David T. Bell

Seed germination ecology in southwestern Western Australia

Germination responses of species from the native plant communities of southwestern Western Australia can be related to syndromes of life history, fire response, and seed storage, and also to factors related to environmental stress. The Mediterranean-type climate of the region with periodic drought and recurrent fires affects the production of viable seeds in plants of limited stature and rooting depth. Fire response ephemerals and species cued to flower by fire tend to produce viable, readily germinable seeds, but there are instances where seed production is aborted in these predominantly herbaceous life forms. Clonal, rhizomatous species often produce mainly inviable seeds. Production of viable seeds in woody species of these highly diverse communities may also be restricted by limitations to cross pollination. Obligate post-fire seeding species tend to produce a greater proportion of viable seeds than species which are capable of resprouting following fire. Serotinous species, whether post-fire re-seeders or post-fire resprouting species, produce mainly viable seeds, which germinate readily once freed from protective fruits. Species of the legume families and a few others of the soil seed bank produce innately dormant seeds which can be germinated following heat shock treatments which simulate the effects of fire. Heat shock in these species appears mainly as a mechanism to crack the hard seed coats, but the effect of heat to denature seed coat inhibitors has not been eliminated. Western Australian species do not seem to break dormancy when exposed to leachates from burned wood as has been observed in comparable habitats in California and South Africa, but further research is advised. Germination in many native southwestern Australian species is cued by temperatures that correspond to the winter rainfall period. There are also indications that an after-ripening period of warm, dry storage increases percentage of germinable seeds. Stimulation of germination by hormones is almost unresearched in Western Australia, but germination percentages have been increased in a small number of species of horticultural potential. Stimulation of germination by soil nutrient concentrations is almost unresearched in Western Australia, except for the inhibitory effect of excess sodium chloride levels inEucalyptus andMelaleuca. These species only germinate when osmotic effects are reduced to lower levels as would occur when winter rains dilute soil salts. Application of research on seed germination has already enhanced the establishment of seedlings in the restoration of mine sites and is becoming important in aspects of the breeding and selection of native plants for the cut flower, bedding plant and essential oil industries.AbstraktDas Keimverhalten von Arten der nativen Pflanzengesellschaften aus dem Südwesten Westaustraliens kann mit Syndromen ihrer Entwicklung, dem Verhalten gegen Waldbrände, der Speicherung in Samen und auch mit Umwelt-Sressfaktoren in Verbindung gebracht werden. Das mediterrane Klima dieser Region mit regelmäßiger Trockenheit und wiederholt auftretenden Bränden beeinflußt die Produktion keimfähiger Samen bei kleineren Pflanzen mit geringer Wurzeltiefe. Wadbrand-Ephemere und Arten, die nach Feuer blühen, produzieren im allgemeinen rasch keimende Samen, jedoch kann die Samenproduktion unter Umständen bei diesen vorwiegend krautigen Lebensformen fehlschlagen. Klonbildende Arten mit Rhizomen erzeugen im wesentlichen nicht keimfähige Samen. Bei holzigen Arten dieser enorm mannigfaltigen Pflanzengesellschaften kann die Produktion keimfähiger Samen auch durch Erfordernis einer Fremdbestäubung begrenzt sein. Obligat nach Feuer aussamende Arten tragen tendenziell einen größeren Anteil keimfähiger Samen als Arten, die nach Feuer regenerieren können. Serotine (bradykarpe) Arten mit langsam öffnenden Früchten, seien es nach Feuer aus Samen keimende ‘reseeders’ oder regenierende ‘resprouters,’ produzieren im wesentlichen keimfähige Samen, die rasch, sobald sie aus den schützenden Früchten befreit sind, keimen. Leguminosenarten und einige andere produzieren von Natur aus ruhende Samen, die nach einer die Wirkung von Feuer simulierenden Hitzeschockbehandlung zum Keimen gebracht weren können. Bei diesen Arten scheint Hitzeschock vor allem ein Mechanismus zu sein, der die harte Testa sprengt, aber eine Auswirkung von Hitze auf eine Denaturierung von Keimungshemmern konnte nicht ausgeschlossen werden. Bei westaustralischen Arten scheint die Keimruhe nicht durch eine Wirkung von Eluaten aus verbranntem Holz gebrochen zu werden, wie dies auf vergleichbaren Standorten in Kalifornien und Südafrika beobachtet wurde; weitere Untersuchungen sind freilich erforderlich. Die Samenkeimung vieler in Südwestaustralien einheimischer Arten wird durch Temperaturen ausgelöst, die der Winterregenperiode entsprechen. Es gibt auch Hinweise, daß eine Nachreifeperiode mit warmer, trockener Lagerung die prozentuale Keimfähigkeit erhöht. Eine hormoneile Keimförderung ist bislang fast nicht in Westaustralien untersucht worden, aber bei einer kleinen Anzahl von gartenbaulich interessierenden Arten konnte die Keimfähigkeit erhöht werden. Auch eine Keimförderung durch Bodennährstoffkonzentrationen ist in Westaustralien so gut wie nicht untersucht, abgesehen von hemmender Wirkung exzessiver Natriumchloridkonzentrationen beiEucalyptus undMelaleuca. Samen dieser Arten keimen nur, wenn die osmotischen Bedingungen soweit erniedrigt werden, wie es geschieht, wenn Winterregen Bodensalze verdünnen. Die Anwendung der Untersuchungen der Samenkeimung hat bereits das erfolgreiche Anwachsen von Sämlingen bei der Wiederbegrünung ehemaliger Gruben verbessert und wird ein wichtiger Aspekt bei Zucht und Auslese einheimischer Pflanzen zur Nutzung als Schnittblumen, Gartenpflanzen und beim Anbau für die Nutzung ätherischer Öle sein.

