Stereoselectivity in the biosynthetic conversion of xanthoxin into abscisic acid

All stereoisomers of xanthoxin (XAN) and abscisic aldehyde (ABA-aldehyde) were prepared from (R) and (S)-4-hydroxy--cyclogeraniol via asymmetric epoxidation. Their stomatal closure activities were measured on epidermal strips of Commelina communis L. Natural (S)-ABA-aldehyde showed strong activity comparable to that of (S)-abscisic acid (ABA). Natural (1S, 2R, 4S)XAN and (1S, 2R, 4R)-epi-XAN also induced stomatal closure at high concentrations. On the other hand, unnatural (1R)-enantiomers of XAN, epi-XAN, and ABA-aldehyde were not effective. To further examine the Stereoselectivity on the biosynthetic pathway to ABA, deuterium-labeled substrates were prepared and fed to Lycopersicon esculentum Mill, under non-stressed or water-stressed conditions. Substantial incorporations into ABA were observed in the cases of natural (1S, 2R, 4S)-XAN, (1S, 2R, 4R)-epi-XAN and both enantiomers of ABA-aldehyde, leading to the following conclusions. The negligible effect of unnatural (1R)-enantiomers of XAN, epi-XAN and ABA-aldehyde can be explained by their own biological inactivity and/or their conversion to inactive (R)-ABA. Even in the isolated epidermal strips, putative aldehyde oxidase activity is apparently sufficient to convert ABA-aldehyde to ABA while the activity of XAN dehydrogenase seems very weak. The stereochemistry of the 1, 2-epoxide is very important for the XAN-dehydrogenase while this enzyme is less selective regarding the 4-hydrdxyl group of XAN and converts both (1S, 2R, 4S)-XAN and (1S, 2R, 4R)-epi-XAN to (S)-ABA-aldehyde. Abscisic aldehyde oxidase can nonstereoselectively convert both (S) and (R)-ABA-aldehyde to biologically active (S) and inactive (R)-ABA, respectively.Abbreviations ABA abscisic acid - ABA-aldehyde abscisic aldehyde - DET diethyl tartrate - epi-XAN xanthoxin epimer - FCC flash column chromatography - GC-EI-MS gas chromatography-electron impact-mass spectrometry - MeABA abscisic acid methyl ester - IR infrared - NMR nuclear magnetic resonance - PCC pyridinium chlorochromate - THF tetrahydrofuran - XAN xanthoxin The authors are very grateful to Mr J.K. Heald (Department of Biological Sciences, University of Wales, Aberystwyth, UK) and Dr. R. Horgan for carrying out GC-EI-MS analyses and advice, respectively.This work was supported by the Japan Society for the Promotion of Science (Fellowship for Young Japanese Researcher No. 0040672).     

Stomatal closure in response to xanthoxin and abscisic acid

Summary The stomata of detached leaves of Commelina communis L., Hordeum vulgare L., Zea mays L., Vicia faba L., Phaseolus vulgaris L. and Xanthium strumarium L. closed when xanthoxin (XAN) was added to the transpiration stream. XAN was approximately half as active as (+)-abscisic acid (ABA) at an equivalent concentration. XAN, like ABA, sensitized stomata of Xanthium strumarium to CO₂. In contrast to ABA, XAN was ineffective in closing stomata of isolated epidermal strips of C. communis or V. faba. This may be because XAN added to the transpiration stream is converted to ABA during passage from the xylem to the epidermis.Abbreviations ABA Abscisic acid - XAN xanthoxin     

Long-term c i /c a response of trees in western North America to atmospheric CO2 concentration derived from carbon isotope chronologies

