Nitens Seedlings Research Articles

Sodium alginate-g-polyacrylamide hydrogel for water retention and plant growth promotion in water-deficient soils

Natural polymer-based hydrogels are preferred as soil water retention agents due to their inherent biocompatibility and biodegradability. Generally, these natural polymers are chemically modified by graft polymerization of vinyl monomers to improve their water absorption and retention properties. Among the polysaccharides used to prepare natural hydrogels, those based on sodium alginate (Alg) stand out for their high-water absorption and retention capacity, as well as for their ability to promote plant growth. The main objective of this study was to develop a biodegradable alginate-based hydrogel as a water retainer to promote plant growth under water deficit conditions. The synthesis was achieved by grafting poly(acrylamide) (PAM) onto Alg backbone, using bisacrylamide as chemical crosslinker and different ratios of alginate:acrylamide (Ac). In addition, Eucalyptus nitens seedlings were used as a model plant to evaluate the effect of alginate-g-polyacrylamide (Alg:PAM) hydrogel application to the growing medium on plant survival, growth, and physiological responses under both well-watered and water-deficient soil conditions. The maximum degree of swelling (65 g/g) was obtained for the hydrogel prepared. The pseudo-second order model described the water uptake kinetics of the hydrogel. The degradability of Alg:PAM hydrogel reached up to 85 % in 5 weeks in soil and occurs by breaking the glycosidic bonds of the alginate. The E. nitens seedlings cultivated with different doses of Alg:PAM hydrogel (4:1 Alg:Ac) showed higher values of height and root diameter relative growth, survival and photosynthetic responses in comparison to non-treated plants. The results indicate that Alg:PAM hydrogel (4:1 Alg:Ac) has promising applications in forestry as a water retention and seedling growth promoter under water deficient conditions.

Nutrient changes in potting mix and Eucalyptus nitens leaf tissue under macadamia biochar amendments

The effect of macadamia nut shell biochar on nitrogen, potassium, phosphorus, magnesium, calcium and sodium concentrations in potting mix used to grow Eucalyptus nitens seedlings was investigated in a glasshouse experiment. The treatments combined two fertiliser rates (50 and 100% rate of the commercial mix commonly used in forestry nurseries) with eight biochar rates (0, 2, 5, 10, 20, 50, 80 and 100 t ha-1) arranged in a randomised complete block with three replicates of four sample plants. Nutrients were quantified in the potting mix and seedling leaves at four destructive harvests 135, 177, 219 and 269 days after planting. Biochar significantly increased nitrate-N, Colwell P, Colwell K and exchangeable Na and reduced ammonium-N, Mg and Ca concentrations in the potting mix. Seedling leaf concentrations of P, K and Na were increased by biochar application, while N remained dependent on fertiliser rate only. Mg and Ca leaf concentrations decreased in response to increasing biochar rates. Elevated nitrate-N and decreased ammonium-N concentrations suggest that biochar might have increased nitrification in the potting mix. We presumed that biochar mediated processes that reduced uptake of P and K when high doses of biochar were combined with full fertilisation. Changes in potting mix K, Na, Mg and Ca were consistent with selective adsorption of ions to biochar surfaces.

Biochar forEucalyptusforestry plantations

Producing robust seedlings for forest plantation establishment is important as early growth influences potential yield and propagation is a significant budget component. The reputed benefits of biochar as a soil amendment may assist the growth and establishment of seedlings while simultaneously allowing for a reduction in fertiliser application. This study focussed on biochar's influence on the establishment of Eucalyptus nitens seedlings, changes to soil and plant chemistry and biomass accumulation. Eight doses of macadamia biochar (0-100 t ha -1 ) combined with two fertiliser rates were applied in a glasshouse pot trial and a 2-year field experiment. Soil Colwell potassium, sodium and nitrogen levels increased in response to biochar treatments while calcium, magnesium and manganese decreased. While soil nutrient dynamics varied under biochar treatments in both experiments, seedling growth did not reveal clear trends in response to biochar application.

