bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure‐activity relationships of biologically active oligosaccharides
Authors: ALDINGTON S. , McDOUGALL G. J. , FRY S. C.
Published: April 2006

Abstract. Oligosaccharides that exert biological effects on higher plants (other than as carbon or energy sources) have been termed‘oligosaccharins'. A limited number of specific oligo‐β‐glucans derived from fungal cell walls exhibit oligosaccharin activity, at very low doses, switching on the synthesis of phytoalexins. The structural requirements for this biological activity are stringent, and there is strong evidence that the oligosaccharins of fungal origin act through a receptor in the plant cell membrane. Chemically unrelated oligosaccharins can also be produced by partial digestion of pectins and xyloglucans from higher plant cell walls. In some cases, for example, the α‐l‐fucosylated oligoxyloglucans that antagonise the growth‐promoting effect of auxin, these plant‐derived oligosaccharins have relatively strict structural requirements for activity and act at ∼10−6 mol m−3; these may act via specific receptors. In other cases, for example, the growth‐promoting action of other oligoxyloglucans, the stringency is somewhat lower and the activity is only seen at ∼10−3 mol m−3, and these oligosaccharins are proposed to influence growth through their ability to modulate the activity of an enzyme, cellulase. A third class of plant‐derived oligosaccharins, the oligo‐α‐galacturonides, appear to have much less stringent structural requirements; a number of unrelated physiological responses are evoked by the same range of oligo‐α‐galacturonides at ∼10−3 to 1 mol m−3. We suggest that these oligosaccharins may act via a mechanism not involving a specific receptor, perhaps by interacting with the plasma membrane to bring about a change in its physical properties.
Environmental productivity indices and productivity for Opuntia ficus‐indica under current and elevated atmospheric CO2 levels*
Authors: NOBEL P. S.
Published: April 2006

Abstract. The productivity of the prickly‐pear cactus Opuntia ficus‐indica, which is cultivated worldwide for its fruits and stem segments, was predicted based on the responses of its net CO2 uptake to soil water status, air temperature and photosynthetic photon flux density (PPFD). Each of these environmental factors was represented by an index with a maximum value of unity when that factor was not limiting net CO2 uptake over a 24‐h period. The water index, the temperature index, and the PPFD index were determined for 87 sites in the contiguous United States using data from 189 weather stations and for 148 sites worldwide using data from 1464 weather stations. The product of these three indices, the environmental productivity index (EPI), was used to predict the productivity of O. ficus‐indica under current climatic conditions and under those accompanying a possible increase in the atmospheric CO2 level to 650μumol mol−1. Sites with temperatures always above ‐10°C and hence suitable for prickly‐pear cultivation numbered 37 in the United States and 110 worldwide; such sites increased by 43 and 5%, respectively, for the global warming accompanying the elevated CO2. Productivity of O. ficus‐indica was at least 15 tonnes dry weight hectare−1 year−1, comparable to that of many agronomic crops, for 20 sites with temperatures always above ‐10°C in the contiguous United States and for 12 such sites worldwide under current climatic conditions; such sites increased by 85 and 117%, respectively, under the elevated CO2 condition, mainly because of direct effects of the atmospheric CO2 level on net CO2 uptake. In summary, simulations based on EPI indicate that O. ficus‐indica may presently be advantageously cultivated over a substantial fraction of the earth's surface, such regions increasing markedly with a future doubling in atmospheric CO2 levels.
A calcium influx precedes organogenesis in Graptopetalum
Authors: HUSH J. M. , OVERALL R. L. , NEWMAN I. A.
Published: April 2006

Abstract. An account is given of an investigation of net ionic currents and specific ion fluxes occuring during the initiation of organogenesis in detached leaves of Graptopetalum paraguayense E. Walther, in which a dramatic change in growth polarity is cytomorphologically evident 3–5 d after leaf detachment from the plant. Using the vibrating probe, it was possible to identify a peak of ionic current which is focused over the area of the leaf base where organogenesis is initiated. This net current is largest during the initial 12h after leaf detachment. With ion‐selective microelectrodes capable of measuring H+, K+ and Ca2+ ion fluxes simultaneously in the same region of the leaf base, H+ and K+ fluxes remain relatively steady during the initial 24 h after detachment, while a large lanthanum‐sensitive Ca2+ influx decreases by 50% from 2 to 12h. By 24h, Ca2+ transport is dominated by an efflux. We present evidence from a quantitative comparison of the ion current data collected using these two techniques, that Ca2+, H+ and K+ transport accounts for the major electrogenic ion fluxes during 2 and 12 but not 24 h after leaf detachment. The possibility is addressed that these ion currents, which precede organogenesis, and in particular the predominant Ca2+ flux, play a role in the establishment of growth polarity in higher plant tissues.
Evaluation of the pressure chamber method for the assessment of water status in chilled plants
Authors: PARDOSSI A. , VERNIERI P. , TOGNONI F.
Published: April 2006

