bluuebunny/crossref_metadata_2025 huggingface dataset:
Changes in chromatin structure associated with germination of maize and their relation with desiccation tolerance
Authors: LEPRINCE O. , COLSON P. , HOUSSIER C. , DELTOUR R.
Published: April 2006

Morphological and physicochemical measurements of chromatin condensation were made on germinating maize (Zea mays L.) radicles to determine whether the loss of genetic activities that occurs during the loss of desiccation tolerance is linked to irreversible changes in chromatin condensation. Chromatin samples were compared at different stages of germination (0, 24 and 72 h after imbibition), before (control) and after 24 h of desiccation. Morphological changes in chromatin structure and condensation were characterized by a qualitative and quantitative electron microscope study of chromatin which was allowed to spread in 0.2 mol m−3 EDTA and then laid on coated microscope grids. The experiments showed similar levels of chromatin condensation in quiescent embryos and 24‐h‐old radicles (desiccation‐tolerant material). After 72 h of imbibition, when radicle emergence and desiccation intolerance had ceased, the chromatin underwent a major decondensation towards various lower order folded structures. Regardless of the desiccation tolerance stage, an in vivo drying treatment of 24‐ and 72‐h‐old radicles before chromatin extraction did not induce significant changes in the extent of condensation compared to their respective controls. Similar conclusions were drawn from measurements of several spectroscopy properties (absorbance ratios: A260/A240, A260/A400; thermal denaturation, and linear electric dichroism) of chromatin fragments that were obtained after nuclease digestion and then dissolved in 0‐2 mol m−3 EDTA. In quiescent and 24‐h‐old material, chromatin fragments were poorly soluble but highly stable during thermal denaturation. Chromatin fragments were 3‐5‐fold more soluble and less thermally stable in 72‐h‐old material than in 24‐h‐old material. In vivo desiccation had no significant effects on these properties compared to the respective controls. Collectively these data suggest that desiccation did not induce irreversible changes in the condensation properties of chromatin. The likelihood that the decondensation process occurring during germination is linked to the loss of desiccation tolerance is discussed.
Quantification of ozone influx and apoplastic ascorbate content in needles of Norway spruce trees (Picea abies L., Karst) at high altitude
Authors: POLLE A. , WIESER G. , HAVRANEK W. M.
Published: April 2006

The objective of the present study was to investigate whether peak concentrations of ozone can deplete the apoplastic ascorbate pool of needles from Norway spruce trees (Picea abies L. Karst.) and, thereby, contribute to damage to forest trees. Twigs of forest trees grown at high altitude (1950m above sea level; Mt Patscherkofel, Austria) were enclosed in situ in chambers and fumigated for 5‐5 or 17 h with ozone concentrations ranging from 60 to 798 nmol mol−1. Adjacent branches were fumigated with filtered air. Ozone influx into the foliage ranging from 1‐7 to 17nmolm−2s−1 had little effect on whole‐needle ascorbate or glutathione contents. However, apoplastic ascorbate decreased by about 30% when the needles were exposed to environmentally relevant ozone concentrations and increased about 3‐fold at higher ozone concentrations. This response suggests the induction of ascorbate as a protective system and may also be important under field conditions. Needles of spruce trees from high altitude that were exposed to chronically increased ozone concentrations contained significantly higher apoplastic ascorbate concentrations than needles from spruce trees from lower altitudes with lower mean atmospheric ozone concentrations. The results show that peak concentrations of ozone do not act in spruce via a depletion of the apoplastic ascorbate pool.
Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica
Authors: MAGNANI F. , BORGHETTI M.
Published: April 2006

The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1‐year‐old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze‐thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.
Lack of C4 photosynthetic metabolism in ears of C3 cereals
Authors: BORT J. , BROWN R. H. , ARAUS J. L.
Published: April 2006

There is continuing controversy over whether a degree of C4 photosynthetic metabolism exists in ears of C3 cereals. In this context, CO2 exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf.) and two six‐rowed barley (Hordeum vulgare L.) cultivars. Three weeks after anthesis, the CO2 compensation concentration at 210 mmol mol−1 O2 in durum wheat and barley ear parts was similar to or greater than that in flag leaves. The O2 dependence of the CO2 compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C3 photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio‐labelled with 14CO2 during a 10s pulse, and the initial products of fixation were studied in various parts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade. All tissues assimilated CO2 mainly by the Calvin (C3) cycle, with little fixation of 14CO2 into the C4 acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C3 cereals C4 photosynthetic metabolism is nil.
The different effects of PEG 6000 and NaCI on leaf development are associated with differential inhibition of root water transport
Authors: CHAZEN O. , HARTUNG W. , NEUMANN P. M.
Published: April 2006