To evaluate how the land carbon reservoir has been responding to the rising CO₂ concentration of the atmosphere, it is important to study how plants in natural forests adjust physiologically to the changing atmospheric conditions. Many experimental studies have addressed this issue, but it has been difficult to scale short-term experimental observations to long-term ecosystem-level responses. This paper derives carbon-isotope-related variables for the past 100–200 years from measurements on trees from natural forests. Calculations show that the c _i/c _a ratios [c _i/c _a is the ratio of the CO₂ concentration (μmol mol⁻¹) in the intercellular space of leaves to that in the atmosphere] of the trees were constant or increased slightly before the 20th century, but changed more rapidly in the 20th century; some increased, some decreased, and some stayed constant. In contrast, the CO₂ concentration inside plant leaves increased monotonically for all trees. Received: 12 June 1997 / Accepted: 29 June 1998     

Regulation of abscisic acid metabolism: towards a metabolic basis for abscisic acid-cytokinin antagonism

The penultimate step in abscisic acid (ABA) biosynthesis involves oxidation of xanthoxal (XAN) catalysed by a molybdenum-cofactor (MoCo)-containing aldehyde oxidase (AO) and represents one potential site of regulation of ABA in plant tissues. In an attempt to understand the biochemical basis for cytokinin-abscisic acid (CK-ABA) antagonism the effect of several CKs, molybdate, tungstate and allopurinol (an inhibitor of xanthine oxidase activity and purine metabolism) on the formation of XAN, ABA and related catabolites in mesocarp of ripening avocado (Persea americana Mill. cv. Hass) was investigated. Treatment with either adenine (Ade), isopentenyladenine (2iP) or zeatin (Z) enhanced conversion of ABA to phaseic acid (PA) and caused a reduction in the amount of radioactivity incorporated from 3R-[2-¹⁴C] mevalonolactone (MVL) into ABA by stimulating overall ABA metabolism. Ancymidol and N-(2-chloro-4-pyridyl)-N-phenylur ea (CPPU), while not affecting formation of PA and DPA, appeared to retard ABA biosynthesis which resulted in the accumulation of XAN. Tungstate caused accumulation of XAN at the expense of ABA and related acidic metabolites while molybdate and allopurinol accelerated ABA metabolism, i.e. formation of XAN, ABA, PA, and DPA. These findings are discussed in terms of the regulation of the ABA biosynthetic pathway in avocado fruit by CK-induced suppression of xanthine dehydrogenase (XDH) activity and a model illustrating the proposed metabolic interrelationship is presented.     

Gas exchange and resource-use efficiency of Leymus cinereus (Poaceae): diurnal and seasonal responses to naturally declining soil moisture

We examined factors that limit diurnal and seasonal photosynthesis in Leymus cinereus, a robust tussock grass from shrub-steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen-use efficiency, 3) a steep leaf-to-air diffusion gradient for carbon dioxide, which enhances intrinsic water-use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO₂ assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (c_i) by ≈40 μl liter^-1. The maximum rate of A achieved during the day for severely stressed plants (predawn water potential = -4 MPa) was one-third and daily carbon gain per unit leaf area was about one-fourth that of well-watered plants. For plants in the natural stand, CO₂-saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect on A at CO₂ ambient levels or on carboxylation efficiency until predawn water potentials reached -1.8 MPa. Nitrogen-use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water-use efficiency as estimated from A vs. c_i curves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated from A vs. c_i curves is relatively constant because maximum stomatal conductance is closely tuned to the CO₂ assimilatory capacity of the mesophyll.     

Application of heat stress in situ demonstrates a protective role of irradiation on photosynthetic performance in alpine plants

The impact of sublethal heat on photosynthetic performance, photosynthetic pigments and free radical scavenging activity was examined in three high mountain species, Rhododendron ferrugineum, Senecio incanus and Ranunculus glacialis using controlled in situ applications of heat stress, both in darkness and under natural solar irradiation. Heat treatments applied in the dark reversibly reduced photosynthetic performance and the maximum quantum efficiency of photosystem II (F_v/F_m), which remained impeded for several days when plants were exposed to natural light conditions subsequently to the heat treatment. In contrast, plants exposed to heat stress under natural irradiation were able to tolerate and recover from heat stress more readily. The critical temperature threshold for chlorophyll fluorescence was higher under illumination (T_c^′) than in the dark (T_c). Heat stress caused a significant de‐epoxidation of the xanthophyll cycle pigments both in the light and in the dark conditions. Total free radical scavenging activity was highest when heat stress was applied in the dark. This study demonstrates that, in the European Alps, heat waves can temporarily have a negative impact on photosynthesis and, importantly, that results obtained from experiments performed in darkness and/or on detached plant material may not reliably predict the impact of heat stress under field conditions.     

