in animals (Knoblich 2001) together withthe possibility of generating knockouts might facilitate theidentification of candidates regulating the process in plants

Perspectives

Plant science can benefit greatly from many concepts modelsand technologies developed in other model organisms andour colleagues working on animal systems are giving us manylessons on how to approach pattern formation and morpho-genesis However note that plant scientists have also openedsome important avenues by working ahead of the animal develop-ment researchers consider the signalosome (Schwechheimeramp Deng 2001) or the field of small RNAs (Hamilton ampBaulcombe 1999 Llave et al 2002) Let us follow theavenues illuminated by research on animal systems but alsonot forget the darker side alleys they hide meaningful treasures

Acknowledgements

Thanks to many colleagues at the meeting for helpfuldiscussions

Laura Serna

Facultad de Ciencias del Medio AmbienteUniversidad de Castilla-La Mancha

Real Faacutebrica de Armas Avda Carlos III sn45071 Toledo Spain

(tel +34 925265715 fax +34 925268840email lsernaamb-touclmes)

ReferencesBerleth T Sachs T 2001 Plant morphogenesis long-distance

coordination and local patterning Current Opinion in Plant Biology 4 57ndash62

Cutler SR Ehrhardt DW 2002 Polarized cytokinesis in vacuolated cells of Arabidopsis Proceedings of the National Academy of Sciences USA 99 2812ndash2817

Goodrich J Tweedie S 2002 REMEMBRANCE OF THINGS PAST Chromatin Remodeling in Plant Development Annual Review of Cell and Developmental Biology 18 707ndash746

Books

wwwnewphytologistcom copy New Phytologist (2003) 157 1ndash7

Forum6

Gray JE Holroyd GH van der Lee FM Bahrami AR Sijmons PC Woodward FI Schuch W Hetherington AM 2000 The HIC signaling pathway links CO2 perception to stomatal development Nature 408 713ndash716

Gurdon JB Bourillot PY 2001 Morphogen gradient interpretation Nature 413 797ndash803

Hamilton AJ Baulcombe DC 1999 A Species of Small Antisense RNA in Posttranscriptional Gene Silencing in Plants Science 286 950ndash952

Holroyd GH Hetherington AM Gray JE 2002 A role for cuticular waxes in the environmental control of stomatal development New Phytologist 153 433ndash439

Knoblich JA 2001 Asymmetric cell division during animal development Nature Reviews in Molecular Cell Biology 2 11ndash20

Llave C Kasschau KD Rector MA Carrington JC 2002 Endogenous and silencing-associated small RNAs in plants Plant Cell 14 1605ndash1619

Lloyd AM Schena M Walbot V Davis RW 1994 Epidermal cell

fate determination in Arabidopsis patterns defined by a stereoid-inducuble regulator Science 266 436ndash439

Meyerowitz EM 2002 Plants Compared to Animals The Broadest Comparative Study of development Science 295 1482ndash1485

Murray AW 1992 Creative blocks cell-cycle checkpoints and feedback controls Nature 359 599ndash604

Nurse P 1990 Universal control mechanism regulating onset of M-phase Nature 344 503ndash508

Scheres B Benfey P 1999 Asymmetric cell division in plants Annual Review in Plant Physiology and Plant Molecular Biology 50 505ndash537

Schwechheimer C Deng XW 2001 COP9 signalosome revisited a novel mediator of protein degradation Trends in Cell Biology 11 420ndash426

Stals H Inzeacute D 2001 When plants cells decide to divide Trends in Plant Science 6 359ndash364

Key words plant animal morphogen asymmetric cell division

Books

Drying without dying

Desiccation and survival in plants drying without dying

Ed by M Black amp H W Pritchard 416 pages Wallingford UK CABI Publishing 2002 pound7500 hb ISBN 0851 99534 9

