bluuebunny/crossref_metadata_2025 huggingface dataset:
Sodium transport measured in plasma membrane vesicles isolated from wheat genotypes with differing K+/Na+ discrimination traits
Authors: ALLEN G. J. , WYN JONES R. G. , LEIGH R. A.
Published: April 2006

Right‐side‐out plasma membrane vesicles were isolated from wheat roots using an aqueous polymer two‐phase system. The purity and orientation of the vesicles were confirmed by marker enzyme analysis. Membrane potential (Ψ)‐dependent 22Na+ influx and sodium/proton (Na+/ H+) antiport‐mediated efflux across the plasma membrane were studied using these vesicles. Membrane potentials were imposed on the vesicles using either K+ gradients in the presence of valinomycin or H+ gradients. The ΔΨ was quantified by the uptake of the lipophilic cation tetraphenylphosphonium. Uptake of Na+ into the vesicles was stimulated by a negative ΔΨ and had a Km for extrav‐esicular Na+ of 34.8 ± 5.9 mol m3. The ΔΨ‐dependent uptake of Na+ was similar in vesicles from roots of hexaploid (cv. Troy) and tetraploid (cv. Langdon) wheat differing in a K+/Na+ discrimination trait, and was also unaffected by growth in 50 mol m−3 NaCl. Inhibition of ΔΨ‐dependent Na+ uptake by Ca2+ was greater in the hexaploid than in the tetraploid.

Sodium/proton antiport was measured as Na+‐dependent, amiloride‐inhibited pH gradient formation in the vesicles. Acidification of the vesicle interior was measured by the uptake of 14C‐methylamine. The Na+/H+ antiport had a Km, for intravesicular Na+ of between 13 and 19 mol m−3. In the hexaploid, Na+/H+ antiport activity was greater when roots were grown in the presence of 50 mol m−3NaCl, and was also greater than the activity in salt‐grown tetraploid wheat roots. Antiport activity was not increased in a Langdon 4D chromosome substitution line which carries a trait for K+/Na+ discrimination. It is concluded that neither of the transport processes measured is responsible for the Na+/K+ discrimination trait located on the 4D chromosome of wheat.
The xanthophyll cycle and sustained thermal energy dissipation activity in Vinca minor and Euonymus kiautschovicus in winter
Authors: ADAMS W. W. , DEMMIG‐ADAMS B.
Published: April 2006

The influence of low temperature on the operation of the xanthophyll cycle and energy dissipation activity, as ascertained through measurements of chlorophyll fluorescence, was examined in two broad‐leaved evergreen species, Vinca minor L. and Euonymus kiautschovicus Loessner. In leaves examined under laboratory conditions, energy dissipation activity developed more slowly at lower leaf temperatures, but the final, steady‐state level of such activity was greater at lower temperatures where the rate of energy utilization (through photosynthetic electron transport) was much lower. The rate at which energy dissipation activity increased was similar to that of the de‐epoxidation of violaxanthin to antheraxanthin and zea‐xanthin at different temperatures. However, leaves in the field examined prior to sunrise on mornings following cold days and nights exhibited a retention of antheraxanthin and zeaxanthin that was associated with sustained decreases in photosystem II efficiency. We therefore suggest that this phenomenon of ‘photoinhibition’ in response to light and cold temperatures during the winter results from sustained photoprotective thermal energy dissipation associated with the xanthophyll cycle. Such retention of the de‐epoxidized components of the xanthophyll cycle responded to day‐to‐day changes in temperature, being greatest on the coldest mornings (when photoprotective energy dissipation might be most required) and less on warmer mornings when photosynthesis could presumably proceed at higher rates.
The role of calcium in turgor regulation in Chara longifolia
Authors: BISSON M. A. , KIEGLE E. , BLACK D. , KIYOSAWA K. , GERBER N.
Published: April 2006

The salt‐tolerant alga Chara longifolia (Robinson) is capable of regulating its turgor in response to hypotonic stress resulting from a decrease in the osmotic pressure of the medium. This regulatory process takes only 40 min in small cells (length ≤ 10 mm), but requires 3d in large cells (length ≥30mm). Turgor regulation in small cells is comprised of two phases, a fast phase reducing the increased turgor by about 25% in the First 5 min, and a second phase reducing the turgor to near the original value within 40 min. The second phase is inhibited by reducing the concentration of Ca2+ in the external medium from 4.6 to 0.01 mol m−3; the first phase is less affected by the reduction of Ca2+. In the first 5 min of stress, the membrane depolarizes in a voltage‐dependent fashion, electrical conductance of the membrane increases transiently and cytoplasmic streaming is inhibited. When the external Ca2+ concentration is lowered, conductance does not increase and streaming continues unaffected. In a low ionic strength medium, Ca2+ is not required in the medium for turgor regulation.