Flavescence dorée phytoplasma deregulates stomatal control of photosynthesis in Vitis vinifera

Flavescence dorée (FD) is among the major grapevine diseases causing high management costs; curative methods against FD are unavailable. In FD‐infected plants, decrease in photosynthesis is usually recorded, but deregulation in stomatal control of leaf gas exchange during FD infection and recovery is unknown. We measured the seasonal time course of gas exchange rates in two cultivars (‘Barbera’ and ‘Nebbiolo’) during the term of 1 year when grapevines experienced a water stress and another with no drought, with difference in gas exchange rates in response to FD infection and recovery as assessed by symptom observation and phytoplasma detection through PCR analysis. Chlorophyll fluorescence was also evaluated at the time of maximum symptom severity in ‘Barbera’, the cultivar showing the most severe stress response to FD infection, causing the highest damage in vineyards of north‐western Italy. In FD‐infected plants, net photosynthesis and transpiration gradually decreased during the season, more during the no drought year than during drought. During recovery, healthy (PCR negative) plants infected 2 years before, but not those infected an year before, regained the gas exchange performances to the level as measured before infection. The relationships between stomatal conductance and the residual leaf intercellular CO₂ concentration (c_i) discriminated healthy versus FD‐infected and recovered plants; at the same c_i, FD‐infected leaves had higher non‐photochemical quenching than healthy ones. We conclude that metabolic, not stomatal, leaf gas exchange limitation in FD‐infected and recovered grapevines is the basis of plant response to FD disease. In addition, we also suggest that such response is dependent upon water stress, by showing that water stress superimposes on FD infection in terms of stomatal and metabolic non‐stomatal limitations to carbon assimilation.     

Internode length and anatomical changes in Pisum genotypes crys and cryc in response to extended daylength and applied gibberellin A1

Dwarf pea (Pisum sativum L.) plants with genotypes cry^c and cry^s responded differently when an 8 h photoperiod (8 h daylight, 16 h dark) was extended to 24 h (8 h daylight, 16 h incandescent light). Genotype cry^c showed up to a 4-fold increase in internode length, sustained by increases in both cell length (particularly of epidermal cells) and cell number (particularly of cortical cells) while cry^s plants showed up to a 2-fold increase in internode length sustained mostly by an increase in cell number. Under an 8 h (daylight) photoperiod the two genotypes did not differ in their sensitivity to applied gibberellin A₁ (GA₁) and they showed a similar pattern of response. GA₁ significantly increased internode length, cell length and cell number in both genotypes. Incandescent light did not increase the size of the response to GA₁ except for cry^s plants at high dose rates of GA₁ (29–58 nmol). At saturating doses of GA₁ the two genotypes attained a similar peak internode length; incandescent light increased the peak by about 40%. GA₁ increased the rate of leaf appearance by up to 33% while incandescent light reduced the rate by 4–7%. The elongation response of the more mature internodes of cry^c plants to GA₁ or incandescent light was due primarily to an increase in cell length whereas increased cell number made a significant contribution in the case of internodes which were relatively immature at the time the stimulus was applied. The progressive increase in internode length of both genotypes during ontogeny was due primarily to an increase in cell number. In conclusion, alleles cry^c and cry^s (background le La) do not confer a difference in sensitivity to GA₁ and the increase in internode length in response to incandescent light is probably not the result of a real or perceived increase in GA₁ level. Allele cry^s may partially block a phytochrome mediated response to light and the key difference between genotypes cry^s and cry^c may lie in the greater elongation (extensibility?) of cry^c epidermal cells in incandescent light.     

