Inhibition of photosynthetic processes in foliose lichens induced by temperature and osmotic stress

Negative effects of osmotically-induced dehydration of two foliose lichen species, Lasallia pustulata and Umbilicaria hirsuta, was studied at physiological (22 °C), low (5 °C) and freezing temperature (−10 °C), using chlorophyll (Chl) fluorescence. In both species, exposure to increasing sucrose concentrations led to a pronounced decrease in potential (F_V/F_M), and actual (Φ₂) quantum yields of photochemical processes in photosystem 2. L. pustulata was more sensitive to osmotic stress, because comparable osmotic dehydration inhibited F_V/F_M and Φ₂ more than in U. hirsuta. Critical concentration of sucrose that fully inhibited photochemical processes of photosynthesis was 2.5 M, which represented water potential (Ψ_w) of −18.8 MPa. Decrease in background Chl fluorescence (F₀) and increase in non-photochemical quenching (qN) revealed two phases of osmotic stress in lichens: phase I with no change (Ψ_w 0 to −6.6 MPa) and phase II (Ψ_w −11.3 to −18.8 MPa) typical by substantial change in Chl fluorescence parameters. Effects of thallus anatomy on species-specific response to osmotic dehydration is discussed and attributed to the results obtained by optical microscopy and Chl fluorescence imaging technique.     

The effect of abscisic acid on chlorophyll fluorescence in lichens under extreme water regimes

Chlorophyll fluorescence measurements were used to test whether externally applied abscisic acid (ABA) has an effect on the desiccation tolerance of lichens and their sensitivity to constant water saturation. A surplus of ABA did not decrease the time required to regain full photosynthetic capacity after prolonged dry periods. However, an effect of ABA could be observed when lichens were permanently hydrated at moderately high temperatures. Lichens suffered less from constant saturation if ABA was added to the incubation medium. These results suggest a positive effect of ABA on membrane function.     

Interspecific variation in the resprouting responses of Acacia species following simulated herbivory in a semi‐arid southern African savannah

Plants have evolved a diverse suite of tolerance traits against herbivory, including compensatory growth, increased photosynthesis and activation of dormant meristems. We studied the responses of five Acacia species to simulated herbivory in a semi‐arid southern African savannah. We clipped terminal shoots of five juvenile Acacia species (Acacia rehmanniana, A. nilotica, A. karroo, A. arenaria and A. gerarrdii) to simulate herbivory. We then determined biomass change after 5 months and also counted the number of resprouts and measured their length and diameter. All clipped shoots produced resprouts, with all the Acacia species compensating for the lost biomass. We found considerable interspecific variation in the compensation for biomass lost to herbivory in the five Acacia species. Resprouts biomass ranged from two times in A. arenaria to four times that removed in A. karroo. Acacia karroo produced many resprouts, while A. arenaria produced very few resprouts (4 vs 15 resprouts). The relationship between the number of resprouts and their growth also varied among the different Acacia species. We conclude that the response of Acacias to herbivory ranges from prolific resprouters (such as A. karroo) to poor resprouters (e.g. A. arenaria).     

Nutritional benefit from leaf litter utilization in the pitcher plant Nepenthes ampullaria

The pitcher plant Nepenthes ampullaria has an unusual growth pattern, which differs markedly from other species in the carnivorous genus Nepenthes. Its pitchers have a reflexed lid and sit above the soil surface in a tighly packed 'carpet'. They contain a significant amount of plant-derived materials, suggesting that this species is partially herbivorous. We tested the hypothesis that the plant benefits from leaf litter utilization by increased photosynthetic efficiency sensu stricto cost/benefit model. Stable nitrogen isotope abundance indicated that N. ampullaria derived around 41.7 ± 5.5% of lamina and 54.8 ± 7.0% of pitcher nitrogen from leaf litter. The concentrations of nitrogen and assimilation pigments, and the rate of net photosynthesis (A(N)), increased in the lamina as a result of feeding, but did not increase in the trap. However, maximal (F(v) /F(m)) and effective photochemical quantum yield of photosystem II (Φ(PSII)) were unaffected. Our data indicate that N. ampullaria benefits from leaf litter utilization and our study provides the first experimental evidence that the unique nitrogen sequestration strategy of N. ampullaria provides benefits in term of photosynthesis and growth.     

