Juvenile Foliage Research Articles

Effect of environment and shoot architecture on floral transition and gene expression in Eucalyptus occidentalis and Metrosideros excelsa

Metrosiderosexcelsa and Eucalyptus occidentalis exhibit different strategies prior to flowering—the former passes through a long juvenile phase and must acquire a degree of architectural complexity to flower, whereas the latter flowers precociously even on stems still exhibiting juvenile foliage. As expediting flowering is of interest to breeders and horticulturalists alike we compared these species by growing plants with two branch architecture treatments in factorial combination with two growth environments. Plants were either allowed to branch freely or constrained to a single stem before subsequently being allowed to branch; one environment was inductive for flowering and the other not. Three meristem identity genes (the equivalents of LEAFY, APETALA1 and TERMINAL FLOWER1) were used as indicators of flowering. Constraining E. occidentalis plants to a single stem delayed the onset of the main flush of flowering in contrast to M. excelsa, although in both species a complex interaction between branching and environment occurred. We show that the complexity of the architecture can impact on production of flowers and can be used to expedite or enhance flowering for breeding purposes, but this is dependent on the species. AP1 appears to be a useful marker not just for floral organ differentiation but also as an indicator of floral induction having occurred.

Quantitative expression analysis of meristem identity genes in Eucalyptus occidentalis: AP1 is an expression marker for flowering

A number of Eucalyptus species exhibit precocious flowering, flowering within a year of germination and often while still exhibiting juvenile foliage. To understand the nature of precocious flowering in Eucalyptus occidentalis, partial homologues of the inflorescence meristem identity gene TERMINAL FLOWER1 and of the floral meristem identity genes LEAFY and APETALA1 (EOTFL1, EOLFY and EOAP1, respectively) were isolated and characterized. The expression patterns of these meristem identity genes during the development of branched and single-stem plants were analysed by quantitative reverse transcriptase PCR. All E. occidentalis plants commenced flowering within 40 weeks of germination. However, the branched plants reached maximum flowering some 5-6 weeks earlier than did single-stem plants. Levels of EOTFL1 and EOLFY expression varied little during the study period irrespective of architecture treatment, whereas expression of EOAP1 reached a peak coincident with peak flowering in both branched and single-stem plants. AP1 is clearly an expression marker for flowering in this species.

Incidence and new records of Mycosphaerella species within a Eucalyptus globulus plantation in Western Australia

Eucalyptus globulus is the predominant exotic hardwood plantation species in Western Australian (WA), and is often planted adjacent to native eucalypt forests. The increase in number of Mycosphaerella species associated with Mycosphaerella leaf disease (MLD) in E. globulus plantations in WA in the past decade has raised concern about the possible movement of pathogens between the native forests and plantations. In order to determine whether the introduction of new E. globulus genetics into WA may have further exacerbated this situation, juvenile and adult foliage were taken from a genetics trial near Albany, WA consisting of 60 full-sib families and Mycosphaerella species identified using morphological and molecular tools. Eleven species of Mycosphaerella were identified from one plantation: Mycosphaerella fori ( Pseudocercospora fori) and Mycosphaerella ellipsoidea are new records for Australia; Mycosphaerella tasmaniensis ( Passalora tasmaniensis) and Mycosphaerella suttoniae ( Kirramyces epicoccoides) are new records for WA; and Mycosphaerella nubilosa, Mycosphaerella cryptica, Mycosphaerella marksii, Mycosphaerella molleriana, Mycosphaerella lateralis, Mycosphaerella aurantia and Mycosphaerella parva, previously recorded for WA. The most frequently isolated species from juvenile foliage was M. marksii (77%) followed by M. nubilosa (33%). M. nubilosa was most frequently isolated from adult leaves (88%) followed by M. parva (7.5%). Three species, M. molleriana, M. lateralis and M. cryptica, were only isolated from adult leaves while M. ellipsoidea was only isolated from juvenile leaves. These records increase the number of known Mycosphaerella species from eucalypts in WA from 10 to 13. The increase in the number, distribution and impact of Mycosphaerella species contributing to MLD in WA is of concern both to the potential productivity of the plantations and the biosecurity of native WA Eucalyptus species. Continued monitoring of the plantation estate is required to understand the dynamics of the host–pathogen interactions.