Leaf form and photosynthesis

M orphological and anatomical features of plant leaves are commonly associated with metabolic type (e.g., Kranz anatomy of C4 species), amount of sun exposure (e.g., sun and shade leaves), or water stress (e.g., xeromorphism). However, although the primary function of the leaf is to absorb and process sunlight and carbon dioxide for photosynthesis, few structural features of leaves have been related mechanistically to these tasks. For example, it has been known for over a century that the internal anatomy of leaves is characterized by different cell layers (e.g., the palisade and spongy mesophyll) and that stomatal pores can be located on one or both sides of a leaf. Yet, only recently has any functional relationship between leaf form and photosynthetic performance been suggested. A variety of ecological studies have correlated numerous leaf structural parameters with photosynthetic performance (e.g., Abrams and Kubiske 1990, 1994, Hinckley et al. 1989,

Ecological response syndromes in the flora of southwestern Western Australia: Fire resprouters versus reseeders

Two fire-response syndromes can be described for species of the vegetation of Mediterranean-climate, southwestern Western Australia. Resprouters survive fires as individuals. Reseeders are killed by fire and must reestablish through germination and establishment of seedlings. Of the Western Australian plant families analyzed for fire-response strategies, 50% of the Proteaceae, 50% of the Restionaceae, 45% of the Orchidaceae, and 25% of the Epacridaceae are resprouter species. Within genera of the Proteaceae, the proportions of resprouters includeAdenanthos (56%),Hakea (52%),Dryandra (35%), andGrevillea (31%). WithinBanksia, 49% are resprouters, and it appears that the reseeding syndrome is the derived character in this genus. The proportion of resprouters within southwestern Western Australian plant communities ranges from 66% to 80%. These percentages are generally higher than in more arid parts of Western Australia and in comparable plant communities from other Mediterranean-type climates of the world. The relatively high proportion of resprouters within plant families and within plant communities probably indicates that the Western Australian vegetation experiences a harsher fire stress regime than do other Mediterranean-type climate areas. Western Australian plant communities have their highest diversity in the early years after fire, when the vegetation contains a higher number of reseeding species and individuals. Seed banks are dominated by the seeds of reseeders.There are no basic differences in mean seed mass, viability, or germinability of seeds between resprouting species and reseeding species, but reseeders tend to have narrower optimum germination temperature regimes. Establishment success is related more to seed mass, seedling size, and leaf ecophysiology and morphology than to fire-response strategy. Reseeder seedlings tend to grow faster than do resprouter seedlings. Basic shrub morphology differs, with reseeders generally being umbrella shaped and resprouters urn shaped. Reseeding species most commonly have a shallow, fibrous root system. Resprouters have a massive, deeply penetrating root system. Shoot:root ratios of first-year seedlings and mature plants are higher for reseeders. Resprouter seedlings store starch in root tissue at a much greater rate than do reseeder seedlings. Although the concentrations of essential nutrients in seedlings are not different between fire-response types, reseeders tend to conserve nutrients to a greater extent through leaf retention. Reseeders tend to produce greater numbers of flowers and greater amounts of floral rewards, but the breeding systems, which lead to the higher seed set in reseeders, can vary between strict outcrossing and considerable selfing. Reseeding species are not likely to be wind pollinated. Species survival in a fire-prone environment can involve a wide range of combinations of attributes. It appears that in Western Australian reseeder species the lack of an ability to resprout is compensated for by a number of other structural and functional features.Knowledge of the fire-response strategies of species of southwestern Western Australia can influence fire-regime management, conservation of rare species, and restoration of vegetation after disturbance. Further