Abstract. Pressure chamber (PC) and thermocouple pyschrometer (TP) were used to determine leaf water potential in tomato (Lycopersicon esculentum Mill.), eggplant (Solanum melongena L.) and bean (Phaseolus vulgaris L.) seedlings subjected to normal watering regime, drought or chilling temperature. The PC readings were corrected for apoplastic osmotic potential. Similar results were obtained in all species. In unchilled plants, the PC and TP measurements corresponded within a potential range of ‐0.1 to ‐2.5MPa, but discrepancies between the two methods often occurred in chilled plants, with PC water potentials 0.2 to 1.4 MPa higher (less negative) than TP values. The PC measurements appeared spuriously high in chilled plants. In droughted plants leaf dehydration occurred in both the blade and the petiole, whereas in chilled plants, water deficit was generally observed only in the blade.
Division of guard cell protoplasts of Nicotiana glauca (Graham) in liquid cultures
Authors: CUPPLES W. , LEE J. , TALLMAN G.
Published: April 2006

Abstract. Guard cells are uniquely differentiated to transduce signals into the metabolic and ion transport processes that result in turgor‐driven stomatal movements. We tested the hypothesis that these highly specialized cells are terminally differentiated. Guard cell protoplasts were isolated from abaxial epidermal tissue of leaves of Nicotiana glauca (Graham) and cultured in a medium designed for culturing mesophyll protoplasts of Nicotiana tabacum. Protoplasts were incubated at densities of 2–5 × 1011 cells m−3 in eight‐well microchamber slides under 50μmol m−2 s−1 of photons of continuous fluorescent light at 25°C. When the medium was modified by the addition of 100mol m−3 of sucrose and by buffering with 10mol m−3 of MES buffer at pH 6.1, cell division began within 96h of the time the culture was initiated. After 9d of culture, 80% of surviving cells had synthesized new cell walls, had dedifferentiated, and were dividing to form small colonies. Callus tissue was visible after 4–5 weeks. We conclude that guard cells of Nicotiana glauca are not terminally differentiated, and that guard cell protoplasts of this species have the capacity to grow, synthesize cell walls and divide.
Reduction of leaf photosynthesis and transpiration rates of potato plants by second‐stage juveniles of Globodera pallida
Authors: SCHANS J.
Published: April 2006

Abstract. Net photosynthesis and transpiration rates of potato plants, grown in pots in the greenhouse, were measured at various light irradiances and ambient CO2 concentrations, 3d after inoculation with second stage juveniles of Globodera pallida. Gas exchange rates, both in darkness and in light, and the initial light use efficiency were strongly reduced by nematodes. Stomatal conductance of infected plants was lower than that of control plants and showed little response to decreasing ambient CO2 concentration. The maximum internal CO2 concentration of infected plants was lower than that of control plants. Globodera pallida reduced photosynthesis also by apparent non‐stomatal effects.

The effects of G. pallida on gas exchange rates are similar to the effects of abscisic acid in the transpiration stream and of abiotic stresses in the root environment. Apparently, there is a general response of plant roots to adverse conditions. The reduction of photosynthesis may be an important factor in yield reduction by potato cyst nematodes.
Ecophysiology of phloem loading in source leaves
Authors: VAN BEL A. J. E. , GAMALEI Y. V.
Published: April 2006

The nature of phloem loading of photosynthesis products – either symplastic or apoplastic – has been a matter of debate over the last two decades. This controversy was reconciled by proposing a multiprogrammed loading mechanism. Different modes of phloem loading were distinguished on the basis of the variety of plasmodesmatal connectivity between the minor vein elements. Physiological evidence for at least two phloem loading mechanisms as well as recent support for coincidence between plasmodesmatal connectivity and the loading mechanism is shortly reviewed. The present paper attempts to correlate the plasmodesmatal connectivity between sieve element/companion cell complex and the adjacent cells (the minor vein configuration) – and implicitly the associate phloem loading mechanisms – with different types of climate. The minor vein configuration is a family characteristic. This enables one to relate vein configuration with ecosystem using the family distribution over the globe. The uneven distribution of vein types between terrestrial ecosystems indicates that apoplastic phloem loading predominates in cold and dry climate zones. Projection of the minor vein configuration on the Takhtajan system of flowering plants suggests evolution from apoplastic to symplastic phloem loading. Accordingly, the distribution of minor vein configurations suggests that drought and temperature stress have led to the transformation of the ancient symplastic mode into the more advanced apoplastic mode of loading.
A new concept of root exudation1
Authors: SCHWENKE H. , WAGNER E.
Published: April 2006

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch‐activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m−3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.
On the role of orientation in reducing photoinhibitory damage in photosynthetic‐twig desert shrubs
Authors: EHLERINGER J. R. , COOPER T. A.
Published: April 2006

Quantum yields for O2 evolution were measured in photosynthetic twigs and leaves of Hymenoclea salsola and Senecio douglasii, two common shrubs of the deserts of western North America. When exposed to long‐term drought, quantum yields of leaves and twigs remained constant up to the point of leaf abscision in both species. As water stress developed further, quantum yields began to decline in twigs; the extent of this quantum yield reduction was dependent on incident photon flux density. The reduction in quantum yield in twigs, which have a near‐vertical orientation, was greatly accelerated when twigs were reoriented to the near‐horizontal inclination typical of leaves. The reductions in quantum yield were not rapidly reversible and are interpreted as indicating photoinhibitory damage. The results are discussed in terms of the role that a near‐vertical orientation might serve in maintaining photosynthetic structures through a drought period.
Depolymerization of cortical microtubules is not a primary cause of chilling injury in corn (Zea mays L. cv Black Mexican Sweet) suspension culture cells
Authors: CHU B. , XIN Z. , LI P. H. , CARTER J. V.
Published: April 2006