Stability of Genetic-Based Defensive Chemistry Across Life Stages in a Eucalyptus Species

Defensive chemistry is a key plant fitness trait, and the investigation of the expression of plant secondary metabolites across life stages is important in understanding the lifetime evolutionary selection pressures on a plant. The expression of genetic-based differences in foliar defensive chemistry, known to influence mammalian herbivore preferences, was studied across two contrasting life phases of the heteroblastic tree, Eucalyptus globulus. With plants from different subraces of E. globulus growing in a field trial, we compared the levels of seven chemical constituents in adult and juvenile foliage from related coppiced plants. Defensive chemistry was generally higher in more vulnerable coppice foliage than adult foliage. Significant, genetic-based differences among subraces were detected for two key defensive chemicals, a sideroxylonal and a macrocarpal, and these differences were stable across life phases. In contrast, significant differences among subraces in adult leaf condensed tannins were not evident in the coppice because of the absence of this group of tannins in this foliage. These findings lend support to hypotheses that suggest condensed tannins may have evolved for reasons other than mammalian herbivore defense.

Differences in the Structure and Gas Exchange Physiology of Juvenile and Adult Leaves inMetrosideros excelsa

In many plant species, ontogeny is characterized by the production of different leaf forms, but the functional significance of this phenomenon is unclear. In Metrosideros excelsa (Myrtaceae), vegetative phase change is characterized by a transition from glabrous juvenile foliage to adult leaves that possess a dense pubescence on their abaxial surface. We examined the changes to anatomical and physiological leaf characteristics that accompany phase change in this species. There was no consistent change to δ13C during ontogeny in M. excelsa, indicating that the ratio of intercellular to ambient CO2 remained relatively constant. Rates of photosynthesis were lower in adult versus juvenile foliage, apparently because of a reduction in resources invested toward carbon gain. The stomatal conductance to water vapor tended to decline as the development of pubescence progressed but increased in fully pubescent adult leaves. The stomatal limitation to photosynthetic carbon gain (Ls) exceeded 40% in juvenile and tran...

How does ontogeny in a Eucalyptus species affect patterns of herbivory by Brushtail Possums?

1 Comparisons of adult and juvenile plant stages are often confounded by other factors such as differences in height between plants. However, these factors can be teased apart using common garden experiments and appropriate sampling designs. 2 Using paired sampling of upper and lower canopies of Eucalyptus nitens trees, which were growing in a common environment trial and only one of which had undergone the transition to adult foliage, this study assessed the effects of ontogeny and canopy position on the feeding preferences of Common Brushtail Possums (Trichosurus vulpecula) in captive feeding trials under choice conditions. 3 Possums preferred juvenile foliage over adult foliage. In contrast, canopy position did not influence preferences. 4 Adult foliage contained significantly less sideroxylonals (a compound known to deter herbivory by Brushtail Possums), but was significantly tougher (with thicker cuticle, more fibre, more lignin and a higher percentage dry mass) than juvenile foliage, suggesting that leaf toughness was more significant than defensive chemistry in influencing intake by possums. Adult foliage did not differ significantly from juvenile foliage in nitrogen, total phenolics or essential oil concentration. 5 Greater resistance to herbivory of adult foliage compared with juvenile foliage by a significant browser of eucalypt tree foliage could contribute to selection for an earlier change from juvenile to adult foliage in areas that are heavily browsed by possums.