knowledge of the fire-response strategies of species of the southwestern Western Australian flora should result in better management of natural and restored plant communities of the region.ZusammenfassungEs können zwei Syndrome in bezug auf Waldbrände für Vegetationsarten des mediterranen Klimas im Südwesten Westaustraliens beschrieben werden. Pflanzen, die nach dem Waldbrand wieder austreiben, songenannte “resprouter,” überleben als Individuen. Pflanzen, die vom Feuer vernichtet werden, müssen sich durch Keimung der Samen und des Aufwachsens der Sämlinge neu etablieren (sogenannte “reseeder”). Eine Analyse der Strategien als Reaktion auf Waldbrände ergab für Pflanzenarten in Westaustralien, daß 50% der Proteaceae, 50% der Restionaceae, 45% der Orchidaceae, und 25% der Epacridaceae “resproter” sind. Innerhalb der Familie der Proteaceae sind Pflanzen, die wieder austreiben, anteilsmäßig zu 56% inAdenanthos, zu 52% inHakea, zu 35% inDryandra, und zu 31% inGrevillea vertreten. 49% derBanksia Arten sind “resprouter,” und es erscheint, daß das Syndrom der Wiederausbreitung durch Samen ein abgeleitetes Merkmal in deiser Familie ist. Der Anteil der “resprouter” beträgt in Pflanzengemeinschaften im Südwesten Westaustraliens 66 bis 80%. Diese prozentualen Anteile sind im allgemeinen höher als in mehr wüstenähnlichen Gebieten Westaustraliens und in vergleichbaren Pflanzengemeinschaften anderer mediterraner Klimate. Der relativ hohe Anteil von “resprouter” innerhalb Pflanzenfamilien un Pflanzengemeinschaften ist wahrscheinlich ein Anzeichen dafür, daß die Vegetation in Westaustralien stärker von der Einwirkung des Feuers geprägt ist als andere Gebiete mediterranen Klimas. Westaustralische Pflanzengemeinschaften haben ihren höchsten Grad an Vielfalt in frühen Jahren nach einem Waldbrand, wenn die Vegetation ein hohe Anzahl von Pflanzenarten und Individuen enthält, die “reseeder” sind. Samenbanken dominieren durch Samen von “reseeder.”Es gibt keine grundlegenden Unterschiede zwischen “resprouter”- und “reseeder”-Pflanzen in bezug auf mittlerer Samenmasse, Überlebensfähigkeit oder Keimfähigkeit der Samen. Allerdings neigen die Samen der “reseeder”-Pflanzen zu einem engeren Temperaturoptimum währen der Keimung. Der Erfolg der Landbesiedelung hängt vornehmlich von Samenmasse, Sämlingsgröße und Blattökophysiologie und Morphologie ab und weniger von der Strategie als Reaktion auf Waldbrände. Sämlinge von “reseeder”-Pflanzen wachsen in der Regel schneller als Sämlinge von “resprouter”-Pflanzen. Die grundlegende Buschmorphologie unterscheidet sich im allgemeinen zwischen “reseeder”-Pflanzen, die eine Regenschirmform, und “resprouter”-Pflanzen, die eine Vasenform annehmen. “Reseeder”-Pflanzenarten besitzen zumeist ein flaches, fibriliäres Wurzelsystem. “Resprouter”-Pflanzen besitzen ein massives, tief durchdringendes Wurzelsystem. Trieb: Wurzel-Verhältnisse von einjährigen Sämlingen und ausgewachsenen Pflanzen sind höher für “reseeder”-Pflanzen. “Resprouter”-Sämlinge speichern Stärke im Wurzelgewebe in höherem Maße als “reseeder”-Sämlinge. Obwohl essentielle Nährstoffkonzentrationen in Sämlingen nicht differieren zwischen “reseeder” und “resprouter,” konservieren “reseeder”-Pflanzen, Nährstoffe in größerem Ausmaß durch Blatterhalt. “Reseeder”-Pflanzen neigen zu einer erhöhten Produktion der Blütenanzahl und einer größeren Menge von bestäubten Blüten. Züchtungssysteme hingenen, die zu einer höheren Samenanlage in “reseeder”-Planzen führen, können zwischen strikter Fremdbestäubung und beträchtlicher Selbstbestäubung variieren. Es ist unwahrscheinlich, daß “reseeder”-Pflanzenarten vom Wind bestäubt werden. Das Überleben von Pflanzenarten in einer von Waldbränden geprägten Umwelt kann eine Reihe von Kombinationen von Attributen einschleißen. Es erscheint, daß in westaustralischen “reseeder”-Pflanzenarten der Mangel an der Unfähigkeit, neu auszutreiben, durch eine Anzahl anderer struktureller und funktioneller Merkmale kompensert wird.Kenntnisse der Strategien von Pflanzen im Südwesten Westaustraliens gegen Waldbrände können das Feuermanagement Regime, die Erhaltung seltener Arten und die Erneuerung der Vegetation nach einer Störung beeinflussen. Zusälzliche Kenntnisse der Strategien gegen Waidenbrände von Arten in der südwest-Avestaustralischen Flora sollen zu besserem Umgang mit natürlichen und erneuerten Pflanzengemeinschaften der Region führen.

