CO2 exchange and thallus nitrogen across 75 contrasting lichen associations from different climate zones

Aiming to investigate whether a carbon-to-nitrogen equilibrium model describes resource allocation in lichens, net photosynthesis (NP), respiration (R), concentrations of nitrogen (N), chlorophyll (Chl), chitin and ergosterol were investigated in 75 different lichen associations collected in Antarctica, Arctic Canada, boreal Sweden, and temperate/subtropical forests of Tenerife, South Africa and Japan. The lichens had various morphologies and represented seven photobiont and 41 mycobiont genera. Chl a, chitin and ergosterol were used as indirect markers of photobiont activity, fungal biomass and fungal respiration, respectively. The lichens were divided into three groups according to photobiont: (1) species with green algae, (2) species with cyanobacteria, and (3) tripartite species with green algal photobionts and cyanobacteria in cephalodia. Across species, thallus N concentration ranged from 1 to 50 mg g^-1 dry wt., NP varied 50-fold, and R 10-fold. In average, green algal lichens had the lowest, cyanobacterial Nostoc lichens the highest and tripartite lichens intermediate N concentrations. All three markers increased with thallus N concentration, and lichens with the highest Chl a and N concentrations had the highest rates of both P and R. Chl a alone accounted for ca. 30% of variation in NP and R across species. On average, the photosynthetic efficiency quotient [K_F=(NP_max+R)/R)] ranged from 2.4 to 8.6, being higher in fruticose green algal lichens than in foliose Nostoc lichens. The former group invested more N in Chl a and this trait increased NP_max while decreasing R. In general terms, the investigated lichens invested N resources such that their maximal C input capacity matched their respiratory C demand around a similar (positive) equilibrium across species. However, it is not clear how this apparent optimisation of resource use is regulated in these symbiotic organisms.     

The influence of UV-B radiation on light-dependent photosynthetic performance in <Emphasis Type="Italic">Sanionia uncinata </Emphasis>(Hedw.) Loeske in Antarctica

Photosynthetic activity of the moss Sanionia uncinata (Hedw.) Loeske was investigated on Léonie Island (67°35'S, 68°20'W, Antarctic Peninsula) in response to short-term changes of UV-B radiation. The UV-environment of natural mat formations dominated by S. uncinata was altered using filter screens. Two filter experiments were conducted in the Antarctic summers 1998 and 1999. A third filter experiment was conducted during springtime ozone depletion in October 1998. Photosynthetic activity of S. uncinata was mainly determined by photosynthetically active photon flux density (PPFD). Light response of relative electron transport rate through photosystem II (rel ETR=jF/Fm'2PPFD) remained unaffected by ambient summer levels of UV-B radiation. The same was found for net photosynthesis and dark respiration. In October 1998, S. uncinata was mainly metabolically inactive due to low temperatures. No significant levels of DNA-damage measured as cyclobutane pyrimidine dimers (CPDs) were induced by ambient summer levels of UV-B. Artificially enhanced UV-B radiation supplying a Setlow-DNA-dose of 8.7 kJ m^ф day^у UV-B led to formation of 7Dž CPD (10⁶ nucleotides)^-1. It is concluded that current ambient summer levels of UV-B radiation do not affect photosynthetic activity in S. uncinata.     

Further evidence that activation of net photosynthesis by dry cyanobacterial lichens requires liquid water

Abstract: In contrast to green algal lichens, cyanobacterial species of different families, growth forms and habitats proved to be unable to attain positive net CO₂ assimilation when the dry thalli were treated with air of high relative humidity; they needed liquid water for the reactivation of their photosynthetic apparatus. Identical behaviour is shown by all of the 47 lichen species with cyanobacterial photobionts, from six different genera, studied so far. This suggests a widely distributed, if not general, characteristic of cyanobacterial lichens. The difference in performance between both groups of photobionts was maintained when the lichen thallus was macerated. Furthermore, cultures of Chroococcidiopsis were unable to make use of water vapour hydration for positive net photosynthesis, and were similar in this respect to some free-living aerophilic cyanohacteria tested earlier. Possible physiological implications as well as ecological consequences for water-relation-dependent habitat selection of green-algal and cyanobacterial lichens are discussed.     