Seed production and germination in two rare and three common co‐occurring Acacia species from south‐east Australia

Abstract Seed set, size, viability and germination requirements were investigated for two rare (Acacia ausfeldii and A. willianisonii) and three common (A. pycnantha, A. genistifolia and A.paradoxa) co‐occurring congeners in box‐ironbark eucalypt forests near Bendigo, south‐east Australia to investigate correlates of rarity. Seed size was significantly smaller for the two rare species and germinants were less able to emerge from deeper sowing depths than were the larger seeded common congeners. All species had a strong heat‐stimulated germination response. While the rare A. ausfeldii showed strong germination only at the highest temperature treatment (100°C), the common and widespread A.pycnantha showed strong germination across a broad range of temperatures (60‐100°C), likely to be experienced by soil‐stored seeds during a fire. Seed viability, number of seeds per plant, and number of firm, aborted and eaten seeds per pod varied between species, but the pattern of variation was not related to rarity. Small seed size and a very specific temperature requirement for germination may help to explain rarity in A. ausfeldii, and to a lesser extent in A. willianisonii. Fires are often patchy and heating of the soil is likely to be highly spatially variable, so species with germination responses to a broad range of temperatures have an advantage over those that respond only to a narrow range. A narrower range of soil depths from which seeds can emerge will further reduce the proportion of the seed bank that might recruit following fire. Human impacts on species habitats, such as fragmentation, loss of topsoil through mining, timber harvesting, grazing and urbanization, and consequent reduction in fire intensity, are likely to have further contributed to rarity in these species. The role of pollination and other factors in relation to population size is the subject of further investigation.     

The impact of nitrogen deposition on photobiont‐mycobiont balance of epiphytic lichens in subtropical forests of central China

Excessive nitrogen (N) deposition can impact lichen diversity in forest ecosystems, and this is a particular situation in China. Here, we examined the N uptake, assimilation, and the impact of excessive N deposition on the symbiotic balance of dominant epiphytic lichens in the subtropical forests in the Mts. Shennongjia of central China. The results show that lichen species took up, assimilated and utilized more ammonium than nitrate in a species‐specific way, following the increase of N availability. The photobiont of the lichens decreased with the increase of N concentration following an initial increase, while the mycobiont response to the N addition was not apparent. Considerable variation in response to excessive N deposition exists among the lichen species. Usnea longissima could regulate its N uptake, resulting in a stable photobiont‐mycobiont ratio among N treatments. In contrast, the photobiont‐mycobiont ratio of other four lichens increased initially but decreased when N concentration exceeded a certain level, and N stress may have broken the balance between photobiont and mycobiont of these lichens. Our results suggest that most epiphytic lichens in subtropical forest of central China could uptake and assimilate more ammonium than nitrate and that the balance between photobiont and mycobiont of many epiphytic lichens might change with the increasing N deposition load, which could impact the lichen diversity of this forest ecosystem.     

Comparative Study on the Changes in Photosynthetic Activity of the Homoiochlorophyllous Desiccation-Tolerant Haberlea Rhodopensis and Desiccation-Sensitive Spinach Leaves During Desiccation and Rehydration

The functional peculiarities and responses of the photosynthetic system in the flowering homoiochlorophyllous desiccation-tolerant (HDT) Haberlea rhodopensis and the non-desiccation-tolerant spinach were compared during desiccation and rehydration. Increasing rate of water loss clearly modifies the kinetic parameters of fluorescence induction, thermoluminescence emission, far-red induced P700 oxidation and oxygen evolution in the leaves of both species. The values of these parameters returned nearly to the control level after 24 h rehydration only of the leaves of HDT plant. PS II was converted in a non-functional state in desiccated spinach in accordance with the changes in membrane permeability, malondialdehyde, proline and H₂O₂ contents. Moreover, our data showed a strong reduction of the total number of PS II centers in Haberlea without any changes in the energetics of the charge recombination. We consider this observation, together with the previously reported unusually high temperature of B-band (S₂Q_B-) emission of Haberlea to reflect some specific adaptive characteristics of the photosynthetic system. As far as we know this is the first time when such adaptive characteristics and mechanism of the photosynthetic system of a flowering HDT higher plant is described. These features of Haberlea can explain the fast recovery of its photosynthesis after desiccation, which enable this HDT plant to rapidly take advantage of frequent changes in water availability.     

