bluuebunny/crossref_metadata_2025 huggingface dataset:
Translocation of nitrogen in osmotically stressed wheat seedlings
Authors: LARSSON M.
Published: April 2006

Wheat (Triticum aestivum L., cv. Drabant) seedlings were grown in a ‘split root’ system where either the whole root system or one root half was subjected to osmotic stress for 24 h, using 200 g polyethylene glycol (PEG, molecular weight 4000) dm−3 nutrient solution. 15N‐Labelled nitrate was fed to one of the root compartments and total N and 15N‐labelling were measured in plant material and xylem sap. Untreated plants translocated 87% of the N taken up to the shoot, and 10% of this was then retranslocated back to the root. Recalculated on a root nitrogen basis, 36% of the label recovered in the root after 24 h had passed through the shoot. Significant labelling of xylem sap collected from non‐labelled roots indicated cycling of organic N through the roots. PEG‐treatment of the whole root system caused significant water loss in both roots and shoots. Uptake of nitrate and retranslocation of N to roots were inhibited, whereas cycling of organic nitrogen through the root was still measurable. Treatment of half the root system with PEG had minor effects on shoot water content, but reduced the water content of the treated root part. The total uptake of nitrate by the root system was unaffected, and the effect on the treated root half was comparatively small. Nitrate reductase activity (NRA) declined in PEG‐treated roots even if high nitrate uptake rates were maintained. Shoot NRA was unaffected by osmotic stress. The data indicate that the reduction in water content of the root per se has only small effects on nitrate uptake. Major inhibition of nitrate uptake was observed only after treatment of a sufficiently large portion of the root system to given an effect on shoot water content.
Development correlations between roots in heterogeneous environments
Authors: GERSANI M. , SACHS T.
Published: April 2006

Roots are known to respond to favourable nutrient conditions by increased initiation and growth of lateral roots. The problem studied here was to what extent does this local developmental response depend on the environments of other roots on the same plant. Such dependence could allow for an optimal allocation of resources required for root growth in unpredictable, heterogeneous soils. Pea seedlings (Pisum sativum var. arvense cv. Dun) were pruned and grown to have two equal root systems, each in an individual container. As expected, these roots responded by increased development to a wide range of nutrient solution concentrations. The local development of these roots, expressed by their dry weight, was a function of the relative rather than the absolute conditions in which they were grown: roots in a given environment developed more rapidly if other roots on the same plant were in poorer than if they were in richer nutrient conditions. The number of lateral initials doubled within 3d after the roots were exposed to optimal nutrient conditions, before any dry weight differences could be detected. This rapid root initiation was also a function of the conditions other roots of the same plant were in. These results mean that root development, and especially lateral root initiation, depends on the integrated effects of the local environment and the internal correlative relations between the roots.
Hydraulic architecture of sugarcane in relation to patterns of water use during plant development*
Authors: MEINZER F. C. , GOLDSTEIN G. , NEUFELD H. S. , GRANTZ D. A. , CRISOSTO G. M.
Published: April 2006

Hydraulic conductance was measured on leaf and stem segments excised from sugarcane plants at different stages of development. Maximum transpiration rates and leaf water potential (ΨL) associated with maximum transpiration were also measured in intact plants as a function of plant size. Leaf specific hydraulic conductivity (Lsc) and transpiration on a unit leaf area basis (E) were maximal in plants with approximately 0.2 m2 leaf area and decreased with increasing plant size. These changes in Fand Lsc were nearly parallel, which prevented φL in larger plants from decreasing to levels associated with substantial loss in xylem conductivity caused by embolism formation. Coordination of changes in E and leaf hydraulic properties was not mediated by declining leaf water status, since φL increased with plant size. Hydraulic constrictions were present at nodes and in the node‐leaf sheath‐leaf blade pathway. This pattern of constrictions is in accord with the idea of plant segmentation into regions differing in water transport efficiency and would tend to confine embolisms to the relatively expendable leaves at terminal positions in the pathway, thereby preserving water transport through the stem.
A theoretical model of hydraulic conductivity recovery from embolism with comparison to experimental data on Acer saccharum
Authors: YANG S. , TYREE M. T.
Published: April 2006

A theoretical model of bubble dissolution in xylem conduits of stems was designed using the finite differential method and iterative calculations via computer. The model was based on Fick's, Henry's and Charles' laws and the capillary equation. The model predicted the tempo of recovery from embolism in small diameter branches of woody plants with various xylem structures under different xylem water pressures. The model predicted the time required to recover conductivity in any position in the stem. Repeated iterative solution of the model for different situations yielded an empirical formula to calculate the time for complete recovery of conductivity in stems from a fully embolised initial state. The time, tp, is given by:

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where α is a temperature coefficient; D is the coefficient of diffusion of air in wood at 25°C; rcs is the ratio of the area of total conduit cross section to the stem cross section; Ψxp is the stem xylem pressure potential (Pa, where 0 Pa equals atmospheric pressure); τ is solution surface tension (0.072 N m−1); and Dc and Ds are diameters of the conduits and the stem, respectively (m). The equation is valid only when Ψxp > –4τ/Dc. The model predicts no recovery of conductivity when Ψxp≤–4τ/Dc. The model agreed with experiments.
Phosphate uptake, efflux and deficiency in the water fern, Azolla
Authors: BIELESKI R. L. , LÄUCHLI A.
Published: April 2006