Cyanolichens can have both cyanobacteria and green algae in a common layer as major contributors to photosynthesis

Background and Aims

Cyanolichens are usually stated to be bipartite (mycobiont plus cyanobacterial photobiont). Analyses revealed green algal carbohydrates in supposedly cyanobacterial lichens (in the genera Pseudocyphellaria, Sticta and Peltigera). Investigations were carried out to determine if both cyanobacteria and green algae were present in these lichens and, if so, what were their roles.

Methods

The types of photobiont present were determined by light and fluorescence microscopy. Small carbohydrates were analysed to detect the presence of green algal metabolites. Thalli were treated with selected strengths of Zn²⁺ solutions that stop cyanobacterial but not green algal photosynthesis. CO₂ exchange was measured before and after treatment to determine the contribution of each photobiont to total thallus photosynthesis. Heterocyst frequencies were determined to clarify whether the cyanobacteria were modified for increased nitrogen fixation (high heterocyst frequencies) or were normal, vegetative cells.

Key Results

Several cyanobacterial lichens had green algae present in the photosynthetic layer of the thallus. The presence of the green algal transfer carbohydrate (ribitol) and the incomplete inhibition of thallus photosynthesis upon treatment with Zn²⁺ solutions showed that both photobionts contributed to the photosynthesis of the lichen thallus. Low heterocyst frequencies showed that, despite the presence of adjacent green algae, the cyanobacteria were not altered to increase nitrogen fixation.

Conclusions

These cyanobacterial lichens are a tripartite lichen symbiont combination in which the mycobiont has two primarily photosynthetic photobionts, ‘co-primary photobionts’, a cyanobacterium (dominant) and a green alga. This demonstrates high flexibility in photobiont choice by the mycobiont in the Peltigerales. Overall thallus appearance does not change whether one or two photobionts are present in the cyanobacterial thallus. This suggests that, if there is a photobiont effect on thallus structure, it is not specific to one or the other photobiont.     

Clearance rates in the Arctic bivalves <Emphasis Type="Italic">Hiatella arctica</Emphasis> and <Emphasis Type="Italic">Mya </Emphasis>sp.

Filtration was studied in two Arctic clams, Hiatella arctica and Mya sp., collected in Young Sound, Northeast Greenland. Clearance rates were determined as a function of ambient temperature and algal cell concentration, using the clearance method and feeding with a unicellular flagellate. For both species, clearance rates increased with increasing temperature from <у up to 4-8°C. At higher temperatures, filtration ceased and the clams closed their valves. Clearance rates were also determined in temperate specimens of H. arctica collected on the west coast of Sweden. For these specimens, clearance rates increased with increasing temperature from 0 to 18-20°C. When weight-specific clearance rates were compared between the two populations and between species, there were no differences at 1°C. Clearance rates in Arctic H. arctica were maximal at algal cell concentrations corresponding to 2.5-8 µg chlorophyll a l^-1. Temperature compensation in Arctic bivalves is discussed and it is concluded that adaptations to constant low temperatures consist of a lower minimum temperature, for active filtration. Low clearance rates due to low temperatures did not seem to limit growth, under the prevailing conditions in Young Sound.     

Photosynthetic capacity in relation to nitrogen content and its partitioning in lichens with different photobionts