Universal chlorophyll equations for estimating chlorophylls <Emphasis Type="Italic">a,b, c</Emphasis>, and <Emphasis Type="Italic">d</Emphasis> and total chlorophylls in natural assemblages of photosynthetic organisms using acetone,methanol, or ethanol solvents

A universal set of equations for determining chlorophyll (Chl) a, accessory Chl b, c, and d, and total Chl have been developed for 90 % acetone, 100 % methanol, and ethanol solvents suitable for estimating Chl in extracts from natural assemblages of algae. The presence of phaeophytin (Ph) a not only interferes with estimates of Chl a but also with Chl b and c determinations. The universal algorithms can hence be misleading if used on natural collections containing large amounts of Ph. The methanol algorithms are severely affected by the presence of Ph and so are not recommended. The algorithms were tested on representative mixtures of Chls prepared from extracts of algae with known Chl composition. The limits of detection (and inherent error, ±95 % confidence limit) for all the Chl equations were less than 0.03 g m⁻³. The algorithms are both accurate and precise for Chl a and d but less accurate for Chl b and c. With caution the algorithms can be used to calculate a Chl profile of natural assemblages of algae. The relative error of measurements of Chls increases hyperbolically in diluted extracts. For safety reasons, efficient extraction of Chls and the convenience of being able to use polystyrene cuvettes, the algorithms for ethanol are recommended for routine assays of Chls in natural assemblages of aquatic plants.     

Natural and anthropogenic forcing of aquatic macrophyte development in a shallow Danish lake during the last 7000 years

Aim To investigate the long‐term changes in aquatic vegetation in a lowland, shallow lake, and to assess the relationship between aquatic vegetation and natural and anthropogenic catchment changes. Location Gundsømagle Sø, Zealand, Denmark: a shallow (mean depth 1.2 m), hypereutrophic lake (mean annual total phosphorus (TP) c. 700 μg TP L^?1) located in a predominantly agricultural catchment (88% cultivated land). The lake is presently devoid of macrophytes. Methods One hundred and forty‐seven contiguous samples from a sediment core (taken in 2000) were analysed for macrofossil remains together with loss‐on‐ignition and dry weight. From an earlier sediment core (taken in 1992), 67 samples were analysed for pollen and the two cores were correlated using the ignition residue profiles. Core chronology was determined by ²¹⁰Pb and ¹³⁷Cs dating of the recent lake sediments, while older sediments were dated by pollen‐stratigraphical correlation, as ¹⁴C dating proved problematical. Aquatic macrofossil abundance was used to reconstruct past changes in the lake's plant community and water‐level. The contemporary catchment land‐use change was inferred from sedimentary pollen data, and soil erosion to the lake was deduced from the minerogenic content of the lake sediments. Results The macrofossil record covers the last 7000 years, but aquatic plant remains were scarce prior to c. 1300 bc . After this date the abundance of submerged and emergent macrophyte remains increased dramatically, paralleled by an increase in sediment minerogenic matter and non‐arboreal pollen (NAP). Aquatic plant remains were abundant for more than 3000 years until the mid 1900s. Macrofossils of Linum usitatissimum (L.) (flax) and high pollen percentages of ‘Cannabis type’ (hemp) were recorded in periods between c. 1150 bc and 1800 ad . Main conclusions Our study suggests that, between c. 5000 bc and 1300 bc , the submerged plant community was confined to the littoral zone. From 1300 bc onwards, the submerged macrophyte vegetation expanded rapidly across the lake bed, presumably as a response to lake shallowing caused by a combination of climatic‐induced water‐level lowering and enhanced erosional infilling of the lake basin due to intensified anthropogenic activities in the catchment. The lake was meso‐eutrophic and had an extensive and diverse aquatic flora for more than 3000 years, until the middle of the twentieth century. In periods between c. 1150 bc and 1800 ad , the lake experienced direct anthropogenic impact from retting of fibre plants (Linum and Cannabis). Over the last 200 years, erosional infilling of the lake basin increased drastically, probably as a result of agricultural intensification. In the twentieth century, the lake was strongly affected by nutrient enrichment from both point sources (sewage from built‐up areas) and diffuse agricultural run‐off which led to hypertrophic conditions and the collapse of the submerged vegetation c. 1950–60. The concept of ‘naturalness’ and the implications for lake conservation are discussed.     