High phosphorus status (High‐P) Azolla mexicana plants (P content 15.5 μmoles g fr wt−1, doubling time ca. 2.2 d) and Low‐P plants with early signs of P‐deficiency (P content 6.2 μmoles g fr wt−1, doubling time ca. 3.2 d) were used to study Pi uptake, efflux and deficiency. When High‐P plants were transferred to medium lacking Pi, uptake capacity increased 1.5‐fold within 12 h and before any detectable change in growth rate (24–48 h). When High‐P and Low‐P plants were compared, uptake rates from 0.3–10000 mmoles m−3 Pi were 2.6–1.7 times higher in Low‐P than High‐P plants (18–1150 vs 7–665 μmoles g fr wt−1 h−1). The relationship of uptake rate to concentration was interpreted as arising from a combined operation of a high‐ and a low‐affinity uptake system. Higher uptake in Low‐P plants involved a 3.4‐fold increase in Vmax (high affinity), no change in Km (high affinity), and a 1.5 to two‐fold increase in both Vmax (low affinity) and Km (low affinity). Rates of P efflux into 1–1000 mmoles m−3 Pi were 1.7 to two times higher from High‐P than Low‐P plants (12–22 vs 7–11 μmoles g fr wt−1 h−1). Below 1 mmole m−3 Pi, uptake and efflux rates were similar: the equilibrium concentration, at which net uptake was zero, was 0.22 mmoles m−3 for High‐P plants and 0.05 mmoles m−3 for Low‐P plants. Similar results were obtained with A. filiculoides. P transport characteristics of Azolla, a fern, are closely comparable with those of higher plants. Its high P requirement in the field arises from its ecological rather than physiological behaviour. We interpret the field behaviour by exploring the relationship between Azolla growth rate in the field, plant P concentration in the field, Pi transport rates required to support such growth, and Pi concentrations in pond waters. The transport characteristics which must operate in the field match those of Low‐P plants in the laboratory, not High‐P plants. Thus, Pi uptake in High‐P plants should be interpreted as repressed from the normal state, instead of that in Low‐P plants being induced.
Rapid calcium exchange for protons and potassium in cell walls of Chara
Authors: RYAN P. R. , NEWMAN I. A. , ARIF I.
Published: April 2006

Net fluxes of Ca2+, H+ and K+ were measured from intact Chara australis cells and from isolated cell walls, using ion‐selective microelectrodes. In both systems, a stimulation in Ca2+ efflux (up to 100 nmol m−2 s−1, from an influx of ∼40 nmol m−2 s−1) was detected as the H+ or K+ concentration was progressively increased in the bathing solution (pH 7.0 to 4.6 or K+ 0.2 to 10mol m−3, respectively). A Ca2+ influx of similar size occurred following the reverse changes. These fluxes decayed exponentially with a time constant of about 10 min. The threshold pH for Ca2+ efflux (pH 5.2) is similar to a reported pH threshold for acid‐induced wall extensibility in a closely related characean species. Application of NH4+ to intact cells caused prolonged H+ efflux and also transient Ca2+ efflux. We attribute all these net Ca2+ fluxes to exchange in the wall with H+ or K+. A theoretical treatment of the cell wall ion exchanges, using the ‘weak acid Donnan Manning’ (WADM) model, is given and it agrees well with the data. The role of Ca2+ in the cell wall and the effect of Ca2+ exchanges on the measured fluxes of other ions, including bathing medium acidification by H+ efflux, are discussed.
Evacuolation and enucleation of mesophyll protoplasts in self‐generating percoll gradients
Authors: WU F.‐S. , TSAI Y.‐Z.
Published: April 2006

Mesophyll protoplasts from Brassica oleracea, B. napus, Nicotiana tobaccum and Solanum tuberosum were isolated and subjected to uttracentrifugation at 65000g for 30 min in percoll solutions containing various strengths of salt and osmotic stabilizing agents. After centrifugation, the self‐generated percoll gradients were evaluated for their effectiveness in protoplast evacuolation and enucleation. The vacuoles, cell debris, evacuolated protoplasts and enucleated protoplasts were separated. Factors that affected evacuolation and enucleation in the percoll gradients were described. Mesophyll protoplasts produced by epidermis peeling and short enzyme incubation periods were more easily evacuolated and enucleated than those produced by leaf‐slicing and long incubation periods. Lower centrifugal force at 25000g for 80 min was also successful in evacuolating and enucleating the mesophyll protoplasts. A green band that contained nearly pure evacuolated protoplasts, of which 45% were enucleated protoplasts, was obtained from the self‐generated percoll gradient. Rhodamine 123 staining of mitochondria indicated that the evacuolated protoplasts were metabolically active and were capable of regenerating the vacuole and cell wall. Cell divisions were also observed when the evacuolated protoplasts were cultured.
Dirunal leaf movement, chlorophyll fluorescence and carbon assimilation in soybean grown under different nitrogen and water availabilities
Authors: KAO W.‐Y. , FORSETH I. N.
Published: April 2006

