bluuebunny/crossref_metadata_2025 huggingface dataset:
Contribution of ornithine aminotransferase to proline accumulation in NaCl‐treated radish cotyledons
Authors: HERVIEU F. , LE DILY F. , HUAULT C. , BILLARD J.‐P.
Published: April 2006

Light‐grown cotyledons of radish (Raphanus sativus L. cv National) subjected to increasing NaCI concentrations displayed a dose‐dependent accumulation of proline at the beginning of their differentiation. Neither erythromycin nor DCMU significantly affected the salt‐induced proline accumulation, which suggests that plastids may not be involved in proline synthesis. Gabaculine has been shown previously to be a powerful and irreversible inhibitor of ornithine aminotransferase [Hervieu et al. (1993) Phyto‐chemistry 34, 231–234]. This inhibitor applied in vivo at a very low concentration (1 mmol m−3) reduced considerably the salt‐induced proline accumulation of radish cotyledons. These results indicate that the ornithine pathway contributes, via an increase of ornithine aminotransferase activity, to proline synthesis, as well as the glutamate pathway, in salt‐treated cotyledons. The use of transcription and translation inhibitors revealed that the salt‐induced increase in ornithine aminotransferase activity may proceed via an activation of the translation of preformed mRNAs. The pre‐existing ornithine aminotransferase mRNAs in radish seeds may be an important adaptation to NaCI, leading to proline accumulation at the beginning of seedling growth.
An open‐air system for exposure of young forest trees to sulphur dioxide and ozone
Authors: McLEOD A. R.
Published: April 2006

The development, construction and operation of an open‐air fumigation system for exposing young forest trees to controlled concentrations of sulphur dioxide and ozone is described. A computer simulation of gas dispersion was used to design an array of pipework sources which minimized spatial variability in exposure concentrations. Five fumigation plots were constructed using the design and were used to fumigate trees during a 7 year study known as the Liphook Forest Fumigation Project. Rates of gas release were controlled by a small computer to follow predetermined patterns of sulphur dioxide concentration and to maintain an elevation above ambient ozone concentration. Effective control of exposure was demonstrated, and examples of experimentally produced concentration frequency distributions are provided. The advantages and shortcomings of the system are discussed with recommendations for future improvements.
The effects of SO2 and O3 fumigation on acid deposition and foliar leaching in the Liphook forest fumigation experiment
Authors: SKEFFINGTON R. A. , SUTHERLAND P. M.
Published: April 2006

The ambient pollution climate at the Liphook forest fumigation site, where coniferous trees were fumigated with SO2 and O3, for 4 years under field conditions, was characteristic of the fringes of the areas where pollutant effects are a problem. Experimental treatments increased SO2 concentrations to levels more characteristic of Eastern Europe, and summer O3 concentrations by 30%. Deposition of SO2 to the soil between the trees (inferred from shallow lysimeters) was significant, the deposition velocity being 2–1 mms−1. Deposition to Scots pine and Sitka spruce canopies was greater, deposition velocities being 8.5 and 9.4 mm s−1, respectively. These high values may perhaps be explained by co‐deposition with NH3. Calculations assume that dry deposition was the sole source of SO42− gain in throughfall, and that there was no significant retention by the trees. There was a trend for O3 to enhance SO2 deposition to both soil and trees. Fumigation with SO2 led to a significant increase in leaching of cations from foliage. Each species neutralized about 63% of the dry‐deposited SO2, predominantly by ion exchange for Ca and K. Equations are provided which allow calculation of foliar leaching given SO2 concentrations or SO42− deposition. Fumigation increased the rate of nutrient cycling considerably, without affecting foliar concentrations or damaging the trees. Ozone treatments did not enhance foliar leaching, calling into question some suggested mechanisms for the causes of forest decline.
The effect of open‐air fumigation with SO2 and O3 on carbohydrate metabolism in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies)
Authors: PEACE E. A. , LEA P. J. , DARRALL N. M.
Published: April 2006

The carbohydrate metabolism of the needles of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) has been examined in trees that were exposed to SO2, and O3, in an open‐air fumigation experiment located in the Liphook forest in southern England. Two‐year‐old seedlings were planted in 1985 in seven experimental plots. Five plots received fumigation treatments of SO2, O3 or a combination of these gases to give a 2 × 3 factorial design with one additional ambient plot Fumigation with SO2, occurred from May 1987 to December 1990 and O3, fumigation occurred from March to December 1988, May to December 1989 and February to December 1990. Five samples of needles for investigation of carbohydrate metabolism were taken between February and July 1989.