The inhibitory effects of PEG on whole‐plant growth can exceed the effects of other osmolytes such as NaCI, and this has been ascribed to toxic contaminants, or to reduced oxygen availability in PEG solutions. We investigated another possibility, namely that PEG has an additional inhibitory effect on root water transport which in turn affects leaf development. The effects on first‐leaf growth of applications of PEG 6000 or isoosmotic NaCI to the roots were determined using hydroponically grown maize (Zea mays L.) seedlings. Leaf growth rates were inhibited within minutes of PEG application to the roots and remained inhibited for days. The inhibitory effects on growth of NaCI, and also of KCl and mannitol, were much smaller. The comparative effects of NaCI and PEG on root water transport were determined by assaying pressurized flow through excised roots. PEG induced a 7‐fold greater inhibition of flow through live roots than NaCI. Killing of the roots by heat treatment, to reduce cell membrane resistances to solute penetration, nearly doubled the flow rate for roots in NaCI, but not for roots in PEG. We suggest that the greater viscosity of PEG solutions, as compared with NaCI, may be a primary factor contributing to the additional inhibition of water flow through live and killed roots. PEG did not have additional effects on leaf turgor but had a 3 times greater inhibitory effect than NaCI on the irreversible extensibility of the leaves and induced 16 times more leaf accumulation of the growth inhibitory stress hormone abscisic acid (ABA). We conclude that greater inhibition of root water transport by PEG 6000, as compared with NaCI, leads to additional reductions in extensibility, additional ABA accumulation, and a greater inhibition of leaf growth.
Foliar maintenance respiration of subalpine and boreal trees and shrubs in relation to nitrogen content
Authors: RYAN M. G.
Published: April 2006

A nitrogen‐based model of maintenance respiration (Rm) would link Rm with nitrogen‐based photosynthesis models and enable simpler estimation of dark respiration flux from forest canopies. To test whether an N‐based model of Rm would apply generally to foliage of boreal and subalpine woody plants, I measured Rm (CO2 efflux at night from fully expanded foliage) for foliage of seven species of trees and shrubs in the northern boreal forest (near Thompson, Manitoba, Canada) and seven species in the subalpine montane forest (near Fraser, Colorado, USA). At 10°C, average Rm for boreal foliage ranged from 0.94 to 6.8μmol kg−1 s−1 (0.18–0.58 μmol m−2 s−1) and for subalpine foliage it ranged from 0.99 to 7.6 μmol kg−1 s−1 (0.28–0.64μmol m−2 s−1). CO2 efflux at 10°C for the samples was only weakly correlated with sample weight (r = 0.11) and leaf area (r = 0.58). However, CO2 efflux per unit foliage weight was highly correlated with foliage N concentration [r = 0.83, CO2 flux at 10°C (mol kg−1 s−1) = 2.62 × foliage N (mol kg−1)J, and slopes were statistically similar for the boreal and subalpine sites (P=0.28). CO2 efflux per unit of foliar N was 1.8 times that reported for a variety of crop and wildland species growing in warmer climates.
Evaluation of the role of damage to photosystem II in the inhibition of CO2 assimilation in pea leaves on exposure to UV‐B radiation
Authors: NOGUÉS S. , BAKER N. R.
Published: April 2006

Mature pea (Pisum sativum L., cv. Meteor) leaves were exposed to two levels of UV‐B radiation, with and without supplementary UV‐C radiation, during 15 h photoperiods. Simultaneous measurements of CO2 assimilation and modulated chlorophyll fluorescence parameters demonstrated that irradiation with UV‐B resulted in decreases in CO2 assimilation that are not accompanied by decreases in the maximum quantum efficiency of photosystem II (PSII) primary photochemistry. Increased exposure to UV‐B resulted in a further loss of CO2 assimilation and decreases in the maximum quantum efficiency of PSII primary photochemistry, which were accompanied by a loss of the capacity of thylakoids isolated from the leaves to bind atrazine, thus demonstrating that photodamage to PSII reaction centres had occurred. Addition of UV‐C to the UV‐B treatments increased markedly the rate of inhibition of photosynthesis, but the relationships between CO2 assimilation and PSII characteristics remained the same, indicating that UV‐B and UV‐C inhibit leaf photosynthesis by a similar mechanism. It is concluded that PSII is not the primary target site involved in the onset of the inhibition of photosynthesis in pea leaves induced by irradiation with UV‐B.
Diurnal water relations of expanding and full‐sized cotton fruits and subtending leaves
Authors: VAN IERSEL M. W. , OOSTERHUIS D. M.
Published: April 2006

Water relations of cotton (Gossypium hirsutum L.) fruits have received less attention than those of leaves, although crop water status has an important influence on fruit physiology. This study was conducted to describe diurnal changes in the water relations of cotton fruits and subtending leaves. Young, expanding fruits and full‐sized fruits were compared because of previously reported changes in xylem maturity with ontogeny. Diurnal changes in relative water content were greater in leaves than in the capsule walls of fruits. The capsule walls of young fruits had a higher relative water content than subtending leaves, and water content was lower in full‐sized (87%) than in expanding (92%) fruits. Water potentials of subtending leaves were always approximately 0‐3 MPa lower than those of capsule walls. Water potential gradients favoured passive water flow from young fruits to branches, but water potentials of branches and the capsule walls of full‐sized fruits were similar (−0.7 MPa). Water potential gradients were consistent throughout the day. These results indicate that xylem transport to young fruits is unlikely, but may occur in older fruits.
Absence of phyB inhibits hypocotyl elongation in dark‐grown Ih cucumber seedlings: an active role for PrB
Authors: SAEFKOW R. L. , ALLISTON T. N. , SHINKLE J. R.
Published: April 2006