Cytokinin treated microcalli of Phelipanche ramosa: an efficient model for studying haustorium formation in holoparasitic plants

The ability of Orobanchaceae to establish a parasitic relationship is based on the development of a specific organ called haustorium. Previous studies in Phelipanche aegyptiaca and P. ramosa have underlined the interest of producing calli from germinated seeds as an efficient genetic transformation system, but they also pointed the need to improve the attachment rate of infectious calli to the host plant as well as the seed yield of the regenerated plants following attachment. It was previously shown that haustorium formation in P. ramosa is triggered by cytokinins. The present work demonstrates that one month-old microcalli produced from germinated seeds displayed a response to c/tZ similar to germinated seeds, as shown by the temporal expression profile of markers genes of cytokinin-related haustorium initiation, including PrRR5, PrCKX2, PrCKX4, PrTRN2 and PrZFP6. In addition, a 48 h treatment of microcalli with c/tZ (10^?7 M) before infestation triggered a 4-fold increase in the attachment rate of microcalli to tomato roots after 18 days in minirhizotrons when compared to untreated microcalli. Finally, it also outlines that increasing the aggressiveness of microcalli with a c/tZ treatment followed by the transfer of parasitized tomato plants into pots significantly improved seed yield of regenerated P. ramosa plants. Indeed, high amounts of viable seeds which germinated at more than 90% in response to GR24 were harvested after a 10 weeks-co-cultivation period. According to these results, cytokinin treated P. ramosa microcalli thus appear to be a good tool for further functional studies in holoparasitic plants, especially on haustorium formation.

     

Circadian clock component,LHY, tells a plant when to respond photosynthetically to light in nature

The circadian clock is known to increase plant growth and fitness, and is thought to prepare plants for photosynthesis at dawn and dusk; whether this happens in nature was unknown. We transformed the native tobacco, Nicotiana attenuata to silence two core clock components, NaLHY (irLHY) and NaTOC1 (irTOC1). We characterized growth and light‐ and dark‐adapted photosynthetic rates (A_c) throughout a 24 h day in empty vector‐transformed (EV), irLHY, and irTOC1 plants in the field, and in NaPhyA‐ and NaPhyB1‐silenced plants in the glasshouse. The growth rates of irLHY plants were lower than those of EV plants in the field. While irLHY plants reduced A_c earlier at dusk, no differences between irLHY and EV plants were observed at dawn in the field. irLHY, but not EV plants, responded to light in the night by rapidly increasing A_c. Under controlled conditions, EV plants rapidly increased A_c in the day compared to dark‐adapted plants at night; irLHY plants lost these time‐dependent responses. The role of NaLHY in gating photosynthesis is independent of the light‐dependent reactions and red light perceived by NaPhyA, but not NaPhyB1. In summary, the circadian clock allows plants not to respond photosynthetically to light at night by anticipating and gating red light‐mediated in native tobacco.     