Variation in Eucalyptus globulus LABILL. and E. nitens DEAN and MAIDEN in Susceptibility of Adult Foliage to Disease Caused by Mycosphaerella cryptica (COOKE) HANSF

SummarySeverity of disease caused by Mycosphaerella cryptica (COOK) HANSF. was assessed on the adult foliage of Eucalyptus globulus LABILL. in two provenance trials (encompassing all four subspecies) and a progeny trial of E. globulus ssp. globulus LABILL. located in Victoria, Australia. Disease was relatively low in all trials (most trees with less than 15% crown severity), except for two provenances at one trial, Judbury (E. globulus ssp. globulus) and Mansfield (E. globulus ssp. bicostata), that had mean crown severities of approximately 25% and 40%, respectively. Eucalyptus globulus ssp. bicostata (MAIDEN et al.) KIRKPATR. was significantly (P < 0.01) more susceptible than E. globulus ssp. globulus, E. globulus ssp. pseudoglobulus (NAUDIN ex MAIDEN) KIRKPATR. and E. globulus ssp. maidenii (F. MUELL.) KIRKPATR., with subspecies maidenii significantly less diseased than all other subspecies. There was significant variation between provenances within subspecies globulus (P < 0.01) but not within subspecies pseudoglobulus, maidenii or bicostata. Subspecies globulus also showed significant (P < 0.01) variation between families. There was a moderate to high genetic correlation between disease of the adult foliage and disease of the juvenile foliage (caused by both M. cryptica and M. nubilosa (COOKE) HANSF.) assessed several years earlier, both at the provenance (rG= 0.67) and family (rG= 0.33) levels. Narrow sense heritability of disease of the adult foliage (M. cryptica) was low (h2= 0.17), compared to that of the juvenile foliage (h2= 0.35) and juvenile defoliation (h2= 0.45) assessed previously. Selection for overall disease resistance (both adult- and juvenile-phase foliage) can be carried out more quickly and accurately at the juvenile stage when trees are 2-3 years old, potentially reducing the time required for resistant trees to be selected and deployed in the field. Mycosphaerella leaf disease on adult E. nitens (DEAN and MAIDEN) MAIDEN was also assessed in two provenance trials; however, there was very little disease observed and no significant differences were found between provenances.

Genetic variation inEucalyptus globulusfor susceptibility toMycosphaerella nubilosaand its association with tree growth

Mycosphaerella species are fungal leaf pathogens of Eucalyptus globulus, one of the major plantation tree species in temperate regions of the world. We examined the quantitative genetic variation in susceptibility to infection by Mycosphaerella nubilosa in a genetically diverse population ofE. globulus families growing in a field trial in north-west Tasmania. Disease incidence and severity were assessed on juvenile foliage following a heavy epidemic where mean leaf area damage was 34%. Disease incidence was uniform across the trial. Significant genetic variation for susceptibility was detected with a narrow sense heritability of disease severity being the highest yet reported (h2=0.60) for aMycosphaerella disease of eucalypts. M. nubilosa damage had a significant deleterious effect on tree growth at both the phenotypic and genetic level. We suggest that E. globulus has at least two mechanisms involved in avoiding the deleterious effects of this disease, one is through resistance of the juvenile foliage per se and the other is through the ontogenetic switch to the resistant adult foliage. There is ample opportunity to select genotypes ofE. globulus that are relatively resistant to damage and if these are deployed in areas of high disease risk, significant benefits in plantation productivity could be obtained.

Creiis lituratus (Froggatt) (Hemiptera: Psyllidae): a new insect pest of Eucalyptus dunnii plantations in sub-tropical Australia