Desiccation and survival in plants drying without dying isa very welcome addition to the plant stress physiologyliterature as it is the first attempt to review comprehensivelythe phenomenon of desiccation tolerance in plants Desiccationtolerance occurs in most seeds pollen and spores in the thalliof many algae lichens and bryophytes and in the vegetativetissues of a few species of vascular plants The book beginswith an overview of desiccation tolerance The clear messageof this and subsequent chapters is that desiccation toleranceinvolves a whole collection of interacting mechanisms

The first section of the book covers methods used to studydesiccation including those to measure water potential andits components practical considerations for example how tocontrol drying rates and how to assess the ability of plants torecover and the advantages and disadvantages of biochemicaland biophysical methods Techniques such as nuclear magneticresonance (NMR) and electron paramagnetic resonance(EPR) promise to tell us about the distribution and state of

water in biological materials the occurrence of free radicalsgenerated by living cells and the nature of membranes Whilethese techniques apparently still cannot explain why someseeds thalli or vegetative tissues are desiccation-sensitiveand others tolerant their full power has yet to be realized inthe study of desiccation tolerance

Desiccation tolerance in relation to seed development inpollen and spores and in bryophytes and higher plants isreviewed in the following chapters Interestingly real desicca-tion sensitivity is uncommon among pollens Evolutionaryaspects of recalcitrance (desiccation intolerance) in seeds arethen reviewed but the book would have been much moreinteresting if this or another chapter had also considered theevolution of desiccation tolerance in cryptograms and higherplants

The way in which desiccation actually damages organismsand tolerance mechanisms are covered in the next group ofchapters Chapter 9 rightly points out that most studies ondesiccation in plants tend to focus on how organismstolerate stress rather than trying to quantify the effects ofstress This review is one of the best I have read on howfrom a structural point of view cell membranes and wallsaccommodate the huge changes in volume that accompanydrying Many proteins are quite stable when desiccated butit is made clear that this is not true for all proteins Therefollows a nice overview of the various mechanisms that havebeen put forward to explain desiccation tolerance My onlycriticism of these two chapters is that they do not sufficiently

Books


Forum 7

detail the involvement of free radical scavenging systems Itis true that at present we can only correlate desiccationtolerance to changes in enzyme activity or concentrations offree-radical scavenging antioxidants and that sometimesthese correlations break down However the same could besaid about some of the more favoured molecules such asdehydrins or sugars or the properties of a cell for example glassformation or the membrane-phase transition temperatureIndeed as the final chapter in the book points out some ofthese correlations for example those with oligosaccharides anddesiccation tolerance in seeds may turn out to be spuriousMolecular genetics of desiccation tolerance are outlined inChapter 11 We now have some indication of which genes areswitched on (and off ) during dehydration and rehydrationWe also know that these genes have considerable similaritiesin such diverse systems as seeds and vegetative moss tissuesWork in this area is just beginning but it will surely not belong before the powerful tools of molecular biology that havebeen developed in other systems are applied to understandingthe mechanism of desiccation tolerance The authors also givea realistic appraisal of the potential for increasing the droughttolerance of crop plants using information derived fromdesiccation-tolerance studies The effects of desiccation onmembranes and the nuclear genome are discussed Clearlycompetent DNA repair is an essential factor for successfulrehydration Reading the section on membranes leaves mewith the impression that we still do not understand how thewidely reported loss of solutes during rehydration occurs

The final lsquoconspectusrsquo chapter is rather disappointingThe opportunity has been lost for vigorous debates on thedefinition of desiccation tolerance and the relationshipbetween desiccation tolerance and drought tolerance I wouldhave liked a thorough comparison of desiccation tolerancein seeds pollen and spores and vegetative material Andwhat about comparing desiccation-tolerance mechanisms inplants with the few animal systems in which it occurs

I have several other criticisms of the book Some chaptersare over long with overlap of subject matter and the textcontains some obvious inconsistencies Finally importantgroups of desiccation tolerant organisms on which highquality work has been carried out (eg lichens and algae) arealmost entirely neglected Having said all this and whilethere are too many omissions for this book to have as the backcover claims a lsquocomprehensive coveragersquo I have no hesitationin recommending it to those studying any aspect of desiccationtolerance in plants and to plant-stress physiologists

