Investigating how nitrogen nutrition and pruning impacts on CBD and THC concentration and plant biomass of Cannabis sativa.

Precise crop fertilization requires an in-depth understanding of plant uptake and utilisation to optimise sustainable production. This study investigated the influence of nitrogen (N) nutrition and pruning on the cannabinoid concentrations and biomass of a commercial cannabis cultivar; the rationale for this study is how N supply and pruning affect cannabinoid yields and concentration in a commercial setting. Clones of a Cannabis sativa L. (CBD-type) were grown in a controlled-environment glasshouse in pots with coarse sand. After five weeks of vegetative growth under 210 mg/L N and an 18 h light regime, rates of 30, 60, 210, and 500 mg/L N were applied to plants for twelve weeks and a light regime set at 12 h. Double stem pruning was applied as an additional treatment to investigate efficacy on biomass increase. Biomass, N concentrations, and cannabinoid concentrations were measured after the final harvest. Pruning treatment did not increase cannabinoid concentrations or affect biomass. It was coincidentally found that plants on the glasshouse edge with higher exposure to sunlight developed more biomass and higher cannabinoid concentrations. Only biomass in leaves was increased significantly via higher nitrogen nutrition. Cannabinoid concentration, as well as cannabinoid yield per plant were decreased with the increase in N supply. High rates of fertilizer are not recommended because of reduced cannabinoid concentration and biomass yield: the ideal N supply is likely to be between 60 and 210 mg/L. This research will benefit growers and advisors in understanding the complexity of effects of nitrogen fertiliser and pruning practices on plant biomass and secondary metabolite production in medicinal cannabis.

Orchard microclimate, tree water uptake and sweet cherry fruit quality under protected cropping.

Protected cropping systems (PCS) de-risk adverse climatic effects in intensive horticultural production but alter the growing environment. The objectives of this study were to investigate the effects of modern, commercial-scale PCS on sweet cherry orchard microclimate, tree water uptake and fruit quality. Sap flow sensors and weather stations were positioned at four locations under a 21 ha PCS at varying elevations (125, 114, 111, 102 m above sea level) and distances from the block boundary (105, 75, 60 or 50 m, referred to hereafter as Locations 1 to 4, respectively). Generalised additive models (GAMs) were used to predict the effect of individual climate parameters (temperature, relative humidity, solar radiation and wind speed) on tree sap flow at each of the four locations. Average and maximum temperatures and average minimum relative humidity (RH) were higher (15.9°C, 26.1°C and 49.0%) at locations with higher elevations and located further from the PCS boundary (locations 1 and 2) in contrast to locations at lower elevations and closer proximity to the PCS boundary (locations 3 and 4) (15.4°C, 24.6°C and 48.1%). Predicted sap flow was strongly correlated (r2 = 0.92) with time across the four locations under the PCS. GAMS modelling indicated that the hourly water uptake by trees within close proximity to the block boundary (locations 3 and 4) responded with greater intensity to increases in temperature and reductions in relative humidity, taking up on average 0.15 L h-1 (at temperatures >30°C) and 0.08 L h-1 (at RH<50%), respectively, in contrast to trees further under the PCS (locations 1 and 2) where average tree water uptake was 0.08 and 0.04 L h-1 at temperatures >30°C and RH<50%, respectively. Highest average predicted hourly tree sap flow was associated with high wind speeds (0.67 L h-1) and low relative humidity levels (0.61 L h-1). Fruit harvested from locations further from the PCS boundary had significantly higher dry matter content (18.2%), total soluble solids (17.8%) and compression firmness (311.3 g mm-1) in contrast to fruit closer to the PCS boundaries (16.1%, 15.7% and 258.3 g mm-1). This study provides greater understanding of the effects of PCS on microclimate and consequences for tree water uptake and fruit quality.

The Effect of the Height of Coppicing and Harvest Season on the Yield and Quality of the Essential Oil of Kunzea ambigua.