Cold‐induced depolymerization of cortical microtubules were examined in suspension culture cells of corn (Zea mays L. cv Black Mexican Sweet) at various stages of chilling. In an attempt to determine whether microtubule depolymerization contributes to chilling injury, experiments were carried out with and without abscisic acid (ABA) pretreatment, since ABA reduces the severity of chilling injury in these cells. Microtubule depolymerization was detectable after 1 h at 4°C and became more extensive as the chilling was prolonged. There was little chilling injury after 1 d at 4°C in either ABA‐treated or non‐ABA‐treated cells. After 3 d at 4°C, there was about 26% injury for ABA‐treated and 40% injury for non‐ABA‐treated cells, as evaluated by 2,3,5‐triphenyl‐tetrazolium chloride reduction and by regrowth. After 1d at 4°C, less than 10% of cells retained full arrays of microtubules in both ABA‐treated and non‐ABA‐treated cells, the remainder having either partial arrays or no microtubules. After 3d at 4°C, about 90% of cells showed complete or almost complete depolymerization of microtubules in both ABA‐treated and non‐ABA‐treated cells. ABA did not stabilize the cortical microtubules against cold‐induced depolymerization. In about 66% of ABA‐treated cells and 57% of non‐ABA‐treated cells that had been held at 4°C for 3d, repolymerization of cortical microtubules occurred after 3h at 28°C. These results argue against the hypothesis that depolymerization of cortical microtubules is a primary cause of chilling injury.
Seasonal fine‐root biomass development of sour orange trees grown in atmospheres of ambient and elevated CO2 concentration
Authors: IDSO S. B. , KIMBALL B. A.
Published: April 2006

Sour orange trees have been grown from the seedling stage out‐of‐doors at Phoenix, Arizona, USA, in open‐top enclosures with clear plastic walls for 3.5 years. For the last 3 years of this period, half of the trees have been continuously exposed to air enriched with CO2 to 300 μmol mol−1 above the ambient concentration. At 2‐month intervals over the last 12 months, we have determined the fine‐root biomass in the top 0.4 m of the soil profile beneath the trees. Results from both treatments define a single relationship between fine‐root biomass and trunk cross‐sectional area. The data also show the CO2‐enriched trees to have approximately 2.3 times more fine‐root biomass in this soil layer than the trees grown in ambient air.
Glucose feeding results in coordinated changes of chlorophyll content, ribulose‐1,5‐bisphosphate carboxylase‐oxygenase activity and photosynthetic potential in photoautrophic suspension cultured cells of Chenopodium rubrum
Authors: SCHÄFER C. , SIMPER H. , HOFMANN B.
Published: April 2006

The effects of an excessive supply of carbohydrates on the photosynthetic apparatus of photoautotrophic suspension cultured cells of Chenopodium rubrum were analysed. Glucose feeding resulted in a massive intracellular accumulation of carbohydrates (glucose, fructose, sucrose and starch) and a delayed but pronounced reduction of the chlorophyll content. Maximum photosynthetic efficiency (determined as relative photon yield of oxygen evolution or calculated from fluorescence parameters) was only slightly affected by this treatment. The maximum values of photosynthetic oxygen evolution and of ribulose‐1,5‐bisphosphate carboxylase‐oxygenase activity also did not change significantly when expressed on a chlorophyll basis, thus indicating that membrane and stroma components were reduced to the same extent. Growth analysis demonstrated that even with an excessive supply of carbohydrates the contribution of photosynthesis to growth was still significant (about 35% of total DW production). The results support the hypothesis that accumulation of carbohydrates leads to a coordinated reduction of the photosynthetic machinery. Comparable changes possibly occur during source/sink transitions in green plant tissues.
Analysis of the electrical coupling of root cells: implications for ion transport and the existence of an osmotic pump
Authors: CORTES P. M.
Published: April 2006

An equivalent circuit was developed to model the radial electrical coupling between root cells. The results of several experiments were analysed using the circuit to determine whether the electrogenic pumps of the inner cortical cells were active. This analysis indicated that, while in some roots electrogenic pumps appear to be active in all cortical layers, they may be inactive in the inner cortical cells in some cases. The circuit was used to show that, in spite of intercellular symplasmic coupling, if the inner cortical cells have inactive electrogenic pumps, their membrane potentials can be significantly less negative than those of the epidermal cells. The radial difference in membrane potential may in part account for observations that the uptake of ions occurs primarily on the root periphery. This implication is developed in an appendix to show that an osmotic water pump may exist in roots to allow them to extract water from soil at a lower water potential. It is hypothesized that, as a result of a radial difference in membrane potential, there is an efflux of solutes from the inner cortical cells as the symplastic solution moves inward. As a result, the water potential of the root interior is elevated with respect to its exterior.
An inexpensive system for exposing plants in the field to elevated concentrations of CO2
Authors: ASHENDEN T.W. , BAXTER R. , RAFAREL C. R.
Published: April 2006