Diurnal heliotropic leaf movements, photosynthetic gas exchange, and the ratio of variable fluorescence to maximum fluorescence (Fv/Fm) of unrestrained and horizontally restrained leaves from soybean (Glycine max cv. Cumberland) plants grown in two different water and two different nitrogen treatments were measured. Leaves of plants grown in low water or low nitrogen availability treatments displayed more pronounced diaheliotropism (solar tracking) in the afternoon and a longer period of paraheliotropism (light avoiding) at midday relative to those of well‐watered, high‐nitrogen‐grown plants. Photosaturated photosynthetic rates and the photon flux required to saturate photosynthesis were reduced by water stress and nitrogen deficiency. Compared to horizontal leaves, irradiance on orienting leaves was nearer to the breakpoint of the photosynthetic light response curve, where photosynthesis is co‐limited by ribulose biphosphate regeneration and carboxylation. This would increase the carbon return on investments of nitrogen into photosynthesis. A positive linear relationship between Fv/Fm and quantum yield of photosynthesis was measured. Leaves of low‐nitrogen‐grown plants had earlier and more prolonged reductions in Fv/Fm at midday compared to leaves of high nitrogen grown plants of the same water treatment. Within the same water and nitrogen treatment, horizontally restrained leaves had lower midday Fv/Fm in relation to orienting leaves. Nitrogen deficiency and water stress enhanced this difference such that horizontally restrained leaves of low water and low nitrogen grown plants had earlier and longer midday depressions in Fv/Fm.
Comparison of salt tolerance and osmotic adjustment of low‐sodium and high‐sodium subspecies of the C4 halophyte, Atriplex canescens
Authors: GLENN E. P. , WATSON M. C. , O'LEARY J. W. , AXELSON R. D.
Published: April 2006

The relationship between Na+ accumulation and salt tolerance was tested by comparing subspecies of the halophyte, Atriplex canescens (fourwing saltbush), that differed markedly in Na+ content and Na:K ratios. Above ground tissues of one low‐sodium and two high‐sodium subspecies were compared with respect to cation accumulation, osmotic adjustment and growth along a salinity gradient in greenhouse trials. Plants of each subspecies were grown for 80 d on 2.2, 180, 540 and 720 mol m−3 NaCl. At harvest, A. canescens ssp. canescens had significantly lower Na+ levels, higher K+ levels and lower Na:K ratios in leaf and stem tissues than A. canescens ssp. macropoda and linearis over the salinity range (P < 0.05 or 0.01). Na:K ratios in leaves of the latter two, high‐sodium, subspecies were approximately 2 on the lowest salinity treatment and ranged from 5 to 10 on the more saline solutions. By contrast, Na:K ratios in leaves of the low‐sodium subspecies canescens, were only 0.4 on the lowest salinity and ranged narrowly from 1.7 to 2.3 at higher salinities. However, despite different patterns of Na+ and K+ accumulation, all three subspecies exhibited equally high salt tolerance and had similar osmotic pressures in their leaves or stems over the salinity range. Contrary to expectations, high salt tolerance was not necessarily dependent on high levels of Na+ accumulation in this species.
Evidence supporting a non‐phloem source of water for export of solutes in the xylem of soybean root nodules
Authors: STREETER J. G. , SALMINEN S. O.
Published: April 2006

The vascular anatomy of soybean nodules [Glycine max (L.) Merr.] suggests that export of solutes in the xylem should be dependent on influx of water in the phloem. However, after severing of stem xylem and phloem by shoot decapitation, export of ureides from nodules continued at an approximately linear rate for 5h. This result was obtained with decapitated roots remaining in the sand medium, but when roots were disturbed by removal from the rooting medium prior to shoot decapitation, export of ureides from nodules was greatly reduced. Stem exudate could not be collected from disturbed roots, indicating that flow in the root xylem had ceased. Thus, ureide export from nodules appeared to be dependent on a continuation of flow in the root xylem. When seedlings were fed a mixture of 3H2O and 14C‐inulin for periods of 14–21 min, nodules had higher 3H/14C ratios than roots from which they were detached. The combined results are not consistent with the proposal that export of nitrogenous compounds from nodules is dependent on import of water via the phloem. The results do support the view that a portion of the water required for xylem export from soybean nodules is supplied via a symplastic route from root cortex to nodule cortex to the nodule vascular apoplast.
Photophysiology of turion germination in Spirodela polyrhiza (L.) Schleiden. X. Role of nitrate in the phytochrome‐mediated response
Authors: APPENROTH K.‐J. , AUGSTEN H. , MOHR H.
Published: April 2006