Cucumber (Cucumis sativus L.) seedlings carrying the long hypocotyl (Ih) mutation, which confers a lack of B‐type phytochrome (phyB), were significantly shorter than their near‐isogenic wild‐type counterparts when grown in complete darkness. Relative growth rates determined for 5 mm hypocotyl regions were lower in Ih seedlings in all growing regions, and the zone of elongation was less extensive in Ih hypocotyls. Digital imaging microscopy revealed that the pattern of epidermal cell lengths along the stem axis differed between the Ih mutant and the iso‐genic wild‐type. These findings (and the fact that experiments were conducted under conditions where phytochrome photoconversion to the far‐red‐absorbing form does not occur) suggest that the red‐absorbing form of phyB (PrB) is an active positive regulator of development in etiolated plants.
Asymmetric responses of adaxial and abaxial stomata to elevated CO2: impacts on the control of gas exchange by leaves
Authors: PEARSON M. , DAVIES W.J. , MANSFIELD T.A.
Published: April 2006

The response of adaxial and abaxial stomatal conductance in Rumex obtusifolius to growth at elevated atmospheric concentrations of CO2 (250 μmol mol−1 above ambient) was investigated over two growing seasons. The conductance of both the adaxial and abaxial leaf surfaces was found to be reduced by elevated concentrations of CO2. Elevated CO2 caused a much greater reduction in conductance for the adaxial surface than for the abaxial surface. The absence of effects upon stomatal density indicated that the reductions were probably the result of changes in stomatal aperture. Partitioning of gas exchange between the leaf surfaces revealed that increased concentrations of CO2 caused increased rates of photosynthesis only via the abaxial surface. Additionally, leaf thickness was found to increase during growth at elevated concentrations of CO2. The tendency for these amphistomatous leaves to develop a distribution of conductance approaching that of hypostomatous leaves clearly reduced their maximum photosynthetic potential. This conclusion was supported by measurements of stomatal limitation, which showed greater values for the adaxial surfaces, and greater values at elevated CO2. This reduction in photosynthesis may in part be caused by higher diffusive limitations imposed because of increased leaf thickness. In an uncoupled canopy, asymmetrical stomatal responses of the kind identified here may appreciably reduce transpiration. Species which show symmetrical responses are less likely to show reduced transpirational rates, and a redistribution of water loss between species may occur. The implications of asymmetrical stomatal responses for photosynthesis and canopy transpiration are discussed.
cDNA cloning and characterization of three genes in the Bet v 1 gene family that encode pathogenesis‐related proteins*
Authors: SWOBODA I. , SCHEINER O. , HEBERLE‐BORS E. , VICENTE O.
Published: April 2006

Bet v 1 isoforms, the major pollen allergens of white birch (Betula verrucosa), show sequence homology to a class of pathogenesis‐related (PR) proteins identified in several other plant species. We recently reported that Bet v 1‐related mRNAs and proteins are induced by microbial pathogens in birch cell suspension cultures, which suggests that these pollen allergens and PR proteins are encoded by the same gene family. However, apart from the genes expressed in pollen, no other Bet v 1 clones have been characterized until now. Here we describe the isolation, by screening with a pollen Bet v 1 cDNA probe, of three full‐length cDNA clones, Bet v 1‐Sc1, Sc2 and Sc3, from a cDNA library prepared from cultured birch cells grown in the presence of Pseudomonas syringae. Sequence analysis indicated that these clones are highly homologous, but different from the genes encoding the pollen isoallergens. Northern blots and RNase protection experiments showed that the three genes are transcriptionally activated, with similar kinetics, in birch cells co‐cultivated with bacteria or fungi, regardless of whether they were pathogenic for birch. Infection of leaves with the fungus Taphrina betulina, a natural pathogen of birch, also led to the coordinated activation of the Bet v 1‐Sc genes, although infection with Fusarium solani did not. These results demonstrate that the Bet v 1 gene family of pollen allergens includes a subset of genes, different from those constitu‐tively expressed in pollen, that are induced in somatic cells upon their interaction with microorganisms. The possible implications of these results for the function of Bet v 1 proteins in the defence reaction against pathogens, as well as for the incidence of Type I allergies, are discussed.
Root architecture in cultivated and wild lettuce (Lactuca spp.)
Authors: JACKSON L. E.
Published: April 2006

Root architecture is described for intact root systems of cultivated (Lactuca sativa L.) and wild (L. serriola L.) lettuce, grown for 5 to 6 weeks in greenhouse pot and cylinder experiments in coarse‐textured soil. L. sativa cv. Salinas and a sclinas line of L. serriola attained the same biomass at 4 to 6 weeks after planting. Root biomass allocation was also similar, but root architecture differed. In the top zone along the tap root (0 to 5 cm), Salinas tended to produce more laterals, a greater total root length, and more external links (segments that originate at a branch point and end in a meristem) than wild lettuce. In the 5 to 55cm zone of the tap root, these measures were greater in the wild species. These patterns of root structure were generally corroborated by a second cylinder study with a different pair of L. sativa and L. serriola. Regressions of root structural traits were made against total root dry weight as a means to compare root architecture independently of the size of the root system. Regressions demonstrated that production of root segments differed between the two species; for example the slope for the regression of summed external link length in the top 0 to 5 cm with total root dry weight was significantly higher for Salinas, indicating that the rate of construction in the top 0 to 5cm was greater for cultivated than for wild lettuce. Yet, from 5 cm depth to the tap root tip, the rate of construction of external link length was greater in L. serriola. For many of these types of regression, r2 and mean slope ± SE suggested that more variation occurred in cultivated than wild lettuce, yet genetic heterogeneity was probably low within the studied taxa. Inadvertent selection may have occurred in the breeding of cultivated lettuce varieties for increased root growth in the surface zone where water and fertilizer are applied, and for greater plasticity in construction of root segments, which might maximize the efficiency of exploitation of soil moisture and nutrients.
Xylem pressure response in maize roots subjected to osmotic stress: determination of radial reflection coefficients by use of the xylem pressure probe
Authors: ZHU J. J. , ZIMMERMANN U. , THÜRMER F. , HAASE A.
Published: April 2006