We tested the hypothesis that lichen species with a photosynthetic CO₂-concentrating mechanism (CCM) use nitrogen more efficiently in photosynthesis than species without this mechanism. Total ribulose bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) and chitin (the nitrogenous component of fungal cell walls), were quantified and related to photosynthetic capacity in eight lichens. The species represented three modes of CO₂ acquisition and two modes of nitrogen acquisition, and included one cyanobacterial ( Nostoc ) lichen with a CCM and N₂ fixation, four green algal ( Trebouxia ) lichens with a CCM but without N₂ fixation and three lichens with green algal primary photobionts ( Coccomyxa or Dictyochloropsis ) lacking a CCM. The latter have N₂-fixing Nostoc in cephalodia. When related to thallus dry weight, total thallus nitrogen varied 20-fold, chitin 40-fold, Chl a 5-fold and Rubisco 4-fold among the species. Total nitrogen was lowest in three of the four Trebouxia lichens and highest in the bipartite cyanobacterial lichen. Lichens with the lowest nitrogen invested a larger proportion of this into photosynthetic components, while the species with high nitrogen made relatively more chitin. As a result, the potential photosynthetic nitrogen use efficiency was negatively correlated to total thallus nitrogen for this range of species. The cyanobacterial lichen had a higher photosynthetic capacity in relation to both Chl a and Rubisco compared with the green algal lichens. For the range of green algal lichens both Chl a and Rubisco contents were linearly related to photosynthetic capacity, so the data did not support the hypothesis of an enhanced photosynthetic nitrogen use efficiency in green-algal lichens with a CCM.     

Photosynthetic performance of Arctic macroalgae after transplantation from deep to shallow waters

Transplantation experiments conducted in the Arctic Kongsfjord (Spitsbergen) in summer 1997 investigated the effects of various types of filtered natural radiation (solar, solar without UV-B, solar without UV-A/B) on photosynthesis of various macroalgae. Two brown algal species (Laminaria solidungula, Saccorhiza dermatodea) and four red algal species (Palmaria palmata, Phycodrys rubens, Phyllophora truncata, Ptilota plumosa) were collected from deeper waters, kept in UV-transparent plexiglass tubes wrapped with different spectral cut-off filter foils and positioned at fixed depths in shallow waters for 7-9 days. At regular intervals, chlorophyll fluorescence of photosystem II (optimum quantum yield, F_v/F_m) was determined, as an indicator of photosynthetic performance. The data demonstrate that shallow-water species such as P. palmata are much less affected by natural photosynthetically active radiation (PAR) and UV radiation near the surface than extremely sensitive deep-water species such as Phyc. rubens which exhibited strong decreases in photosynthetic performance, as well as photobleaching of part of the thallus. The other species showed intermediate response patterns. In most species investigated inhibition of photosynthesis was mainly caused by the UV-B wavelengths. Interpretation of the data clearly indicates species-specific tolerances of photosynthesis to ambient solar radiation which can be explained by broad physiological acclimation potentials and/or genetic adaptation to certain (low or high) irradiances. The species-specific photosynthetic performance under radiation stress is in good accordance with the vertical distribution of the macroalgae on the shore.     

薇甘菊叶和茎的光合特性

薇甘菊(Mikania micranth)是世界性的入侵有害植物,对其入侵特性的理解将有助于我们更进一步揭示入侵机理和开展植物入侵的防治工作。叶片的光合作用是入侵植物薇甘菊入侵特性的研究内容之一,但到目前为止,仍没有开展对薇甘菊非同化器官茎的同化特性的研究。该文采取对比的研究方法,使用LICOR-6400气体交换系统和荧光系统对其幼嫩的绿色茎和成熟叶片的气体交换和叶绿素荧光特性进行测定,并对测定结果进行了对比分析,同时应用激光共聚焦显微镜对薇甘菊茎中叶绿体分布进行观察。使用叶绿素荧光系统测量瞬时叶绿素荧光特性表明,茎和叶的电子传输速率(Electron transport rate, ETR)和光系统Ⅱ实际光化学量子产量(Φ_PSⅡ)存在较好的正比例关系,相关系数达到0.97,说明茎中存在和叶中类似的光合结构。但在应用LICOR-6400气体交换系统测量稳定状态下的CO₂的气体交换速率时,观察到叶的气体交换速率稳定性较好,而茎的气体交换速率出现较大的波动,这可能是由于茎的气孔因素引起。综合来看,在相同面积和饱合光强下(光通量密度(Photosynthetic photo flux density, PPFD)=2 000 μmol·m ^-2·s^-1),叶的ETR为42.44 μmol·m ^-2·s^-1,茎的ETR为30.32 μmol·m ^-2·s^-1。在相同面积和低光强(PPFD=10 μmol·m ^-2·s^-1)下,叶的Φ_PSⅡ为0.69;茎的Φ_PSⅡ为0.57。在单位SPAD下,茎中ETR是每单位SPAD 4.24 μmol·m ^-2·s^-1,是叶的2.3倍,实际光化学量子产量是每单位SPAD 0.08,是叶的3倍。在比较茎和叶ETR中观察到茎比叶有更强的强光适应能力。激光共聚焦图像观察到薇甘菊茎的叶绿体主要分布在两个区域:皮层区和维管束周围。对照以往关于茎中叶绿体功能的研究表明,可能分布在两个区域中的叶绿体功能上存在差别。如上,薇甘菊茎中存在一定的光合作用能力,且在叶绿体的光能利用瞬时效率上茎明显强于叶,但在茎中这些光合作用的具体作用仍不清楚。     