To test the hypothesis that there is increased Ca2+ entry from the medium during turgor regulation, we measured the influx of 45Ca2+ into the cell. We found an increased influx of Ca2+, from 18 to 36 nmol m−2 s−1 during the first 30 to 90 s following osmotic stress. This increase was evident only in cells below about 7 mm in length, and was more marked in smaller cells.
Effects of soil fertility and mycorrhizal infection on pollen production and pollen grain size of Cucurbita pepo (Cucurbitaceae)
Authors: LAU T.‐C. , LU X. , KOIDE R. T. , STEPHENSON A. G.
Published: April 2006

The effects of soil fertility (two levels of soil nitrogen and two levels of soil phosphorus) and mycorrhizal infection on pollen production and pollen grain size were studied in two cultivars of the common zucchini (Cucurbita pepo). Overall, soil fertility and mycorrhizal infection had significant effects on traits affecting the male function of plants (staminate flower production, pollen production per flower and pollen grain size). There were also differences between the cultivars for these male traits in all three experiments. In addition, pollen grain size decreased toward the end of the growing season. In the mycorrhiza experiment, both phosphate concentration per pollen grain and total phosphate content per anther were greater but not significantly greater in the mycorrhizal plants than in the non‐mycorrhizal plants. A significant negative relationship between pollen production and pollen grain size was found in the mycorrhiza and soil phosphorus experiments, indicating that there was a trade‐off between pollen production and pollen size. This study is the first to show that mycorrhizal infection has an effect on male function (pollen production and size) in addition to the well‐documented effects on female function (fruit/seed production and size).
The mechanism of water‐stress‐induced embolism in two species of chaparral shrubs
Authors: JARBEAU J. A. , EWERS F. W. , DAVIS S. D.
Published: April 2006

The mechanism of water‐stress‐induced embolism of xylem was investigated in Malosma laurina and Heteromeles arbutifolia, two chaparral shrub species of southern California. We tested the hypothesis that the primary cause of xylem dysfunction in these species during dehydration was the pulling of air through the pores in the cell walls of vessels (pores in pit membranes) as a result of high tensions on xylem water. First, we constructed vulnerability‐to‐embolism curves for (i) excised branches that were increasingly dehydrated in the laboratory and (ii) hydrated branches exposed to increasing levels of external air pressure. Branches of M. laurina that were dehydrated became 50% embolized at a xylem pressure potential of ‐1.6 MPa, which is equal in magnitude but opposite in sign to the +1.6 MPa of external air pressure that caused 50% embolism in hydrated stems. Dehydrated and pressurized branches of H. arbutifolia reached a 50% level of embolism at ‐6.0 and +6.4 MPa, respectively. Secondly, polystyrene spheres ranging in diameter from 20 to 149 nm were perfused through hydrated stem segments to estimate the pore size in the vessel cell walls (pit membranes) of the two species. A 50% or greater reduction in hydraulic conductivity occurred in M. laurina at perfusions of 30, 42, 64 and 82 nm spheres and in H. arbutifolia at perfusions of 20 and 30 nm spheres. Application of the capillary equation to these pore diameters predicted 50% embolism at xylem tensions of ‐2.2 MPa for M. laurina and ‐6.7 MPa for H. arbutifolia, which are within 0.7 MPa of the actual values. Our results suggest that the size of pores in pit membranes may be a factor in determining both xylem efficiency and vulnerability to embolism in some chaparral species. H. arbutifolia, with smaller pores and narrower vessels, withstands lower water potentials but has lower transport efficiency. M. laurina, with wider pores and wider vessels, has a greater transport efficiency but requires a deeper root system to help avoid catastro‐phically low water potentials.
Ethylene and carbon dioxide: regulation of oat mesocotyl growth
Authors: NISHIZAWA T. , SUGE H.
Published: April 2006

Either ethylene or carbon dioxide stimulated the growth of oat mesocotyls in darkness, although the effect was much greater for carbon dioxide. Maximum elongation was obtained in the presence of both gases. Ethylene also induced lateral expansion of the mesocotyl: the volume of the mesocotyl was increased more in air with added ethylene than in air depleted of ethylene. Ethylene also stimulated mesocotyl growth under red light. Gibberellin only slightly increased mesocotyl length under red light and acted cooperatively with ethylene in the promotion of growth. The oat mesocotyl is a unique organ in terrestrial plants because ethylene simultaneously stimulates not only longitudinal growth but also lateral expansion. The ecological significance of oat mesocotyl growth in relation to the response to ethylene and carbon dioxide is discussed.
Phenotypic plasticity of the phenology of seven European tree species in relation to climatic warming
Authors: KRAMER K.
Published: April 2006

To evaluate the potential responses of individual trees to climatic warming, phenological observations of clones of Larix decidua (Mill.), Betula pubescens (Ehrh.), Tilia cor‐data (Mill.), Populus canescens (Ait.), Quercus robur (L.), Fagus sylvatica (L.) and Picea abies (L.) relocated over a large latitudinal range in Europe were analysed. The magnitude of the response of the clone was compared to that of genetically different trees of the same species in part of the latitudinal range, which were assumed to have adapted to their local climates. It was found that the responses of the date of leaf unfolding and the date of leaf fall in the clones to temperature were similar in magnitude to those in the genetically different trees. This demonstrates that trees possess considerable plasticity and are able to respond phenotypically to a major change in their local climate.

For the clones of Larix decidua and Quercus robur the duration of the growing season may decrease with increasing temperature, because leaf fall is advanced more than leaf unfolding. In Betula pubescens and Populus canescens, leaf unfolding and leaf fall are advanced equally, whereas in Tilia cordata and Fagus sylvatica the date of leaf fall seems to be unaltered but the date of leaf unfolding advances with increasing temperature. These differences in the duration of the growing season at increased temperature may alter the competitive balance between the species.