Effect of additional food and water on house mice in a semi‐arid agricultural environment in Australia

Abstract We tested the hypothesis that providing high‐quality food and water would increase reproduction, survival and population size in house mice living in a semi‐arid cropping environment in the mallee region of western Victoria, Australia, where outbreaks of house mice (Mus domesticus) occur irregularly. We employed a factorial design and applied treatments along internal fencelines (16 sites), adjacent to cropping areas, from November 2003 to July 2004. Population abundance was low during the experiment (0–26 mice per site), and the summer rainfall was below average. We confirmed that mice used the supplementary food and water through a reduction in weight of food containers over time and for water through the presence of Rhodamine B in blood samples and positive bands in whiskers. Abundant food was also available through grain spilt on the ground after harvest of the wheat and barley crops. There was some evidence of increased breeding on sites where water was added, but no effect of food or food and water in combination. Sites where free water was available had marginally higher populations over summer (～2 more mice on average; P = 0.07). This difference was well below any biologically meaningful effect. Mice were 0.9 g heavier on sites where water was added (P = 0.04), and were in better condition (P = 0.03). The addition of high‐quality food did not affect mouse population dynamics, and the addition of water resulted in only marginal responses for some demographic characteristics. We conclude that other factors appear to be important for limiting mouse population growth in summer and autumn.     

Germination sensitivities to water potential among co‐existing C3 and C4 grasses of cool semi‐arid prairie grasslands

An untested theory states that C₄ grass seeds could germinate under lower water potentials (Ψ) than C₃ grass seeds. We used hydrotime modelling to study seed water relations of C₄ and C₃ Canadian prairie grasses to address Ψ divergent sensitivities and germination strategies along a risk‐spreading continuum of responses to limited water. C₄ grasses were Bouteloua gracilis, Calamovilfa longifolia and Schizachyrium scoparium; C₃ grasses were Bromus carinatus, Elymus trachycaulus, Festuca hallii and Koeleria macrantha. Hydrotime parameters were obtained after incubation of non‐dormant seeds under different Ψ PEG 6000 solutions. A t‐test between C₃ and C₄ grasses did not find statistical differences in population mean base Ψ (Ψ_b(50)). We found idiosyncratic responses of C₄ grasses along the risk‐spreading continuum. B. gracilis showed a risk‐taker strategy of a species able to quickly germinate in a dry soil due to its low Ψ_b(50) and hydrotime (θ_H). The high Ψ_b(50) of S. scoparium indicates it follows the risk‐averse strategy so it can only germinate in wet soils. C. longifolia showed an intermediate strategy: the lowest Ψ_b(50) yet the highest θ_H. K. macrantha, a C₃ grass which thrives in dry habitats, had the highest Ψ_b(50), suggesting a risk‐averse strategy for a C₃ species. Other C₃ species showed intermediate germination patterns in response to Ψ relative to C₄ species. Our results indicate that grasses display germination sensitivities to Ψ across the risk‐spreading continuum of responses. Thus seed water relations may be poor predictors to explain differential recruitment and distribution of C₃ and C₄ grasses in the Canadian prairies.     

Reproductive biology of two co‐occurring mugilids,Liza argentea and Myxus elongatus,in south‐eastern Australia

The reproductive biology of Liza argentea and Myxus elongatus occurring in two estuaries (Lake Macquarie and St Georges Basin) was found to differ. Gonado‐somatic index values and macroscopic staging of gonads identified the peak spawning period of L. argentea occurred between March and November in Lake Macquarie and January and April in St Georges Basin. In contrast, peak spawning of M. elongatus was concentrated between January and March in both estuaries. Spawning of L. argentea probably occurred in the lower reaches of estuaries as well as in nearshore coastal waters, whereas evidence indicated M. elongatus spawned only in ocean waters. The mean fork length at maturity (L_F50) was greater for females than males in both species, and it also occurred at a larger mean L_F in M. elongatus (males = 230 mm and females = 255 mm) than L. argentea (males = 180 mm and females = 207 mm). Estimates of total potential fecundity were also greater for M. elongatus (425 484–1 157 029) compared to L. argentea (159 933–548 954). Both species had determinate fecundity and displayed a group synchronous pattern of oocyte development, with two distinct size classes of oocytes present in mature ovaries. Liza argentea probably release the larger class of oocytes in one spawning event, but this could not be established for M. elongatus.     