Photosynthetic Depression at High Thallus Water Contents in Lichens: Concurrent Use of Gas Exchange and Fluorescence Techniques with a Cyanobacterial and a Green Algal Peltigera Species

Abstract: Lichens, being poikilohydric, have varying thallus water contents (WC) and show a complex interaction between net photosynthesis (NP) and WC. NP can be depressed at low WC (desiccation effects) and, in some species, also at high WC. In the latter case the depression is normally ascribed to increased CO₂ diffusion resistances through water blockage. Recently, an earlier explanation, that the depression at high WC is due to recycling of CO₂ from increased dark respiration processes (DR), has been given renewed prominence. The two explanations were distinguished by the concurrent use of gas exchange and chlorophyll fluorescence techniques to investigate NP: WC relationships in the lichens Peltigera leucophlebia (green algal) and P. neckeri (cyanobacterial). Both species had a distinct optimal WC for NP with depressed values at low and high WC. The maximal quantum yield for both CO₂ fixation (initial slope of light response curves of NP) and photosystem II (fluorescence signals of dark-adapted thalli) was depressed only at low WC and remained high at optimal and greater WC. In contrast, the relative electron transport rate (ETR, derived from fluorescence signals of thalli in the light) tracked NP and was depressed at low and high WC. The depression of both NP and ETR at high WC (not that at low WC) could be prevented by using elevated external CO₂ concentrations. A single, linear relationship was found between all values of gross photosynthesis (NP + DR) and ETR regardless of external CO₂ concentration or WC. Our results show that, for these lichens, the depression in NP at high WC is a real fall in photosynthetic rate of the photobionts and is not due to recycling of CO₂. The removal of the depression in NP and ETR at high WC by using elevated external CO₂ levels allows us to conclude that an additional CO₂ diffusion resistance is present.     

In situ oxygen microelectrode measurements of bottom-ice algal production in McMurdo Sound, Antarctica

In-situ estimates of fast-ice algal productivity at Cape Evans, McMurdo Sound, in 1999 were lower than at the same site in previous years. Under-ice irradiance was between 0 and 8 µmol photons m^-2 s^-1; the ice was between 1.9 and 2.0 m thick and the algal biomass averaged 150 mg chl a m^-2, although values as high as 378 mg chl a m^-2 were recorded. Production on 11 and 12 November was between 0.053 and 1.474 mg C m^-2 h^-1. When the data from 11 November were fitted to a hyperbolic tangent function, a multilinear regression gave estimates for P_max of 0.571 nmol O₂ cm^-2 s^-1, an ! of 0.167 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1 and an E_k of 3.419 µmol photons m^-2 s^-1. A P_max of 2.674 nmol O₂ cm^-2 s^-1, an ! of 0.275 nmol O₂ cm^-2 s^-1 µmol^-1 photons m^-2 s^-1, r of 0.305 nmol O₂ cm^-2 s^-1 and an E_k of 9.724 µmol^-1 photons m^-2 s^-1 were estimated from the 12 November data. The sea-ice algal community was principally comprised of Nitzschia stellata, Entomoneis kjellmanii and Berkeleya adeliensis. Other taxa present included N. lecointei, Fragilariopsis spp., Navicula glaciei, Pleurosigma spp. and Amphora spp. Variations in the method for estimating the thickness of the diffusive boundary layer were not found to significantly affect the measurements of oxygen flux. However, the inability to accurately measure fine-scale variations in biomass is thought to contribute to the scatter of the P versus E data.     