Descriptive dynamic models showed that most of the variance in the date of leaf unfolding can be accounted for by temperature. However, a generally applicable model of leaf fall based on temperature and/or photoperiod could not improved the null model, i.e. the mean date of leaf fall, because of variability in other environmental factors.

The lowest temperatures around the dates of leaf unfolding and leaf fall differed among the clones. The hypothesis that the survival of the clones is curtailed by spring frosts was supported. Thus, these lowest temper‐tures around leaf unfolding may represent thresholds below which the species cannot survive. It is argued that these thresholds may be a particularly sensitive means to evaluate the impact of climatic warming on the geographical distribution of tree species.
The effects of SO2 and O3 on the foliar nutrition of Scots pine, Norway spruce and Sitka spruce in the Liphook open‐air fumigation experiment
Authors: SHAW P. J. A. , McLEOD A. R.
Published: April 2006

Foliar elements were analysed in Scots pine, Sitka spruce and Norway spruce over a 6 year period before and during continuous exposure to SO2 and O3 in an open‐air fumigation experiment. Sulphur dioxide treatment elevated foliar sulphur concentration in all species, and there were increases in foliar nitrogen in the two spruce species but not in pine. The concentrations of cations were frequently increased by SO2 treatment, but there was no correlation between the sulphur concentration of needles and their total cation charge. SO2‐related elevations of foliar magnesium were correlated with the concentration of this element in soil solution, but the mechanism by which other cations were enhanced remains unclear. The only consistent effects on nutrient ratios were for SO2 treatments to increase sulphur/cation ratios.
Photosynthetic performance, chloroplast pigments and mineral content of Norway spruce (Picea abies (L.) Karst.) exposed to SO2 and O3 in an open‐air fumigation experiment
Authors: WEDLER M. , WEIKERT R. M. , LIPPERT M.
Published: April 2006

Photosynthetic performance, mineral content and chloroplast pigments were investigated in August‐September 1988 and 1989 in Norway spruce trees (Picea abies (L.) Karst.) exposed to SO2, and O3 in an open‐air fumigation facility at Liphook, England. The data do not suggest a treatment effect on the mineral content of the needles in terms of nutrient leaching from the foliage. In addition, there were no direct SO2 and/or O3 effects on the content and/or composition of the chloroplast pigments. However, the long‐term application of SO2 resulted in a depression of net photosynthesis under light saturation and ambient CO2 (A 340) which was probably caused by a treatment‐related depression of the carboxylation efficiency (CE). In 1989, the supposed treatment effects were apparently masked by an insufficient N‐supply and probably also by low water availability during summer. However, fumigation appeared to accelerate an N‐deficiency‐related decrease of CE, stomatal closure and the age‐dependent development of the chlorophyll content of the needles. In 1989, an observed depression of the photosynthetic capacity (A2500) was in part accompanied by a decrease in light use efficiency (α), suggesting an enhanced photosensitivity resulting from the impact of several possible interacting stresses (drought, N deficiency and fumigation). The results support the general conclusion that long‐term low‐level SO2 dosage adversely affects the photosynthetic performance of the needle, whether directly or indirectly, and may also interact with other environmental stresses. The findings of our investigations are discussed with regard to the hypothesis of forest decline in the mountain regions of the Fichtelgebirge (north‐eastern Bavaria, Germany).
Leaf surface properties of Norway spruce needles exposed to sulphur dioxide and ozone in an open‐air fumigation system at Liphook
Authors: CAPE J. N. , SHEPPARD L. J. , BINNIE J.
Published: April 2006

Samples of current‐year and 1‐year‐old foliage were taken from Norway spruce (Picea abies (L.) Karst.) trees in April 1991, 4 months after a 3–4 year controlled fumigation with O3 and SO2 in the open at Liphook, south‐east England. Trees were grown in seven plots, and treated in a factorial design with three levels of SO2 and two levels of O3 (ambient and c. 1.3 × ambient), with an extra ambient air plot. All statistical analyses were made on plot means.

Leaf wettability, as measured by the contact angle of water droplets, was significantly affected by needle age and by SO2 treatment (P≤0–05. in older needles, decreasing with increasing SO2 concentration. There was no effect of O3 on wettability, and no effect of any treatment on amounts of surface wax extracted by immersion of needles in chloroform. Electrolyte leakage rates from detached current‐year needles were not affected by prior exposure to O3, but decreased significantly (P= 0.034) with increasing exposure to SO2. There was no detectable effect of fumigation on the rate of water loss from detached needles. Similarly, there was no effect of fumigation on the dry weight/fresh weight ratio of needles. The total sulphur content of needles increased significantly (P≤0.0001) with exposure to SO2 and with needle age. Amounts of water‐extractable sulphate, however, varied greatly among plots, but with no pattern with respect to fumigation treatment.