Differences in leaf thermoregulation and water use strategies between three co‐occurring Atlantic forest tree species

Given anticipated climate changes, it is crucial to understand controls on leaf temperatures including variation between species in diverse ecosystems. In the first study of leaf energy balance in tropical montane forests, we observed current leaf temperature patterns on 3 tree species in the Atlantic forest, Brazil, over a 10‐day period and assessed whether and why patterns may vary among species. We found large leaf‐to‐air temperature differences (maximum 18.3 °C) and high leaf temperatures (over 35 °C) despite much lower air temperatures (maximum 22 °C). Leaf‐to‐air temperature differences were influenced strongly by radiation, whereas leaf temperatures were also influenced by air temperature. Leaf energy balance modelling informed by our measurements showed that observed differences in leaf temperature between 2 species were due to variation in leaf width and stomatal conductance. The results suggest a trade‐off between water use and leaf thermoregulation; Miconia cabussu has more conservative water use compared with Alchornea triplinervia due to lower transpiration under high vapour pressure deficit, with the consequence of higher leaf temperatures under thermal stress conditions. We highlight the importance of leaf functional traits for leaf thermoregulation and also note that the high radiation levels that occur in montane forests may exacerbate the threat from increasing air temperatures.     

Will rising CO2 protect plants from the midday sun? A study of photoinhibition of Quercus myrtifolia in a scrub‐oak community in two seasons

Over a large part of the photoperiod, light energy absorbed by upper canopy leaves saturates photosynthesis and exceeds the energetic requirements for light‐saturated linear electron flow through photosystem II (J_PSII), so that photoinhibition results. From a theoretical consideration of the response of light‐saturated photosynthesis to elevated atmospheric CO₂ partial pressure (pCO₂) it may be predicted that, where light‐saturated photosynthesis is Rubisco‐limited, an increase in pCO₂ will stimulate J_PSII. Therefore, the proportion of absorbed quanta dissipated photochemically will increase and the potential for photoinhibition of photosynthesis will decrease. This was tested by measuring modulated chlorophyll a fluorescence from Quercus myrtifolia Willd. growing in the field in open‐top chambers, at either current ambient or elevated (ambient + 35 Pa) pCO₂ on Merritt Island, Florida, USA. During spring and summer, light‐saturated photosynthesis at current ambient pCO₂ was Rubisco‐limited. Consistent with theoretical prediction, J_PSII was increased and photoinhibition decreased by elevated pCO₂ in spring. In the summer, when growth had largely ceased, an acclimatory decrease in the maximum Ribulose 1,5 bisphosphate saturated carboxylation capacity (V_{c max}) removed the stimulation of J_PSII seen in the spring, and photoinhibition was increased in elevated pCO₂. It is concluded that, for Q. myrtifolia growing in the field, the effects of elevated pCO₂ on J_PSII and photoinhibition will reflect seasonal differences in photosynthetic acclimation to elevated pCO₂ in a predictable manner.     

Floodplain ecosystem dynamics under extreme dry and wet phases in semi‐arid Australia

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Linking stream ecology with morphological variability in a native freshwater fish from semi‐arid Australia

Environmental variation is a potent force affecting phenotypic expression. While freshwater fishes have provided a compelling example of the link between the environment and phenotypic diversity, few studies have been conducted with arid‐zone fishes, particularly those that occur in geographically isolated regions where species typically inhabit intermittent and ephemeral creeks. We investigated morphological variation of a freshwater fish (the western rainbowfish, Melanotaenia australis) inhabiting creeks in the Pilbara region of northwest Australia to determine whether body shape variation correlated with local environmental characteristics, including water velocity, habitat complexity, predator presence, and food availability. We expected that the geographic isolation of creeks within this arid region would result in habitat‐specific morphological specializations. We used landmark‐based geometric morphometrics to quantify the level of morphological variability in fish captured from 14 locations within three distinct subcatchments of a major river system. Western rainbowfish exhibited a range of morphologies, with variation in body depth accounting for a significant proportion (>42%) of the total variance in shape. Sexual dimorphism was also apparent, with males displaying deeper bodies than females. While the measured local habitat characteristics explained little of the observed morphological variation, fish displayed significant morphological differentiation at the level of the subcatchment. Local adaptation may partly explain the geographic patterns of body shape variation, but fine‐scale genetic studies are required to disentangle the effects of genetic differentiation from environmentally determined phenotypic plasticity in body shape. Developing a better understanding of environment–phenotype relationships in species from arid regions will provide important insights into ecological and evolutionary processes in these unique and understudied habitats.     