It was shown previously that light‐dependent germination of turions of Spirodela polyrhiza (Lemnaceae) is mediated by the photoreceptor phytochrome [Appenroth & Augsten (1990) Photochemistry and Photobiology 52, 61–65]. In the present study, we found that this photoresponse depends on nitrate in the surrounding medium both during after‐ripening (under natural conditions occurring in winter) and during germination after light‐induction (in spring). The action of nitrate in the germination response is neither related to the induction of nitrate reductase nor to the rate of uptake of 15NO3−. Moreover, two‐factor analysis (phytochrome, nitrate) revealed a multiplicative coaction, i.e. independent action of both factors in mediation of germination. The notion that nitrate is a nutritional prerequisite in phytochrome‐mediated germination of turions, is supported by the following facts: (1) Nitrate‐requirement during germination was strongly increased by nitrate starvation during after‐ripening prior to germination. (2) Ammonium could substitute for nitrate. (3) Nitrate uptake by the turions was unaffected by phytochrome and very pronounced even at low concentrations (0.07 mol m−3) in the medium. With regard to the phytochrome‐induced chain of events, it is concluded that nitrate is a prerequisite during a specific developmental phase. Nitrate is not a regulatory element within the chain. In an ecological sense, however, nitrate contents of the aquatic system regulate the germination of turions.
Response of plant roots to elevated atmospheric carbon dioxide
Authors: ROGERS H. H. , PETERSON C. M. , McCRIMMON J. N. , CURE J. D.
Published: April 2006

Plant root response to atmospheric CO2 enrichment can be great. Results from this controlled environment investigation demonstrate substantial effects on root system architecture, micromorphology and physiology. The most pronounced effects were an increase in root length (110%) and root dry weight (143%). Root diameter, stele diameter, cortex width, root/shoot and root weight ratios all increased; root numbers did not increase. The long‐term implications for belowground processes could be enormous.
Effects of elevated carbon dioxide concentration on photosynthesis and growth of small birch plants (Betula pendula Roth.) at optimal nutrition
Authors: PETTERSSON R. , McDONALD A. J. S.
Published: April 2006

Small birch plants (Betula pendula Roth.) were grown from seed for periods of up to 70d in a climate chamber at optimal nutrition and at present (350 μmol mol−1) or elevated (700 μmol mol−1) concentrations of atmospheric CO2. Nutrients were sprayed over the roots in Ingestad‐type units. Relative growth rate and net assimilation rate were slightly higher at elevated CO2, whereas leaf area ratio was slightly lower. Smaller leaf area ratio was associated with lower values of specific leaf area. Leaves grown at elevated CO2 had higher starch concentrations (dry weight basis) than leaves grown at present levels of CO2. Biomass allocation showed no change with CO2, and no large effects on stem height, number of side shoots and number of leaves were found. However, the specific root length of fine roots was higher at elevated CO2. No large difference in the response of carbon assimilation to intercellular CO2 concentration (A/Ci curves) were found between CO2 treatments. When measured at the growth environments, the rates of photosynthesis were higher in plants grown at elevated CO2 than in plants grown at present CO2. Water use efficiency of single leaves was higher in the elevated treatment. This was mainly attributable to higher carbon assimilation rate at elevated CO2. The difference in water use efficiency diminished with leaf age. The small treatment difference in relative growth rate was maintained throughout the experiment, which meant that the difference in plant size became progressively greater. Thus, where plant nutrition is sufficient to maintain maximum growth, small birch plants may potentially increase in size more rapidly at elevated CO2.
Ultraviolet‐B and visible light penetration into needles of two species of subalpine conifers during foliar development
Authors: DeLUCIA E. H. , DAY T. A. , VOGELMAN T. C.
Published: April 2006

The depth of penetration of Ultraviolet‐B (UV‐B, 300 and 320 nm) and visible (680 nm) light was measured in foliage of Abies lasiocarpa and Picea engelmannii using a fibre‐optic microprobe. Measurements were made on foliage at four times during development: needles were sampled from within expanding buds (in bud); within 72 h of emergence from the bud scales (emergent); from elongating branches (elongating); and from foliage that emerged the previous summer (mature). Light attenuation in pre‐emergent needles of both species was steep and showed strong wavelength dependence. Short wavelength 300‐nm light was attenuated strongly in the developing epidermal layer, but a significant proportion of this potentially damaging UV‐B radiation penetrated into the mesophyll. For A. lasiocarpa and P. engelmannii, 99% attenuation of 300‐nm light occurred at 51 and 96 μm, respectively, well within the mesophyll. At this stage, however, the bud scales were opaque to light below 400nm. As the epidermal cell walls and cuticle continued to develop and chlorophyll accumulated following emergence from the bud scales, light attenuation, particularly of UV‐B radiation, increased. Although no UV‐B is transmitted through the epidermis‐hypodermis of mature needles, small but measurable quantities of 300‐ and 320‐nm light were measured in the photosynthetic mesophyll of post‐emergent and elongating needles. Thus, shortly after emergence from the bud scales in mid‐June to mid‐July, when incident UV doses are highest, absorption of UV‐B radiation by potentially sensitive chromophores in the mesophyll may disrupt physiological and developmental processes in these species. Soluble UV‐absorbing pigments accumulated during needle maturation for P. engelmannii but not A. lasiocarpa, suggesting that, for A. lasiocarpa at least, the development of effective UV screening properties in the epidermis may not be related to the induction of soluble flavonoids.
Water relations of the tomato during fruit growth
Authors: JOHNSON R. W. , DIXON M. A. , LEE D. R.
Published: April 2006