It is concluded that leaf wettability and electrolyte leakage rates may be good indicators of the persistent effects of SO2 on Norway spruce growing in the open air, and that the observed changes in leaf surface properties in response to SO2 fumigation have implications for the processes, both biotic and abiotic, that occur on leaf surfaces.
Effect of open‐air fumigation with sulphur dioxide and ozone on phyllosphere and endophytic fungi of conifer needles
Authors: MAGAN N. , KIRKWOOD I. A. , MCLEOD A. R. , SMITH M. K.
Published: April 2006

The phyllosphere microbial populations inhabiting the needles of three conifer species, Scots pine (Pinus sylvestris L.), Sitka spruce (Picea sitchensis L.) and Norway spruce (Picea abies (L.) Karst.), exposed to SO2 and O3, in an open‐air fumigation experiment were analysed over a 3 year period using serial dilution after washing, direct plating and a fluorescein diacetate (FDA) enzyme assay. Total fungal populations ranged from 102 to 105 colonyforming units (CPU) g−1 fresh weight of needles. The dominant fungi isolated from needles varied with tree species and isolation technique; Aureobasidium pullulans (de Bary) Arnaud was most common on Scots pine and Norway spruce and white yeasts on Sitka spruce using the dilution plating method. However, direct plating of needle segments onto culture media indicated that Sclerophoma pythiophila (Corda) Hohnel was dominant on Scots pine and A. pullulans on Sitka and Norway spruce. Green needles of Sitka spruce were found to be endophytically colonized by Rhizosphaera kalkhoffii Bubak, but seldom by Lophodermium piceae (Fuckel) Hohn during extensive sampling in 1990. Statistical analyses revealed significant differences (P<0.05) between plots in the 3 year mean of the total fungal populations or the fungal biomass (FDA assay) on all three tree species. Differences between plots were also observed for a number of dominant component species. Data were also analysed for treatment effects. A significant effect of SO2 treatment was observed on the total fungal populations on Sitka spruce (P<0.05) which were reduced markedly by the low‐SO2 treatment, while the O3 treatment caused a significant increase in total fungal numbers on Scots pine (P<0.05). The FDA activity on needles of both Scots pine and Sitka spruce was noticeably higher in the 03‐only treatment plot, but the overall O3 effect was not significant. Treatment effects were also detected on the occurrence of component species. The serial dilution method revealed an SO2 effect (P<0.05) of a reduction in the occurrence of pink yeasts on Sitka spruce and an O3 effect (P<0.05) of an increase in the occurrence of S. pythiophila on Sitka spruce (P<0.01) but a decrease of Epicoccum nigrum Link and Cladosporium spp. on Scots pine. The direct‐plating method revealed an SO2 effect of an increase in S. pythiophila on Norway spruce (P<0.05). Ozone treatment caused a significant increase in the isolation of a black strain of A. pullulans on Norway spruce (P<0.05). Endophytic colonization of Sitka spruce needles by R. kalkhoffii was found to be increased on two occasions by O3 exposure.
Effects of sulphur dioxide and ozone on stilbenes and disease resistance in spruce trees from the Liphook Forest Fumigation Project
Authors: PEARCE R. B. , McLEOD A. R.
Published: April 2006

Levels of the stilbene glucosides astringin and isorhapontin, the main constitutive antifungal compounds in bark tissues of spruce trees, were not altered in young Picea sitchensis (Bong.) Carr. or Picea abies (L.) Karst. trees exposed to sulphur dioxide and ozone in the Liphook Forest Fumigation Project. These trees had received computer‐controlled fumigation treatments with two levels of SO2 (long‐term means 13 and 22nmol mol−1) or one level of O3 (1–3.times ambient), or a combination of these treatments, from spring until December.

Resistance of bark tissues from these trees to colonization by the root‐ and butt‐rot pathogen Heterobasidion annosum (Fr.) Bref., assessed in vitro using excised stem lengths, was not significantly altered in fumigated plants compared with those exposed to ambient pollutant levels only. This study therefore provided no evidence for altered disease resistance in P. abies and P. sitchensis trees exposed to SO2 and O3.
The effects of open‐air fumigation with sulphur dioxide on the decomposition of sycamore (Acer pseudoplatanus L.) leaf litters from polluted and unpolluted woodlands
Authors: NEWSHAM K. K. , INESON P. , FRANKLAND J. C.
Published: April 2006

Sycamore (Acer pseudoplatanus L.) leaf litters from 15 woodlands exposed to a broad range of ambient sulphur dioxide (SO2) concentrations were fumigated with environmentally realistic concentrations (ll‐20nmol mol−1) of SO2, for 166 d in an open‐air fumigation experiment. Fumigation of the sycamore litters significantly increased sulphate‐S and proton leaching, and decreased calcium, magnesium and potassium concentrations in leachates and leaf tissues. Leaf litters from relatively unpolluted woodlands showed a tendency to lose higher amounts of sulphate‐S, calcium, magnesium and nitrate‐N in leachates than litters from polluted sites when exposed to elevated levels of SO2 in treatment plots.

Fumigation inhibited the decomposition rates (CO2 evolution) of the leaf litters. Marked changes in the composition of the saprotrophic fungal communities in SO2‐fumigated leaf litters were also recorded, but fungal communities and responses to SO2, were similar between woodlands. There was no evidence from our data to suggest that resistance to SO2, was developed in decomposer mycofloras in woodlands more frequently polluted by the gas.
Populations of root‐feeding aphids in the Liphook forest fumigation experiment
Authors: SALT D. T. , WHITTAKER J. B.
Published: April 2006

