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The Ecophysiology of Foliar Anthocyanin

The accumulation of foliar anthocyanins can be consistently attributed to a small range of contexts. Foliar anthocyanin accumulates in young, expanding foliage, in autumnal foliage of deciduous species, in response to nutrient deficiency or ultraviolet (UV) radiation exposure, and in association with damage or defense against browsing herbivores or pathogenic fungal infection. A common thread through these causative factors is low photosynthetic capacity of foliage with accumulated anthocyanin relative to leaves at different ontogenetic stages or unaffected by the environmental factor in question.The ecophysiological function of anthocyanin has been hypothesized as: 1) a compatible solute contributing to osmotic adjustment to drought and frost stress; 2) an antioxidant; 3) a UV protectant; and 4) protection from visible light. Review of the internal leaf distribution of anthocyanin, of experimental evidence using seedlings, and of studies that directly investigated light absorption by anthocyanin and its development relative to recognized processes of photoprotection support the hypothesis that anthocyanins provide protection from visible light.

Temporal variation of tannins (galloylglucoses), flavonols and anthocyanins in leaves of Eucalyptus nitens seedlings: implications for light attenuation and antioxidant activities

Leaves of Eucalyptus nitens (Deane & Maiden) Maidenwere sampled during establishment of seedlings that were deprived of nutrientsor fertilised at planting. Extracts were analysed for polyphenolic compoundsusing high pressure liquid chromatography (HPLC) in conjunction with UV diodearray and electrospray ionisation mass spectrometry (ESI-MS) with negative iondetection as well as direct MS injection. The presence of quercetin, rutin,sideroxylonal A and B andpenta-galloylglucose were confirmed by comparison with standards. Othergalloylglucoses were identified by first principles. Galloylglucoses comprisedabout 75% of polyphenolics. Quercetin and various quercetin glycosidescomprised ca 85% of flavonols. The anthocyanins,cyanidin-3-glycoside and cyanidin-3,5-diglycoside were identified from directinjection MS data. HPLC–ESI-MS was found to be a superior analyticaltechnique compared to conventional HPLC–UV diode array detection due toco-elution of peaks under UV. Temporal variation of anthocyanins may berelated to the severity of cold-induced photoinhibition. Changes ingalloylglucoses and flavonols were similar with time and related to levels offoliar nitrogen. The suitability of HPLC–ESI-MS for the study of foliarpolyphenolics, the phenolic constitution of E. nitensfoliage, and temporal variation of the phenolic constituents during a seedlingestablishment period of 29 weeks are demonstrated.

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.

Premature Decline of Eucalyptus and Altered Ecosystem Processes in the Absence of Fire in Some Australian Forests

We propose a model of ‘premature tree decline’ whereby an absence of fire hastens the mortality of overstorey eucalypts in some forests. This model is relevant to some temperate Australian forests in which fire regimes have shifted from relatively frequent before European settlement to infrequent following settlement. The increased development of midstorey vegetation and litter accumulation has occurred since European settlement in some specific examples of Australian forests and woodlands. Our model proposes that in the long absence of fire: 1. midstorey vegetation reduces the availability of soil water for eucalypts and; 2. Eucalypts have less access to P and/or cations as these elements become locked up in soil, litter and midstorey biomass. We highlight important knowledge gaps and argue that research into ecological burning, for eucalypt health and other values such as biodiversity, is urgently required.

Cold-induced photoinhibition and foliar pigment dynamics of Eucalyptus nitens seedlings during establishment

The effects of cold-induced photoinhibition on chlorophyll and carotenoid dynamics and xanthophyll cycling in Eucalyptus nitens (Deane and Maiden) Maiden were assessed between planting and 32 weeks after planting. The seedlings were fertilised or nutrient-deprived (non-fertilised) before planting and shaded or not shaded after planting. The experimental site was 700 m a.s.l., which is considered marginal for establishment of E. nitens plantations in Tasmania due to low mean annual minimum temperatures. Low temperature–high light conditions caused a reduction in variable to maximal chlorophyll fluorescence ratio (F v /F m ), which was more pronounced in non-fertilised than in fertilised seedlings. Shadecloth shelters alleviated this depression. Except in shaded fertilised seedlings, F v /F m did not recover to the level before planting until after 20 weeks. Total chlorophyll content was initially reduced in shaded treatments but subsequently increased with increasing temperatures and F v /F m. Total xanthophyll content and xanthophylls per unit chlorophyll remained relatively constant in fertilised seedlings but decreased in non-fertilised seedlings within 2 weeks after planting. Total xanthophyll and xanthophylls per unit chlorophyll subsequently recovered in non-shaded, non-fertilised seedlings with increasing temperatures and F v /F m. Diurnal [yield and non-photochemical quenching (NPQ) and seasonal (F v /F m) variation in chlorophyll fluorescence parameters were not reflected in xanthophyll cycling during the period of most severe photoinhibition. This result may indicate that chlorophyll–xanthophylls protein complexes form in winter-acclimated E. nitens foliage as have been demonstrated to occur in Eucalyptus pauciflora Sieb. ex Spreng. (Gilmore and Ball 2000, Proceedings of the National Academy of Sciences USA 97, 11098–11101).