Effects of temperature and CO2 enrichment on kinetic properties of phospho-enol-pyruvate carboxylase in two ecotypes of Echinochloa crus-galli (L.) Beauv., a C4 weed grass species

Summary Two populations of Echinochloa crus-galli (Québec, Mississippi) were grown at the Duke University Phytotron under 2 thermoperiods (28°/22°C, 21°/15°C day/night) and 2 CO₂ regimes (350 and 675 l l^-1). Thermostability, energy of activation (E _a ),K _m (PEP), K _m (Mg⁺⁺), and specific activity of phospho-enol-pyruvate carboxylase (PEP_c) were analyzed in partially purified enzyme preparations of plants grown for 5 weeks. Thermostability of PEP_c from extracts (in vitro) and leaves (in situ) was significantly higher in Mississippi plants. In vitro denaturation was not appreciably modified by thermal acclimation but CO₂ enrichment elicited higher thermostability of PEP_c. In situ thermostability was significantly higher than that of in vitro assays and was higher in Mississippi plants acclimated at 28°/22°C and in plants of the two ecotypes grown at 675 l l^-1 CO₂. E _a (Q ₁₀ 30°/20°C) for PEP_c was significantly lower in Québec plants as compared to Mississippi and no acclimatory shifts were observed. Significantly higher K _m's (PEP) in 20°C assays were obtained for Mississippi as compared to Québec plants but values were similar at 30°C and 40°C assays. K _m (Mg⁺⁺) decreased at higher assay temperatures and were significantly lower for PEP_c of the Québec ecotype. No significant changes in K _m (Mg⁺⁺) values were associated with modifications in temperature on CO₂ regimes. PEP_c activity measured at 30°C was significantly higher for Québec plants when measured on a leaf fresh weight, leaf area or protein basis but not on a chlorophyll basis. Significantly higher PEP_c activity for both genotypes was observed for plants acclimated at 21°/15°C or grown at 675 l l^-1 CO₂. Net photosynthesis (Ps) and net assimilation rates (NAR) were higher in Québec plants and were enhanced by CO₂ enrichment. NAR was higher in plants acclimated at low temperature, while an opposite trend was observed for Ps. PEP_c activities were always in excess of the amounts required to support observed rates of CO₂ assimilation.     