Three species of root aphid (Pachypappa vesicalis Koch, Pachypappa tremulae L. and Prociphilus (Stagona) xylostei De Geer) were found feeding on Norway spruce [Picea abies (L.) Karsten] that were exposed to elevated concentrations of O3, and/or SO2 in a field manipulation experiment at Liphook, England. The experiment exposed conifer trees to controlled concentrations of these gases between 1987 and 1990. Root‐aphid populations feeding on Norway spruce were monitored in autumn 1990, the third year of fumigation and in autumn 1991, 10 months after fumigation had ceased. In 1990 the numbers of aphids in the high‐SO2, high‐SO2+ O3 and O3 treatments were significantly less than in ambient plots. Although after fumigation had ceased in 1991 the ambient plots still had higher numbers than the treatment plots, most of these differences were no longer statistically significant. In all plots sampled, total numbers of root aphids were higher in 1991 than in 1990. These results indicate that the numbers of root aphids in the experiment were negatively related to pollution levels. This relationship is the same as that shown in controlled laboratory experiments and is the converse of that generally reported in experiments using shoot‐feeding aphids.
A reinterpretation of stomatal responses to humidity
Authors: MONTEITH J. L.
Published: April 2006

The stomatal conductance (g) for single leaves and the equivalent canopy conductance for stands of vegetation are often represented in models as empirical functions of saturation vapour pressure deficit or relative humidity. The mechanistic basis of this dependence is very weak. A reanalysis of 52 sets of measurements on 16 species supports the conclusion of Mott & Parkhurst (1991, Plant, Cell and Environment 14, 509–515) that stomata respond to the rate of transpiration (E) rather than to humidity per se. In general, ∂g/∂E is negative and constant so that the relation between g and E can be defined by two parameters: a maximum conductance gm obtained by extrapolation to zero transpiration, and a maximum rate of transpiration Em obtained by extrapolation to zero conductance. Both parameters are shown to be functions of temperature, CO2 concentration, and soil water content. Exceptionally, transpiration rate and conductance may decrease together in very dry air, possibly because of patchy closure of stomata.
The cellular pathway of photosynthate transfer in the developing wheat grain. II. A structural analysis and histochemical studies of the pathway from the crease phloem to the endosperm cavity
Authors: WANG H. L. , OFFLER C. E. , PATRICK J. W.
Published: April 2006

In the developing wheat grain, photosynthate is transferred longitudinally along the crease phloem and then laterally into the endosperm cavity through the crease vascular parenchyma, pigment strand and nucellar projection. In order to clarify this cellular pathway of photosynthate unloading, and hence the controlling mechanism of grain filling, the potential for symplastic and apoplastic transfer was examined through structural and histochemical studies on these tissue types. It was found that cells in the crease region from the phloem to the nucellar projection are interconnected by numerous plasmodesmata and have dense cytoplasm with abundant mitochondria. Histochemical studies confirmed that, at the stage of grain development studied, an apoplastic barrier exists in the cell walls of the pigment strand. This barrier is composed of lignin, phenolics and suberin. The potential capacity for symplastic transfer, determined by measuring plasmodesmatal frequencies and computing potential sucrose fluxes through these plasmodesmata, indicated that there is sufficient plasmodesmatal cross‐sectional area to support symplastic unloading of photosynthate at the rate required for normal grain growth. The potential capacity for membrane transport of sucrose to the apoplast was assessed by measuring plasma membrane surface areas of the various cell types and computing potential plasma membrane fluxes of sucrose. These fluxes indicated that the combined plasma membrane surface areas of the sieve element–companion cell (se–cc) complexes, vascular parenchyma and pigment strand are not sufficient to allow sucrose transfer to the apoplast at the observed rates. In contrast, the wall ingrowths of the transfer cells in the nucellar projection amplify the membrane surface area up to 22‐fold, supporting the observed rates of sucrose transfer into the endosperm cavity. We conclude that photosynthate moves via the symplast from the se–cc complexes to the nucellar projection transfer cells, from where it is transferred across the plasma membrane into the endosperm cavity. The apoplastic barrier in the pigment strand is considered to restrict solute movement to the symplast and block apoplastic solute exchange between maternal and embryonic tissues. The implications of this cellular pathway in relation to the control of photosynthate transfer in the developing grain are discussed.
Effects of continuous leaf wetness on photosynthesis: adverse aspects of rainfall
Authors: ISHIBASHI M. , TERASHIMA I.
Published: April 2006

Above‐ground parts of Phaseolus vulgaris L. plants were treated with artificial misty rain (‘rain’) in a growth chamber to investigate the effects of leaf wetness on photosynthetic performance. The following results were obtained. (1) Stomata closed completely within 2 min of the onset of continuous ‘rain’ application and gradually opened to half the original aperture by 60 min. The rate of CO2 exchange measured on such wet leaves changed in parallel with the changes in stomatal aperture and attained 60 to 70% of the control level by 1h. (2) The dependence of the rate of leaf photosynthesis, A, on the intercellular CO2 concentration, ci [A(ci) relationship], examined in thoroughly dried leaves which had been treated with ‘rain’ did not change until after 4 h of treatment. However, leaves treated for 6h showed discernible decreases in A at high ci (ci>500μmolmol −1). The photosynthetic rate of leaves treated with ‘rain’ for 24 h was reduced at all ci, and A at the ambient CO2 concentration of 350μmolmol−1 was 60 to 70% of that of the control level. The rate of photosynthesis did not recover even after 3 d of treatment of the plants in a dry environment. These results clearly indicate that leaf wetness causes not only instantaneous suppression of photosynthesis but also chronic damage to the photosynthetic apparatus. Potential effects of leaf wetness on photosynthetic performance in nature are also discussed.
Two modes of bicarbonate utilization in the marine green macroalga Ulva lactuca
Authors: AXELSSON L. , RYBERG H. , BEER S.
Published: April 2006