Kunzea ambigua is a small shrub belonging to the Myrtaceae family and the leaves are steam-distilled to produce a therapeutically active essential oil. With production moving from wild-harvested to orchardised stands, there is a need for harvest management of kunzea oil. This study compared the regrowth, essential oil content and composition of kunzea plants after harvesting vegetative material to a depth of 0.2 m above ground level (shallow-cut), relative to plants cut to a depth of 0.1 m above ground level (deep-cut) over the 2018/2019 growing season. Increased vegetative biomass accounted for the increased oil yield and was caused by consistently higher growth rates of 50 to 60% across all seasons in shallow-cut crops relative to those subject to deep-cut. Total soluble sugar concentrations were higher in the leaves and lower in the roots of deep-cut treated plants compared to the other treatments, indicating defoliated K. ambigua responds by mobilising sugars into above-ground biomass. The overall essential oil content of leaves was constant regardless of season, though the oil yield for shallow-cut was 1.9-fold higher at 11.79 ± 0.23 g/m2 compared to deep-cut (6.24 ± 0.18 g/m2). An interactive effect of harvest intensity with season was recorded for all major components except for a non-significant effect of season on terpinen-4-ol. Bicyclogermacrene and α-pinene were elevated in both shallow- and deep-cut treatments relative to control (un-cut) in spring, possibly due to the plant defense response after de-foliation. The highest percentage of bioactive compounds (1,8-cineole and viridiflorol) were present in autumn. Therefore, the recovery of biomass post-harvest is optimised by shallow-cut harvests, and the profile of kunzea oil can be manipulated to elevate levels of specific bioactive components by selecting to crop in autumn/spring.

Focus on the role of seed tannins and pectolytic enzymes in the color development of Pinot noir wine.

Maceration techniques which promote the extraction of color pigments and tannin from grapes are often sought in Pinot noir winemaking to optimise color stability; alternatively, exogenous grape tannins may be included during fermentation. To examine the effect of seed-derived tannins and the use of pectolytic enzymes on color development in wines, conventional must preparations of Vitis vinifera L. cv Pinot noir grapes were compared with wines made using a supplementary addition of either a commercial seed tannin product or previously fermented seeds, while in a complementary experiment, seeds were sequentially removed during fermentation. After 6 months bottle aging, wines supplemented with either a commercial seed tannin solution (0.4 â€‹g/L), or fermented seeds (20% w/w seeds) had from 60% to 95% higher tannin concentration than the untreated wine, and up to 60% more monomeric anthocyanins. Conversely, when a third of the seeds were removed from the fermenting wine, the concentration of both tannin and non-bleachable pigments was 20-30% lower than in untreated wines and the wine hue had more red-purple tones. Exploration of the use of pectolytic enzymes in conjunction with seed removal was also found to have a significant impact on wine color parameters. Further insights on the timing of egress of tannin precursors from seeds was obtained from histochemical examination of the seeds that had been removed during alcoholic fermentation.

Nitrogen Use Efficiency, Allocation, and Remobilization in Apple Trees: Uptake Is Optimized With Pre-harvest N Supply.

Optimizing the utilization of applied nitrogen (N) in fruit trees requires N supply that is temporally matched to tree demand. We investigated how the timing of N application affected uptake, allocation, and remobilization within 14-year-old "Gala"/M26 apple trees (Malus domestica Borkh) over two seasons. In the 2017-2018 season, 30 g N tree-1 of 5.5 atom% 15N-calcium nitrate was applied by weekly fertigation in four equal doses, commencing either 4 weeks after full bloom (WAFB) (pre-harvest) or 1-week post-harvest, or fortnightly, divided between pre- and post-harvest (50:50 split). Nitrogen uptake derived from fertilizer (NDF) was monitored by leaf sampling before whole trees were destructively harvested at dormancy of the first season to quantify N uptake and allocation and at fruit harvest of the second season to quantify the remobilization of NDF. The uptake efficiency of applied N fertilizer (NUpE) was significantly higher from pre-harvest (32.0%) than from the other treatments (~17%). The leaf NDF concentration, an indicator of N uptake, increased concomitantly only when pre-harvest N was applied. Pre-harvest treated trees allocated more than half of the NDF into fruit and leaves and stored the same amount of NDF into perennial organs as the post-harvest treatment. Subsequent spring remobilization of NDF was not affected by the timing of N fertigation from the previous season. A seasonal effect of remobilization was observed with a decrease in root N status and a reciprocal increase in branch N status at fruit harvest of season two. These findings represent a shift in the understanding of dynamics of N use in mature deciduous trees and indicate that current fertilizer strategies need to be adjusted from post-harvest to primarily pre-harvest N application to optimize N use efficiency. This approach can provide adequate storage N to support early spring growth the following season with no detriment to fruit quality.