The full text of the book is available for viewing at httpwwwcabi-publishingorgBookshopReadingroombrowseA-Zasp but note it cannot be printed

Richard Beckett

School of Botany and Zoology University of NatalPietermaritzburg South Africa

(tel +27 33 2605141 fax +27 33 2605105email beckettnuacza)

Commentary


copy

New Phytologist

(2003)

157

1ndash7

Forum2

environments on the acclimation potential of the examinedspecies but what makes the paper by Cunningham amp Readspecial is that it is the first attempt to separate the effectsof constant and fluctuating temperature regimes onacclimation potential of rainforest tree species originatingfrom different latitudes Mean temperatures and annual anddiurnal temperature ranges vary along latitudinal gradientsbut temperature fluctuations may also vary seasonally inthe same site In deserts seasonal and diurnal temperatureranges are greater in winter than in summer and winter desertannuals have been shown to have a greater capacity forthermal acclimation than summer annuals (Downton

et al

1984)

Acclimation to light regimes

Are there other examples where tree species from fluctuatingenvironments show greater potential for acclimation thansimilar species from more constant environments Differencesin light regime provide such an example Species that growin high light environments are subjected to greaterfluctuations in light intensity than those that grow in shadeor climax communities where the fluctuations in lightintensity are less Trees from open environments are typicallyprimary colonisers and early successional species whereastrees from shaded environments are usually late successionalspecies from climax vegetation

Do early successional species show greater plasticity inresponse than late successional or shade-adapted species Thisquestion has been addressed by investigations of Australianrainforest species (Read amp Hill 1985)

Ceratopetalum apetalum

and

Doryphora sassafras

are subsidiary species that co-occur

in temperate rainforests in south-eastern Australia whichmay be dominated by either

Nothofagus moorei

or

Eucryphiamoorei

These canopy dominants show acclimation ofphotosynthesis (on an area basis) to high and low light regimeswhereas the photosynthesis of the subsidiary species showslittle acclimation to differing light regimes Similar differencesin acclimation are shown in Chilean

Nothofagus

species theshade tolerant

N alpina

shows little acclimation to highlight regimes while the canopy dominants

N dombeyi

and

N obliqua

show greater acclimation to changed light regimesSimilar observations have been made in North Americawhere late successional shade-adapted tree species such as

Tilia americana

and

Acer saccharum

showed little alterationof photosynthesis rates when grown in contrasting lightregimes and even showed enhanced photosynthesis aftershade treatment By contrast mid-successional tree speciessuch as

Quercus imbricaria

and

Platanus occidentalis

showedlarge differences in photosynthesis rates after being grown insun and shade (Bazzaz amp Carlson 1982)

Acclimation to freezing temperatures

A final example is concerned with frost resistance of treespecies along a latitudinal gradient It is well known thatspeciesrsquo frost resistance is well correlated with latitude speciesfrom high latitudes have high frost resistance those fromlow latitudes have low frost resistance (Sakai amp Larcher1987) As mentioned previously high latitude temperateenvironments are characterised by lower mean temperaturesbut greater seasonal and diurnal variation than low latitudetropical environments If fluctuating environments leadto the selection of species with greater acclimationpotential then species from lower latitudes would be expectedto show lesser acclimation potential than those from higherlatitudes

Read amp Hope (1989) provide an appropriate Australasianexample They measured the frost resistance of species of

Nothofagus

from Tasmania south-eastern Australia and PapuaNew Guinea These were artificially hardened in growthchambers with a short (8 h) photoperiod at 15

deg

C and a 16-hdark period at 2

deg

C The maximum hardiness obtained inthe growth cabinets was similar to that in plants that hadbeen hardened naturally outside Frost resistance increasedwith latitude and decreased with both accumulated temper-ature (day-degrees) and the estimated temperature at the sitesof origin of the species and was also associated with photo-synthetic parameters Both the temperature span for max-imum photosynthesis of the species and the degree of frosthardening increase with latitude (Fig 2) indicating a similarresponse to environmental temperatures