The green marine macroalga Ulva lactuca L. was found to be able to utilize HCO3− from sea water in two ways. When grown in flowing natural sea water at 16°C under constant dim irradiance, photosynthesis at pH8.4 was suppressed by acetazolamide but unaffected by 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonate. These responses indicate that photosynthetic HCO3− utilization was via extracellular carbonic anhydrase (CA) ‐mediated dehydration followed by CO2 uptake. The algae were therefore described as being in a ‘CA state’. If treated for more than 10 h in a sea water flow‐through system at pH9.8, these thalli became insensitive to acetazolamide but sensitive to 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonate. This suggests the involvement of an anion exchanger (AE) in the direct uptake of HCO3−, and these plants were accordingly described as being in an ‘AE state’. Such thalli showed an approximately 10‐fold higher apparent affinity for HCO3− (at pH9.4) than those in the ‘CA state’, while thalli of both states showed a very high apparent affinity for CO2. These results suggest that the two modes of HCO3− utilization constitute two ways in which inorganic carbon may enter the Ulva lactuca cells, with the direct entry of HCO3−, characterizing the ‘AE state’, being inducible and possibly functioning as a complementary uptake system at high external pH values (e.g. under conditions conducive to high photosynthetic rates). Both mechanisms of entry appear to be connected to concentrating CO2 inside the cell, probably via a separate mechanism operating intracellularly.
Water movement through Quercus rubra I. Leaf water potential and conductance during polycyclic growth
Authors: REN Z. , SUCOFF E.
Published: April 2006

Two experiments examined simultaneous changes in leaf area (AL), root length (Lr), stomatal conductance (gs), leaf water potential (ΨL), transpiration and hydraulic plant conductance per unit leaf area (G) during the first three shoot cycles of northern red oak (Quercus rubra L.) grown under favourable and controlled conditions. Each shoot cycle consisted of bud swell, stem elongation, leaf expansion and rest; roots grew almost continuously. The gs of all leaves decreased substantially while leaves of the newest flush were expanding and increased modestly when seedling leaf area remained constant. Overall, gs decreased. The ΨL of mature leaves decreased during leaf expansion and increased by an equivalent amount during intervening periods. Possible explanations for the paired changes in gs and ΨL are considered. Changes in G closely paralleled those of canopy gs. These parallel changes during polycyclic seedling growth should act to keep seedling ΨL relatively constant as plant size increases and thereby help prevent ΨL from dropping to levels that would cause runaway embolism.
Evaluation of the heat pulse velocity technique for measurement of sap flow in rainforest and eucalypt forest species of south‐eastern Australia
Authors: BARRETT D. J. , HATTON T. J. , ASH J. E. , BALL M. C.
Published: April 2006

Sap flow in the stems of two cut saplings each of Eucalyptus maculata (a canopy eucalypt forest tree), Doryphora sassafras and Ceratopetalum apetalum (both canopy rainforest trees of south‐eastern coastal Australia) was measured by the heat pulse velocity technique and compared with water uptake from a potometer. Scanning electron micrographs of wounding caused by implantation of temperature sensor and heater probes into the sapwood showed that wounding was similar in rainforest and eucalypt species and was elliptical in shape. A circular wound has been implicitly assumed in previous studies. Accurate measurements of sapling water use were obtained using the smaller transverse wound dimension rather than the larger longitudinal dimension because maximum disruption of sap flow through the xylem vessels occurred in the transverse plane. Accurate measurements of sap flux were obtained above a minimum threshold sap velocity. These velocities were 15·7,10·9 and 9·4 cm h−1 for E. maculata, C. apetalum and D. sassafras, respectively. Below the threshold sap velocity, however, sap flow could not be accurately calculated from measurements of heat pulse velocity. The minimum threshold sap velocity appeared to be determined by probe construction and xylem anatomy. Despite the elliptical wounding and inaccurate measurement of sap flow below the threshold sap velocity, total sap flow over the experimental period for two saplings of each species was within 7% of water use measured by the potometer.
Temperature‐dependent changes of electric potential differences on opposite sides of the tepals of Eranthis in relation to thermonastic responses
Authors: HEJNOWICZ Z. , TREBACZ K. , SIEVERS A.
Published: April 2006

Electric potential differences (PDs) across the plasma membrane in cells of tepals of Eranthis hyemalis (L.) Salisb. were recorded under a regime in which the temperature was changed rapidly between two levels of approximately 8 and 19°C. The warming of an intact tepal resulted in a transient small increase in the magnitude of PD followed by a substantial long‐lasting depolarization. Upon cooling the reverse response occurred: a small transient depolarization was followed by a substantial hyperpolarization. These responses were more pronounced in the epidermis on the abaxial side of the tepal than in that on the adaxial side, indicating an electrophysiological dorsiventrality of the tepals which may be related to thermonastic movements through differential proton secretion on the two sides.
Abscisic acid concentrations and fluxes in droughted conifer saplings
Authors: JACKSON G. E. , IRVINE J. , GRACE J. , KHALIL A. A. M.
Published: April 2006

We present the first study of abscisic acid (ABA) concentrations and fluxes in the xylem sap of conifers during a drought cycle. In both Pinus sylvestris and Picea sitchensis the concentration of ABA in the sap rose 11‐fold as the drought progressed. There were clear diurnal trends in this concentration, which reached its maximum (6–8.ininol ABA m−3) near the middle of the day. The fluxes of ABA were calculated by multiplying the xylem ABA concentration by the sap flow rate. The ABA fluxes in the droughted plants in the middle of the day were usually no higher than those of the controls, as a result of the very low sap flow in the droughted plants at that time. However, the ABA flux in the droughted plants was higher than in the controls in the morning, and we postulate that the stomata are responding to these ‘morning doses’

Stomatal conductance, gs, could not be related statistically to leaf turgor or to the ABA flux. However, £s did display a negative exponential relationship with ABA concentration in the xylem.