Heat resistance and energy budget in different Scandinavian plants

• 1 Alpine vascular plants seem to use other strategies in surviving a cold environment than reducing the reflectance in level leaves. Pubescence in alpine plants has small effect upon total reflectance, but may increase the amount of photosynthetic active radiation within the sheltered canopy. Alpine cushion plants like Silene acaulis, Diapensia lapponica and Loiseleuria procumbens maximize the absorptance of radiant energy with minimum heat losses, probably as an effect of the dense canopy structure. The young inflorescences of Eriophorum vaginatum were found to be extremely efficient absorbators, while the reflectance in Salix catkins was close to that of green leaves.
• 2 In lichens, a great variation both in visible (400–700 nm) and infrared (700–1400 nm) reflectance was found: (A) The Pseudephebe pubescence group consists of species with very low reflectance at all measured wavelengths. The species are chionophobous, probably because of the high absorptance which makes growth possible during the cold season. (B) The Sticta sylvatica group, characterized by very low visible reflectance and very high infrared reflectance, is well adapted to shade. (C) The Cetraria nivalis group consists of fruticose species with high reflectance both in the visible and the near infrared. The intense visible reflectance probably makes net photosynthesis possible in well protected layers of the canopy. (D) The Nephroma arcticum group with spectral properties resembling green leaves in vascular plants. (E) The Haematomma ventosum group and the Parmelia perlata group with spectral properties intermediate between group C and D.
• 3 A modified method determining lethal temperatures and energies of activation in the process leading to death during a heat shock, is described. The two parameters are rather species specific in many of the 118 Scandinavian plants investigated. The lethal temperatures completely overlap the values in hotter parts of the world. However, habitat specific lethal temperatures were found; low values in wet- or shade-growing species and high values in dry-growing species. In Picea abies lethal temperatures and energies of activation showed pronounced, but diverging, year cycles in 12 ecotypes from different parts of Europe. Only negligible differences between the ecotypes exist, and cycles are probably photoperiodically determined.
• 4 Heat hardening can be achieved quickly, both in an active and dormant stage, by increasing the temperature. A linear correlation between hardening temperature, both in the optimal and supraoptimal temperature range, and hardening capability was found. In most species, but especially in cold adapted species, the hardening capability at supraoptimal temperatures decreases with increasing cultivation temperature.
• 5 Diffusion resistances with open (r_s) and closed stomata (r_c) are measured on excised leaf samples in 72 species. A positive correlation between r_s and r_c was found. r_c ranges from 2.2-62.5 s cm⁻¹ in mesophytes and from 19-425 s cm⁻¹ in xerophytes, the highest values were found in succulents. Some of the alpine species had extremely low r_c, falling within the r_s-range. Some habitat specific differences in r_c were found, but the relatively few significant differences in r_s between different habitats indicate that a lot of different drought avoidance mechanisms exist. The greatest variation in r_c between different species was found in periodically dry habitats, though a few species like Epilobium alsinifolium (r_c= 62.5 s cm⁻¹) growing in constantly wet habitats had remarkably high r_c.
• 6 In Saussurea alpina leaf size increases with improved moisture conditions. Calculations of leaf temperatures with closed stomata and somewhat extreme meteorological conditions showed that the mean leaf size in the wettest part of the transect was below, but very close to the size giving lethal leaf temperatures. In Rubus chamaemorus leaf size increases with increasing artificial shading. The leaves growing in sunexposed sites will be only 0.5°C below the lethal limit when the stomata are closed. All the shade-leaves would exceed the lethal limit if the screen was removed and closing of stomata occurred. The northern distribution of this species is probably due to its low ability to avoid heat stress.
• 7 In Silene acaulis heat damage was observed under natural conditions at an air temperature of only 21°C. Leaf temperatures about 20°C above air temperature was often found in prostrate alpine vascular plants during sunny periods. The highest overtemperature (25.5°C) was observed in the broad leaves of Rubus chamaemorus. A comparison with maximum leaf temperatures measured in different parts of the world revealed rather uniform maximum leaf temperatures in spite of very contrasting air temperatures. Thus, vascular plants seem to control the leaf temperatures to a great extent by means of morphological modifications.
• 8 Leaf temperatures in a hot and dry period were calculated and compared with the heat resistance in 69 Scandinavian, mainly alpine, plants. In 14 wet growing species the lethal limits were exceeded if closing of the stomata occurred. In the remaining species calculated temperatures in single leaves never exceeded the lethal limit. Most of these species have leaves densely crowded in cushions or prostrate rosettes. Hence they get warmer than indicated by the calculated temperatures in single leaves, and will probably be heated close to the lethal limit. A highly significant correlation between lethal temperatures and cuticular diffusion resistances was found, probably illustrating the importance of transpirational cooling during a hot period. A combination of cuticular diffusion resistances and lethal temperatures segregates the species better in natural groups than only one parameter alone.
• 9 Factors involved in limiting the downward distribution of alpine plants are discussed. Some species avoid lowlands since they are drought sensitive (low cuticular diffusion resistance), others, mainly cushion plants with low heat exchange capacity, are probably overheated in lowlands.

     

Influences of habitat complexity on the diversity and abundance of epiphytic invertebrates on plants

SUMMARY 1. The compound influence of habitat complexity and patch size on stream invertebrate assemblages associated with submerged macrophytes was investigated through field sampling of two natural macrophyte species with contrasting leaf morphologies (complex, Ranunculus yezoensis; simple, Sparganium emersum) and an experiment with two artificial plants with different levels of morphological complexity. 2. The artificial plant experiment was designed to separate the effects of habitat area (patch size) and habitat complexity, thus enabling a more rigorous assessment of complexity per se than in previous studies where only a single patch size was used. Simple and complex artificial plants were established with five different patch sizes corresponding to the range found in natural plants. 3. Invertebrates occurred on both complex and simple forms of natural and artificial plants at similar abundances with dipterans and ephemeropterans being predominant. Taxon richness was higher on structurally complex Ranunculus than on simple Sparganium and was similarly higher on the complex artificial plant than on the simple one, over the entire range of habitat patch sizes. Thus, architectural complexity affected the taxon richness of epiphytic invertebrates, independently of habitat scale. 4. On the natural plants there was no difference in the abundance (both number of individuals and biomass) of invertebrates between simple and complex forms, while on artificial plants more invertebrates occurred on complex than on simple forms. The amount of particulate organic matter, >225 μm (POM) and chlorophyll a showed mixed patterns on natural and artificial plants, suggesting that the availability of these resources is not an overriding proximate factor controlling invertebrate abundance on plants. The difficulty of extrapolating from experimental results involving use of artificial plants is discussed, especially when considering the relationship between habitat structure and the occurrence of epiphytic invertebrates on natural plants.     