Seasonal flight activity, phenology and physiological status of the major establishment pests ofEucalyptus nitensplantations,Heteronyx dimidiataandH. crinitus(Coleoptera: Scarabaeidae: Melolonthinae)

Heteronyx dimidiata and H. crinitus are significant pests of young Eucalyptus spp. plantations in Tasmania, but little is known about their biology. Adult beetles feeding on the leaves and developing leaf buds of young seedlings can severely affect their growth and form or even kill them. Yellow-vaned funnel traps were used to monitor the flight activity of Heteronyx in a commercial plantation over 3 years. Visual searches were also conducted to determine the incidence of inactive beetles in the soil underneath trees and the presence of active beetles on leaves. Over 12 000 H. dimidiata and 1300 H. crinitus were collected during the study. The pattern of trap captures was similar in both species with over 90% of beetles being caught in spring and low numbers being caught throughout summer and autumn. High numbers of H. dimidiata were found in the soil surrounding the roots of Eucalyptus nitens seedlings, but very few larvae were present, indicating that oviposition occurs outside these plantations. Trap data suggest that spring flight activity is principally due to the emergence of overwintering adult beetles and that newly eclosed beetles typically invade plantations during late summer/autumn. Spring trap captures were strongly male biased, suggesting a scramble mating system where males were more active as they competitively searched for females. Over 65% of the variation in H. dimidiata activity in spring was accounted for by levels of solar radiation and wind. Females mated up to three times and ovarian development usually commenced in spring. The incidence of food in the crop was highest during autumn/winter, but the presence of fat in the abdomen was similar throughout the year. The use of yellow-vaned funnel traps to monitor the flight activity of diurnally active Heteronyx species in commercial Eucalyptus spp. plantations is recommended.

Spatial scale and opportunities for choice influence browsing and associational refuges of focal plants

1. Foraging decisions by herbivores depend on variation in food types, the scale(s) at which this variation occurs and the opportunity and capacity for herbivores to respond to such variation. These decisions affect not only the herbivores themselves, but also the vulnerability of individual plants to being eaten. Associational plant refuges, in which neighbouring plants alter focal plant vulnerability, are an emergent property of foraging decisions. 2. Using the red-bellied pademelon (Thylogale billardierii) as a model generalist mammalian herbivore, we investigated the spatial scale(s) at which animals made foraging decisions and the resultant effect on focal plant vulnerability. In a replicated design, we varied vegetation at the individual plant scale, generating intraspecific differences in Eucalyptus nitens seedlings by altering their nutrient status (high, low). We varied vegetation at the patch scale, in which seedlings were planted, using high- (grass) and low- (herbicided) quality patches. Animals were allowed to choose where they fed and what they ate. Animal behaviour was recorded and intake of seedlings measured. 3. We found that animals made foraging decisions first at the patch scale then at the scale of individual plants; both patch and focal seedling characteristics influenced browsing. Pademelons spent most of their time in high-quality patches, and seedlings were consequently more vulnerable there than in low-quality patches. Pademelons also ate more foliage from high- than from low-nutrient status seedlings. This behaviour concentrated resources, increasing foraging efficiency and making focal plants more vulnerable to browsing. 4. The opportunity and capacity to choose at both plant and patch scales resulted in a pattern of focal plant vulnerability consistent with the repellent-plant hypothesis. This contrasts with our previous study, in which animals were only provided with choice at the plant level and plant vulnerability followed the attractant-decoy hypothesis. These combined results demonstrate that the influence of neighbouring vegetation on consumption of a focal plant depends on the spatial scale of selection and on opportunities (and capacity) for herbivores to choose.