Fruit and stem water potentials of tomato plants were measured continuously for several days using automated psychrometers. A linear voltage displacement transducer was used to simultaneously measure diameter changes on an adjacent fruit. A strong correlation was observed between the water potential gradient of the fruit and stem, and changes in fruit diameter. Fruit diameter increased when the apoplasmic water potential gradient favoured solution flow into the fruit and fruit shrinkage occurred only when the water potential gradient was inverted. Based on our data and other published data (Ehret & Ho 1986; Lee 1989a) on phloem transport in tomato, we have concluded that low stem water potentials have an immediate and direct effect on phloem turgor; reducing the driving force for sap flow into the fruit. Since fruit water potential remained relatively constant, the diurnal variation in stem water potential was sufficient to account for the correlation with changes in fruit diameter. There are consequences with respect to predicting the accumulation of dry matter in tomato fruit.
Osmotic relations of the drought‐tolerant shrub Artemisia tridentata in response to water stress
Authors: EVANS R. D. , BLACK R. A. , LOESCHER W. H. , FELLOWS R. J.
Published: April 2006

Turgor maintenance, solute content and recovery from water stress were examined in the drought‐tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above −2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to −5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.
Cadmium — citric acid — xylem cell wall interactions in tomato plants
Authors: SENDEN M. H. M. N. , VAN PAASSEN F. J. M. , VAN DER MEER A. J. G. M. , WOLTERBEEK H. Th.
Published: April 2006

Abstract. Mutual interactions between cadmium ions, citric acid and xylem cell walls were examined. Cadmium and citric acid were measured as 115Cd and [1,5‐14C] citric acid, respectively. Xylem cell walls were obtained by bacterial degradation of tomato stem sections (Lycopersicon esculentum Mill, cv. Tiny Tim), and applied as ion‐exchange columns. The xylem column material carried 2·4 dm3 H2O kg−1 dry weight, and was temporarily capable of buffering perfusates at pH 5·7. Sorbed cadmium and citric acid were determined from H2O and HCl rinses after perfusion periods. In all experiments, total cadmium and/or citric acid recoveries were better than 98%, indicating both the effectiveness of the rinses applied and the possibility of full regeneration of the xylem column. The results indicate that the presence of 2·45 mol m−3 citric acid causes an approximately 50% reduction of adsorbed cadmium levels, irrespective of the applied total cadmium concentrations (0·04–0·4 mol m−3 Cd(NO3)2.4H2O). This reduction is probably related to a corresponding reduction to approximately 2% of the control applied free Cd2+ concentration, the latter also independent of the total cadmium concentrations. Furthermore, without inducing positively charged citrate complexes in the applied solution, the presence of cadmium resulted in increased levels of citric acid absorbed in the xylem column. The Donnan Free Space accumulation of citric acid in the presence of Cd(NO3)2.4H2O, observed in the experiments described, could be expressed by its distribution coefficient, as approximately 15 times the control accumulation. These data indicate that the xylem column may operate as a ligand exchanger, suggesting the importance of metal ions for the longitudinal and lateral movement of organic complexing compounds in the xylem.
Absorption of nitrate and ammonium ions by Lolium perenne from flowing solution cultures at low root temperatures
Authors: CLARKSON D. T. , JONES L. H. P. , PURVES J. V.
Published: April 2006

Lolium perenne L. cv. 23 (perennial ryegrass) plants were grown in flowing solution culture and acclimatized over 49 d to low root temperature (5°C) prior to treatment at root temperatures of 3, 5, 7 and 9°C for 41 d with common air temperature of 20/15°C day/night and solution pH 5·0. The effects of root temperature on growth, uptake and assimilation of N were compared with N supplied as either NH4 or NO3 at 10 mmol m−3. At any given temperature, the relative growth rate (RGR) of roots exceeded that of shoots, thus the root fraction (Rf) increased with time. These effects were found in plants grown with the two N sources. Plants grown at 3 and 5°C had very high dry matter contents as reflected by the fresh weight: freeze‐dried weight ratio. This ratio increased sharply, especially in roots at 7 and 9°C. Expressed on a fresh weight basis, there was no major effect of root temperature on the [N] of plants receiving NHJ but at any given temperature, the [N] in plants grown with NHJ was significantly greater than in those grown with NO3. The specific absorption rate (SAR) of NH+4 was greater at all temperatures than SAR‐NO3. In plants grown with NH+, 3–5% of the total N was recovered as NH+4, whereas in those grown with NO−3 the unassimilated NO−3 rose sharply between 7 and 9°C to become 14 and 28% of the total N in shoots and roots, respectively. The greater assimilation of NH+4 lead to concentrations of insoluble reduced N (= protein) which were 125 and 20% greater, in roots and shoots, respectively, than in NO−3‐grown plants. Plants grown with NH+4 had very much greater glutamine and asparagine concentrations in both roots and shoots, although other amino acids were more similar in Concentration to those in NO−3 grown plants. It is concluded that slow growth at low root temperature is not caused by restriction of the absorption or assimilation of either NH+4 or NO−3. The additional residual N (protein) in NH+4 grown plants may serve as a labile store of N which could support growth when external N supply becomes deficient.
Non‐destructive analysis of root growth in porous media
Authors: GORDON D. C. , HETTIARATCHI D. R. P. , BENGOUGH A. G. , YOUNG I. M.
Published: April 2006