Chlorophyll fluorescence and photosynthetic response to light in 1-year-old needles during spring and early summer in <Emphasis Type="Italic">Pinus leucodermis</Emphasis>

Ten-year-old trees from four Italian populations of Pinus leucodermis (populations A, B, C and D), which were collected from different sites at different altitudes, were grown near Florence, Italy. Needle CO₂ gas exchange and chlorophyll fluorescence response to increasing light intensities were evaluated; gas exchange and chlorophyll fluorescence variation between April and July were also monitored. Populations A, B and C showed a similar photosynthetic response to increasing photosynthetic photon flux density (PPFD) intensities, while at various light intensities population D, which originated from the highest altitude, showed the highest photosynthetic rates. In this population photosynthesis was saturated at PPFDs higher than 900 µmol m^-2s^-1 and a slow decrease of effective photosystem II quantum yield and F'_V/F'_M in response to increasing PPFDs were found. The same trees also showed a faster recovery in photosynthesis from limitations induced by winter temperatures than the other three populations. This work showed that photosynthetic response to light in population D was different from the other populations; trees from this population were probably naturally selected to prevent photoinhibition due to excess light.     

Effects of high nitrogen load on growth, photosynthesis and nutrient status of Cryptomeria japonica and Pinus densiflora seedlings

To evaluate the sensitivity of Japanese cedar (Cryptomeria japonica D. Don) and Japanese red pine (Pinus densiflora Sieb. et Zucc.) to high N deposition, 1-year-old seedlings were grown in brown forest soil treated with N as NH₄NO₃ at 0, 25, 50, 100 and 300 mg l^-1 fresh soil volume, equivalent to 0, 28, 57, 113 and 340 kg N ha^-1. Net photosynthetic rate and whole-plant dry mass of C. japonica seedlings were increased by the N treatment, whilst those of P. densiflora seedlings were significantly reduced by the highest N treatment. The reduction in the net photosynthesis of P. densiflora seedlings was mainly due to a depression of carboxylation efficiency accompanied by a decrease in concentration and activity of Rubisco in the needles. In P. densiflora seedlings, needle concentrations of P and Mg were decreased, and the concentrations of N and Mn were increased by the highest N treatment. The reductions in needle protein concentration and Rubisco activity were negatively correlated with the ratios of N/P and Mn/Mg in the needles. These results suggest that nutrient imbalances of these elements may be induced in P. densiflora seedlings grown under high N deposition. We conclude that P. densiflora is more sensitive to high N deposition than C. japonica, and that the relatively high atmospheric N deposition to Japanese forest ecosystems may adversely affect the health of N-sensitive tree species such as P. densiflora.     

In situ photosynthetic differentiation of the green algal and the cyanobacterial photobiont in the crustose lichen Placopsis contortuplicata

In situ photosynthetic activity in the green algal and the cyanobacterial photobionts of Placopsis contortuplicata was monitored within the same thallus using chlorophyll a fluorescence methods. It proved possible to show that the response to hydration of the green algal and the cyanobacterial photobionts is different within the same thallus. Measurements of the photochemical efficiency of PS II, Fv/Fm, reveal that in the dry lichen thallus photosynthetic activity could be induced in the green algal photobiont by water vapour uptake, in the cyanobacterial photobiont only if it was hydrated with liquid water. However, rates of apparent electron flow through PS II as well as rates of CO₂ gas exchange were suboptimal after hydration with water vapour alone and maximum rates could only be observed when the thallus was saturated with liquid water. The differences in the waterrelated photosynthetic performance and different light response curves of apparent electron transport rate through PS II indicate that the two photobionts act highly independently of each other. It was shown that the cyanobacteria from the cephalodia in P. contortuplicata act as photobiont. The rate of electron flow through PS II was found to be saturated at 1500 mol photon m^–2 s^–1, despite a considerable increase of non-photochemical quenching in the green algal photobiont which is lacking in the cyanobacterial photobiont. No evidence of photoinhibition could be found in either photobiont. Pronounced competition between the green algal and the cyanobacterial thallus can be observed in the natural habitat, indicating that the symbiosis in P. contortuplicata should be regarded as a very variable adaptation to the extreme environmental conditions in the maritime Antarctic.Abbreviations DR dark respiration - ETR apparent rate of electron flow of PS II (=F/Fm×PFD) - F difference in yield of fluorescence and maximal Fm and steady state Fs under ambient light - Fo minimum level of fluorescence yield in dark-adapted state - Fo minimum level of fluorescence yield after transient darkening and far-red illumination - Fm maximum level of dark-adapted fluorescence yield - Fm maximum yield of fluorescence under ambient light - Fs yield of fluorescence at steady state - Fv difference in minimum fluorescence and maximum fluorescence in dark-adapted state - NP net photosynthesis - NPQ coefficient for non-photochemical quenching - PAR photosynthetically active radiation (400–700 nm) - PFD photon flux density in PAR - PS II photosystem II - qN coefficient for non-photochemical quenching - qP coefficient for photochemical quenching     