Summary The sap-sucking psyllid Creiis lituratus has become a significant pest of young Eucalyptus dunnii plantations in north-eastern New South Wales (NSW) and south-eastern Queensland. Previously recorded from native forests and amenity trees, but not as a significant pest or from E. dunnii, it has recently (1998–2004) caused severe damage in plantations 1–5 y old. This paper reports observations on C. lituratus from routine and specific forest health surveys, as well as preliminary results from a detailed study on the behaviour and pest management of the insect, with the objective of highlighting the presence of this new insect pest in commercial plantations. Damage symptoms associated with C. lituratus are chlorosis, reddening and necrosis of leaves. Moderately—to severely-affected trees take on a purple-brown colour and damaged (necrotic) leaves are often retained on trees until re-foliation occurs over summer. Severely-affected trees, where >75% of the tree has been damaged, may experience severe defoliation, tip dieback and the sprouting of epicormic shoots the following growing season. Damage can occur on both immature and mature (fully expanded) juvenile, intermediate and adult foliage. Populations are most active over autumn—winter when nymphs can occur in large numbers, virtually covering the entire (abaxial) leaf surface. Creiis lituratus was first observed causing severe damage in several E. dunnii plantations south of Bonalbo in north-eastern NSW in 1998. In 1999 and 2000, severe crown damage was observed in more plantations around Bonalbo, as well as in plantations around Casino. In 2003 C. lituratus caused severe crown damage in up to 25 E. dunnii plantations within 100 km of Bonalbo, with almost 500 ha moderately to severely affected. About 450 ha were sprayed with insecticide (dimethoate). Damage from C. lituratus was more localised in 2004, with less than 50 ha significantly affected; most of this was sprayed with insecticide. Creiis lituratus has been observed in E. dunnii and E. grandis plantations from Bulahdelah to south-eastern Queensland, but significant damage has occurred only in E. dunnii plantations from Grafton north. The occurrence of C. lituratus in plantations south of Grafton is sporadic.

Comparison of Terpene Composition in Engelmann Spruce (Picea engelmannii) Using Hydrodistillation, SPME and PLE

Terpenes emitted by conifer trees are generally determined by analysing plant extracts or essential oils, prepared from foliage and cones using steam distillation. The application of these procedures limits experiments to cut plant materials. Recently headspace techniques have been adopted to examine terpene emission by living plants. This paper deals with the application of solid-phase micro-extraction (SPME) for the analysis of terpenes emitted by conifers foliage of Engelmann spruce (Picea engelmannii), including its seedlings. The compositions of SPME extracts obtained for destroyed and non-destroyed old and juvenile spruce needles were compared with the compositions of essential oils and pressurised liquid extraction (PLE) extracts corresponding to the same plant materials. No substantial differences have been found in the qualitative terpene composition estimated by analysing essential oil and PLE and SPME extracts from non-destroyed old and juvenile foliage. The disintegration of spruce needles results in the formation of a significant amount of myrcene in the case of the old conifer foliage and non-terpenoic compounds in the case of juvenile conifer foliage. This phenomenon can be attributed to enzymatic reactions occurring in the destroyed plant cells.

Genetic variation of Eucalyptus globulus in relation to autumn gum moth Mnesampela privata (Lepidoptera: Geometridae) oviposition preference

The autumn gum moth Mnesampela privata Guenée is a major defoliator of Australia’s commercially important pulpwood plantation species, Eucalyptus globulus Labill. However, little is known about genetic variability of E. globulus to M. privata oviposition preference. We examined oviposition preference by exposing juvenile foliage sprigs of different genetic origins to gravid female moths in cage bioassays. Foliage sprigs were collected from 40 E. globulus families grown in a common environment field trial in southern Tasmania. These families were derived from open-pollinated seed collected in native stands and represented four geographic races of E. globulus. Significant differences in the level of oviposition was detected between foliage sprigs from the different races, with those from the Furneaux race receiving over twice as many egg batches compared to those from either the Strzelecki Ranges or northeastern Tasmania races. The southern Tasmania race received an intermediate egg load. No significant variability in oviposition preference was identified between different localities within races or between different open-pollinated families within localities. Selections from both the Strzelecki Ranges and Furneaux races comprise a large component of the Australian E. globulus breeding population. There is therefore likely to be significant genetic variation for M. privata oviposition preference within the breeding population and by screening before deployment gains in plantation productivity in areas subject to M. privata outbreaks may be possible.