Crystal structure of plant light‐harvesting complex shows the active,energy‐transmitting state

Plants dissipate excess excitation energy as heat by non‐photochemical quenching (NPQ). NPQ has been thought to resemble in vitro aggregation quenching of the major antenna complex, light harvesting complex of photosystem II (LHC‐II). Both processes are widely believed to involve a conformational change that creates a quenching centre of two neighbouring pigments within the complex. Using recombinant LHC‐II lacking the pigments implicated in quenching, we show that they have no particular role. Single crystals of LHC‐II emit strong, orientation‐dependent fluorescence with an emission maximum at 680 nm. The average lifetime of the main 680 nm crystal emission at 100 K is 1.31 ns, but only 0.39 ns for LHC‐II aggregates under identical conditions. The strong emission and comparatively long fluorescence lifetimes of single LHC‐II crystals indicate that the complex is unquenched, and that therefore the crystal structure shows the active, energy‐transmitting state of LHC‐II. We conclude that quenching of excitation energy in the light‐harvesting antenna is due to the molecular interaction with external pigments in vitro or other pigment–protein complexes such as PsbS in vivo, and does not require a conformational change within the complex.     

Sources of protein in two semi‐arid zone mistletoe specialists: Insights from stable isotopes

Obtaining adequate levels of dietary protein is essential for the physiology of consumers. This presents potential problems for frugivorous birds because fruit is generally low in protein rendering it nutritionally inadequate and potentially explaining the rarity of exclusive frugivory in birds. We addressed this issue by determining the isotope composition (¹⁵N/¹⁴N) in the whole blood of two mistletoe consumers, that is, painted honeyeater (Grantiella picta, Meliphagidae) and mistletoebird (Dicaeum hirundinaceum, Dicaeidae) during the grey mistletoe (Amyema quandang, Loranthaceae) fruiting peak in a semi‐arid woodland, NSW, Australia. Grey mistletoe fruit pulp and arthropods were isotopically distinct (mean δ¹⁵N fruit 4.4‰vs. arthropods 7.1‰), thus readily discriminated using the stable isotope approach. Painted honeyeaters and mistletoebirds formed a single group based on their mean δ¹⁵N values and, on average, assimilated approximately half of their nitrogen from mistletoe fruit although individual variation was high. The importance of nitrogen derived from mistletoe fruit did not track its abundance in the environment, suggesting that at least during peak fruiting, this resource is not limiting at this site. Researchers should account for intraspecific variation and take a cautious approach when reconstructing diets using stable isotopes by incorporating individual‐based analyses rather than presenting mean values alone. This is the first study to use the isotope approach to investigate the dietary relationship of mistletoe frugivores and mistletoe fruit and has implications for our understanding of the nutritional ecology of frugivores and its functional relationship to ecosystem processes such as seed dispersal.     

Effect of water content components on desiccation and recovery in Sphagnum mosses

BACKGROUND AND AIMS: The basic parameters of water relations were measured in Sphagnum mosses. The relationships of these parameters to the photosynthetic response to desiccation and the ecology of these mosses were then tested. METHODS: The water relations parameters of six Sphagnum species (mosses typical of wet habitats) and Atrichum androgynum (a moss more typical of mesophytic conditions) were calculated from pressure-volume isotherms. Photosynthetic properties during and after moderate desiccation were monitored by chlorophyll fluorescence. KEY RESULTS: When desiccated, the hummock-forming species S. fuscum and S. magellanicum lost more water before turgor started dropping than other sphagna inhabiting less exposed habitats (73 % compared with 56 % on average). Osmotic potentials at full turgor were similar in all species, with an average value of -1.1 MPa. Hummock sphagna had clearly more rigid cell walls than species of wet habitats (epsilon = 3 x 55 compared with 1 x 93 MPa). As a result, their chlorophyllous cells lost turgor at higher relative water contents (RWCs) than species of wet habitats (0 x 61 compared with 0 x 46) and at less negative osmotic potentials (-2 x 28 compared with -3 x 00 MPa). During drying, Phi(PSII) started declining earlier in hummock species (at an RWC of 0 x 65 compared with 0 x 44), and F(v)/F(m) behaved similarly. Compared with other species, hummock sphagna desiccated to -20 or -40 MPa recovered more completely after rehydration. Atrichum androgynum responded to desiccation similarly to hummock sphagna, suggesting that their desiccation tolerance may have a similar physiological basis. CONCLUSIONS: Assuming a fixed rate of desiccation, the higher water-holding capacities of hummock sphagna will allow them to continue metabolism for longer than other species. While this could be viewed as a form of 'desiccation avoidance', hummock species also recover faster than other species during rehydration, suggesting that they have higher inherent tolerance. This may help them to persist in drought-exposed hummocks. In contrast, species growing in wet habitats lack such strong avoidance and tolerance mechanisms. However, their turgor maintenance mechanisms, for example more elastic cell walls, enable them to continue metabolizing longer as their water contents fall to the turgor-loss point.