Effects of temperature, light and gibberellic acid on the germination of seeds of 43 species native to Western Australia

Species native to the southwest of Western Aus- tralia, representing a range of plant families, life-history strat- egies, fire-response syndromes, seed-store types and seed weights, were tested for viability using tetrazolium chloride and for germination under combinations of constant tempera- tures of 15 ?C or 23 ?C, constantly dark or 12 h diurnal white- light conditions, and with, or without, addition of gibberellic acid (GA3, 50 mg/1). Species previously known to require a heat-shock treatment to overcome dormancy due to an imper- vious testa were pre-treated prior to imposition of tempera- ture, light and GA3 conditions. The test environmental condi- tions related to differences between winter and autumn tem-

Australian trees for the rehabilitation of waterlogged and salinity-damaged landscapes.

The revegetation of damaged agricultural landscapes requires a detailed knowledge of appropriate species and their adaptations to cope with the stresses of environments altered by humans. A range of Australian species has a role in the restoration of water and salt balances of catchments and can provide income diversity to agricultural properties damaged by increased frequencies of flooding, rising groundwaters and increased salinities. This review concentrates on the ecologically significant attributes of Australian woody species in waterlogged and saline habitats, and responses of species particularly suited to the restoration of water balance in cleared catchments. Australian catchments yield little water under natural vegetation, the trees and shrubs being especially resourceful in utilising much of the annual rainfall input. Replacing native, deep-rooted perennial species with annual crops always results in a net gain in catchment water. To redress these problems, cleared landscapes must be partially restored to tree and shrub cover to utilise the excess water remaining when crops are harvested or lie dormant over summer. Upland regions of restored landscapes should be planted to tree crops, particularly those that are luxuriant water users, of commercial value to farmers. Tree plantations for paper pulp, soft-wood timber and eucalypt oils are possibilities. Lowland sites in damaged catchments must be revegetated with trees which have waterlogging adaptations, such as aerenchyma, and tolerance to the products of anaerobic respiration. Areas of waterlogging that are additionally affected by excess salts must have exceptional trees. Australia has a number of native species which are well suited to survive these conditions, produce biomass and utilise excess water, while restricting or coping with the uptake of over-abundant salts. Most tolerant Australian species have a range of anatomical, morphological and physiological attributes to contribute to these adaptive qualities. This review highlights some of these features and describes various combinations that are successful. Australia now has a range of genotypes to bring to bear in the battle to rehabilitate landscapes damaged by disruption of the soil–salt–water balance. Only by redressing these problems can we ensure that future generations will have land capable of retaining economic value and producing potable water.