Effectiveness of repellents for reducing damage to eucalypt seedlings by browsing mammals

Summary Marsupial herbivores cause significant damage to seedlings in forestry operations. Damage can lead to changes in tree form, reduced growth rates and seedling death. Non-lethal tactics, as a component of an integrated browsing management strategy, are currently sought to assist in reducing this damage. One such tactic involves making seedlings deterrent or unpalatable through the application of chemical repellents. We investigated the effect of three chemical repellents, Plant Plus, Sentree and Hot Shot, upon browsing of Eucalyptus nitens seedlings by two marsupial herbivores known to browse newly established plantations in Tasmania: the common brushtail possum and the red-bellied pademelon. One repellent was designed to deter herbivores through an unpleasant odour (Plant Plus), one to be unpalatable (Hot Shot), and the third to be both deterrent and unpalatable (Sentree). We ran a combination of paired feeding trials (one repellent + control) and cafeteria trials (all three repellents at once with no control) with captive possums and pademelons, for three nights per trial. Both species consumed significantly more foliage from control seedlings than those treated with Plant Plus or Sentree repellents. Pademelons also preferred controls to Hot Shot treated seedlings but, interestingly, possums consumed significantly more foliage from seedlings treated with Hot Shot than from control seedlings. The most effective repellent against both herbivores was Sentree, and the marked reduction in browsing indicates that further testing in the field is warranted.

Phenolic acclimation to ultraviolet-A irradiation in Eucalyptus nitens seedlings raised across a nutrient environment gradient

We investigated the effects of long-term acclimation of Eucalyptus nitens seedlings to ultraviolet-A (UV-A) irradiation (320–400 nm) on phenolic compounds (gallotannins, stilbenes, and flavonols), photochemical efficiency, and chlorophyll and carotenoid contents. Seedlings were raised under four nutrient regimes, ranging from low to high application rates, in an environment that included or excluded UV-A irradiance. Our aims were: to classify phenolic compounds that absorb in the UV-A and their relative contribution to total UV-A absorption; to identify how phenolic compounds respond to UV-A exposure and exclusion, and to determine how plant nutrient status affects acclimation of photo-and pigment-chemistry to UV-A exposure and exclusion. Gallotannins contributed to only a minor fraction of total absorption within the lower range (320–360 nm) of the UV-A spectrum. Stilbene and flavonol compounds dominated absorption within the 320–360 and 360–400 nm ranges, respectively. Contents of gallotannin were generally high in UV-A-exposed seedlings. Although there was a significant effect of UV-A on contents of stilbenes, a general response (across nutrient treatment comparisons) was not evident. Contents of flavonols were not affected by UV-A exposure. Contents of gallotannin, stilbene, and flavonols decreased from low to high nutrient-application treatments. There were no effects of UV-A on photochemical efficiency or pigment-chemistry.

How do soil nutrients affect within-plant patterns of herbivory in seedlings of Eucalyptus nitens?

This study assessed how the palatability of leaves of different age classes (young, intermediate and older) of Eucalyptus nitens seedlings varied with plant nutrient status, based on captive feeding trials with two mammalian herbivores, red-bellied pademelons (Thylogale billardierii), and common brushtail possums (Trichosurus vulpecula). Seedlings were grown under three nutrient treatments (low, medium and high), and we determined how palatability was related to chemical and physical characteristics of the leaves. Pademelons ate more older leaves than young and intermediate leaves for all treatments. This pattern was best explained by sideroxylonals (formylated phloroglucinol compounds known to deter herbivory by other marsupials), and/or essential oil compounds that were present in lower concentrations in older leaves. In the low-nutrient treatment, possums also ate more of the older leaves. However, in the medium- and high-nutrient treatments, possums ate more intermediate leaves than older leaves and showed a behavioural preference for young leaves (consuming younger leaves first) over intermediate and older leaves, in spite of high levels of sideroxylonals and essential oils. The young leaves did, however, have the highest nitrogen concentration of all the leaf age classes. Thus, either sideroxylonals and essential oils provided little or no deterrent to possums, or the deterrent was outweighed by other factors such as high nitrogen. This study indicates that mammalian herbivores show different levels of relative use and damage to leaf age classes at varying levels of plant nutrient status and, therefore, their impact on plant fitness may vary with environment.