An inexpensive, potentially mobile field exposure system is described which may be easily constructed by a small workshop. It may be operated as an open‐top with a frustrum or covered with a polycarbonate ‘lid’. The system is cost‐effective for CO2 exposure work because the small size allows provision of CO2‐enriched atmospheres over prolonged periods at relatively low cost. A preliminary assessment of the chambers has been made and concentrations can be maintained at ±6% for a target atmosphere of 680 cm3 m−3 CO2 under normal operating conditions. Other chamber environmental conditions are reported.
The antioxidant effects of thylakoid Vitamin E (α‐tocopherol)
Authors: FRYER M. J.
Published: April 2006

Vitamin E (α‐tocopherol) is essential for the prevention of photo‐oxidative deterioration of biomembranes. The occurrence, relative biological activity and distribution of tocopherols in photosynthetic membranes is considered together with the possible biochemical and biophysical mechanisms by which tocopherols confer protection upon illuminated membranes. The common protective effects of Vitamin E in photo‐synthetic membranes and in medically important light‐induced diseases and conditions of the skin and eye in animal cell membranes are discussed. The importance of the Vitamin E‐Vitamin C thylakoid antioxidant system is also examined, in terms of susceptibility to photo‐oxidative damage under stress conditions including chilling, ageing and senescence, drought, atmospheric pollutants, herbicides and photosensitizing fungal toxins.
Water balance in the arborescent palm,Sabal palmetto.II. Transpiration and stem water storage
Authors: HOLBROOK N. M. , SINCLAIR T. R.
Published: April 2006

The contribution of stem water storage to the water balance of the arborescent palm,Sabal palmetto, was investigated using greenhouse studies, field measurements and a tree‐cutting experiment. Water balance studies of greenhouse trees (1.5 to 3 m tall) were conducted in which transpiration was measured by weight loss, and changes in soil and stem water content by time‐domain reflectometry. When the greenhouse plants were well‐watered (soil moisture near saturation), water was withdrawn from the stem during periods of high transpiration and then replenished during the night so that the net transpirational water loss came primarily from the soil. As water was withheld, however, an increasing percentage of daily net transpirational water loss came from water stored in the stem. However, studies on palms growing in their natural environment indicated that during periods of high transpiration leaf water status was somewhat uncoupled from stem water stores. In a tree‐cutting experiment, the maintenance of high relative water content of attached leaves was significantly correlated with stem volume/leaf area. Leaves of a 3‐m tree remained green and fully hydrated for approximately 100d after it had been cut down, whereas those of a 1‐m‐tall plant turned brown within one week. The significance of stem water storage may be in buffering stem xylem potentials during periods of high transpiration and in contributing to leaf survival during extended period of low soil water availability.
Carotenoid composition in sun and shade leaves of plants with different life forms
Authors: DEMMIG‐ADAMS B. , ADAMS W. W.
Published: April 2006

The carotenoid composition of sun leaves of nine species of annual crop plants (some with several varieties) was compared with sun and shade leaves of several other groups of plants, among those sun and shade leaves of several species of perennial shrubs and vines and deep‐shade leaves of seven rainforest species. All sun leaves contained considerably greater amounts of the components of the xanthophyll cycle violaxanthin, antheraxanthin and zeaxanthin as well as of β‐carotene than the shade leaves, as had previously been reported for a variety of other species by Thayer & Björkman (Photosynthesis Research, 1990, 23, 331–343). Therefore, high light specifically stimulated β,β‐carotenoid synthesis. The sun leaves of these crop species did not contain α‐carotene which was, however, present in large amounts in all shade leaves and in smaller amounts in sun leaves of three of the four species of perennial shrubs and vines. There was no difference in neoxanthin content on a chlorophyll basis between sun and shade leaves, and there was no consistent general difference in the lutein content between all sun and all shade leaves. The zeaxanthin (and antheraxanthin) content at peak irradiance and the xanthophyll cycle pool size were compared for sun leaves from the different groups of plants with different life forms and different metabolic activities. When growing in full sunlight the annual crop species and a perennial mesophyte had high rates of photosynthesis whereas the perennial shrubs and vines had relatively low photosynthesis rates. More zeaxanthin (and antheraxanthin) were accumulated at noon in full sunlight in those species with the lower photosynthesis rates. However, it was not such that those species also possessed the larger pools of violaxanthin + antheraxanthin + zeaxanthin. Instead, the xanthophyll cycle pools of sun leaves of the annual crop species and the perennial mesophyte were not smaller, and were even possibly larger, than those of sun leaves of the perennial shrubs and vines with low photosynthesis rates. This was so in spite of the fact that the crop species experienced much lesser degrees of excessive light at full sun than the shrubs and vines. Thus, many of the crop species converted only about 30–50% of their xanthophyll cycle pool to zeaxanthin at noon, whereas the shrubs and vines typically converted more than 80% of their pool into zeaxanthin. The crop species also had larger pools of β‐carotene than the shrubs and vines but smaller pools of lutein than the majority of the latter species.
Defining leaf area index for non‐flat leaves
Authors: CHEN J. M. , BLACK T. A.
Published: April 2006