The absolute pressure in conducting xylem vessels of roots of 2‐week‐old, slowly transpiring intact maize plants (bathed in nutrition medium) was determined to be +0·024 ± 0·044 MPa using the xylem pressure probe. When the roots were subjected to osmotic stress (NaCI, KCI or sucrose), the xylem pressure decreased immediately and became more negative. However, the response of xylem pressure to osmotic stress was considerably attenuated, indicating that the radial reflection coefficients, σ13 of the maize root for these solutes were rather low (between 0·2 and 0·4 depending on the concentration of the osmoticum). The low values of a, may be caused (partly) by unstirred layer effects. In repeated osmoticum/nutrition regimes a complex pattern of changes in xylem pressure was observed which was apparently linked to the interplay between transpiration and (passive and/or active) solute loading of the xylem. These processes were not observed when the roots were subjected to osmotic stress after excision. In this case, a biphasic response was observed comparable to that found for excised roots using the root pressure probe.
The active uptake of carbon dioxide by the marine diatoms Phaeodactylum ticornutum and Cyclotella sp.
Authors: ROTATORE C. , COLMAN B. , KUZMA M.
Published: April 2006

Mass spectromelry has been used to investigate the uptake of CO2 by two marine diatoms, Phaeodactylum tricornutum and Cyclotella sp. The time course of CO2 formation in the dark after addition of 100 mmol m−3 dissolved inorganic carbon (DIC) to cell suspensions showed that external carbonic anhydrase (CA) was not present in cells of P. tricornutum but was present in Cyclotella sp. In the absence of external CA, or when it was inhibited by 5 mmol m−3 acetazolamide, cells of both species preincubated with 100 mmol m−3 DIG rapidly depleted almost all of the free CO2 (3·2mmol m−31 at pH7·5) from the suspending medium within seconds of illumination and prior to the onset of steady‐state photosynthesis. Addition of bovine CA quickly restored the HCO3−–CO2 equilibrium in the medium, indicating that the initial depletion of CO2 resulted from the selective uptake of CO2 rather than uptake of all DIG species. Transfer of cells to the dark caused a rapid increase in the CO2 concentration in the medium, largely as a result of the efflux of unfixed inorganic carbon from the cells. The measured CO2 uptake rates for both species accounted for 50% of the total DIG uptake at HCO3−–CO2 equilibrium, indicating that HCOHCO3− was also being taken up. These results indicate that both Phaeodactylum tricornutum and Cyclotella sp. have the capacity to transport CO2 actively against concentration and pH gradients.
Respiration and growth of tomato fruit
Authors: GRANGE R.I. , ANDREWS J.
Published: April 2006

The respiration rate and diameter expansion growth of young tomato fruit were measured simultaneously and related to changes in carbon import and plant water status. Respiration rate was directly proportional to the volume expansion rate of fruit growing on isolated plant tops at a positive water potential, whether the growth rate was changed by changing the fruit temperature or by manipulating the source:sink ratio of the plants. From the latter relationship, the maintenance respiration rate was estimated by extrapolation to zero growth and was found to be about 25% of the respiration rate of the average fruit at 21°C. Alternatively, when carbon import was prevented by heat‐ringing the fruit peduncle, the respiration rate of the fruit declined to about 40% of the control rate and remained steady, while the expansion rate then declined steadily to >10% of the control rate. These results show that fruit expansion was not contributing significantly to fruit respiration. Indeed, large fluctuations in fruit expansion rate could also be induced by repeated darkening and illumination of potted plants without a corresponding change in fruit respiration. Most significantly, fruit expansion was considerably reduced when plants were allowed to wilt, hut there was no change in fruit respiration rate unless the fruit peduncle was subsequently heat‐ringed. We conclude that a major part of the respiration of young tomato fruit was determined by the rate of carbon import, or associated processes, and that fruit expansion per se can occur with relatively low respiratory costs.
Developmental regulation of anoxic stress tolerance in maize
Authors: VANTOAI T. T. , SAGLIO P. , RICARD B. , PRADET A.
Published: April 2006