The physiological basis of containerised tree seedling ‘transplant shock’: a review

Summary Tree seedlings are planted on sites of widely differing climatic, edaphic and vegetative characteristics. Seedling transplant shock, defined as seedling mortality or impaired growth soon after planting, has been reported across this spectrum of planting conditions. Thus, transplant shock is used to describe a phenomenon that embraces many distinct physiological responses to stress. This review lists and discusses the potential sources of transplant shock for containerised tree seedlings and suggests options for minimising its detrimental effects for a range of specific causes. Through an understanding of the physiological basis underlying transplant shock under a given set of conditions, it may be possible to eliminate, or at least minimise, the effects of transplant shock on containerised tree seedlings soon after planting.

Strategies of light energy utilisation, dissipation and attenuation in six co-occurring alpine heath species in Tasmania

Alpine environments are characterised by low temperatures and high light intensities. This combination leads to high light stress owing to the imbalance between light energy harvesting and its use in photochemistry. In extreme cases, high light stress can lead to the level of photo-oxidative damage exceeding the rate of repair to the photosynthetic apparatus. Plant species may vary in the mechanisms they use to prevent photodamage, but most comparisons are of geographically and ecologically distinct species. Differences in leaf colouration suggested that photoprotective strategies might differ among Tasmanian evergreen alpine shrub species. We compared chlorophyll fluorescence and leaf pigment composition in six co-occurring alpine shrub species on the summit of Mt Wellington, southern Tasmania, Australia, during spring and autumn. We found marked differences among species in light energy utilisation, attenuation and dissipation. Ozothamnus ledifolius maintained a large capacity for photosynthetic light utilisation and thus, had a low requirement for light dissipation. All five of the other species relied on xanthophyll-cycle-dependent thermal energy dissipation. In addition Epacris serpyllifolia, Richea sprengelioides and Leptospermum rupestre had foliar anthocyanins that would attenuate photosynthetically active light in the leaf. During spring, all species retained de-epoxidised xanthophylls through the night and the pre-dawn concentration of antheraxanthin and zeaxanthin was significantly correlated with reductions in pre-dawn Fv / Fm. We propose that these species use three photoprotective strategies to cope with the combination of high light and low temperature.

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.

Effect of shadecloth tree shelters on cold-induced photoinhibition, foliar anthocyanin and growth of Eucalyptus globulus and E. nitens seedlings during establishment

The effects of shadecloth tree shelters on cold-induced photoinhibition, foliar anthocyanin and growth of Eucalyptus globulus Labill. and Eucalyptus nitens (Deane & Maiden) Maiden seedlings were assessed between planting (in early spring) and the age of 23 weeks. The experimental site was at 350 m above sea level (asl), which is considered marginal for establishment of E. globulus (but not E. nitens) plantations in Tasmania because of low mean minimum temperatures. Conditions within 3 weeks of planting induced severe photoinhibition in non-shaded seedlings. This was associated with increased anthocyanin and photodamage in non-shaded E. nitens and E. globulus. As a result, there was 20p mortality in non-shaded E. globulus. In contrast, shaded seedlings of both species had levels of photoinhibition and anthocyanin that were largely similar to those before planting and there was no photodamage. Levels of anthocyanin indicated that its synthesis responded to the severity of photoinhibition. Height growth and levels of mortality indicated that cold-induced photoinhibition, and not frost tolerance alone, determines the range of environments where E. globulus can be successfully planted. In contrast, the tolerance of E. nitens seedlings to cold-induced photoinhibition may be a factor in the demonstrated success of this species as a high-altitude plantation species.

Interactive effects of nitrogen and irradiance on sustained xanthophyll cycle engagement in Eucalyptus nitens leaves during winter

Abstract.Eucalyptus nitens is a species that is adapted to low temperature. This study examines xanthophyll-cycle engagement in E. nitens seedlings exposed to cold-induced photoinhibitory conditions under different levels of irradiance and nutrient status. Xanthophyll-cycle pool size indicated an increased requirement for light energy dissipation under high irradiance and low nutrient status. Greater sensitivity to photoinhibition of non-shaded seedlings indicated that sustained xanthophyll-cycle engagement may occur in response to damaged chlorophyll. Within irradiance treatments, fertilised seedlings had higher photochemical efficiency and faster recovery from photoinhibition than unfertilised seedlings. These results demonstrate that fertilised compared to unfertilised seedlings can utilise a greater proportion of incident light under cold temperature conditions