Marine reserves demonstrate top-down control of community structure on temperate reefs

Replicated ecological studies in marine reserves and associated unprotected areas are valuable in examining top-down impacts on communities and the ecosystem-level effects of fishing. We carried out experimental studies in two temperate marine reserves to examine these top-down influences on shallow subtidal reef communities in northeastern New Zealand. Both reserves examined are known to support high densities of predators and tethering experiments showed that the chance of predation on the dominant sea urchin, Evechinus chloroticus, within both reserves was approximately 7 times higher relative to outside. Predation was most intense on the smallest size class (30-40 mm) of tethered urchins, the size at which urchins cease to exhibit cryptic behaviour. A high proportion of predation on large urchins could be attributed to the spiny lobster, Jasus edwardsii. Predation on the smaller classes was probably by both lobsters and predatory fish, predominantly the sparid Pagrus auratus. The density of adult Evechinus actively grazing the substratum in the urchin barrens habitat was found to be significantly lower at marine reserve sites (2.2ǂ.3 m^-2) relative to non-reserve sites (5.5ǂ.4 m^-2). There was no difference in the density of cryptic juveniles between reserve and non-reserve sites. Reserve populations were more bimodal, with urchins between 40 and 55 mm occurring at very low numbers. Experimental removal of Evechinus from the urchin barrens habitat over 12 months lead to a change from a crustose coralline algal habitat to a macroalgal dominated habitat. Such macroalgal habitats were found to be more extensive in both reserves, where urchin densities were lower, relative to the adjacent unprotected areas that were dominated by urchin barrens. The patterns observed provide evidence for a top-down role of predators in structuring shallow reef communities in northeastern New Zealand and demonstrate how marine reserves can reverse the indirect effects of fishing and re-establish community-level trophic cascades.     

Using combined measurements of gas exchange and chlorophyll fluorescence to investigate the photosynthetic light responses of plant species adapted to different light regimes

One broad-leaved pioneer tree, Alnus formosana, two broad-leaved understory shrubs, Ardisia crenata and Ardisia cornudentata, and four ferns with different light adaptation capabilities (ranked from high to low, Pyrrosia lingus, Asplenium antiquum, Diplazium donianum, Archangiopteris somai) were used to elucidate the light responses of photosynthetic rate and electron transport rate (ETR). Pot-grown materials received up to 3 levels of light intensity, i.e., 100%, 50% and 10% sunlight. Both gas exchange and chlorophyll (Chl) fluorescence were measured simultaneously by an equipment under constant temperature and 7 levels (0?C2,000 ??mol m^?2 s^?1) of photosynthetic photon flux density (PPFD). Plants adapted to-or acclimated to high light always had higher light-saturation point and maximal photosynthetic rate. Even materials had a broad range of photosynthetic capacity [maximal photosynthetic rate ranging from 2 to 23 ??mol(CO₂) m^?2 s^?1], the ratio of ETR to gross photosynthetic rate (P _G) was close for A. formosana and the 4 fern species when measured under constant temperature, but the PPFD varied. In addition, P. lingus and A. formosana grown under 100% sunlight and measured at different seasonal temperatures (15, 20, 25, and 30°C) showed increased ETR/P _G ratio with increasing temperature and could be fitted by first- and second-order equations, respectively. With this equation, estimated and measured P _G were closely correlated (r ² = 0.916 and r ² = 0.964 for P. lingus and A. formosana, respectively, p<0.001). These equations contain only the 2 easily obtained dynamic indicators, ETR and leaf temperature. Therefore, for some species with near ETR/P _G ratio in differential levels of PPFD, these equations could be used to simulate dynamic variation of leaf scale photosynthetic rate under different temperature and PPFD conditions.     