Effects of within‐patch characteristics on the vulnerability of a plant to herbivory

Consumption of a focal plant by herbivores depends, not only on the physical and chemical characteristics of that plant, but also on the characteristics of the neighbouring vegetation. Consumption of focal plants has been related to their own characteristics and to the quality of the neighbouring vegetation, but the two have not been combined to examine the relative importance of focal plant and neighbouring vegetation characteristics.We conducted a series of feeding trials to examine the relative importance of focal plant and various characteristics of neighbouring vegetation to browsing of a focal plant within vegetation patches. We planted Eucalyptus nitens seedlings of high and low nutrient status amongst vegetation patches differing in palatability, abundance and height. Generalist mammalian herbivores, red‐bellied pademelons (Thylogale billardierii), were allowed to feed in each of these patches one at a time, and seedling consumption was recorded. Results were considered in light of the attractant‐decoy and apparency hypotheses, which focus on the outcome to plants, and in terms of foraging theory, which is process‐focussed.Seedling and vegetation characteristics were both important. Seedlings of high nutrient status were preferred over those of low nutrient status. The relative quality, abundance and height of neighbouring vegetation all influenced browsing of a focal plant. Seedlings were more vulnerable amongst vegetation that was of low palatability, of low abundance, or was short. Seedling and vegetation effects were additive in two of three trials.Results were consistent with both the attractant‐decoy and apparency hypotheses, and could be explained in terms of maximising foraging efficiency. They demonstrate the need to consider characteristics of both the focal plant and its neighbouring vegetation when predicting the vulnerability of the former to browsing by generalist herbivores.

Physiology and anatomy of lenticel-like structures on leaves of Eucalyptus nitens and Eucalyptus globulus seedlings

Intumescences or abnormal, non-pathogenic, blister-like protuberant growths, form on Eucalyptus globulus Labill. and, to a much lesser extent, Eucalyptus nitens (Deane and Maiden) Maiden leaves when plants are grown in a high relative humidity environment. We examined the histology of intumescences and their effects on leaf photosynthetic processes. Intumescences were induced by placing E. globulus and E. nitens seedlings in a relative humidity of 80% in a greenhouse for 5 days. Symptomatic and asymptomatic leaves of plants with intumescence development were compared with leaves of control plants. Light-saturated carbon dioxide (CO(2)) assimilation (A(max)) and responses of CO(2) assimilation (A) to varying intercellular CO(2) partial pressure (C(i)) were measured. Symptomatic and asymptomatic leaf samples were fixed and sectioned and cellular structure was examined. Intumescences greatly reduced the photosynthetic capacity of E. globulus leaves and were associated with reduced electron transport rate and ribulose bisphosphate (RuBP) regeneration capacity. Tissue necrotization and cellular collapse of the palisade mesophyll and deposition of phenolic compounds in the affected areas, probably reduced light penetration to photosynthesizing cells as well as reducing the amount of photosynthesizing tissue. Photosynthetic capacity of E. nitens was unaffected. The intumescences resembled simple lenticels, both morphologically and developmentally. To our knowledge, this is the first time that lenticel-like structures developed in response to environmental conditions have been described on leaves.

Leaf angle responds to nitrogen supply in eucalypt seedlings. Is it a photoprotective mechanism?

We examined the adjustment of leaf angle (L theta) and foliar chlorophyll and xanthophyll chemistry in Eucalyptus nitens (Deane and Maiden) Maiden seedlings maintained in various nitrogen (N)-supply treatments over a 6-month period. Adjustment of L theta toward the vertical was greatest under conditions of foliar N deficiency and became incrementally more horizontal with increasing foliar N concentration. Photochemical efficiency (Fv/Fm) and quantum yield were lower in seedlings with low foliar N (low-N seedlings) in winter, but not in autumn. Low-N seedlings generally had low area-based chlorophyll concentrations and high xanthophyll-cycle conversion ratios, particularly during months of low temperature. Under mild temperature conditions, high concentrations of zeaxanthin and antheraxanthin were associated with lower electron transport rates (ETR). Incident light, Fv/Fm, ETR and total chlorophyll concentration were negatively correlated with L theta, with horizontal leaf orientation measured as 0 degrees and vertical leaf orientation as 90 degrees . Xanthophyll conversion ratio was positively correlated with L theta. Adjustments in L theta may play a role in photoprotection of E. nitens seedlings by assisting the leaf to balance its utilization and dissipation of energy.