The concentrations of soluble carbohydrates (including sucrose and hexoses) were greatly reduced in the needles taken from Scots pine growing in the treated plots, and were also reduced, but to a lesser extent, in the needles taken from Norway spruce. Little variation in the concentration of starch in the needles of either species was detected. The activities of the two final enzymes of sucrose synthesis, sucrose phosphate synthase and sucrose 6‐phos‐phate phosphatase, were greatly reduced in the needles of Scots pine and were also reduced, but to a lesser extent, in the needles of Norway spruce in the fumigated plots. These reductions could be correlated with decreases in rates of photosynthetic CO2 assimilation determined by independent groups of researchers working on the Liphook site.
A critical appraisal of a combined stomatal‐photosynthesis model for C3 plants
Authors: LEUNING R.
Published: April 2006

Gas‐exchange measurements on Eucalyptus grandis leaves and data extracted from the literature were used to test a semi‐empirical model of stomatal conductance for CO2

gSc=go+a1A/(cs‐I) (1+Ds/Do)]

where A is the assimilation rate; Ds and cs are the humidity deficit and the CO2 concentration at the leaf surface, respectively; g0 is the conductance as A → 0 when leaf irradiance → 0; and D0 and a1 are empirical coefficients. This model is a modified version of gsc=a1A hs/cs first proposed by Ball, Woodrow & Berry (1987, in Progress in Photosynthesis Research, Martinus Mijhoff, Publ., pp. 221–224), in which hs is relative humidity. Inclusion of the CO2 compensation point, τ, improved the behaviour of the model at low values of cs, while a hyperbolic function of Ds for humidity response correctly accounted for the observed hyperbolic and linear variation of gsc and ci/cs as a function of Ds, where Ci is the intercellular CO2 concentration. In contrast, use of relative humidity as the humidity variable led to predictions of a linear decrease in gsc and a hyperbolic variation in ci/cs as a function of Ds, contrary to data from E. grandis leaves. The revised model also successfully described the response of stomata to variations in A, Ds and cs for published responses of the leaves of several other species. Coupling of the revised stomatal model with a biochemical model for photosynthesis of C3 plants synthesizes many of the observed responses of leaves to light, humidity deficit, leaf temperature and CO2 concentration. Best results are obtained for well‐watered plants.
The cellular pathway of photosynthate transfer in the developing wheat grain. III. A structural analysis and physiological studies of the pathway from the endosperm cavity to the starchy endosperm
Authors: WANG H. L. , PATRICK J. W. , OFFLER C. E. , WANG X. ‐D.
Published: April 2006

The cellular pathway of sucrose transfer from the endosperm cavity to the starchy endosperm of developing grains of wheat (Triticum turgidum) has been elucidated. The modified aleurone and sub‐aleurone cells exhibit a dense cytoplasm enriched in mitochondria and endoplasmic relicilium. Significantly, the sub‐aleurone cells are characterized by secondary wall ingrowths. Numerous plasmodesmata interconnect all cells between the modified aleurone and starchy endosperm. The pro‐tonophore carbonylcyanide‐m‐chlorophenyl hydrazone (CCCP) slowed [14C]sucrose uptake by grain tissue slices enriched in modified aleurone and sub‐aleurone cells but had no effect on uptake by the starchy endosperm. The fluorescent weak acid sulphorhodamine G (SRG) was preferentially accumulated by the modified aleurone and sub‐aleurone cells, and this uptake was sensitive to CCCP. The combined plasma membrane surface areas of the modified aleurone and sub‐aleurone cells appeared to be sufficient to support the in vivo rates of sucrose transfer to the starchy endosperm. Plasmolysis of intact excised grain inhibited [14C]sucrose transfer from the endosperm cavity to the starchy endosperm. The sulphydryl group modifier p‐chloromercuribenzenesulphonie acid (PCMBS) decreased [14C]sucrose uptake by the modified aleurone and sub‐aleurone cells but had little effect on uptake by the starchy endosperm. In contrast, when PCMBS and [14C]sucrose were supplied to the endosperm cavity of intact excised grain, PCMBS slowed accumulation by all tissues equally. Estimates of potential sucrose fluxes through the interconnecting plasmodesmata were found to be within the published range. It is concluded that the bulk of sucrose is accumulated from the endosperm cavity by the modified aleurone and sub‐aleurone cells and subsequently transferred through the symplast to the starchy endosperm.
The concentration of xylem sap constituents in root exudate, and in sap from intact, transpiring castor bean plants (Ricinus communis L.)
Authors: SCHURR U. , SCHULZE E. ‐D.
Published: April 2006

Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura‐type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura‐type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration‐flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.
Mitochondria contribute to increased photosynthetic capacity of leaves of winter rye (Secale cereale L.) following cold‐hardening
Authors: HURRY V. , TOBIÆSON M. , KRÖMER S. , GARDESTRÖM P. , ÖQUIST G.
Published: April 2006

Cold‐hardening of winter rye (Secale cereale L. cv. Musketeer) increased dark respiration from −2.2 to −3.9 μmol O2 m−2s−1 and doubled light‐and CO2‐saturated photosynthesis at 20°C from 18.1 to 37.0μmol O2 m−2 s−1 We added oligomycin at a concentration that specifically inhibits oxidative phosphorylation to see whether the observed increase in dark respiration reflected an increase in respiration in the light, and whether this contributed to the enhanced photosynthesis of cold‐hardened leaves. Oligomycin inhibited light‐ and CO2‐saturated rates of photosynthesis in non‐hardened and cold‐hardened leaves by 14 and 25%, respectively, and decreased photochemical quenching of chlorophyll a fluorescence to a greater degree in cold‐hardened than in non‐hardened leaves. These data indicate an increase both in the rate of respiration in the light, and in the importance of respiration to photosynthesis following cold‐hardening. Analysis of metabolite pools indicated that oligomycin inhibited photosynthesis by limiting regeneration of ribulose‐1,5‐bisphosphate. This limitation was particularly severe in cold‐hardened leaves, and the resulting low 3‐phospho‐glycerate pools led to a feed‐forward inhibition of sucrose‐phosphate synthase activity. Thus, it does not appear that oxidative phosphorylation supports the increase in photo‐synthetic O2 evolution following cold‐hardening by increasing the availability of cytosolic ATP. The data instead support the hypothesis that the mitochondria function in the light by using the reducing equivalents generated by non‐cyclic photosynthetic electron transport.
Photomorphogenesis and canopy dynamics. Phytochrome‐mediated proximity perception accounts for the growth dynamics of canopies of Populus trichocarpa × deltoides‘Beaupré’
Authors: GILBERT I. R. , SHAVERS G. P. , JARVIS P. G. , SMITH H.
Published: April 2006

The phytochrome family of signal‐transducing photoreceptors provides plants with the capacity to perceive variations in the relative fluxes of red (R) and far‐red (FR) radiation. This capacity has been proposed to be of ecological value in the perception of the proximity of neighbouring plants and the consequent induction of shade avoidance responses. The work reported here has evaluated this potential by determining quantitatively the effect of neighbour proximity on the growth of canopies of Populus trichocarpa×deltoides‘Beaupré’ trees, and relating the measured variables to the long‐term vectoral radiation quality inside each canopy. The spectral distribution of radiation inside four canopies of Populus trichocarpa×deltoides‘Beaupre’ of different densities was monitored throughout the growing season. Spectral distributions inside the canopies were measured in 10° wedges at different heights and angles. The results are presented as PFD over 400–700 nm (PFD400–700) and PFD over 400–800 nm (PFD400–700). Results are also presented for the calculated phytochrome photoequilibrium (Pfr/P) and red:far‐red ratio (R:FR). Data are presented as in‐canopy angular and height profiles, and as diurnal and seasonal variations. PFD400–700 and Pfr/P were found to be reduced inside each canopy, the reduction being greatest in the most dense canopy, and least in the most open canopy. At any height within each canopy, calculated Pfr/P decreased linearly with time throughout the growing season, until leaf senescence began. The reduction was greater in the denser canopies and was found to be similar for three consecutive field seasons. Linear relationships were found between plant stem growth rate, plant spacing and Pfr/P calculated from radiation propagated approximately horizontally within the canopies. The findings support the role of phytochrome in proximity perception in the natural environment and provide a quantitative basis for investigating the competitive interactions between plants growing in dense stands. The hypothesis is proposed that the dynamics of developing or regenerating canopies can be accounted for on the basis of phytochrome‐mediated perception of the proximity of neighbouring plants.
Comparative microscopic and enzymatic characterization of the leaf necrosis induced in Arabidopsis thaliana by lead nitrate and by Xanthomonas campestris pv. campestris after foliar spray
Authors: LUMMERZHEIM M. , SANDRONI M. , CASTRESANA C. , DE OLIVEIRA D. , VAN MONTAGU M. , ROBY D. , TIMMERMAN B.
Published: April 2006