Genetic variation of Eucalyptus globulus in relation to autumn gum moth Mnesampela privata (Lepidoptera: Geometridae) oviposition preference

The autumn gum moth Mnesampela privata Guenée is a major defoliator of Australia’s commercially important pulpwood plantation species, Eucalyptus globulus Labill. However, little is known about genetic variability of E. globulus to M. privata oviposition preference. We examined oviposition preference by exposing juvenile foliage sprigs of different genetic origins to gravid female moths in cage bioassays. Foliage sprigs were collected from 40 E. globulus families grown in a common environment field trial in southern Tasmania. These families were derived from open-pollinated seed collected in native stands and represented four geographic races of E. globulus. Significant differences in the level of oviposition was detected between foliage sprigs from the different races, with those from the Furneaux race receiving over twice as many egg batches compared to those from either the Strzelecki Ranges or northeastern Tasmania races. The southern Tasmania race received an intermediate egg load. No significant variability in oviposition preference was identified between different localities within races or between different open-pollinated families within localities. Selections from both the Strzelecki Ranges and Furneaux races comprise a large component of the Australian E. globulus breeding population. There is therefore likely to be significant genetic variation for M. privata oviposition preference within the breeding population and by screening before deployment gains in plantation productivity in areas subject to M. privata outbreaks may be possible.

Phosphorus fertiliser can induce earlier vegetative phase change in Eucalyptus nitens

Many eucalypts are heteroblastic, exhibiting a distinct change in leaf morphology through their ontogeny. The physiological and chronological age at which the tree switches from the production of juvenile foliage to adult foliage (vegetative phase change) can be under strong genetic control and influenced to some extent by environment. We studied the effect of nitrogen (N) and phosphorus (P) fertiliser on the growth and vegetative phase change in Eucalyptus nitens. Whilst neither fertiliser treatment affected tree height at 3.5 years of age, P fertiliser significantly reduced the length of the juvenile vegetative phase. In contrast, N, but not P, has been shown to promote flowering. These findings support the argument that vegetative and reproductive maturity are genetically and physiologically uncoupled in Eucalyptus.

RELATIVE IMPORTANCE OF PLANT ONTOGENY, HOST GENETIC VARIATION, AND LEAF AGE FOR A COMMON HERBIVORE

Using an experimental eucalypt forest of known pedigree and laboratory feeding trials, we examined the relative importance of plant ontogeny (heteroblasty), genetic variation among host trees, canopy height, and leaf age as potential drivers that could affect the distribution and feeding preference of a common insect herbivore, Chrysophtharta agricola. We found that ontogeny is a major factor affecting this insect. Its importance rivaled leaf physiological age, a well-documented factor, which served as our standard for judging the relative importance of other effects. Three patterns emerged: (1) In the field, beetle feeding was nine times greater in the adult zone than in the juvenile zone of heteroblastic trees (i.e., trees with both adult and juvenile foliage). (2) Laboratory feeding trials confirmed their strong preference for adult foliage. (3) Although eucalypt species, hybrid cross type (F1, F2, and backcrosses), and canopy height also exhibited significant effects on beetle feeding in laboratory trials, their relative importance was much less than ontogeny and leaf physiological age. We conclude that beetles perceive greater variation in host quality within individual plants than between different eucalypt species and their hybrids. The magnitude of these effects argues that ontogeny may rival other better studied plant traits that affect herbivores. We discuss how genetic regulation of phenotypic expression in plants may affect herbivore populations and structure communities.

Intra‐plant host selection, oviposition preference and larval survival of Chrysophtharta agricola (Chapuis) (Coleoptera: Chrysomelidae: Paropsini) between foliage types of a heterophyllous host