Pinus displayed more acclimation to drought than Picea, Its ABA concentration rose and its stomatal conductance fell at day 6 of the drought, as opposed to day 17 for Picea, and its osmotic potential fell during the drought treatment.
Isolation and characterization of variant wheat cultivars for ABA sensitivity
Authors: BLUM A. , SINMENA B.
Published: April 2006

Genetic variants for abscisic acid (ABA) sensitivity are important for investigating the role of ABA sensitivity in conditioning plant response to environmental stress, and especially to those soil conditions that may elicit a root‐mediated hormonal signal. This study was performed in order to isolate variation in ABA sensitivity among wheat (Triticum aestivum and T. durum) cultivars, as characterized by two plant responses: (i) shoot growth reduction in response to 5×10−2mol m−3 ABA (racemic) in the root medium of hydroponically grown plants, and (ii) changes in transpiration and gas exchange in a bioassay of detached leaves (leaflaminac) infused with 10−4mol m−3 ABA.

Very significant (P≤0.01) and repeatable differences were found among 36 wheat cultivars and 19 landraces in the growth rate in ABA‐containing nutrient solutions, expressed as a percentage of the growth rate in control nutrient solutions (ABA/control ratio). In duplicate experiments, the ABA/control ratio ranged between 60 and 83% for the least sensitive cultivars (V2151‐3, Bethlehem, K1056 and Sunstar) and between 9 and 19% for the most sensitive cultivars (Sundor, Comet, Barkaec and V5). In the transpiration bioassay, performed with seven selected cultivars, it was found that the reductions in transpiration of ABA‐infused leaves corresponded very well with the reductions in growth in response to ABA in the root media. Measurement of gas exchange in the detached leaves of two cultivars differing in ABA sensitivity (Bethlehem and Sundor) showed that variable ABA sensitivity was expressed very well in the stomatal conductance, carbon exchange rate (CER) and photosynthetic capacity (CER/Ci ratio) of the leaf. These results therefore allowed us to isolate wheat variants for ABA sensitivity and to conclude that, while ABA sensitivity is expressed in the growth of plants challenged by ABA in the root medium, the control of sensitivity resides, at least partly, in the leaf.
15N natural abundance of vascular rainforest epiphytes: implications for nitrogen source and acquisition
Authors: STEWART G. R. , SCHMIDT S. , HANDLEY L.L. , TURNBULL M. H. , ERSKINE P. D. , JOLY C. A.
Published: April 2006

The foliar natural abundance of 15N was analysed to compare the potential nitrogen sources of vascular rainforest epiphytes and associated soil‐rooted trees. Leaves of epiphytes collected from six rainforest communities in Brazil, Australia and the Solomon Islands were depleted in 15N relative to the trees at each site. Epiphyte δ15N was as low as ‐6.4%o, while trees were generally enriched in 15N (0.7 to 3.5%o). These results indicate either that epiphytes use nitrogen sources depleted in 15N or that discrimination against 15N is an intrinsic function of epiphyte physiology. At three sites, epiphytes could be grouped into those having both low δ15N and low leaf‐nitrogen content and those possessing both high δ15N and high leaf‐nitrogen content. The second group had δ15N values in the range sometimes attributable to N2 fixation (‐2 to 0%o). There was no correlation between growth form and δ15N. It is concluded that epiphytes may utilize 15N‐depleted nitrogen from atmospheric deposition and N2 fixation.
ABA xylem concentrations determine maximum daily leaf conductance of field‐grown Vitis vinifera L. plants
Authors: CORREIA M. J. , PEREIRA J. S. , CHAVES M. M. , RODRIGUES M. L. , PACHECO C. A.
Published: April 2006

Differences in maximum leaf conductance in grapevine plants growing in soils with contrasting water availabilities during mid‐summer in Portugal could be accounted for by differences in the concentration of ABA in xylem sap. This conclusion is reinforced by the observation that the relationship between leaf conductance and endogenous ABA concentration can be mimicked by the application of exogenous ABA to leaves detached from irrigated plants. During the day, leaf conductance decreased after a morning peak, even when the leaves remained in a constant environment at a moderate temperature and leaf‐to‐air vapour pressure difference. This decline in leaf conductance was not a consequence of an increase in the xylem ABA concentration or the rate of delivery of this compound by the transpiratory stream. The afternoon depression in leaf conductance was associated with an apparent limitation in stomatal opening potential, which persisted even when detached leaves were fed with water and rehydrated. The reason for this inhibition has still to be identified.
Water in the stems of sessile oak (Quercus petraea) assessed by computer tomography with concurrent measurements of sap velocity and ultrasound emission
Authors: RASCHI A. , TOGNETTI R. , RIDDER H. ‐W. , BÉRES C.
Published: April 2006