The heavy metal lead was administered to Arabidopsis thaliana plants by foliar spray. At a concentration of l4mol m−3, the lead nitrate suspension induced densely distributed necrotic lesions on A. thaliana leaves. A number of Arabidopsis ecotypes were tested and a differential response to heavy‐metal toxicity was noted. The necrosis provoked as a result of the phytotoxic effect of lead had a similar appearance to the necrotic lesions observed in a hypersensitive response of A. thaliana to inoculation with Xanthomonas campestris pv. campestris (Lummerzheim et al. 1993, Molecular Plant‐Microbe Interactions 6, 532–544). In addition to this phenotypic resemblance, accumulation of polyphenols and callose depositions observed by microscopic analysis, as well as increases in the activities of the stress‐related proteins β1,3‐glucanases, chitinases and peroxidases, revealed significant similarities in the plant response to the two treatments examined, lead toxicity and bacterial infection. The results allow the establishment of markers for both types of stress.
Multiple xenobiotic‐inducible P450s are involved in alkoxycoumarin and alkoxyresorufin metabolism in higher plants
Authors: BATARD Y. , ZIMMERLIN A. , LE RET M. , DURST F. , WERCK‐REICHHART D.
Published: April 2006

O‐Dealkylation of two series of fluorescent 7‐alkoxy‐coumarins and 7‐alkoxyphenoxazones by plant cytochrome P450s was investigated in Helianthus tuberosus tuber tissues treated with prototype P450 inducers, environmental pollutants or agrochemicals. Methoxy‐, ethoxy‐, propoxy‐and butoxycoumarins and methoxy‐ and ethoxyresorufins were metabolized by fplant microsomes. Dealkylation of pentoxy‐ and benzyloxyresorufins was not detected. All dealkylating activities were enhanced by aging plant tissues in the presence of xenobiotics, in some cases up to 20‐fold relative to the activities detected in control tissues. Increases in total P450 in the same tissues never exceeded 3‐fold. The isozymes induced by prototype P450 inducers clearly differed from those in mammalian liver. That multiple P450s with overlapping substrate specificities were involved in the metabolism of both alkoxycoumarins and alkoxyresorufins was demonstrated by (1) the differential induction of the activities in response to exposure to xenobiotics, (2) the differential inhibition of the activities by clotrimazole, paclobutrazole and tetcyclacis in aminopyrine and benzo(a)pyrene‐treated tissues, and(3) the selective inhibition observed with antibodies raised against purified ethoxycoumarin deethylase fractions. Our results suggest that the measurement of the dealkylation of such fluorescent substrates in plants might be useful to monitor environmental pollution.
Magnetic resonance imaging (MRI) of water during cold acclimation and freezing in winter wheat
Authors: MILLARD M. M. , VEISZ O. B. , KRIZEK D. T. , LINE M.
Published: April 2006

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) were used to analyse changes in the physical state of water in wheat crowns during cold acclimation and during the freezing/thawing cycle. Spectroscopically measured average spin‐spin relaxation times (T2) decreased during cold acclimation and increased when plants were grown at normal temperature. Spin‐spin relaxation images whose contrast is proportional toT2, times were calculated allowing association of water relaxation with regions of tissue in spin‐echo images during acclimation and freezing. Images taken during freezing revealed nonuniform freezing of tissue in crowns and roots. Acclimated and non‐acclimated wheat crowns were imaged during freezing and after thawing. Spin‐echo image signal intensity andT2times decreased dramatically between ‐4°C and ‐8°C as a result of a decrease in water mobility during freezing. Images collected during thawing were diffuse with less structure and relaxation times were longer, consistent with water redistribution in tissue after membrane damage.
The effect of carbon dioxide concentration on respiration of growing and mature soybean leaves
Authors: BUNCE J. A.
Published: April 2006