Relations between efficiency of water transport and duration of leaf growth in some deciduous and evergreen trees

Shoot and leaf growth rate as well as shoot hydraulic conductance per unit leaf area (K_SL) were measured on three evergreen (Viburnum tinus L., Prunus laurocerasus L., Laurus nobilis L.) and three deciduous (Corylus avellana L., Juglans regia L., Castanea sativa L.) trees growing under the same environmental conditions. The times required to complete shoot growth (27 days for P. laurocerasus to 51 days for V. tinus) and leaf expansion (24 days for C. sativa to 42 days for C. avellana) were very different among the studied species. These species also differed in K_SL that ranged between 1.5 and 3.5 e-4 kg s^-1 m^-2 MPa^-1 in C. avellana and C. sativa, respectively, with intermediate values recorded in the other species. A strong, negative and statistically significant correlation was found to exist between K_SL and the time required for complete leaf expansion. This suggests that duration of leaf growth is shortened by the high hydraulic efficiency of the shoot. In contrast, no statistically significant relationship was found to exist between K_SL and shoot growth rate. Whether a high leaf growth rate can be interpreted as advantageous to plants or it is only an epiphenomenon of the high efficiency in the vertical water transport is discussed.     

Embryogenesis and regeneration of green plantlets from oat (Avena sativa L.) leaf-base segments: influence of nitrogen balance, sugar and auxin

The nitrogen composition and sugar and auxin concentrations of callus induction medium were optimized in order to improve the regeneration of green plants from two elite oat cultivars, Aslak and Veli. For both cultivars, the production of green plantlets was doubled by optimization. However, the results obtained also clearly demonstrated that cultivars of the same species may differ drastically in their requirements for essential media components. Veli clearly required higher total amounts of nitrogen (67.8 mM) than Aslak (44.9 mM) but less maltose and 2,4-dichlorophenoxyacetic acid (28 g l^-1 and 0.6 mg l^-1) than Aslak (38 g l^-1 and 2 mg l^-1). This result indicates that the optimal production of green plantlets through embryogenesis requires that media be optimized for each cultivar separately.     

Exploring Phenotypic Plasticity in the LichenRamalina capitata: Morphology, Water Relations and Chlorophyll Content in North- and South-facing Populations

The present work analyses the morphology, anatomy, water relationsand chlorophyll content of thalli of the lichenRamalina capitatavar.protectafromtwo different populations exposed to contrasting microclimaticconditions due to differences in the orientation of the rocksurface. The population on the north-facing rock surface (NFS)was exposed to lower photosynthetic photon flux densities (PPFD),remained at high relative humidities for longer periods of timeand was exposed to lower temperatures than the population onthe south-facing surface (SFS). We proposed the hypothesis thatthe shadier the habitat the greater the ecological advantagefor enhanced light harvesting. Thalli from the SFS had shorterand wider lacinia, thicker thalli, mostly due to increased medullathickness, a higher water-retention capacity, a higher percentageof thallus volume occupied by the algal cells and a higher chlorophyllcontent than thalli from NFS. The phenotypic plastic responseof the traits studied inR. capitatavar.protectawas not directlyrelated to differences in the light availability, at least forthe range of PPFD experienced by the two populations studied,since the population exposed to higher PPFD exhibited largeramounts of light harvesting pigments. Both populations exhibitedthe same intrathalline distribution of algal cells and chlorophylls,which were more abundant in the apical than in the basal zonesof all thalli studied. Periods of water-induced metabolic activitywere shorter in the SFS than in the NFS, and structural andchlorophyll data indicated that thalli from the SFS were betterprepared for the photosynthetic exploitation of these brieferperiods and for maintaining thallus hydration into dry periods.These results suggest that differences in selective pressurebetween the two populations ofR. capitatavar.protectastudiedinvolved maximization of the photosynthetic exploitation ofthe periods of metabolic activity when they are brief, as hasbeen described for certain vascular plants from xeric environments. Ramalina capitatavar.protecta; algal cells; chlorophylls; water relations; microclimate; morphology; intrathalline variation; lichen; phenotypic plasticity     