To eliminate the confusion in the definition of leaf area index (L) for non‐flat leaves, the projection coefficients of several objects including spheres, cylinders, hemicircular cylinders, and triangular and square bars are investigated through mathematical derivation and numerical calculation for a range of ellipsoidal angular distributions. It is shown that the projection coefficient calculated based on half the total intercepting area is close to a constant of 0.5 when the inclination angle of the objects is randomly (spherically) distributed, whereas the calculated results based on the object's largest projected area are strongly dependent on the shape of the objects. Therefore, it is suggested that the leaf area index of non‐flat leaves be defined as half the total intercepting area per unit ground surface area and that the definition ofLbased on the projected leaf area be abandoned.
The influence of water stress on leaf and stem photosynthesis in Spartium junceum L.
Authors: NILSEN E. T.
Published: April 2006

Stem and leaf photosynthetic responses to environmental parameters were studied in Spartium junceum L., a legume with chlorophyllous stems. Stem net photosynthesis (Pn) was consistently lower than leaf Pn. The low stem Pn was due to lower quantum yield, lower mesophyll conductance and lower CO2‐saturated Pn than that of leaf Pn. Stomatal limitations to leaf and stem Pn were similar (25%). Water stress caused a greater reduction in leaf Pn than that of stems. Leaf Pn was also reduced in water‐stressed plants following rehydration. The reduced leaf Pn was associated with a reduced photon saturated Pn rate and a reduced CO2 saturated Pn rate. Apparent quantum yield, mesophyll conductance and stomatal limitation of leaves were unaffected by water‐stress. Stem Pn following rehydration was not influenced by the water‐stress treatment. In general, leaf Pn was more responsive to environmental parameters and more sensitive to water stress than stem Pn. These data support the hypothesis that stem Pn has greater tolerance of water stress, but is limited to low Pn by biochemical means compared to leaves.
Phytochrome‐mediated changes in the ATP content of Kalanchoë blossfeldiana seeds
Authors: DEDONDER A. , RETHY R. , FREDERICQ H. , DE GREEF J.A.
Published: April 2006

One short red (R) irradiation increases the ATP content of Kalanchoë blossfeldiana Poelln. cv Feuerblüte seeds before onset of germination. Phytochrome control is demonstrated by the full R/far‐red light (FR) reversibility of the effect in water imbibed seeds. In seeds imbibed in the presence of gibberellin A3 (GA3, one short R exposure already increases the ATP content when given 2h after start of imbibition, showing phytochrome control at the energy‐metabolic level when one R pulse cannot yet induce germination. After longer imbibition periods in the presence of GA3, one short FR irradiation also increases the ATP content of ungerminated Kalanchoë seeds. The time course of the ATP levels after a R or FR germination inducing irradiation shows an initial increase that clearly preceeds germination. A second increase starts about 15 h after irradiation and is most probably the consequence of the germination itself. The results suggest that, in Kalanchoë seeds, the increase in ATP levels, induced by irradiation(s) and preceding germination, is a phytochrome‐mediated process, supplying energy, required for germination.
A method for inducing xylem emboli in situ: experiments with a field‐grown tree
Authors: SALLEO S. , HINCKLEY T. M. , KIKUTA S. B. , LO GULLO M. A. , WEILGONY P. , YOON T.‐M. , RICHTER H.
Published: April 2006

A pressure collar, assembled around 25cm sections of 4‐year‐old willow twigs, was used to examine cavitation events under field conditions. When the air pressure inside the collar was raised to between 1–8 and 2–8MPa, ultrasound acoustic emission signals were triggered which indicated the breaking of water columns in the xylem. The hydraulic conductivity of the twig portion inside the chamber decreased markedly. As a result, water potentials and conductances in leaves at the end of the twig decreased. Similar changes were induced at comparable pressures in detached twigs. The equipment used is described in detail, and evidence is presented that the mechanism of this artificial production of emboli follows the air‐seeding principle hypothesized for natural cavitation events.
The role of calcium in salt toxicity
Authors: RENGEL Z.
Published: April 2006

Salt toxicity comprises osmotic and ionic components both of which can severely affect root and shoot growth. Uptake of Na+ across the plasma membrane is very fast resulting in physiological effects on extracellular as well as intracellular sites. Sodium reduces binding of Ca2+ to the plasma membrane, inhibits influx while increasing efflux of Ca2+, and depletes the internal stores of Ca2+ from endomembranes. These changes in the cell Ca2+ homeostasis are suggested here to be the primary responses to salt stress that are perceived by root cells. Salt would almost instantly reduce the amount of Ca2+ being transferred to the leaf cells, with Ca2+ activity dropping and Na+ activity rising in the apoplasm of leaf cells. This Ca2+ signal would be transported to leaves together with, if not preceding, the signal of limited water supply. Hormonal signals are likely to be secondary in nature and caused by the Na+‐related disturbance of the root cell Ca2+ homeostasis. Ameliorative effects of supplemental Ca2+ on salt stress are exerted through preventing Na+‐related changes in the cell Ca2+ homeostasis.
The use of 3‐amino‐1,2,4‐triazole to investigate the short‐term effects of oxygen toxicity on carbon assimilation by Pisum sativum seedlings
Authors: AMORY A. M. , FORD L. , PAMMENTER N. W. , CRESSWELL C. F.
Published: April 2006