Novel Methods to Manipulate Autolysis in Sparkling Wine: Effects on Yeast.

Sparkling wine made by the traditional method (Méthode Traditionelle) develops a distinct and desirable flavour and aroma profile attributed to proteolytic processes during prolonged ageing on lees. Microwave, ultrasound and addition of ß-glucanase enzymes were applied to accelerate the disruption of Saccharomyces cerevisiae, and added to the tirage solution for secondary fermentation in traditional sparkling winemaking. Scanning electron microscopy and flow cytometry analyses were used to observe and describe yeast whole-cell anatomy, and cell integrity and structure via propidium iodide (PI) permeability after 6-, 12- and 18-months post-tirage. Treatments applied produced features on lees that were distinct from that of the untreated control yeast. Whilst control yeast displayed budding cells (growth features) with smooth, cavitated and flat external cell appearances; microwave treated yeast cells exhibited modifications like 'doughnut' shapes immediately after treatment (time 0). Similar 'doughnut'-shaped and 'pitted/porous' cell features were observed on progressively older lees from the control. Flow cytometry was used to discriminate yeast populations; features consistent with cell disruption were observed in the microwave, ultrasound and enzyme treatments, as evidenced by up to 4-fold increase in PI signal in the microwave treatment. Forward and side scatter signals reflected changes in size and structure of yeast cells, in all treatments applied. When flow cytometry was interpreted alongside the scanning electron microscopy images, bimodal populations of yeast cells with low and high PI intensities were revealed and distinctive 'doughnut'-shaped cell features observed in association with the microwave treatment only at tirage, that were not observed until 12 months wine ageing in older lees from the control. This work offers both a rapid approach to visualise alterations to yeast cell surfaces and a better understanding of the mechanisms of yeast lysis. Microwave, ultrasound or ß-glucanase enzymes are tools that could potentially initiate the release of yeast cell compounds into wine. Further investigation into the impact of such treatments on the flavour and aroma profiles of the wines through sensory evaluation is warranted.

Grape skins as supplements for color development in Pinot noir wine.

A particular challenge to making wine from Pinot noir grapes is the delicate flavor, light color and poor ageing potential of the wine. Conventional Pinot noir must preparations were compared with those made using a skin-based supplement to assess the impact on non-bleachable (sulfur resistant) pigments in the wine. When supplemented with either fresh grape pomace of Pinot noir, Pinot gris or Chardonnay grapes; Pinot noir grape marc or a commercial liquid grape skin extract, the additional seeds and pulp from the supplements were shown to compromise the development of stable pigments in the wine. To compare the relative merits of tannin derived from grape skins and seeds, the supplements used in a parallel experiment were the skins alone of the same three grape varieties and at six months bottle age, the stable pigment concentration was found to exceed the amount attributable to the supplement. A third experiment used fermented grape skins as the supplement, with 85% of the supplementary anthocyanin recovered as stable pigment complexes in the wine. Notably, this series of experiments showed that supplements containing grape seeds appeared to compromise non-bleachable pigment formation in the wine while skin only supplements stimulated their development.