Soybean plants were grown continuously at 350 and 700cm3m−3 CO2 at constant temperature. Respiration rates of third trifoliolate leaves were measured at the growth CO2 concentration for the whole dark period from 5d before through to 5d after full area expansion. The short‐term response of respiration rate to the measurement CO2 concentration was also determined at each age. Respiration rates per unit of dry mass declined with age and were significantly less at a given age or RGR in leaves grown and measured at the elevated CO2. The difference in respiration rate was largest in mature leaves and resulted from the different measurement CO2 concentrations. The respiratory costs of the tissue synthesis, estimated from the elemental composition of the tissue, did not differ substantially between CO2 treatments. The response of respiration rate to carbon dioxide concentration was not strongly affected by the form of nitrogen supplied. Maintenance respiration calculated by subtracting growth respiration from total respiration was negative in rapidly growing leaves for both CO2 treatments. This indicates that CO2 efflux in the dark does not accurately reflect the average 24 h rate of energy expenditure on growth and maintenance for soybean leaves.
Utilization of adenine and guanine as nitrogen sources by Chlamydomonas reinhardtii cells
Authors: LISA T. A. , PIEDRAS P. , CÁRDENAS J. , PINEDA M.
Published: April 2006

Chlamydomonas reinhardtü Dangeard, adenine or guanine can be used as the sole nitrogen source for growth by means of an inducible system which is repressed by ammonia. Cells grown on either adenine or guanine were able to take up both purines, although the adenine uptake rate was always about 40% of the guanine uptake rate. Both adenine and guanine were taken up by an inducible system(s) exhibiting hyperbolic kinetics with identical apparent A, values of 3‐2 mmol m−3 for adenine and 3‐2mmol m−3 for guanine. Adenine and guanine utilization depended on pH, with similar optimal pH values of 7·3 and 7·4, respectively. Adenine and guanine each acted as a competitive inhibitor of the other's uptake, and their utilization was also inhibited by hypoxanthine, xanthine and urate. Inhibition of adenine uptake by guanine and hypoxanthine was competitive, with A′, values of 5·5 and 1. 6 mmol m−3 respectively. Guanine uptake was also inhibited competitively by adenine (K1= 1·3mmol m−3) and hypoxanthine (K1= 3. 3 mmol m−3). Utilization of both adenine and guanine was inhibited by cyanide, azide, 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea, 2,4‐dinitrophenol and carbonylcyanide m‐chlorophenylhydrazone, and was also sensitive to p‐hydroxymercuribenzoate and N‐ethyl‐maleimide. On the basis of these results, taken together, the possibility that adenine and guanine are translocated into Chlamydomonas by a common system is discussed.
CO2 responsiveness of plants: a possible link to phloem loading
Authors: KÖRNER CH. , PELAEZ‐RIEDL S. , VAN BEL A. J. E.
Published: April 2006

Of the many responses of plants to elevated CO2, accumulation of total non‐structural carbohydrates (TNC in % dry weight) in leaves is one of the most consistent. Insufficient sink activity or transport capacity may explain this obvious disparity between CO2 assimilation and carbohydrate dissipation and structural investment. If transport capacity contributes to the problem, phloem loading may be the crucial step. It has been hypothesized that symplastic phloem loading is less efficient than apoplastic phloem loading, and hence plant species using the symplastic pathway and growing under high light and good water supply should accumulate more TNC at any given CO2 level, but particularly under elevated CO2. We tested this hypothesis by carrying out CO2 enrichment experiments with 28 plant species known to belong to groups of contrasting phloem‐loading type. Under current ambient CO2 symplastic loaders were found to accumulate 36% TNC compared with only 19% in apoplastic loaders (P=0.0016). CO2 enrichment to 600 μmol mol−1 increased TNC in both groups by the same absolute amount, bringing the mean TNC level to 41% in symplastic loaders (compared to 25% in apoplastic loaders), which may be close to TNC saturation (coupled with chlornplast malfunction). Eight tree species, ranked as symplastic loaders by their minor vein companion cell configuration, showed TNC responses more similar to those of apoplastic herbaceous loaders. Similar results are obtained when TNC is expressed on a unit leaf area basis, since mean specific leaf areas of groups were not significantly different. We conclude that phloem loading has a surprisingly strong effect on leaf tissue composition, and thus may translate into alterations of food webs and ecosystem functioning, particularly under high CO2.
A model of the acclimation of photosynthesis in the leaves of C3 plants to sun and shade with respect to nitrogen use
Authors: HIKOSAKA K. , TERASHIMA I.
Published: April 2006