A technique is described for non‐destructive observations and analysis of root growth in granular media, using time‐lapse video in conjunction with a pressure cell. The pressure cell consists of one half of a traditional triaxial cell with a flat clear perspex front allowing growth of the root to be monitored using a video camera. The cell is connected to an external pressure supply, which is used to regulate precisely the confining pressure, and hence, the physical impedance to root growth. Preliminary results of the growth of root axes and the emergence of laterals of peas over a range of physical impedances are presented to illustrate the potential of this technique in studying root growth.
The cyanobacterium Synechococcus R‐2 (Anacystis nidulans, S. leopoliensis) PCC 7942 has a sodium‐dependent chloride transporter
Authors: RITCHIE R. J.
Published: April 2006

Synechococcus R‐2 (PCC 7942) actively accumulated Cl− in the light and dark, under control conditions (BG‐11 media: pHo, 7·5; [Na+]o, 18 mol m−3; [Cl−]o, 0·508 molm−3). In BG‐11 medium [Cl−], was 17·2±0·848 mol m−3 (light), electrochemical potential of Cl− (ΔμCl−i,o) =+211±2mV; [Cl−]i= 1·24±0·11 mol m−3(dark), ΔμCl−i,o=+133±4mV. Cl− fluxes, but not permeabilities, were much higher in the light: ϕCl−i,o= 4·01±5·4 nmol m−2 s−1, PCl−i,o= 47±5pm s−1 (light); ϕCl−i,o= 0·395±0·071 nmol m−2 s−1, PCl−i,o= 69±14 pm s−1 (dark). Chloride fluxes are inhibited by acid pHo (pHo 5; ϕCl−i,o= 0·14±0·04 nmol m−2 s−1); optimal at pHo 7·5 and not strongly inhibited by alkaline pHo (pHo 10; ϕCl−1i,o= 1·7±0·14 nmol m−2 s−1). A Cl−in/2H+in coporter could not account for the accumulation of Cl− alkaline pHo. Permeability of Cl− is very low, below 100pm s−1 under all conditions used, and appears to be maximal at pHo 7·5 (50–70 pm s−1) and minimal in acid pHo (20pm s−1). DCCD (dicyclohexyl‐carbodiimide) inhibited ϕCl−i,o in the light about 75% and [Cl−]i fell to 2·2±0·26 (4) mol m−3. Valinomycin had no effect but monensin severely inhibited Cl− uptake ([Cl−]i= 1·02±0·32 mol m−3; ϕCl−i,o= 0·20±0·1 nmol m−2 s−1). Vanadate (200 mmol m−3) accelerated the Cl− flux (ϕCl−i,o= 5·28±0·64 nmol m−2 s−1) but slightly decreased accumulation of Cl− ([Cl−], = 13·9±1·3 mol m−3) in BG‐11 medium but had no significant effect in Na+‐free media. DCMU (dichlorophenyldimethylurea) did not reduce [Cl−], or ϕCl−i,o to that found in the dark ([Cl−]i= 8·41±0·76 mol m−3; ϕCl−i,o= 2·06±0·36 nmol m−2 s−1). Synechococcus also actively accumulated Cl− in Na+‐free media, [Cl−]i was lower but ΔΨi,o hyperpolarized in Na+‐free media and so the ΔμCl−i,o was little changed ([Cl−]i= 7·98±0·698 mol m−3; ΔμCl−i,o=+203±3 mV). Net Cl− uptake was stimulated by Na+; Li+ acted as a partial analogue for Na+. Synechococcus has a Na+ activated Cl− transporter which is probably a primary 2Cl−/ATP pump. The Cl− pump is voltage sensitive. ΔμCl−i,o is directly proportional to ΔΨi,o(P»0·01%): ΔμCl−i,o= ‐1·487 (±0·102) ×ΔΨi,o, r= ‐0·983, n= 31. The ΔμCl−i,o increased (more positive) as the Δμi,o became more negative. The ΔμCl−i,o has no known function, but might provide a driving force for the uptake of micronutrients.
Xylem ABA controls the stomatal conductance of field‐grown maize subjected to soil compaction or soil drying
Authors: TARDIEU F. , ZHANG J. , KATERJI N. , BETHENOD O. , PALMER S. , DAVIES W. J.
Published: April 2006

Stomatal conductance of individual leaves was measured in a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration in the same leaves. Stomatal conductance showed a tight relationship with xylem ABA, but not with the current leaf water status or with the concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non‐compacted and irrigated soil, and to plant‐to‐plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions.
ATP‐driven proton pumping in two species of Chara differing in salt tolerance
Authors: YAO X. , BISSON M. A. , BRZEZICKI L. J.
Published: April 2006