Interactions between herbivores, vegetation and eucalypt tree seedlings in a plantation forestry environment

Summary Non-lethal management methods for reducing mammal browsing damage to Eucalyptus seedlings during plantation establishment are currently being investigated in Tasmania, Australia. One method is use of selective retention of non-seedling vegetation in plantations. To assess its potential, information is needed on how herbivores use such vegetation and whether it has positive or negative effects on browsing and growth of commercial tree seedlings. We quantified the populations of five mammalian herbivore species in and around a spot-herbicided E. nitens plantation during its establishment. We also investigated several aspects of the interactions between those mammalian herbivores, insect herbivores, tree seedlings and non-seedling vegetation. We found that overall population densities were high for red-bellied pademelons (Thylogale billardierii) and red-necked wallabies (Macropus rufogriseus rufogriseus), but very low for common brushtail possums (Trichosurus vulpecula), common wombats (Vombatus ursinus) and the introduced European rabbit (Oryctolagus cuniculus). Population densities of the five mammalian species did not change in response to planting E. nitens seedlings. Dominant vegetation in the plantation was grass, then forbs and non-grass monocots. Herbivores ate a large proportion of available plantation vegetation in unfenced plots. Mammal browsing had little effect on survival of E. nitens seedlings 15 weeks after planting, but significantly reduced their growth. In unfenced plots, insects, mainly Heteronyx spp., caused twice as much damage to seedlings as browsing mammals. The absence of mammalian browsing in fenced plots, which resulted in high grass cover, was associated with reduced insect damage to seedlings.

Xanthophyll-cycle dynamics and rapid induction of anthocyanin synthesis in Eucalyptus nitens seedlings transferred to photoinhibitory conditions

The effects of rapid induction and subsequent relaxation of cold-induced photoinhibition on pigment chemistry of Eucalyptus nitens (Deane and Maiden) Maiden seedling leaves was assessed. The seedlings were subjected to four treatments in a nursery, fertilised or non-fertilised and shaded or non-shaded, before induction of photoinhibition in a growth chamber. Within 2 days, growth chamber conditions decreased photochemical efficiency ( F v / F m ) and increased xanthophyll-cycle conversion ratio. This was associated with decreased levels of total chlorophyll and increased levels of xanthophyll-cycle pigments. After 8 days, F v / F m of all treatments rose. Anthocyanin levels gradually increased until day eight, except in non-shaded, non-fertilised seedlings, which had high levels before the induction of photoinhibition. Visible spectroscopy indicated increased absorption between 500 and 590 nm, indicative of anthocyanin absorption. Electron transport rate after xanthophyll-cycle relaxation was half that measured before induction of photoinhibition. The findings indicate that anthocyanins may provide a photoprotective role in E. nitens seedlings under conditions of photoinhibition.

Growth responses, physiology and decay associated with pruning plantation-grown Eucalyptus globulus Labill. and E. nitens (Deane and Maiden) Maiden