Concurrent measurements of heat pulse velocity and ultrasound acoustic emission were performed on the trunks of adult Quercus petraea plants under different water stress conditions. On the trunk section of the plants the wood density was measured non‐destructively using a mobile computer tomograph which measures the attenuation of a collimated beam of radiation which traverses the trunk. By scanning the trunk in different directions, it is possible to map wood density in the section. As wood density is strictly correlated with water content, this method allows evaluation of the water content in the trunk section and the water conditions in the different parts of the section. The computer‐tomograph technique is non‐invasive and is not influenced by climatic fluctuations. A close agreement was found between wood density and heat pulse velocity; the relationship between these two parameters and ultrasound acoustic emission is discussed. Trunks of sessile oak appear well suited as water storage reservoirs playing an important role in tree survival during extended periods of low soil water availability, especially in the context of global climatic change. Here the computer‐tomograph methodology is described and suggestions arc made for further research development.
Temporal variability and scaling behaviour in the fine‐root system of kiwifruit (Actinidia deliciosa)
Authors: REID J. B. , BOWDEN M.
Published: April 2006

We show that temporal variability in root populations can depend upon the scale of measurement (particularly the sampled soil volume). The presence of roots in a range of volumes of soil was studied using perspex tubes installed horizontally into the soil around three mature kiwifruit vines. Roots intercepting lines scored on the tubes were counted using a periscope. For small volumes of soil (c. 2–4 cm3) the root counts varied with time in a very irregular manner, and as the interval between measurements increased the autocorrelation between the measurements decayed rapidly. At about half of the locations monitored there was no significant autocorrelation between measurements 27 d apart. Linear interpolation in these time series was unreliable, and where the correlation dimension could be resolved it was usually non‐integer (suggesting chaotic behaviour). The time series measured at different locations were poorly correlated, indicating weak coordination. As the observed soil volume increased, the coordination between locations improved, the autocorrelation function increased, and linear interpolation errors decreased (although these remained substantial). Clearly there are considerable fundamental constraints on our ability to predict the root behaviour of kiwifruit vines at scales that are appropriate for mechanistic models of nutrient and water uptake. We discuss the need for a new conceptual model of the fine‐root systems of kiwifruit and similar species.
A field study of the effects of elevated CO2 on carbon assimilation, stomatal conductance and leaf and branch growth of Pinus taeda trees
Authors: TESKEY R. O.
Published: April 2006

A study was conducted in 21‐year‐old loblolly pine (Pinus taeda L.) trees growing in plantation in north central Georgia, USA. The experiment used branch chambers to impose treatments of ambient, ambient +165 and ambient + 330 μmol mol−1 CO2. After one growing season there was no indication of acclimation to elevated CO2. In August and September, carbon assimilation, measured by two different methods, was twice as high at ambient +330 μmol mol−1 CO2 than at ambient. Dark respiration was suppressed by 6% at ambient +165 and by 14% at ambient + 330 μmol mol−1 CO2. This suppression was immediate, and not an effect of exposure to elevated CO2 during growth, since respiration was reduced by the same amount in all treatments when measured at a high CO2 concentration. Elevated CO2 increased the growth of foliage and woody tissue. It also increased instantaneous transpiration efficiency, but it had no effect on stomatal conductance. Since the soil at the study site had low to moderate fertility, these results suggest that the growth potential of forests on many sites may be enhanced by global increases in atmospheric CO2, concentration.
Predominant localization of mitochondria enriched with glycine‐decarboxylating enzymes in bundle sheath cells of Alternanthera tenella, a C3–C4 intermediate species
Authors: DEVI M. T. , RAJAGOPALAN A. V. , RAGHAVENDRA A. S.
Published: April 2006

Mesophyll protoplasts and bundle sheath cells were prepared by enzymatic digestion of leaves of Alternanthera tenella, a C3‐C4 intermediate species. The intercellular distribution of selected photosynthetic, photorespiratory and respiratory (mitochondrial) enzymes in these meso‐phyll and bundle sheath cells was studied. The activity levels of photosynthetic enzymes such as PEP carboxylase (EC 4.1.1.31) or NAD‐malic enzyme (EC 1.1.1.39) and photorespiratory enzymes such as glycolate oxidase (EC 1.1.3.1) or NADH‐hydroxypyruvate reductase (EC 1.1.1.29) were similar in the two cell types. The activity levels of mitochondrial TCA cycle enzymes such as citrate synthase (EC 4.1.3.7) or fumarase (EC 4.2.1.2) were 2‐ to 3‐fold higher in bundle sheath cells. On the other hand, the activity levels of mitochondrial photorespiratory enzymes, namely glycine decarboxylase (EC 2.1.2.10) and serine hydroxymethyltransferase (EC 2.1.2.1), were 6‐9‐fold higher in bundle sheath cells than in mesophyll protoplasts. Such preferential localization of mitochondria enriched with the glycine‐decarboxylating system in the inner bundle sheath cells would result in efficient refixa‐tion of CO2 from not only photorespiration but also dark respiration before its exit from the leaf. We propose that predominant localization of mitochondria specialized in glycine decarboxylation in bundle sheath cells may form the basis of reduced photorespiration in this C3‐C4 intermediate species.