Anoxia associated with flooding stress is detrimental to plant growth and productivity. When maize seedlings 2 to 7 d old were exposed to anoxic stress, 3‐d‐old seedlings were found to have much lower tolerance than 2‐d‐old seedlings. Ninety per cent of 2‐d‐old seedlings survived 72 h of anoxic stress compared with 0% of the 3‐d‐old seedlings. Since 2‐d‐old isolated root tips survived anoxic stress better than 3‐d‐old tips, the anoxic tolerance of 2‐d‐old seedlings was independent of the translocation of nutrient reserves from the endosperm to the root. The addition of glucose to the medium improved the anoxia tolerance of 2‐d‐old seedlings by 25% but had no effect on 3‐d‐old seedlings. Acclimation by pre‐cxposure to 4% oxygen and pre‐treatment with 100mmol m−1 abscisic acid (ABA) improved the anoxia tolerance of 3‐d‐old seedlings by 2‐ and 4‐fold, respectively. However, acclimation and ABA treatment had no effect on 2‐d‐old seedlings. The results indicate that anoxia tolerance in maize is develop‐mentally regulated. The mechanism of anoxia tolerance innate to 2‐d‐old seedlings was inducible in 3‐d‐old seedlings by acclimation or treatment with ABA.
Transmembrane potential‐mediated coupling between H+ pump operation and K+ fluxes in Elodea densa leaves hyperpolarized by fusicoccin, light or acid load
Authors: BELLANDO M. , MARRÈ M.T. , SACCO S. , TALARICO A. , VENEGONI A. , MARRÈ E.
Published: April 2006

In isolated Elodea densa leaves, the relationships between H+ extrusion (‐ΔH+), K+ fluxes and membrane potential (Em) were investigated for two different conditions of activation of the ATP‐dependent H+ pump. The ‘basal condition’ (darkness, no pump activator present) was characterized by low values of‐ΔH+ and K+ uptake (ΔK+), wide variability of the −ΔH+/ΔK+ ratio, relatively low membrane polarization and Em values more positive than EK for external K+ concentrations (|K+]o of up to 2mol m−3. A net K+ uptake was seen already at [K+]o below 1 mol m−3, suggesting that K+ influx in this condition was a thermodynamically uphill process involving an active mechanism. When the H+ pump was stimulated by fusicoccin (FC), by cytosol acidification, or by light (the ‘high polarization condition’), K+ influx largely dominated K+ and C− efflux, and the −ΔH+/ΔK+ ratio approached unity. In the range 50 mmol m−3−5 mol m−3 [K+]0, Em was consistently more negative than EK. The curve of K+ influx at [K+]0 ranging from 50 to 5000mmol m−3 fitted a monophasic, hyperbolic curve, with an apparent half saturation value = 0–2 mol m−3. Increasing |K+]0 progressively depolarized Em, counteracting the strong hyperpolarizing effect of FC. The effects of K+ in depolarizing Em were well correlated with the effects on both K+ influx and −ΔH+, suggesting a cause‐effect chain: K+0 influx → depolarization → activation of H+ extrusion. Cs+ competitively inhibited K+ influx much more strongly in the ‘high polarization’ than in the ‘basal’ condition (50% inhibition at [Cs+]/[K+]0 ratios of 1:14 and 1:2, respectively) thus confirming the involvement of different K+ uptake systems in the two conditions. These results suggest that in E. densa leaves two distinct modes of interactions rule the relationships between H+ pump, membrane polarization and K+ transport. At low membrane polarization, corresponding to a low state of activation of the PM H+‐ATPase and to Em values more positive than EK, K+ influx would mainly
Site‐specific isotope fractionation of hydrogen in the biosynthesis of plant fatty acids
Authors: QUEMERAIS B. , MABON F. , NAULET N. , MARTIN G. J.
Published: April 2006

The overall deuterium content of plant lipids has been investigated by isotope ratio mass spectrometry (IRMS), and the site‐specific natural isotope fractionation of hydrogen has been studied by 2H‐NMR at natural abundance (SNIF‐NMR). An analytical strategy has been developed in order to exploit the isotopomeric composition determined in clusters associated with different chemical sites of one or several fatty acid components. The method, which combines spectrometric and chromatographic data, enables isotopic criteria to be directly derived from raw vegetable oils containing in general two saturated and two unsaturated fatty acids. These results provide new information on isotopic fractionation caused by biochemical, physiological and natural environmental effects. Some alternation in the molecular deuterium distribution has been detected which may be related to the mechanism of fatty acid elongation. The successive methylene groups introduced through malonyl CoA are the subjects of different kinetic isotope effects since one of them is exclusively derived from NADH whereas the other has a contribution from pyruvate. A discriminant analysis of the cluster isotopic parameters enables several kinds of botanical precursors to be distinguished. The authenticating performances can be improved by taking into account the influence of climatic effects related to the region in which the plant grew.
The isolation and characterization of novel low‐amylose mutants of Pisum sativum L.
Authors: DENVER K. , BARBER L. M. , BURTON R. , HEDLEY C. L. , HYLTON C. M. , JOHNSON S. , JONES D. A. , MARSHALL J. , SMITH A. M. , TATGE H. , TOMLINSON K. , WANG T. L.
Published: April 2006