Regulation of xylem sap flow in an evergreen, a semi-deciduous, and a deciduous Meliaceae species from the Amazon

The significance of phenological characteristics, stomatal conductance of the leaves, and stem water storage fluctuations for the regulation of xylem sap flow in an evergreen (Carapa guianensis Aubl.), in a semi-deciduous (Swietenia macrophylla King), and in a deciduous (Cedrela odorata L.) Meliaceae species was studied in a 7-year-old plantation near Manaus, Brazil. The study responds to the increasing demand for knowledge on the water relations of highly exploited timber trees of the Amazon. Xylem sap flow measurements indicated that the daily sap flow of Carapa (3.8 l day^-1 tree^-1 to 16.4 l day^-1 tree^-1) exceeded the daily sap flow of Swietenia (2.4 l day^-1 tree^-1 to 7.0 l day^-1 tree^-1) and Cedrela (1.6 l day^-1 tree^-1 to 11.6 l day^-1 tree^-1) during the entire year, although the highest flux densities were measured in Cedrela. The decrease in xylem sap flow observed in periods with low soil water potentials and high atmospheric vapor saturation deficits was more pronounced in the deciduous (Cedrela) and semi-deciduous species (Swietenia) than the evergreen species (Carapa). Carapa, which has the highest daily sap flow, had the highest biomass and sapwood portion. The high flux densities measured in Cedrela most likely result from the large earlywood vessels in this species. The seasonal variation of xylem sap flow of the three species was correlated with the stomatal conductance of the leaves measured by infiltration experiments. Stem water storage fluctuations in Carapa and Swietenia were predominantly due to transpiration; in Cedrela it was predominantly due to evaporative water loss on the stem surface during dry periods.     

Water availability and carbon isotope discrimination in conifers

The stable C isotope composition ('¹³C) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and '¹³C. A significant component of the variation in '¹³C was related to altitude (discrimination decreased with altitude in stemwood, 2.53‰ km^-1 altitude, r²=0.49, and in foliage, 1.91‰ km^-1, r²=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO₂ partial pressure into the leaf (P_a-P_i) and altitude were generally unrelated. The ratio of precipitation to evaporation (P/E) explained significant variation in P_a-P_i of stemwood (r²=0.45) and foliage (r²=0.27), but only at low (<0.8) P/E. At greater P/E there was little or no relationship, and other influences on '¹³C probably dominated the effect of water availability. We also examined the relationship between plant drought stress (O) and '¹³C within annual rings of stemwood from Pinus radiata and Pinus pinaster in south-western Australia. Differential thinning and fertiliser application produced large differences in the availability of water, nutrients and light to individual trees. At a density of 750 stems ha^-1, O and '¹³C were less (more negative) than at 250 stems ha^-1 indicating greater drought stress and less efficient water use, contrary to what was expected in light of the general relationship between discrimination and P/E. The greater '¹³C of trees from heavily thinned plots may well be related to an increased interception of radiation by individual trees and greater concentrations of nutrients in foliage - attributes that increase rates of photosynthesis, reduce P_i and increase '¹³C. '¹³C was thus modified to a greater extent by interception of radiation and by nutrient concentrations than by water availability and the '¹³C-O relationship varied between thinning treatments. Within treatments, the relationship between '¹³C and O was strong (0.38<r²<0.58). We conclude that '¹³C may well be a useful indicator of water availability or drought stress, but only in seasonally dry climates (P/E<1) and where variation in other environmental factors can be accounted for.