To study the in vivo short‐term effect of hydrogen peroxide on plant metabolism, 2 mol m−3 3‐amino‐1,2,4‐triazole, a catalase inhibitor, was applied through the transpiration stream to Pisum sativum seedlings, and gas exchange characteristics, ascorbate peroxidase, glutathione reductase and catalase activities, and levels of hydrogen peroxide and formate were determined. Carbon dioxide assimilation rates were inhibited after the addition of aminotriazole: photorespiratory conditions exacerbated this inhibition. Carbon dioxide response curves showed that aminotriazole reduced both the RuBP regeneration rate and the efficiency of the carboxylation reaction of Rubisco. Catalase activity was completely inhibited 200 min after the application of this inhibitor, but no concomitant increase in H2O2 concentration was found. Under enhanced photorespiratory conditions, H2O2 concentrations increased. This suggests that under normal environmental conditions hydrogen peroxide is metabolized via alternative mechanisms. The aminotriazole treatment had no effect on the ascotbate peroxidase and glutathione reductase activities, but caused a substantial increase in the formate pool size. These results suggest that hydrogen peroxide is metabolized by reacting with glyoxylate to produce formate and CO2. The increased production of formate may reduce the flow of carbon through the normal photorespiratory pathway and may also be used anaplerotically as a precursor of products of 1‐C metabolism other than serine. This would prevent the return of photorespiratory carbon to the RPP pathway, leading to a smaller RuBP pool size which would in turn result in a decrease in carboxylation conductance (carboxylation efficiency) and regeneration rate of RuBP.
Nickel toxicity and peroxidase activity in seedlings of Triticum aestivum L.
Authors: PANDOLFINI T. , GABBRIELLI R. , COMPARINI C.
Published: April 2006

Ni2+ toxicity was evaluated in Triticum aestivum L. by its effects on root and shoot length, dry matter production and water content. Over a threshold value of 20 mmol m−3 Ni2+ the degree of toxicity increases as a function of the Ni2+ concentration in the medium. Ni2+‐treated roots show enhanced lipid peroxidation; the higher Ni2+ treatment (40mmol m−3) also increases leakage of K+. In roots and shoots, Ni2+ enhances both guaiacol and syringaldazine extracellular peroxidase activity. The increase in extracellular peroxidase activity is also associated with an increase in the phenolic contents of roots and shoots. The observed growth inhibition might be partly the result of the effect of Ni2+ on cell turgor and cell‐wall extensibility. Intracellular soluble peroxidases are also stimulated by Ni2+; such effects, independently of the substrate, were detected in extracts of Ni2+‐treated shoots at a lower Ni2+ concentration than in the roots. Intracellular peroxidases might act as scavengers of peroxide radicals produced as a result of nickel toxicity.
Calcium‐salinity interactions affect ion transport in Chara corallina
Authors: WHITTINGTON J. , SMITH F. A.
Published: April 2006

Detached internodes of Chara corallina survived in solutions containing 100 mol m−3 NaCl when the external concentration of Ca2+ was greater than 1 mol m−3. Na+ influx was roughly proportional to external Na+ up to 100 mol m−3 NaCl. Na+ influx involved two components: a Ca2+‐insensitive influx which allowed the passage of Na+ independently of external Ca2+; and a Ca2+‐inhibitable mechanism where Na+ influx was inversely proportional to external Ca2+. The Ca2+‐inhibitable Na+ influx was similar to the Ca2+‐inhibitable K+ influx. Mg2+ and Ba2+ were able to substitute for Ca2+ in partially inhibiting Na+ influx in the absence of external Ca2+. The effect of Ca2+ appears specific to Na+ and K+ influx since the effects of a Ca2+‐free solution on the influx of some other cations, anions and neutral compounds is small. It is suggested that Na+ influx via the Ca2+‐inhibitable mechanism represents Na+ leakage through K+ channels and that cell death at high salinity occurs due to a cytotoxic Na+ influx via this mechanism.
Low conductances for CO2 diffusion from stomata to the sites of carboxylation in leaves of woody species
Authors: LLOYD J. , SYVERTSEN J. P. , KRIEDEMANN P. E. , FARQUHAR G. D.
Published: April 2006