Mutants of Pisum sativum L. with seeds containing low‐amylose starch were isolated by screening a population derived from chemically mutagenized material. In all of the mutant lines selected, the low‐amylose phenotype was caused by a recessive mutation at a single locus designated lam. In embryos of all but one mutant line, the 59 kDa granule‐bound starch synthase (GBSSI) was absent or greatly reduced in amount. The granule‐bound starch synthase activity in developing embryos of the mutants was reduced but not eliminated. These results provide further evidence that amylose synthesis is unique to GBSSI. Other granule‐bound isoforms of starch synthase cannot substitute for this protein in amylose synthesis. Examination of iodine‐stained starch granules from mutant embryos by light microscopy revealed large, blue‐staining cores surrounded by a pale‐staining periphery. In this respect, the low‐amylose mutants of pea differ from those of other species. The differential staining may indicate that the structure of amylopectin varies between the core and peripheral regions.
Water deficit in developing endosperm of maize: cell division and nuclear DMA endoreduplication
Authors: ARTLIP T. S. , MADISON J. T. , SETTER T. L.
Published: April 2006

Water deficit severely decreases maize (Zea mays L.) kernel growth; the effect is most pronounced in apical regions of ears. The capacity for accumulation of storage material in endosperms is thought to he partially determined by the extent of cell division and endoreduplication (post‐mitotic nuclear DNA synthesis). To gain a better understanding of the regulatory mechanisms involved, we have examined the effect of water deficit on cellular development during the post‐fertilization period. Greenhouse‐grown maize was subjected to water‐limited treatments during rapid cell division [from 1 to 10days after pollination (DAP)] or rapid endoreduplication (9 to 15 DAP). The number of nuclei and the nuclear DNA content were determined with flow cytometry. Water deficit from 1 to 10 DAP substantially decreased the rate of endosperm cell division in apical‐region kernels, but had little effect on middle‐region endosperms. Rewatcring did not allow cell division to recover in apical‐region endosperms. Water deficit from 9 to 15 DAP also decreased cell division in apical‐region endosperms. Endoreduplication was not affected by the late treatment in either region of the car, but was inhibited by the early treatment in the apical region. In particular, the proportion of nuclei entering higher DN A‐content size classes was reduced. We conclude that cell division is highly responsive to water deficit, whereas endoreduplication is less so. We also conclude that the reduced proportion of nuclei entering higher DNA‐content size classes during endoreduplication is indicative of multiple control points in the mitotic and endoreduplication cycles.
Temperature gradient chambers for research on global environment change. II. A twin‐wall tunnel system for low‐stature, field‐grown crops using a split heat pump
Authors: HADLEY P. , BATTS G. R. , ELLIS R. H. , MORISON J. I. L. , PEARSON S. , WHEELER T. R.
Published: April 2006

A temperature gradient chamber (TGC) is described which enables elevated CO2 concentrations and a dynamic temperature gradient to be imposed on field crops throughout their life cycle under standard husbandry. Air is circulated through two double‐walled polyethylene‐covered tunnels connected to a split heat pump system to give a near‐linear temperature gradient along each tunnel. Solar energy gain along each tunnel and exchange with outer tunnel air flow contribute to the temperature gradient and also produce diurnal and seasonal temperature fluctuations corresponding to ambient conditions. Mean temperature gradients of between 3 and 5°C have been recorded throughout the growing seasons of crops of lettuce, carrot, cauliflower and winter wheat. Elevated or present CO2 concentrations are maintained in each of two pairs of tunnels throughout the cropping season using pure CO2 injected through motorized needle valves. This system can realistically simulate aspects of the effects of projected future environmental change on crop growth, development and yield, and in particular tin‐possible interaction of the effects of increased CO2 and temperature.
Temperature gradient chambers for research on global environment change. III. A system designed for rice in Kyoto, Japan
Authors: HORIE T. , NAKAGAWA H. , NAKANO J. , HAMOTANI K. , KIM H. Y.
Published: April 2006

Synthesis and validation of crop models for assessment of of the impact of elevated atmospheric CO2 concentration and anticipated global warming on crop production require crop response data obtained under field‐like conditions. The temperature gradient chamber (TGC) with the facility for CO2 enrichment allows the creation of various CO2 and temperature regimes for crops over the entire growth period with relatively inexpensive construction and running costs. The TGC develops a temperature gradient along its longitudinal axis using solar energy during the day and heating at night while maintaining the natural diurnal cycle. The temperature gradient and the CO2 concentration in the TGC are regulated by computer control of the air ventilation rate through the TGC and of the CO2 release rate. Longitudinal gradients of CO2 concentration and water vapour pressure deficit of air in the TGC were generally less than 5% and ±0.2 kPa, respectively.

A CO2 enrichment experiment on rice in the TGC showed that a doubling of the CO2 concentration markedly enhanced crop dry matter production. Temperature had less effect on dry matter production, although panicle dry weight was greatly decreased at higher temperature as a result of high‐temperature‐induccd sterility of rice spikelets. Since rice spikclets are most sensitive to high temperature at the moment of flowering, and their flowering habit is highly synchronized with the diurnal courses of environmental conditions, the TGC is a useful tool in understanding rice responses to changes in atmosphere and temperature.
A computer‐controlled system for studying ammonia exchange, photosynthesis and transpiration of plant canopies growing under controlled environmental conditions
Authors: HUSTED S. , SCHJOERRING J. K.
Published: April 2006