Is the mean temperature or the annual or diurnal temperat-ure range the controlling factor At low altitudes temperat-ures decrease and diurnal and seasonal ranges of temperatureincrease with latitude ndash consequently tropical latitudes are

Fig 1 Acclimated temperature span (AT span degC) for 80 of maximum photosynthesis in relation to latitude of origin of the species Filled circles indicate acclimation to the lsquoconstantrsquo temperature regime and open circles represent acclimation to the fluctuating regime Data from Tables 2 and 5 (Cunningham amp Read pp 55ndash64)

Commentary

copy

New Phytologist

(2003)

157

1ndash7


Forum 3


et al

1982 Goldstein

et al

1985 Squeo

et al





Perspectives


Peter Bannister



References



Oecologia

54

313ndash316




5

215ndash222



New Phytologist

157

55ndash64



Plant Physiology

74

786ndash790



Oecologia

68

147ndash152

Grime JP 2001


2nd edn


Hill R

S

Rea

d

J Busby JR 1988



15

453ndash465


Meetings


copy

New Phytologist

(2003)

157

1ndash7

Forum4




114

185ndash112

Read J 1990


Nothofagus

species

Journal of Ecology

78

100ndash112




38

185ndash205

Read J Hill RS 1985


Nothofagus


New Phytologist

101

731ndash742

Read J Hope GS 1989



Nothofagus

species


37

361ndash373

Read J Hope GS 1996

Ecology of

Nothofagus








Oecologia

86

378ndash382

Key words


Meetings




Arabidopsis


Past progress



Morphogens




Meetings


Forum 5




Cell diversity





Perspectives


Acknowledgements


Laura Serna








Books


Forum6
















Books









Books


Forum 7





Richard Beckett



Commentary

copy

New Phytologist

(2003)

157

1ndash7


Forum 3


et al

1982 Goldstein

et al

1985 Squeo

et al





Perspectives


Peter Bannister



References



Oecologia

54

313ndash316




5

215ndash222



New Phytologist

157

55ndash64



Plant Physiology

74

786ndash790



Oecologia

68

147ndash152

Grime JP 2001


2nd edn


Hill R

S

Rea

d

J Busby JR 1988



15

453ndash465


Meetings


copy

New Phytologist

(2003)

157

1ndash7

Forum4




114

185ndash112

Read J 1990


Nothofagus

species

Journal of Ecology

78

100ndash112




38

185ndash205

Read J Hill RS 1985


Nothofagus


New Phytologist

101

731ndash742

Read J Hope GS 1989



Nothofagus

species


37

361ndash373

Read J Hope GS 1996

Ecology of

Nothofagus








Oecologia

86

378ndash382

Key words


Meetings




Arabidopsis


Past progress



Morphogens




Meetings


Forum 5




Cell diversity





Perspectives


Acknowledgements


Laura Serna








Books


Forum6
















Books









Books


Forum 7





Richard Beckett



Meetings


copy

New Phytologist

(2003)

157

1ndash7

Forum4




114

185ndash112

Read J 1990


Nothofagus

species

Journal of Ecology

78

100ndash112




38

185ndash205

Read J Hill RS 1985


Nothofagus


New Phytologist

101

731ndash742

Read J Hope GS 1989



Nothofagus

species


37

361ndash373

Read J Hope GS 1996

Ecology of

Nothofagus








Oecologia

86

378ndash382

Key words


Meetings




Arabidopsis


Past progress



Morphogens




Meetings


Forum 5




Cell diversity





Perspectives


Acknowledgements


Laura Serna








Books


Forum6
















Books









Books


Forum 7





Richard Beckett



Meetings


Forum 5




Cell diversity





Perspectives


Acknowledgements


Laura Serna








Books


Forum6
















Books









Books


Forum 7





Richard Beckett



Books


Forum6
















Books









Books


Forum 7





Richard Beckett



Books


Forum 7





Richard Beckett



plant development – lessons from animals

Documents