Associations between leaf structure, orientation, and sunlight exposure in five Western Australian communities.

Five plant communities in Western Australia, as well as selected desert and Rocky Mountain species of the western USA, were surveyed to evaluate associations among leaf structure, orientational properties, and the sunlight exposure and precipitation characteristic of each community. Selected leaf structural features have been associated previously with photosynthetic function and included shape, thickness, the ratio of thickness to width, stomatal distribution, leaf surface coloration, and the number and distribution of palisade cell layers. Decreases in annual precipitation (<4 to over 15 cm/yr) and increases in total daily sunlight (4.2 to 29.2 mol photons/m1) corresponded strongly to an increase in the percentage of species in a given community with more inclined (more inclined than +/- 45 degrees from horizontal) or thicker leaf mesophyll (>0.4 mm) leaves. Also, the percentage of species with a leaf thickness to width ratio >0.1, which were amphistomatous, or which had palisade cell layers beneath both leaf surfaces, increased from >20% in the highest rainfall and lowest sunlight community to >80% in the community with least rainfall but greatest sunlight exposure. Over 70% of the species in the most mesic, shaded community had lighter abaxial than adaxial leaf surfaces (leaf bicoloration). All of the above structural features were positively associated with a more inclined leaf orientation (r1 = 0.79), except for leaf bicoloration, which was negatively associated (r1 = 0.75). The ratio of adaxial to abaxial light was more strongly associated with leaf bicoloration (r1 = 0.83) and the presence of multiple adaxial and isobilateral palisade cell layers(r1 = 0.80) than with total incident sunlight on just the adaxial leaf surface (r1 = 0.69 and 0.73, respectively). These results provide field evidence that leaf orientation and structure may have evolved in concert to produce a photosynthetic symmetry in leaf structure in response to the amount of sunlight and other limiting factors of the community. This structural symmetry may serve fundamentally to regulate the distribution of both light and CO2 levels inside the leaf and, thus, increase photosynthetic CO2 uptake per unit leaf biomass.

Screening for salt and waterlogging tolerance in Eucalyptus and Melaleuca species

Abstract Seedlings of forty Eucalyptus and twenty Melaleuca species from within Australia were tested in three separate glasshouse experiments for their tolerance to the individual stresses of salinity and waterlogging as well as a combined salt/waterlogging stress. Waterlogging with freshwater had little effect on growth and survival of Melaleuca species but caused growth reduction for most Eucalyptus species. Under freely-drained conditions, salinity caused growth reduction in all species, while percentage survival remained high. Survival and growth for all species were reduced most in saline/ waterlogged conditions. Using a tolerance index combining survival and growth, the most tolerant Eucalyptus species to salt/waterlogging in the first experiment were E. occidentalis, E. sargentii and E. spathulata , and in the second experiment E. intertexta, E. microtheca, E. raveretiana, E. striaticalyx and E. tereticornis. Melaleuca lateriflora, M. sp. aff. lanceolata and M. thyoides were the most tolerant Melaleuca species for both freely-drained and saline/waterlogging stresses. Although these species showed greatest tolerance to salt/waterlogging conditions, there was wide variation for tolerance between provenances of most species in all treatments. Matching of species with sites should provide suitable choices for reclamation of saline seeps in Australia and other countries, and provide renewable resources of timber and fuelwood from habitats currently unavailable to agricultural or silvicultural production.

Effect of salinity on growth of four strains of Rhizobium and their infectivity and effectiveness on two species of Acacia