A fully automatic growth chamber system was built in order to study NH3, exchange and NH3, compensation points of plant canopies growing under controlled environmental conditions in which atmospheric NH3, concentrations corresponded to those naturally occurring over terrestrial ecosystems. The system included plant cuvettes with separate root and shoot compartments constructed of coated polycarbonate. This material did not change the spectral composition of photosynthetically active light and had a low adsorption of NH3, and water vapour. Atmospheric NH3, concentrations in the inlet of the cuvettes were controlled by mass‐flow controllers. Inlet and outlet NH3, concentrations were measured on‐line with a modified chemiluminescent NH3, monitor. At airflow rates per unit leaf area of about 3 dm3 m−2 s−1, the system allowed accurate determinations of NH3, exchange rates down to about 0.1 nmol NH3, m−2 s−1. The NH3, compensation points at anthesis for barley cultivars Laevigatum and Golf were 4.2±2.8 and 4.6±2.9nmol mol−1 of NH3, in air (SE, n=4), respectively. NH3, absorption in both cultivars increased linearly with atmospheric NH3, concentration in the range 0–30 nmol mol−1 of NH3, in air. NH3, absorption was much higher in the light than in the dark, indicating a strong stomatal and/or metabolic control of NH3, exchange. Photosynthesis and transpiration were not affected by exposure to NH3, concentrations in the range 0–30nmol mol−1 for 7d.
Scaling the seas
Authors: RAVEN J. A.
Published: April 2006

Primary producers in aquatic environments cover a range of living biomasses from 10−13 to 104 g dw. Benthic plants at the high end of this range contribute 5 Gt C year,1 to global primary productivity. Plankton at the lower end (up to 10−4 g dw) contributes about 30 Gt C year−1. While many problems of interpretation remain, in general terms the size of the organisms which dominate particular habitats can be related to the physics of water movement and its interaction with the availability of light and nutrients, the generation time of the organism, and the attentions of grazers. A second scaling problem is that of methods of studying the global energy flow and nutrient cycling roles of aquatic primary producers. Problems with scaling up from small‐scale and mesoscale to regional or global scale, and the prospects of more direct estimates of large‐scale productivity, are discussed.
Towards a better experimental basis for upscaling plant responses to elevated CO2 and climate warming
Authors: KÖRNER Ch.
Published: April 2006

Few of the most common assumptions used in models of responses of plants and ecosystems to elevated CO2 and climate warming have been tested under realistic life con‐ditions. It is shown that some unexpected discrepancies between predictions and experimental findings exist, suggesting that a better empirical basis is required for predictions. The following ten suggestions may improve our potential to scale up from experimental scales to the real world.

(1) Experiments should be timed to account for non‐linearity in system responsiveness, asynchrony of responses and developmental differences. (2) By altering mineral nutrient supply, a wide range of CO2 responses can be ‘produced’, thus requiring realistic soil conditions. (3) Distinctions should be made between ‘doubling CO2 sup‐ply’ and biologically effective degrees of CO2 enrichment. (4) Because of the non‐linearity of plant responses to CO2, studies of at least three instead of two CO2 concentrations are necessary to describe future trends adequately. (5) Edge effects, in particular unscreened side light, may lead to allometric anomalies, strongly constraining up‐scaling to stand‐scale CO2 responses. (6) Variables such as growth, yield, net primary production and C turnover are often confused with carbon pools, carbon sequestration or net ecosystem production. (7) Mono‐ and interspecific interactions between individuals may lead to completely unpredictable CO2 responses. (8) Experiments with seedlings benefit from the absence of prehistory effects but are likely to be irrelevant for the responses of larger trees which, on the other hand, may be constrained by carry‐over effects. Tree ring research indicates immediate sensitivity of large trees to environmental changes, supporting their usefulness in short‐term CO2‐enrichment experiments. (9) In predicting temperature responses, acclimation deserves more attention. (10) The significance of developmental responses is largely under‐represented in experimental research, although these responses may overrule many of the other effects of atmospheric change. Results of more realistic experiments which account for these problems will provide a better basis for modelling the future of the biosphere.
Scaling gross ecosystem production at Harvard Forest with remote sensing: a comparison of estimates from a constrained quantum‐use efficiency model and eddy correlation
Authors: WARING R.H. , LAW B.E. , GOULDEN M.L. , BASSOW S.L. , McCREIGHT R.W. , WOFSY S.C. , BAZZAZ F.A.
Published: April 2006

Two independent methods of estimating gross ecosystem production (GEP) were compared over a period of 2 years at monthly integrals for a mixed forest of conifers and deciduous hardwoods at Harvard Forest in central Massachusetts. Continuous eddy flux measurements of net ecosystem exchange (NEE) provided one estimate of GEP by taking day to night temperature differences into account to estimate autotrophic and heterotrophic respiration. GEP was also estimated with a quantum efficiency model based on measurements of maximum quantum efficiency (Qmax), seasonal variation in canopy phenology and chlorophyll content, incident PAR, and the constraints of freezing temperatures and vapour pressure deficits on stomatal conductance. Quantum efficiency model estimates of GEP and those derived from eddy flux measurements compared well at monthly integrals over two consecutive years (R2= 0–98).