Internal eutrophication in peat soils through competition between chloride and sulphate with phosphate for binding sites

Inputs of surface waters high in chloride and sulphateincreased the availability of nutrients in fenpeatlands. This `internal eutrophication' wasdemonstrated with test plants (`phytometers') andthrough water and soil analysis. Three experiments arepresented in which the level of chloride and/orsulphate was increased to 3 mmol_c l^–1. Inexperiment 1 chloride levels were increased from 0.5to 3 mmol_c l^–1 as CaCl₂ or NaCl. Inexperiment 2 and 3 similar increased levels forchloride and sulphate (3 mmol_c l^–1; as NaCland Na₂SO₄) were used. The following resultswere found:(i) No differences in soil total-N and total-P werefound before and after the treatments in any of thethree experiments.(ii) Experiment 1 showed a significant increase inBio-Available P (BAP) in pots planted with Anthoxanthum odoratum as well as in bare pots for theNaCl and CaCl₂ treatments. The plants in thesetreatments had taken up much more P.(iii) Experiment 2 showed an increase in soil BAPafter treatment with chloride and sulphate in potsplanted with Anthoxanthum odoratum. The chloridetreatment had no effect on plant biomass, whereas thesulphate treatment resulted in a reduction in rootbiomass and root N and P content. The shoots showedan increase in P content in the sulphate and chloridetreatments, while N content remained the same.(iv) In experiment 3, treatments with chloride andsulphate led to significantly increased biomass and Puptake of Anthoxanthum odoratum. Again, noeffects on N uptake were found.These experiments provide evidence for distinctlyincreased availability of phosphate in peat soils whenthese come into contact with water with evenmoderately increased sulphate or chloride levels.Surface water originating from the Rhine river, whichis enriched in chloride and sulphate, is oftensupplied to fen reserves in The Netherlands, tocompensate for water losses due to agriculturaldrainage in the region. The results of this study showthat phosphate availability to the vegetation may risedrastically, with detrimental effects on the speciesdensity and the occurrence of rare species in thevegetation. Hence, supply of this water should beavoided.     

Changes of Some Mitochondrial Enzyme Activities of Cucumber Leaves during Ammonium Toxicity

The following enzyme activities were determined in the mitochondria of cucumber leaves (Cucumis sativus L. cv. Suisei No. 2) during ammonium toxicity: malate dehydrogenase, succinate dehydrogenase, glutamate dehydrogenase, cytochrome c oxidase, NADH diaphorase, NADH oxidase, succinate: cytochrome c oxidoreductase, NADH: cytochrome c oxidoreductase and adenosine triphosphatase. The activities of all enzymes except ATPase increased more or less during ammonium toxicity. Generally speaking the marked increase was found at 7 days treatment with 200 mg/1 NH₃-N. The adenosine triphosphatase activity of injured plants was lower than that of normal plants through treatment. The addition of various organic acids (15 mM) to the culture solution contaning 200 mg/1 NH₃-N (14.3 mM NH₄Cl) suppressed the ammonium toxicity. The accumulation of free ammonia in the leaves was also repressed by the addition of organic acids. The results of present and previous reports suggest that the increase of respiratory metabolism due to ammonium toxicity is required for the supply of organic acids, specially δ-ketoglutaric acid, to counteract ammonia. Uncoupling in mitochondria resulting in the increase of respiration does not seem to occur during ammonium toxicity.