Chara corallina is an obligate freshwater alga, while C. buckellii can be grown in salt and freshwater culture. When grown in fresh water, C. buckellii has electrophysiological properties similar to C. corallina, but when cultured in salt water, it has a less negative membrane potential and has a higher conductance. We show in internally perfused, tonoplast‐free cells that the ATP‐dependence of the two species cultured in fresh water is similar, although C. buckellii hyperpolarizes at lower ATP concentrations. We determined the pump parameters in perfused and intact cells. Using both techniques, C. corallina and C. buckellii cultured in fresh water show similar values of Ep, Gp and Ip. However, there is a significant difference between the two techniques: Ep is more negative (–400 to –700 mV) in perfused cells than in intact cells (–220 to –260mV); Gp is lower (0·1–0·2 versus 0·3–0·9 S m−2); and Ip is higher (40–60 versus 10–18 mA m−2). Salt‐cultured C. buckellii was compared with freshwater C. buckellii using intact cells; Gp and Ep were similar, but Ip was much higher in salt‐cultured cells (60 versus 15mA m−2). This higher pump rate is due to the depolarization of the membrane of salt‐cultured algae, which is caused by a higher passive conductance. The significance of the less negative membrane potential and the higher rate of proton pumping is discussed with respect to the banding pattern and salt stress.
Effects of daytime carbon dioxide concentration on dark respiration in rice
Authors: BAKER J. T. , LAUGEL F. , BOOTE K. J. , ALLEN L. H.
Published: April 2006

Rising atmospheric carbon dioxide concentration ([CO2]) has generated considerable interest in the response of agricultural crops to [CO2]. The objectives of this study were to determine the effects of a wide range of daytime [CO2] on dark respiration of rice (Oryza sativaL. cv. IR‐30). Rice plants were grown season‐long in naturally sunlit plant growth chambers in subambient (160 and 250), ambient (330), or super‐ambient (500, 660 and 900 μmol CO2mol−1air) [CO2] treatments. Canopy dark respiration, expressed on a ground area basis (Rd) increased with increasing [CO2] treatment from 160 to 500 μmol mol−1treatments and was very similar among the superambient treatments. The trends in Rdover time and in response to increasing daytime [CO2] treatment were associated with and similar to trends previously described for photosynthesis. Specific respiration rate (Rdw) decreased with time during the growing season and was higher in the subambient than the ambient and superambient [CO2] treatments. This greater Rdwin the subambient [CO2] treatments was attributed to a higher specific maintenance respiration rate and was associated with higher plant tissue nitrogen concentration.
Interactions of blue light and inorganic carbon supply in the control of light‐saturated photosynthesis in brown algae
Authors: FORSTER R. M. , DRING M. J.
Published: April 2006

Photosynthetic capacities of five species of brown algae in red light were found to be strongly limited by the inorganic carbon supply of natural sea water. Under these conditions, pH 8·2 and dissolved inorganic carbon concentration (DIG) of 2·1 mol m−3, a short pulse of blue light was found to increase the subsequent rate of photosynthesis in saturating red light. The degree of blue light stimulation varied between species, ranging from an increase of over 200% of the original rate in Colpomenia peregrins to only 10% in Dictyota dichotoma. Increasing the DIG concentration of sea water by bicarbonate addition resulted in carbon saturation of photosynthesis in all five species. Blue light stimulation was greatly reduced at these higher DIG concentrations. The response in Laminaria digitata was examined in more detail by manipulation of pH and DIG to produce solutions with different concentrations of dissolved CO2. At a CO2 concentration typical of normal sea water (12·4 mmol m−3), blue light treatment increased photosynthetic rate by approximately 50%. Blue light stimulation was increased to over 150% at CO2 concentrations below that of sea water, whereas at concentrations above that of sea water, the effect was diminished. Therefore, the effect of blue light on photosynthetic capacity appears to involve an increase in the rate of supply of carbon dioxide to the plant.
Carbonate ions appear to neither inhibit nor stimulate use of bicarbonate ions in photosynthesis by Ulva lactuca
Authors: MABERLY S. C.
Published: April 2006

Rates of photosynthesis by the marine macroalga Ulva lactuca were measured in a factorial experiment at five concentrations of HCO3− and CO32‐ between 0·20 and 1·26 mol m−3, but very low concentrations of CO2. The results demonstrated that HCO3− was available for use, but an analysis of variance showed that CO32‐ had neither an inhibiting nor a stimulating effect on rates of photosynthesis over this concentration range. Over the experiment, pH varied from 8·46 to 10·06 and this also had no significant effect on rates of photosynthesis. The lack of a stimulatory effect of high concentrations of CO32‐ on the rate of photosynthesis at low concentrations of HCO3− was taken as circumstantial evidence for direct uptake of HCO3− rather than proton extrusion and external production of CO2. In the rockpools in which U. lactuca grows, pH values up to 10·35 have been recorded, and for much of the time, CO32‐ was the major form of inorganic carbon available. The apparent lack of an ability to use CO32‐ under these conditions suggests that direct use of CO32‐ as a source of inorganic carbon for photosynthesis is unlikely to be widespread.
Intercellular communication in Chara: factors affecting transnodal electrical resistance and solute fluxes
Authors: REID R. J. , OVERALL R. L.
Published: April 2006

The permeability of plasmodesmata in the nodal complex of branch cells of Chara corallina was examined by measuring both the transnodal electrical resistance and transnodal fluxes of 36CI and 14C‐buty‐rate. Under normal circumstances, the resistance across the node was low, but increased rapidly in response to metabolic inhibition, pressure gradients across the node or excision of one of the cells. For each of these treatments, there was a substantial reduction in solute transport between the cells. Acidification of the cytoplasm by weak acids or alkalinization by amines did not affect either the electrical resistance or the flux of solutes through the node between whorl cells. The transnodal resistance was significantly higher in older cell pairs, but was unaffected by large transnodal voltage differences or by the passage of action potentials. There was no evidence that short‐term increases in cytoplasmic calcium have any effect on plasmodesmatal permeability.
Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. I. The concentration of cations, anions, amino acids in, and pH of, the xylem sap
Authors: GOLLAN T. , SCHURR U. , SCHULZE E.‐D.
Published: April 2006