Concurrent measurements of leaf gas exchange and on‐line 13C discrimination were used to evaluate the CO2 conductance to diffusion from the stomatal cavity to the sites of carboxylation within the chloroplast (internal conductance; gi). When photon irradiance was varied it appeared that gi and/or the discrimination accompanying carboxylation also varied. Despite this problem, gi, was estimated for leaves of peach (Prunus persica), grapefruit (Citrus paradisi), lemon (C. limon) and macadamia (Macadamia integrifolia) at saturating photon irradiance. Estimates for leaves of C. paradisi, C. limon and M. integrifolia were considerably lower than those previously reported for well‐nourished herbaceous plants and ranged from 1.1 to2.2μmol CO2 m−2 s−1 Pa−1, whilst P. persica had a mean value of 3.5 μmol CO2 m−2 s−1 Pa−1. At an ambient CO2 partial pressure of 33Pa, estimates of chloroplastic partial pressure of CO2 (Cc) using measurements of CO2 assimilation rate (A) and calculated values of gi, and of partial pressure of CO2 in the stomatal cavity (Cst) were as low as 11.2 Pa for C. limon and as high as 17.8Pa for peach. In vivo maximum rubisco activities (Vmax) were also determined from estimates of Cc. This calculation showed that for a given leaf nitrogen concentration (area basis) C. paradisi and C. limon leaves had a lower Vmax than P. persica, with C. paradisi and C. limon estimated to have only 10% of leaf nitrogen present as rubisco. Therefore, low CO2 assimilation rates despite high leaf nitrogen concentrations in leaves of the evergreen species examined were explained not only by a low Cc but also by a relatively low proportion of leaf nitrogen being used for photosynthesis. We also show that simple one‐dimensional equations describing the relationship between leaf internal conductance from stomatal cavities to the sites of carboxylation and carbon isotope discrimination (Δ) can lead to errors in the estimate of gi. Potential effects of heterogeneity in stomatal aperture on carbon isotope discrimination may be particularly important and may lead to a dependence of gi upon CO2 assimilation rate. It is shown that for any concurrent measurement of A and Δ, the estimate of Cc is an overestimate of the correct photosynthetic capacity‐weighted value, but this error is probably less than 1.0 Pa.
The consequences of delayed greening during leaf development for light absorption and light use efficiency
Authors: KURSAR T. A. , COLEY P. D.
Published: April 2006

Many species of rainforest plants have an unusual form of leaf development such that leaves delay greening until after full leaf expansion. Chlorophyll accumulation was measured during leaf development in five woody rainforest species, three with white young leaves, and two with ‘normal’ greening. In the three species with white leaves, the chlorophyll content of the expanding leaves was about 0.4mg dm−2, whereas in the two species with green young leaves, chlorophyll content was about 2.1 mg dm−2. Chlorophyll accumulation in greenhouse and field experiments was independent of light level. During leaf expansion, species with delayed chloroplast development only absorb 18–25% of the maximum possible light, compared with 80% for species with normal greening. Furthermore, species with delayed greening have low chlorophyll contents and reduced absorption for at least 30 d after full expansion. At a PPFD typical of the forest under story, the photosynthetic light use efficiency based upon incident radiation was 0.030–0.036 for species with delayed chloroplast development and 0.068–0.085 for the two species with normal greening. The lower light use efficiency of white species was primarily due to decreased light absorption. However, they also had a slightly lower light use efficiency based upon absorbed radiation, suggesting that development of other components of the photo‐synthetic apparatus also may be delayed. Despite the fact that delayed greening decreases light absorption and light use efficiency during leaf development, it is extremely common in shade‐tolerant species. We suggest that an advantage of delayed greening is that resources are not invested in the leaf until it is fully expanded and better defended from herbivores.
Disturbance of cell Ca2+ homeostasis as a primary trigger of Al toxicity syndrome
Authors: RENGEL Z.
Published: April 2006

The primary toxic effects of Al are fast (taking only seconds to several minutes to develop). These effects may be due to the presence of Al ions on the apoplasmic side of the plasma membrane; Al ions may not need to enter the cytosol for a range of primary deleterious Al effects to occur. Aluminium may disturb the symplasmic Ca2+ homeostasis by altering the pattern of Ca2+ fluxes across the plasma membrane. Disturbance of transmembrane Ca2+ fluxes may prevent an increase in cytosolic Ca2+ activity necessary to trigger spindle formation during prophase as well as to initiate the metaphase/anaphase transition. The mechanism behind Al‐related inhibition of mitosis and polar cell growth is suggested to be disturbance of stimulus/response coupling effected through Ca2+ as a second messenger. Involvement of calmodulin in the Al‐related phenomena could be indirect, at least in the initial stages of the Al treatment when Al is probably confined to the apoplasm only. Al‐related disturbance of the Ca2+ homeostasis in the cytosol would alter the patterns of Ca2+ ‐calmodulin binding, resulting in the absence of activation or only partial activation of calmodulin.
Growth of Sesbania rostrata (Brem) with stem nodules under controlled conditions
Authors: KWON D.‐K. , BEEVERS H.
Published: April 2006

A series of experiments was carried out in an attempt to produce nodulated plants of Sesbania rostrata with qualities more closely resembling those in the wild than has been achieved to date. When groups of five plants were grown in a controlled climate chamber in pipes containing ∼12dm3 modified Jensen's medium with 6mol m−3 nitrate, the daily growth in height reached 5 cm and at 30 d the plants were ∼40cm high. At this time, the stems were inoculated with Azorhizobium caulinodans ORS 571 and the medium replaced with Jensen's medium without nitrate. In the subsequent 19‐d period ∼300 nodules (representing >50% of the potential infection sites) developed on each stem. The nodules increased linearly in size over this time to ∼15mg. Specific acetylene reduction activity, ARA ((μmol C2H4 mg−1 h−1) rose to 45 between days 5 and 10 after inoculation and plateaued; total ARA rose to ∼200 μmol C2H4 plant−1 h−1. Under the conditions described the plants grew vigorously, and reproducibly uniform yields of nodules with high ARA activities were obtained. As outlined, the procedure offers a standard system in which, within a 2‐week period after inoculation, individual strains of bacteria can be quantitatively compared in their ability to induce nodulation and N2‐fixation. Physiological and biochemical aspects of the nodulated system can be much more readily approached than with plants producing only root nodules. The inhibitory effects of stem nodules induced by wild type and two mutant strains of Azorhizobium on the development and activity of root nodules are described.
Magnesium deficiency enhances resistance to paraquat toxicity in bean leaves
Authors: CAKMAK I. , MARSCHNER H.
Published: April 2006