Detailed studies have been undertaken to define pruning regimes for Eucalyptus nitens, but little is known of E. globulus responses to pruning although this is a more commonly planted species. This paper describes experiments that aimed to identify (a) levels of pruning that reduce E. globulus growth, (b) physiological processes contributing to those growth responses, (c) the incidence of decay and factors influencing decay incidence following pruning of E. globulus, and (d) comparative responses of E. nitens and E. globulus to live branch pruning. Results of a field experiment indicated that removal of between 30 and 50% of the crown length was appropriate for E. globulus plantations verging on canopy closure. The significant reduction in height growth associated with removal of 50 or 70% of crown length suggested pruning should remain below 50% of crown length if reduced stem growth of pruned trees was to be avoided. Stem volume was only significantly reduced over the period of the experiment by 70% pruning, but it was estimated that standing volume following removal of 50% of crown length would be reduced by 82 m 3 ha −1 over a 20-year rotation if there were no other silvicultural interventions. The growth responses observed were probably related to large reductions in leaf area following 50 or 70% removal of crown length. Trees responded to pruning by changing patterns of biomass partitioning to favour stem growth at the expense of branch growth. A glasshouse study determined that light-saturated net CO 2 uptake ( A max) increased following pruning. E. nitens seedlings had both a higher baseline A max and higher A max following pruning than did E. globulus, which could partially explain the greater effect of pruning on E. globulus growth than has been observed for E. nitens in other studies. This result, as well as apparently different patterns of foliage distribution through the crowns of E. globulus and E. nitens, suggested that models of pruning responses need to be parameterised for both species. In addition, a more conservative pruning regime may be appropriate for E. globulus than E. nitens. Pruning increased the frequency of branch traces with decay infection, and there was a trend towards increasing decay outbreaks with increasing pruning severity. Decay outbreaks were more likely to occur following pruning of high angle or larger diameter branches.

Evaluating effects of nursery and post-planting nutrient regimes on leaf chemistry and browsing of eucalypt seedlings in plantations

We examined the effects of nursery and field nutrient treatments on leaf chemistry and browsing during establishment of Eucalyptus globulus (two trials; Cressy and Dunalley) and Eucalyptus nitens (one trial; Tyenna) seedlings. Seedlings had high- or low-fertilizer application treatments in the nursery, were fertilized or not in the field and were individually fenced (used for leaf chemistry sampling) or not (used for browsing assessment). The high-fertilizer nursery treatment resulted in relatively high foliar nitrogen (N) and low tannin content (all seedling types), and high total essential oils (seedling batch planted at Cressy only), but nursery fertilizer level did not affect dry matter, neutral detergent (NDF) or acid detergent fibre (ADF) or lignin. After planting, levels of N, tannins and total essential oils changed over time, but the degree differed between sites. Field fertilizing generally had no effect on leaf chemistry during 10 weeks after planting. Across sites, the pattern of browsing was the same. Seedlings with the high-fertilizer nursery treatment were browsed more than the low-fertilizer nursery treatment, though the difference was only significant at two of the three sites. Thus trends in browsing damage were consistent with differences in foliar N and tannins as a result of nursery- and not field fertilizing treatment. Manipulation of seedling chemistry, using particular fertilizer application regimes in the nursery, may be one tool for managers wishing to decrease seedling damage by browsers, particularly when browsing severity is not extreme.

Foliar anthocyanin accumulation may be a useful indicator of hardiness in eucalypt seedlings

Anthocyanin as an indicator of seedling hardiness was tested in field trials at 350 m above sea level (a.s.l.) (Watson’s Block with Eucalyptus globulus Labill. and E. nitens [Deane and Maiden] Maiden) and at 700 m a.s.l. (Moory Rd. with E. nitens only). Nutrients were withheld (nutrient-deprived) or added (nutrient-loaded) to seedlings in the nursery and shaded or non-shaded in the field. The photochemical efficiency of photosystem II ( F v/ F m) decreased with average minimum temperature in non-shaded, but not shaded, treatments. At Watson’s Block this decrease was most pronounced in nutrient-loaded E. globulus and nutrient-deprived E. nitens seedlings and at Moory Rd. in nutrient-deprived non-shaded E. nitens. Shading reduced the rate of decrease of F v/ F m, or there was no change, with average minimum temperature at Moory Rd. Trends of changes in light-saturated net photosynthesis ( A max) with decreasing average minimum temperature at Moory Rd. were generally similar to that of F v/ F m. Boundary-line analysis indicated that F v/ F m was optimal (ca. 0.8) whilst foliar anthocyanin concentration was ≤0.5 mmol m −2 and then decreased exponentially with increasing foliar anthocyanin concentration. A similar analysis showed that A max decreased exponentially from a maximum of 20 μmol m −2 s −1 at 0.2 mmol m −2 to a minimum of 1 μmol m −2 s −1 at 0.8 mmol m −2 foliar anthocyanin concentration. Seedlings high in anthocyanin were associated with low stomatal conductance and low specific leaf area before planting. It was concluded that foliar anthocyanin concentration in the nursery is a potential indicator of eucalypt seedling hardiness to low temperatures experienced after transplanting.