Sunflower plants (Helianthus annuus L.) were subjected to soil drying with their shoots either kept fully turgid using a Passioura‐type pressure chamber or allowed to decrease in water potential. Whether the shoots were kept turgid or not, leaf conductance decreased below a certain soil water content. During the soil drying, xylem sap samples were taken from individual intact and transpiring plants. Xylem sap concentrations of nitrate and phosphate decreased with soil water content, whereas the concentrations of the other anions (SO42 and Cl−) remained unaltered. Calcium concentrations also decreased. Potassium, magnesium, manganese and sodium concentrations stayed constant during soil drying. In contrast, the pH, the buffering capacity at a pH below 5 and the cation/anion ratio increased after soil water content was lowered below a certain threshold. Amino acid concentration of the xylem sap increased with decreasing soil water content. The effect of changes in ion concentrations in the xylem sap on leaf conductance is discussed.
Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. II. Stomatal sensitivity to abscisic acid imported from the xylem sap
Authors: SCHURR U. , GOLLAN T. , SCHULZE E.‐D.
Published: April 2006

Sunflower plants [Helianthus annuus L.) were subjected to soil drought. Leaf conductance declined with soil water content even when the shoot was kept turgid throughout the drying period. The concentration of abscisic acid in the xylem sap increased with decreasing soil water content. No general relation could be established between abscisic acid concentration in the xylem sap and leaf conductance due to marked differences in the sensitivity of leaf conductance of individual plants to abscisic acid from the xylem sap. The combination of these results with data from Gollan, Schurr & Schulze (1992, see pp. 551–559, this issue) reveals close connection of the effectiveness of abscisic acid as a root to shoot signal to the nutritional status of the plant.
The relationship between vascular differentiation and the distribution of ice within Forsythia flower buds
Authors: ASHWORTH E. N. , WILLARD T. J. , MALONE S. R.
Published: April 2006

The relationship between vascular development and the distribution of ice within overwintering forsythia flower buds was examined. Previous experiments demonstrated that ice formed in dormant buds within the bud scales, sepals, and in the peduncle and lower portions of the developing flower. The current study demonstrated that these tissues contained columns of primary xylem forming a continuous network with the subtending stem tissues. The vascular traces within the developing petals, anther filaments and pistil were not fully differentiated. Xylem vessels were not present and only procambial cells were observed. Large ice crystals did not accumulate in these tissues. When vascular development resumed in the spring, coincident changes in the distribution of ice within buds were noted. Observations were consistent with the hypothesis that ice propagates into buds via the vascular system, and that the segregation of ice within bud tissues reflects the distribution of xylem.
Photoadaptation, photoinhibition and productivity in the blue‐green alga, Spirulina platensis grown outdoors
Authors: VONSHAK A. , GUY R.
Published: April 2006

Two Spirulina platensis strains, SP‐G and SP‐RB, resistant and sensitive to photoinhibition of photosynthesis, respectively, were grown outdoors in dense cultures and under different photon fluxes provided by shading. Cultures of both strains grown under full sunlight were more resistant to photoinhibition than those grown under nets with 15–50% decreases in the incident photon flux. Cultures grown outdoors were more resistant to photoinhibition than the laboratory ones. At noon, the photosynthetic activity, as expressed by O2 evolution, was higher for cultures grown under 50% shade, as compared with unshaded cultures. Productivity of the shaded cultures, in terms of biomass produced per day, was always higher when the cultures were protected from photoinhibition.
Variations in the natural abundance of oxygen‐18 and deuterium in plant carbohydrates
Authors: YAKIR D.
Published: April 2006

Variations in the natural abundance of 18O and 2H in plant cellulose are influenced by the isotopic composition of the water directly involved in metabolism—the metabolic water fraction. The isotopic distinction between the metabolic source water and total tissue water must reflect the formation of isotopic gradients within the tissue that are influenced by the rate of water turnover, by properties of the water conducting system and by environmental conditions. It seems that the 18O abundance in the metabolic water is conserved in cellulose with a relatively constant isotope effect. The relationship of the 2H abundance between metabolic water and cellulose is more complex. Hydrogen incorporated into photosynthetic products during primary reduction steps is highly depleted in 2H. However, a large proportion of these hydrogens are subsequently replaced by exchange with water, leading to 2H enrichment during heterotrophic metabolism. Deciphering the oxygen isotope ratio of cellulose could help in providing insights into the carbon and oxygen fluxes exchanged between plants and the atmosphere. This is because the 18O abundance in cellulose records the 18O abundance in the metabolic water, which in turn, controls the oxygen isotopic signatures of the CO2 and O2 released by plants into the atmosphere. The hydrogen isotope effects associated with carbohydrate metabolism provide insights into the autotrophic state of a plant tissue. This is because the hydrogen isotope ratio of carbohydrates must reflect the net effects of the two opposing isotope effects associated with photosynthesis and heterotrophic metabolism.