Abstract 1 Paropsine chrysomelid beetles defoliate commercial eucalypt plantations in Australia. Adults and larvae feed on the same host, with the larval food source determined by the oviposition choice of females. Most eucalypt species are heterophyllous, with their foliage undergoing distinct morphological and chemical changes between adult and juvenile growth.2 The intra‐plant foliage feeding and oviposition preference adults and the larval development of Chrysophtharta agricola were examined using adult and juvenile foliage of a heterophyllous plantation species, Eucalyptus nitens. The foliage types differ in chemistry, toughness, waxiness and timing of production.3 In the field, feeding damage caused by adult beetles was 15% more frequent on adult foliage than on juvenile foliage; however, egg batches were three times more common on juvenile than on adult foliage.4 Oviposition preference for juvenile foliage over adult foliage was confirmed in choice trials in the laboratory, with adult fecundity and longevity not significantly different between foliage types.5 Larval survival, development time and subsequent pupal weight were also unaffected by foliage type, suggesting that neither foliage type is nutritionally superior for adults or for larvae. However, adult foliage was significantly thicker than juvenile foliage and this may prove a physical constraint to larval establishment. Biotic and abiotic factors (including interactions with natural enemies, competition, microclimate and mate location) that may affect patterns of host plant utilization are discussed.

Spectrometric prediction of secondary metabolites and nitrogen in fresh Eucalyptus foliage: towards remote sensing of the nutritional quality of foliage for leaf-eating marsupials

Near-infrared reflectance spectroscopy provides an excellent means of assessing the chemical composition of Eucalyptus foliage but the standard methods of drying and grinding the samples limit the speed at which spectra can be collected and thus are unsuitable for measurements in the field. We investigated whether reliable spectra could be collected from whole fresh and dry leaves of E. melliodora and E. globulus and whether we could predict the concentration of total nitrogen, the volatile terpene, 1,8 cineole and the phenolic antifeedant compound, sideroxylonal A, from these spectra. Water absorbance peaks did not obscure the absorption spectrum of 1,8 cineole and so cineole concentration was readily predicted from spectra of whole, fresh E. melliodora leaves. Similarly, both total nitrogen and sideroxylonal A could be predicted from spectra of fresh leaf in E. melliodora even though water absorption obscured some spectral features. The predictions of cineole and total nitrogen concentration in E. globulus were not as good as those in E. melliodora, possibly due to interference from waxes on the leaf surface of E. globulus juvenile foliage. Overall, these results suggest that certain important ecological attributes of Eucalyptus foliage can be predicted from spectra of whole fresh leaves. Thus, it is feasible to investigate the collection of spectra by portable or airborne spectrophotometry.

Canopy position and needle age affect photosynthetic response in field-grown Pinus radiata after five years of exposure to elevated carbon dioxide partial pressure

Photosynthesis of tree seedlings is generally enhanced during short-term exposure to elevated atmospheric CO2 partial pressure, but longer-term studies often indicate some degree of photosynthetic adjustment. We present physiological and biochemical evidence to explain observed long-term photosynthetic responses to elevated CO2 partial pressure as influenced by needle age and canopy position. We grew Pinus radiata D. Don. trees in open-top chambers for 5 years in sandy soil at ambient (36 Pa) and elevated (65 Pa) CO2 partial pressures. The trees were well watered and exposed to natural light and ambient temperature. In the fourth year of CO2 exposure (fall 1997), when foliage growth had ceased for the year, photosynthetic down-regulation was observed in 1-year-old needles, but not in current-year needles, suggesting a reduction in carbohydrate sink strength as a result of increasing needle age (Turnbull et al. 1998). In 5-year-old trees (spring 1997), when foliage expansion was occurring, photosynthetic down-regulation was not observed, reflecting significantly large sinks for carbohydrates throughout the tree. Net photosynthesis was stimulated by 79% in trees growing in elevated CO2 partial pressure, but there was no significant effect on photosynthetic capacity or Rubisco activity and concentration. Current-year needles were more responsive to elevated CO2 partial pressure than 1-year-old needles, exhibiting larger relative increases in net photosynthesis to elevated CO2 partial pressure (98 versus 64%). Lower canopy and upper canopy leaves exhibited similar relative responses to growth in elevated CO2 partial pressure. However, needles in the upper canopy exhibited higher net photosynthesis, photosynthetic capacity, and Rubisco activity and concentration than needles in the lower canopy. Given that the ratio of mature to juvenile foliage mass in the canopy will increase as trees mature, we suggest that trees may become less responsive to elevated CO2 partial pressure with increasing age. We conclude that tree response to elevated CO2 partial pressure is based primarily on sink strength and not on the duration of exposure.