Total, and chemical fractions, of nitrogen and phosphorus in Eucalyptus seedling leaves: Effects of species, nursery fertiliser management and transplanting

Total and nitrogen (N)- and phosphorus (P)-containing chemical fractions extracted in trichloroacetic acid (TCA) were assessed after nursery production (pre-planting) and during establishment of E. globulus Labill. and E. nitens (Deane and Maiden) Maiden seedlings in the field and following transfer of E. nitens seedlings from warm temperatures in a nursery to cool temperatures in a growth chamber. In field experiments, seedlings of both species were transplanted in spring onto a site at 350 m above sea level (asl) and E. nitens in early winter onto a site at 700 m asl. E. nitens seedlings received either high- or low-nutrient, treatments in the nursery before planting at 700 m asl or before transfer to the growth chamber. E. globulus had greater foliar concentrations of total N and P at planting than E. nitens after nursery production under the same fertiliser regime. In both field trials and the growth chamber experiment a decrease in N, and generally P, concentration in the leaves was observed between pre-planting and the first post-planting measurement. Decreased N was observed after a period of one week and 15 weeks at 350 and 700 m asl, respectively. E. globulus seedlings had higher concentrations of insoluble P complexes and lower concentrations of inorganic P than E. nitens. Increased levels of soluble N across treatments were associated with warm spring temperatures in both field experiments. N and P concentrations with time during establishment were similar in all experiments. This may be related to retranslocation associated with growth after transplanting. Despite differences in N and P concentrations, partitioning of total N to soluble (nitrate, ammonia and amino acid), nucleic acid and protein N and of total P to nucleic acid, sugar, inorganic and insoluble P were similar between E. nitens high- and low-nutrient treatments, and with time during establishment. The TCA method yielded insights additional to those of total N and P. This study showed that the plant nutrition associated with eucalypt seedling transplanting is highly dynamic and complex.

PHOTOINHIBITION: A LINK BETWEEN EFFECTS OF THE ENVIRONMENT ON EUCALYPT LEAF CHEMISTRY AND HERBIVORY

We investigated effects of light and nutrients on whole plant growth, ecophysiology, and leaf chemistry of Eucalyptus nitens seedlings, and the effect of these characteristics on herbivory by common brushtail possums (Trichosurus vulpecula) and red-bellied pademelons (Thylogale billardierii). We compared results of the response of plant secondary chemistry to changes in environmental conditions against two contrasting hypotheses: (1) the carbon:nutrient balance hypothesis (CNBH) and (2) the photoinhibition hypothesis. Nutrient and light levels affected relative growth rate and leaf area ratio of seedlings. Electron transport rates and optimum photochemical efficiencies indicated different rates of carbon fixation between treatments, consistent with levels of chlorophyll and xanthophyll pigments. Nonphotochemical quenching and xanthophylls per unit chlorophyll were negatively correlated with total chlorophyll content. Consistent with the photoinhibition hypothesis, levels of leaf phenolics (flavonols, hydrolyzable tannins, and sideroxylonals) per unit chlorophyll were also negatively correlated with total chlorophyll content. In contrast, phenolic levels on a dry-mass basis were not consistent with predictions of the CNBH; they were better explained as a response to severity of photoinhibition than by reallocation of resources from growth to defense. Total essential oil and cineole levels were positively correlated with nitrogen levels, but there was an interaction in their response to nutrients and light. Thus, levels of essential oils were not well predicted by the CNBH. Sideroxylonal and cineole levels were not correlated, and we hypothesize that different processes and requirements drive their production. Intake of seedlings by possums and pademelons was not directly negatively correlated with levels of hydrolyzable tannins, sideroxylonals, or essential oils. An interaction of the costs of these compounds against benefits of nitrogen levels may explain patterns in intake. We suggest that, while environmentally induced levels of carbon-based plant secondary metabolites have consequences for herbivory, photoinhibition may drive the adjustment of levels of leaf phenolics in response to variation in light and nutrients. This contrasts with the view that these adjustments represent a trade-off in resource allocation between growth and defense.