Effects of deficient (20mmol m−3) and sufficient (1000 mmol m−3) magnesium (Mg) supply and of varied light intensity (100 μmol m−2 s−1 to 580 μmol m−2 s−1) on paraquat‐dependent chlorophyll destruction in bean (Phaseolus vulgaris) plants grown in nutrient solution were studied over a 12‐d period using leaf discs or intact primary leaves. Treatment of leaf discs with 10mmol m 3 paraquat for 15h caused severe chlorophyll loss, especially with increasing light intensity. This chlorophyll destruction by paraquat was very much higher in Mg‐sufficient than Mg‐deficient leaves. The occurrence of paraquat resistance in Mg deficient leaves was already apparent after 6d growth in nutrient solution, i.e. before any decrease in chlorophyll or growth by Mg deficiency was evident. Also, following foliar application of paraquat (10–140 mmol m−3) to intact plants, Mg‐deficient plants were much more resistant to paraquat, even following longer exposure duration (72 h) and four to 14 times higher paraquat concentrations than those received by Mg sufficient plants. From experiments where exogenous scavengers of superoxide radical (O2.‐), hydroxyl radical (OH·) and singlet oxygen (1O2) were applied to leaf discs, it appears that O2.‐, and partly, OH· are the main O2 species which contribute to chlorophyll destruction by paraquat. The results demonstrate that Mg‐deficient bean plants become highly resistant to O2.‐‐mediated and light‐induced paraquat injury. The mode of this paraquat resistance is attributed to well‐known stimulative effects of Mg deficiency on O2.‐ and H2O2 scavenging enzymes and antioxidants.
Self‐assembly of plant cell walls
Authors: JARVIS M. C.
Published: April 2006

The object of this paper is to define criteria for distinguishing between self‐assembly and template‐based assembly in plant cell walls. The example of cellulose shows that cell wall polymers biosynthesized at a membrane may retain parallel chain packing arrangements that are thermodynamically unstable and cannot be reproduced in vitro, making the experimental testing of the self‐assembly hypothesis difficult. Also, natural cellulose is ordered on a number of scales of pattern, each of which may be constructed by either self‐ or template‐based assembly independently of the rest. These conceptual problems apply equally to the self‐assembly of complete cell walls and other cell wall polymers. It is suggested that the self‐assembly concept should be applied only to one stage or level in the synthesis of a cell wall, and that an additional concept of parallel assembly may be useful for understanding the synthesis of some polysaccharides.
Zinc and salinity effects on membrane transport in Chara connivens
Authors: RYGOL J. , ARNOLD W. M. , ZIMMERMANN U.
Published: April 2006

Pressure‐probe measurements showed that the pressure relaxation of internodal cells of the freshwater alga Chara connivens slowed considerably when 1–5 mol m−3 Zn2+, or more especially Zn2+ and 75 mol m−3 NaCl, were present in the medium for periods of 1 h or longer. These results indicate that the water permeability of the Chara membrane is decreased by Zn2+, and that this effect is enhanced by 75 mol m−3 NaCl. Specific values taken after 375 min exposure were: 5 mol m−3 Zn2+ and 75 mol m−3 NaCl caused the half‐time for bulk water movement to increase from 7·8±2·3 to 79·5±5·4s, corresponding to a decrease in the hydraulic conductivity (Lp) from (13·0±3·3) × 10−7 m s−1 mPa−1 to (1·25±0·23) × 10−7 m s−1 MPa−1 (mean±S.D., n= 10). These changes are not seen in the presence of NaCl alone, and to a reduced extent in the presence of 5 mol m−3Zn2+ alone (after 375 min, Lp was (2·4±0·1) × 10−7 m s−1 MPa−1, mean±S.D., n = 6). Ca2+ cannot substitute for Zn2+, but seems to competitively inhibit Zn2+. There was another, kinetically distinct effect of Zn2+: the ingress of Na+ within 15 min of exposure to 75 mol m−3 NaCl is halved by the presence of 1–5 mol m−3 Zn2+, although internal osmolality is little changed by Zn2+. In spite of this, Zn2+ does not exert the long‐term protection against NaCl that has been reported for Ca2+. Depending on the concentration of Zn2+ and the duration of the exposure, the effects on water permeability were fully or partly reversible within 24–48 h. The mechanism of these changes is difficult to identify. One possibility is a zinc‐induced restriction of trans‐membrane channels to give single‐file channels which can be blocked by salt.