Two Rhizobium strains (WU1001 and WU1008) were isolated from nodules of Acacia redolens growing in saline areas of south-west Australia, and two strains selected from the University of Western Australias culture collection (WU429 isolated from A. saligna and WU433 from A. cyclops). The growth of each in buffered, yeast extract mannitol broth culture was largely unaffected by salt up to 300 mM NaCl. A slight increase in lag time occurred at concentrations of 120 mM NaCl and above, but cell number at the static phase was not affected. Each of the four Rhizobium strains tested accumulated Na+ but showed decreasing levels of sugar with increasing salt in the external medium. Amino acid levels also increased, in some cases by more than tenfold. However, the relative proportion of each remained fairly constant in the bacteria, irrespective of salt treatment. Only trace quantities of proline were detected and there was no increase in this amino acid with salt. Acidic amino acids (glutamate and aspartate) remained as a constant proportion.Rhizobium strains WU429, WU1001 and WU1008 produced effective nodules on both A. cyclops and A. redolens grown in sand with up to 80 mM NaCl (added in nutrient solutions free of nitrogen). Strain WU433 was highly infective on both Acacia species tested at low salt concentrations (2–40 mM NaCl), but infection was sensitive to salt levels at 120 mM NaCl and above. Nodules formed with strain WU433 were, however, ineffective on both A. redolens and on A. cyclops and showed nil or negligible rates of acetylene reduction at all salt concentrations. Strains WU429, WU1001 and WU1008 in combination with a highly salt-tolerant provenance of A. redolens formed symbioses which did not vary significantly in nodule number and mass, specific nodule activity or total N content irrespective of salt level up to 160 mM NaCl. On a more salt sensitive provenance of A. redolens and on A. cyclops the infectivity and effectivity of the Rhizobium strains tested usually decreased as the external salt concentration increased. These data are interpreted to indicate that tolerance of the legume host was the most important factor determining the success of compatible Rhizobium strains in forming effective symbioses under conditions of high soil salinity.

Response to Salt and Waterlogging Stress of Ten Taxa of Acacia Selected from Naturally Saline Areas of Western Australia

Ten taxa of Acacia were selected from areas of moderate to high soil salinity (electrical conductivities of saturated soil paste extracts (ECe) between 1000 mS m--1 and 4800 mS m-1 at 50-600 mm depth) and sodicity to test the tolerance of young, symbiotic plants to increasing levels of salinity both with and without waterlogging. Nodulated plants, 3 months old, were grown in glasshouse experiments which consisted of four treatments: non-saline drained control (12 weeks); saline drained (12 weeks); non-saline waterlogged (5 weeks); and saline waterlogged (5 weeks). Acacia cyclops, A. brumalis, A. redolens (Ravensthorpe) and A. aff. lineolata had 100% survival after 12 weeks irrigation with saline solution (final ECw = 9500 mS m-1). Generally, the species tested were sensitive to waterlogging with A. patagiata, A. cyclops and A. brumalis being the most sensitive, having 19-44% mortality with no salt in the solution. The combined treatment of salt and waterlogging greatly increased the mortality of plants, with four species having > 70% dead after 5 weeks treatment (ECw = 3900 mS m-1). A. aff. lineolata and A. mutabilis subsp. ,stipulifera were highly tolerant of salt plus waterlogging, with 100% and 96% survival respectively. In salt plus waterlogged treatments, Na+ concentration in phyllodes of all taxa exceeded (0.37-2.13 mmol g-1 dry wt) that taken up by plants in freely drained salt treatments (0.03-0.42 mmol g-1 dry wt). Taxa with the slowest rates of growth tended to accumulate the highest concentrations of Na+ in the uppermost phyllodes. Provenances of A. redolens and A. patagiata collected from sites of high soil salinity (ECe > 2200 mS m-1) had less than half the Na+ concentration in uppermost phyllodes (0.22 mmol -1 dry wt) at the termination of the salt treatment, compared with provenances of the same species collected from moderately saline areas (ECe = 1100 mS m-1). This indicates that Acacia provenances collected from the most saline sites had greater potential to survive high levels of external salinity in the longer term than those from less saline sites.

Comparisons of selected and cloned plantlets against seedlings for rehabilitation of saline and waterlogged discharge zones in Australian agricultural catchments

Summary Clonal lines of Australian tree species have been developed for tolerance to saline and/or waterlogged conditions. These clonal plants have been shown to be more tolerant under glasshouse and field conditions when compared with seedling lines. Selection procedures included the initial collection of seed from trees growing naturally in seasonally waterlogged and/or saline soils. Following germination and establishment, three-month-old seedlings were stressed in glasshouse trials using progressively increased levels of salinity, either in freely drained or saturated conditions, and the most tolerant individuals were micropropagated. Under conditions of saturation and salinity stress, in both glasshouse trial conditions and under field situations, selected and cloned Eucalyptus cawaldulensis. E. spathulata subspecies spathulata, Casuarina obesa and C. glauca plants showed higher survival rates and the surviving plants grew faster than provenance-matched seedlings.