An assessment of ectomycorrhizal fungal communities in Tasmanian temperate high-altitude Eucalyptus delegatensis forest reveals a dominance of the Cortinariaceae.

Fungal diversity of Australian eucalypt forests remains underexplored. We investigated the ectomycorrhizal (EcM) fungal community characteristics of declining temperate eucalypt forests in Tasmania. Within this context, we explored the diversity of EcM fungi of two forest types in the northern highlands in the east and west of the island. We hypothesised that EcM fungal community richness and composition would differ between forest type but that the Cortinariaceae would be the dominant family irrespective of forest type. We proposed that EcM richness would be greater in the wet sclerophyll forest than the dry sclerophyll forest type. Using both sporocarps and EcM fungi from root tips amplified by PCR and sequenced in the rDNA ITS region, 175 EcM operational taxonomic units were identified of which 97 belonged to the Cortinariaceae. The Cortinariaceae were the most diverse family, in both the above and below ground communities. Three distinct fungal assemblages occurred within the wet and dry sclerophyll forest types and two geographic regions that were studied, although this pattern did not remain when only the root tip data were analysed. EcM sporocarp richness was unusually higher than root tip richness and EcM richness did not significantly differ among forest types. The results are discussed in relation to the importance of the Cortinariaceae and the drivers of EcM fungal community composition within these forests.

Effect of Biochar on Nutrient Leaching in a Young Apple Orchard.

Nutrient leaching from agricultural soils is a worldwide problem that has been implicated in deleterious impacts on the environment. Application of biochar to soil has been proposed as a means to reduce nutrient leaching and improve fertilizer use efficiency. The potential for biochar to reduce nutrient leaching and increase fertilizer use efficiency was tested by applying 47 Mg ha hardwood biochar before replanting a commercial apple () orchard, in the Huon Valley, Tasmania. Passive wick flux meters were installed at the base of the A1 horizon at a depth of 25 cm to monitor leachate volume and the concentration of nutrients leached below the A1 soil horizon over a 38 mo period. Biochar application significantly increased the concentration of phosphorous in the leachate, while having no significant effect on nitrate or potassium concentration. The volume of leachate collected in the flux meters was significantly higher in the biochar treatment, which resulted in significantly higher amounts of potassium and phosphorous being leaching from the biochar treatment than the control. Biochar application had no significant effect on either the concentration or the flux of nitrate leached from the A1 horizon. Nonetheless, nutrient application was well in excess of tree requirements, such that between 53 to 78% of the applied nitrogen, 5 to 11% of the applied phosphate, and 69 to 112% of the applied potassium were leached below the A1 horizon.

Yeast effects on Pinot noir wine phenolics, color, and tannin composition.

Extraction and stabilization of wine phenolics can be challenging for wine makers. This study examined how yeast choice affected phenolic outcomes in Pinot noir wine. Five yeast treatments were applied in replicated microvinification, and wines were analyzed by UV-visible spectrophotometry. At bottling, yeast treatment Saccharomyces cerevisiae RC212 wine had significantly higher concentrations of total pigment, free anthocyanin, nonbleachable pigment, and total tannin and showed high color density. Some phenolic effects were retained at 6 months' bottle age, and RC212 and S. cerevisae EC1118 wines showed increased mean nonbleachable pigment concentrations. Wine tannin composition analysis showed three treatments were associated with a higher percentage of trihydroxylated subunits (skin tannin indicator). A high degree of tannin polymerization was observed in wines made with RC212 and Torulaspora delbruekii , whereas tannin size by gel permeation chromatography was higher only in the RC212 wines. The results emphasize the importance of yeast strain choice for optimizing Pinot noir wine phenolics.

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.

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.

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 photo-inhibition 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 (Fv/Fm) 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, Fv/Fm 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 590nm, 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.

Differential distribution of leaf chemistry in eucalypt seedlings due to variation in whole-plant nutrient availability.