A model of leaf photosynthesis of C3, plants has been developed to describe their nitrogen economy. In this model, photosynthetic proteins are categorized into five groups depending on their functions. The effects of investment of nitrogen in each of these groups on the maximal rate of photosynthesis and/or the initial slope of the light‐response curve are described as simple equations. Using this model, the optimal pattern of nitrogen partitioning which maximizes the daily rate of CO2 exchange is estimated for various light environments and leaf nitrogen contents. When the leaf nitrogen content is fixed, the amount of nitrogen allocated to Calvin cycle enzymes and electron carriers increases with increasing irradiance, while that allocated to chlorophyll‐protein complexes increases with decreasing irradiance. For chlorophyll‐proteins of photosystem II, the amount of light‐harvesting complex II relative to that of the core complex increases with decreasing irradiance. At any irradiance, partitioning into ribulose bisphosphate carboxylase increases with increasing leaf nitrogen content Taking the total leaf nitrogen content and the daily CO2 exchange rate as ‘cost’ and ‘benefit’, respectively, the optimal amount and partitioning of nitrogen are examined for various conditions of light environment and nitrogen availability. The leaf nitrogen content that maximizes the rate of daily carbon fixation increases with increasing growth irradiance. It is also predicted that, at low nitrogen availabilities, low leaf nitrogen contents are advantageous in terms, of nitrogen use efficiency. These trends predicted by the present model are largely consistent with those reported for actual plants. The differences in the total amount of leaf nitrogen and in the organization of photosynthetic components that have been reported for plants from different environments would therefore be of adaptive significance, because such differences can contribute to realization of efficient photosynthesis. These results are fürther discussed in an ecological context.
In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply
Authors: VALENTINI R. , EPRON D. , DE ANGELIS P. , MATTEUCCI G. , DREYER E.
Published: April 2006

Diurnal time courses of net CO2 assimilation rates, stomatal conductance and light‐driven electron fluxes were measured in situ on attached leaves of 30‐year‐old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O2 concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O2 to compute total light‐driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light‐driven electron flow, net CO2 assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non‐photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R1) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de‐excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.
Can perennial C4 grasses attain high efficiencies of radiant energy conversion in cool climates?
Authors: BEALE C. V. , LONG S. P.
Published: April 2006

Although C4 plants are considered to have higher conversion efficiency and productivity than C3, plants, this advantage may not be realized under sub‐optimal conditions. Two perennial C4 rhizomatous grasses of cool temperate origin, Miscanthus × giganteus and Spartina cyno‐suroides, have been suggested as potential fuel crops for north‐western Europe. The conversion efficiencies of these species were examined for 2 years in fertilized, irrigated, replicated plots in south‐eastern England. In the second year, the energy conversion efficiencies for shoot and total biomass production were 0.040 and 0.051, respectively, for S. cynosuroides, with significantly higher values of 0.060 and 0.078, respectively, for M. × giganteus. The M. × giganteus crop attained shoot productivity of 2.87 kg m−2 between April and September, exceeding the highest values typically obtained with intensively managed C3, crops. Canopy development was early, and high interception and conversion efficiencies were maintained over most of the growing season. This study provides the evidence that the superior potential light conversion efficiencies associated with C4 photosynthesis can be realized under cool temperate conditions and that such climatic conditions do not Inherently impair the C4 process.
Evidence that the partial resistance of theArabidopsis 5‐2mutant to fusicoccin depends on a decreased capacity of the H+pump to respond to activating factors
Authors: MARRÉ M. T. , VENEGONI A. , TALARICO A. , SOAVE C. , MARRÉ E.
Published: April 2006

