bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ecophysiology of phloem loading in source leaves
Authors: VAN BEL A. J. E. , GAMALEI Y. V. 
Published: April 2006

ABSTRACTThe 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

ABSTRACTAfter 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

ABSTRACTQuantum 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

ABSTRACTCold‐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

ABSTRACTSour 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

ABSTRACTThe 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

ABSTRACTAn 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

ABSTRACTAn 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

ABSTRACTVitamin 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

ABSTRACTThe 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

ABSTRACTThe 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

ABSTRACTTo 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

ABSTRACTStem 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

ABSTRACTOne 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

ABSTRACTA 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

ABSTRACTSalt 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

ABSTRACTTo 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.