Chilling-dependent photoinhibition, nutrition and growth analysis of Eucalyptus nitens seedlings during establishment.

Effects of chilling-dependent photoinhibition on gas exchange, chlorophyll fluorescence, growth and nutrition of Eucalyptus nitens (Deane and Maiden) Maiden seedlings were assessed for 70 weeks after transplanting 9-month-old seedlings in early winter. One month before transplanting, the seedlings were assigned to fertilized or nutrient-deprived treatments. Immediately after transplanting, half the seedlings in each nutrient treatment were placed in shadecloth tree shelters. The experimental site was at an altitude of 700 m, which is considered marginal for the establishment of E. nitens plantations in Tasmania because of low mean annual minimum temperatures. Overnight frosts followed by sunny morning conditions in the first 20 weeks after transplanting (early June to early October) caused severe photoinhibition. Predawn maximal photochemical efficiency (Fv/Fm) and maximum net photosynthesis (Amax) were depressed in nutrient-deprived seedlings compared with fertilized seedlings, although shading partially alleviated this difference. Neither Fv/Fm nor Amax recovered to values observed before transplanting until > 20 weeks after transplanting. During this period, non-photochemical quenching (NPQ) was high in seedlings in all treatments, although NPQ was lower in shaded, fertilized seedlings than in seedlings in the other treatments. Total foliar nitrogen (N) concentration increased up to 42 weeks after transplanting in the nutrient-deprived seedlings in parallel with increasing relative growth rate (RGR). Fractionation of N- and phosphorus (P)-containing compounds indicated that differences in protein N accounted for the treatment differences in total seedling N. Nucleic acid P increased and inorganic P decreased during growth periods, although total seedling P remained constant. Among treated seedlings, height growth was greatest in shaded seedlings: this was probably a result of apical dominance effects because RGR was higher in unshaded seedlings than in shaded seedlings. Thus, the shade treatment alleviated chilling-dependent photoinhibition and maximized growth during winter, but limited growth during warmer periods and therefore overall growth.

Gene flow between introduced and native Eucalyptus species: exotic hybrids are establishing in the wild

F1 hybrids between exotic Eucalyptus nitens plantations and native E. ovata have previously been reported among seedlings grown from open-pollinated seed collected from E. ovata, on the island of Tasmania. Such exotic hybrid seedlings have now been found in the wild adjacent to plantations at three locations. The proportion of exotic hybrids in open-pollinated seed collected from nearby mature E. ovata was 5.5%. This level compares with only 0.4% for natural hybrids between native species at these sites (E. ovata, E. viminalis and E.�rodwayi). Detection of hybrids was initially based on their deviant morphology, which was generally intermediate between parental species. This subjective classification was then successfully verified by morphometric and allozyme analyses. Pure E. nitens seedlings (wildlings) were restricted to within 30 m of these plantations, whereas established hybrids were found up to 310 m from the plantations. This pattern of establishment reflects dispersal of exotic seed and pollen respectively. It is likely that the recent expansion of the eucalypt plantation estate in Australia will cause an increase in the frequency of exotic hybrids. However, the long-term impact of such hybridisation is yet to be assessed.