Measurements of H+extrusion activity K+influx, andEs bm in 3‐d‐old seedlings of the 5‐2 mutant ofArabidopsis thaliana(which is partially insensitive to fusicoccin) showed the following, (i) The reduced response of5‐2to fusicoccin (FC) does not depend on the penetration of FC to its site of action, or on decreased affinity of the FC receptor, (ii) The reduced response of H+and K+transport to FC does not depend on an impairment of the K+absorption system, (iii) The mutation can influence the H+extrusion system independently of the presence of FC. In the presence of factors other than FC known to activate the plasma membrane H+‐ATPase (e.g. a cytosol‐acidifying treatment), the response in 5‐2 is about 50% lower than inwt.(iv) When both genotypes grow in optimal conditions, the rate of fresh weight increase and stem elongation is higher inwtthan5‐2. These data indicate that the5‐2mutation affects some intrinsic component of the H+‐extrusion machinery, the limiting effect of which becomes considerable when either the physiological or the experimental conditions induce a high level of proton pump activity. An alteration either of the ATPase itself or of a factor controlling its activity is compatible with our observations.
Base and optimum temperatures vary with genotype and stage of development in wheat
Authors: SLAFER G. A. , RAWSON H. M.
Published: April 2006

Some assumptions concerning development in wheat (Triticum aestivum, L.) were examined. These are that (i) the rate of development towards anthesis increases linearly with temperature, (ii) the base temperature is 0°C, (iii) the optimum temperature is above the range at which wheat is normally grown, (iv) base and optimum temperatures do not change with development, and (v) the relationships for different cultivars are similar. We tested these assumptions in studies using a naturally lit phytotron with four cultivars and six temperature regimes between 10 and 25°C. Seedlings were vernalized for 50 d and then grown under a photoperiod of 18 h to avoid confounding the responses to vernalization and photoperiod with those to temperature. In cultivars Sunset and Rosella, the rate of development for the full period to anthesis increased linearly between base and optimum temperatures. However, in cultivars Condor and Cappelle Desprez, a linear fit was not statistically acceptable. For these cultivars, the rate of development towards anthesis increased rapidly with increase in temperature from 10 to 19°C, but temperatures higher than 19°C had little or no fürther accelerating effect. When a linear relationship was fitted by ignoring data for temperatures above 19 7deg;C, base temperatures calculated for the full period to anthesis were c. 5.5, 5.5,4.0 and 2.5°C for Sunset, Condor, Rosella and Cappelle Desprez, respectively (i.e. an average value of c. 4 7deg;C). The full period to anthesis was subdivided into three phases for fürther analysis. These were (i) from the beginning of the experiment to terminal spikelet initiation, (ii) from terminal spikelet initiation to heading, and (iii) from heading to anthesis. When these sub‐phases were analysed a linear relationship was found to be appropriate for all combinations of cultivar and developmental phase. However, both base and optimum temperatures calculated from the relationships increased as development progressed from (i) to (iii). Averaging across cultivars, base temperatures for the three phases were ‐1.9, %1.2 and %8.1°C, respectively, while optimum temperatures were <22, 25 and >25°C, respectively. Cultivars differed substantially in all these parameters. The progressive increase in optimum temperature with phasic development was apparently the main reason why linear fits for the three sub‐phases became a curvilinear fit for the full phase to anthesis.
The expression of cellulase gene family members during induced avocado fruit abscission and ripening
Authors: TONUTTI P. , CASS L. G. , CHRISTOFFERSEN R. E.
Published: April 2006

The expression of specific cellulase (endo‐l,4‐β‐glucanase; EC 3‐2‐1‐4) gene family members was monitored in the mesocarp and abscission zone of mature avocado fruit. Fruit abscission was induced by either girdling of the stem or treatment with 2‐chloroethyl phosphonic acid (CEPA). Gene‐specific probes that hybridized to the 3′ untranslated region of either the cel1 or the cel2 gene of avocado were used to distinguish between RNA transcripts from these two closely related genes. Only cel1 transcripts were detected in ripe mesocarp. An accumulation of cel1 mRNA has been observed in the activated fruit abscission zone. Using antibody raised against cellulase protein isolated from ripe fruit, a cross‐reacting cellulase antigen was detected in extracts from the abscission zones of CEPA‐treated and girdled fruit. Cellulase enzyme activity was detected in these tissues. These results suggest that the same cellulase gene (cel1) that has been shown to be expressed in the mesocarp during fruit ripening might also be involved in the mature fruit abscission process.
Microaerobic respiration and oxidative phosphorylation by soybean nodule mitochondria: implications for nitrogen fixation
Authors: MILLAR A. H. , DAY D. A. , BERGERSEN F. J.
Published: April 2006