We investigated the effects of whole-plant nutrient-availability on the degree of distribution of some plant primary and secondary (nitrogen, fibre, flavonols, gallotannins and cineole) chemicals across young, mature and old leaves of seedlings of Eucalyptus nitens. Four treatments that ranged from low to high nutrient-application rates resulted in mean whole-plant foliar concentrations of 0.63%, 0.85%, 1.11% and 1.82% nitrogen dry matter (N%DM) for treatments A, B, C and D, respectively. Within-plant distribution (across the leaf age profile of young, mature and old leaves within a eucalypt seedling) of N%DM ranged from zero in treatment A to a wide range of distribution in treatment D (low N%DM concentrations in old leaves to high N%DM concentrations in young leaves). Similarly, the distribution of fibre ranged from zero in treatment A to a wide range of distribution in treatment D, but with high concentrations in old leaves and low concentrations in young leaves. In contrast, flavonols (weakly) and gallotannins had a wide range of distribution in treatment A (low concentrations in old leaves to high concentrations in young leaves) but were little or not distributed in the other treatments. Again in contrast, cineole was strongly distributed between old and young leaves (low concentrations in old leaves to high concentrations in young leaves) across all treatments while concentrations in mature leaves reflected one or other leaf age depending on treatment. Protein precipitable phenols in treatments A, B and C were high in young, and low in old leaves; whereas in treatment D they were low and similar between leaves of different ages.

Pigment dynamics during cold-induced photoinhibition of Acacia melanoxylon.

Long-term acclimation of photo- and pigment-chemistry was investigated in a naturally-regenerating stand of Acacia melanoxylon R.Br. ex Ait. A pronounced decrease in photochemical efficiency of A. melanoxylon saplings was observed between autumn and winter in both thinned and unthinned treatments, but the decrease was more severe in the thinned treatment. Associated pigment changes in the unthinned treatment included a decrease in total chlorophyll content and a rise in chlorophyll a : b. Similar acclimation occurred in the thinned treatment with additional increase in zeaxanthin per unit chlorophyll observed. Saplings in the thinned treatment were exposed to lower minimum temperatures, more hours of frost and higher light intensities in the mid- to lower-crown. Growth chamber studies of the short-term acclimation of photo- and pigment-chemistry were conducted in a low / high light and cold / warm temperature factorial experiment. Photochemical efficiency and quantum yield adjusted within one day and then remained constant for 10 d in response to the imposed treatments. Chlorophyll concentration had decreased in all treatments by day 2 in the growth chambers, and subsequently increased in warm, but not in cold, treatments, irrespective of light level by day 10 in the growth chambers. The concentration of lutein-5,6-epoxide decreased in response to the cold-high light treatment and increased in response to other treatments by day 10 in the growth chambers, consistent with a function in sustained photoprotection in leaves of shade-adapted species. Our experiments indicated that A. melanoxylon is susceptible to cold-induced photoinhibition under cool temperatures (2-8°C) and moderate light intensities (450 µmol m-2 s-1).

Alternate energy dissipation? Phenolic metabolites and the xanthophyll cycle.

The dynamics of phenolic galloylglucoses (di-, tri-, tetra- and penta-galloylglucose), flavonoids (quercitin and quercitin glycosides) and sideroxylonal were compared with that of xanthophyll cycle-dependent energy dissipation during rapid induction of chilling-dependent photo-inhibition. Pre-dawn xanthophyll cycle engagement of seedlings of Eucalyptus nitens transferred from mild nursery conditions to a low temperature controlled environment increased logarithmically during eight days of treatment. Photochemical efficiency and flavonoids decreased after four days of treatment and non-photochemical quenching after two days of treatment. Galloylglucoses and sideroxylonal decreased linearly during treatment. These results demonstrate that rapid changes in foliar phenolic levels are associated with abrupt changes in the plant environment. It is argued that under these growth-chamber conditions, the xanthophyll cycle facilitated dissipation of excess light energy, lessening the requirement for the dissipation of energy or antioxidant activity through phenolic metabolites.

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

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

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.

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.