The Miena cider gum, Eucalyptus gunnii subsp. divaricata (Myrtaceae): a taxon in rapid decline

The new combination Eucalyptus gunnii subsp. divaricata (McAulay & Brett) B.M.Potts comb. & stat. nov. is introduced for the Miena form of the Tasmanian cider gum Eucalyptus gunnii Hook.f., once described as a separate species, E. divaricata McAulay & Brett. This subspecies occurs on the Central Plateau of Tasmania, where it intergrades clinally with E. gunnii subsp. gunnii and E. archeri. Core populations of this subspecies are among the most frost-resistant of E. gunnii sens. lat., have juvenile foliage of interest for floriculture and have been exploited historically for their sweet sap. However, high mortality of trees in the last decade, coupled with only rare seedling recruitment and poor seed crops, is threatening the long-term survival of this taxon in the wild. E. gunnii subsp. divaricata qualifies as Endangered under Commonwealth and Tasmanian legislation.

Leaf morphological and anatomical characteristics of heteroblastic Eucalyptus globulus ssp. globulus (Myrtaceae)

Leaf characteristics of Eucalyptus globulus Labill. ssp. globulus vary in response to plant genotype, ontogenetic position and environmental conditions. Glasshouse-grown seedlings from provenances at St Marys, Tasmania, and Wilsons Promontory, Victoria, produced seedling leaves for 10 nodes before producing leaves of juvenile form. Tasmanian provenance seedlings began to produce juvenile leaves after 18 weeks, 4 weeks earlier than Wilsons Promontory seedlings. Tasmanian seedlings continued to produce juvenile foliage, whereas Wilsons Promontory seedlings began producing transitional leaves at 33 weeks. Successive transitional leaves ranged from the juvenile to the adult leaf form owing to variability in the rate of change of particular morphological and anatomical leaf characteristics. Retention of broad, thin, sessile, horizontally oriented, dorsiventral, hypostomatal juvenile leaves of Tasmanian seedlings assists in increasing growth rates under mesic conditions. Early production of thick, narrow, petiolate, vertically oriented, isobilateral, amphistomatal adult leaves by Wilsons Promontory seedlings appears to be related to the stressful conditions within its local habitat. An increase in amphistomy and the distribution of palisade mesophyll on both leaf surfaces with ontogenetic development was strongly related to leaf orientation and light interception, increasing the supply of CO2 for photosynthesis.

JUVENILE FOLIAGE AND THE SCALING OF TREE PROPORTIONS, WITH EMPHASIS ONEUCALYPTUS

Juvenile foliage on seedlings and saplings occurs across a wide range of tree species and is particularly notable among eucalypts. Juvenile leaves generally have a lower leaf mass per area (LMA) and are often more horizontal in orientation than adult leaves. This paper examines the hypothesis that the shift from juvenile to adult foliage is related to the scaling of mechanical support costs. A simple model of stem allometry and production was used to show that low values of LMA and leaf area per unit crown area (LAI) should increase the growth rates of small, open-grown seedlings, but not larger saplings or trees. This result occurs because relatively little biomass need be invested in stems and branches to spread out the leaf mass of a seedling, favoring the option of increasing light interception by increasing crown area. In contrast, the large support requirements of trees make increasing crown area expensive and favor shifts to higher LAI and LMA, thereby increasing within-crown light interception and light use efficiency. Reported LMA values for juveniles are lower than for older trees under similar light levels, and there is evidence for similar trends in LAI. However, juvenile foliage may have a variety of other functions, including the enhancement of shade tolerance, and trade-offs may also occur between efficient light interception and protection against herbivores and mechanical damage. Model simulations suggest that the nature and duration of the juvenile phase can have large impacts on early growth, particularly in favorable environments.