Remotely sensed data were acquired seasonally with an ultralight aircraft to provide a means of scaling the leaf area and leaf pigmentation changes that affected the light absorption of photosynthetically active radiation to larger areas. A linear correlation between chlorophyll concentrations in the upper canopy leaves of four hardwood species and their quantum efficiencies (R2= 0–99) suggested that seasonal changes in quantum efficiency for the entire canopy can be quantified with remotely sensed indices of chlorophyll. Analysis of video data collected from the ultralight aircraft indicated that the fraction of conifer cover varied from < 7% near the instrument tower to about 25% for a larger sized area. At 25% conifer cover, the quantum efficiency model predicted an increase in the estimate of annual GEP of < 5% because unfavourable environmental conditions limited conifer photosynthesis in much of the non‐growing season when hardwoods lacked leaves.
Carbon balance in the tundra, boreal forest and humid tropical forest during climate change: scaling up from leaf physiology and soil carbon dynamics
Authors: WANG Y. P. , POLGLASE P. J.
Published: April 2006

Carbon exchange by the terrestrial biosphere is thought to have changed since pre‐industrial times in response to increasing concentrations of atmospheric CO2 and variations (anomalies) in inter‐annual air temperatures. However, the magnitude of this response, particularly that of various ecosystem types (biomes), is uncertain. Terrestrial carbon models can be used to estimate the direction and size of the terrestrial responses expected, providing that these models have a reasonable theoretical base. We formulated a general model of ecosystem carbon fluxes by linking a process‐based canopy photosynthesis model to the Rothamsted soil carbon model for biomes that are not significantly affected by water limitation. The difference between net primary production (NPP) and heterotrophic soil respiration (Rh) represents net ecosystem production (NEP). The model includes (i) multiple compartments for carbon storage in vegetation and soil organic matter, (ii) the effects of seasonal changes in environmental parameters on annual NEP, and (iii) the effects of inter‐annual temperature variations on annual NEP. Past, present and projected changes in atmospheric CO2 concentration and surface air temperature (at different latitudes) were analysed for their effects on annual NEP in tundra, boreal forest and humid tropical forest biomes. In all three biomes, annual NEP was predicted to increase with CO2 concentration but to decrease with warming. As CO2 concentrations and temperatures rise, the positive carbon gains through increased NPP are often outweighed by losses through increased Rh, particularly at high latitudes where global warming has been (and is expected to be) most severe. We calculated that, several times during the past 140 years, both the tundra and boreal forest biomes have switched between being carbon sources (annual NEP negative) and being carbon sinks (annual NEP positive). Most recently, significant warming at high latitudes during 1988 and 1990 caused the tundra and boreal forests to be net carbon sources. Humid tropical forests generally have been a carbon sink since 1960. These modelled responses of the various biomes are in agreement with other estimates from either field measurements or geochemical models. Under projected CO2 and temperature increases, the tundra and boreal forests will emit increasingly more carbon to the atmosphere while the humid tropical forest will continue to store carbon. Our analyses also indicate that the relative increase in the seasonal amplitude of the accumulated NEP within a year is about 0–14% year−1 for boreal forests and 0–23% year−1 in the tundra between 1960 and 1990.
‘Photoinhibition’ of Heliconia under natural tropical conditions: the importance of leaf orientation for light interception and leaf temperature
Authors: HE J. , CHEE C. W. , GOH C. J.
Published: April 2006

The influence of irradiance on photosynthesis under natural conditions was studied in aseasonal Singapore using three Heliconia taxa: H. rostrata, H. psittacorum × H. spathocircinata cv. Golden Torch and H. psittacorum cv. Tay. When grown under full sunlight, all three heliconias exhibited reduced phatosynthetic capacities and lowered chlorophyll content per leaf area as compared with those grown under intermediate and deep shade. A marked decrease in the chlorophyll fluorescence Fv/Fm ratio and an increase in photochemical quenching (1‐ qp) and non‐photochemical quenching (qN) were observed in upper leaves of plants grown under full sunlight. Increases in qN suggest that ‘photoinhibition’ (decreases in Fv/Fm) in Heliconia grown under natural tropical conditions are probably due to photoprotective energy dissipation processes. The quantum yield, the maximum photosynthetic rate, Fv/Fm and the chlorophyll content of upper leaves were lower than those of lower leaves on the same plants grown under full sunlight. Similarly, lower values were obtained for the tip (sun) portion than for the base (shaded) portion of the leaves. The changes in Fv/Fm and in the levels of (1 –qp) in leaves grown under intermediate and deep shade were negligible in plants during the course of day. However, there was a steep decrease in Fv/Fm and an increase in the levels of (1 –qp), along with an increase in incident light in the sun leaves. The lowest Fv/Fm and the highest level of (1 –qp) indicated minimum PSII efficiency at midday in full sun. These results indicate that, in Heliconia, the top leaves (particularly leaf tips) experienced sustained decreases in PSII efficiency upon exposure to full sunlight. Although all three taxa exhibited sustained decreases in photosynthetic capacity in full sunlight, the sun leaves of ‘Tay’ showed higher photosynthetic capacity than those of the other two taxa. This could be due, at least in part, to the vertical leaf angle and smaller lamina area. When the upright leaves of ‘Tay’ were constrained to a horizontal angle, they exhibited lower PSII efficiency (FvIFm ratio), while horizontal leaves of ‘Rostrata’ and ‘Golden Torch’ inclined lo near‐vertical angles showed increased efficiency. Thus, an increase in leaf angle helps to achieve a reduction in the sustained decrease in PSII efficiency by decreasing the levels of incident sunlight and subsequently the leaf temperature.