The infected cells of soybean (Glycine max) root nodules require ATP production for ammonia assimilation and purine synthesis under microaerobic conditions. It is likely that the bulk of this demand is supplied through mitochondrial oxidative phosphorylation. Mitochondria purified from root nodules respired and synthesized ATP in sub‐micromolar oxygen concentrations as measured by leghaemoglobin spectroscopy and luciferase luminescence. Both oxygen uptake and the apparent ATP/O ratio declined significantly as the oxygen concentration fell below 100 μmol m−3. Cytochrome‐pathway respiration by root nodule mitochondria had a higher apparent affinity for oxygen (Km 50 μmol m−3) than did mitochondria isolated from roots (Km 125 μmol m−3). Electron micrographs showed that mitochondria predominated at the periphery of infected cells adjacent to gas‐filled intercellular spaces, where the oxygen concentration is predicted to be highest. Calculations of oxygen concentration and nitrogen fixation rates on an infected cell basis suggest that the measured rates of ATP production by isolated mitochondria are sufficient for the quantifiable in vivo requirements of ammonia assimilation and purine synthesis. The possible roles of mitochondrial respiration in the control of infected cell metabolism are also discussed.
Effects of elevated CO2 concentration and increased temperature on winter wheat: test of ARCWHEAT1 simulation model
Authors: MITCHELL R.A.C. , LAWLOR D.W. , MITCHELL V.J. , GIBBARD C.L. , WHITE E.M. , PORTER J.R.
Published: April 2006

Winter wheat (Triticum aestivum L., ev. Mercia) was grown in a controlled‐environment facility at two CO2 concentrations (targets 350 and 700 μmol mol−1), and two temperature regimes (tracking ambient and ambient + 4°C). Observations of phenology, canopy growth, dry matter production and grain yield were used to test the ARCWHEAT1 simulation model. Dry‐matter production and grain yield were increased at elevated CO2 concentration (27 and 39%, respectively) and reduced at increased temperature (−16 and −35%, respectively). ARCWHEAT1 substantially underestimated canopy growth for all treatments. However, differences in the facility environment from field conditions over the winter, indicated by the unusually rapid canopy growth observed in this period, meant that empirical model relationships were being used outside the conditions for which they were developed. The ARCWHEAT productivity submodel, given observed green area indices as inputs, overestimated the effect of CO2 on productivity. An alternative, more mechanistic submodel of productivity, based on the SUCROS87 and Farquhar & von Caemmerer models, simulated observed crop biomass very closely. When these productivity simulations were inputed into the ARCWHEAT1 partitioning and grain‐fill submodels, grain yield was predicted poorly, mainly as a result of the assumption that the number of grains is proportional to total growth during a short pre‐anthesis phase. While yield was not correlated with growth in this phase, it was correlated with growth in longer pre‐anthesis phases, indicating that ARCWHEAT1 could be improved by taking into account the contribution of earlier growth in determining yield.
Changes in trace metal species and other components of the rhizosphere during growth of radish
Authors: HAMON R. E. , LORENZ S. E. , HOLM P. E. , CHRISTENSEN T. H. , McGRATH S. P.
Published: April 2006

Changes in the properties of soil solution in the rhizosphere of developing radish plants were investigated. Variations in these properties were expected to affect the distribution and speciation of metals in the soil and soil solution. Applications of essential nutrients were linked to plant transpiration rates and prevented excess addition of nutrient ions, so that subtle changes in soil solution composition would not be obscured. Soil solution pH, the concentration of dissolved organic carbon (DOC) and the concentrations of major and trace elements in solution were found to vary over time. Strict control of fertilizer additions led to the maintenance of a relatively low ionic strength in the soil solution, and under such conditions trace metal solubility appeared to be highy influenced by the concentration of DOC. A chemical speciation analysis was performed which showed that, while dissolved Cd and Zn were largely uncomplexed in unplanted soil, Cd and Zn in the rhizosphere existed mainly as complexed forms. It is hypothesized that this is partly a result of Ca‐metal‐ligand equilibrium in solution, with higher Ca concentrations in unplanted soil leading to more of the Cd and Zn in solution existing in the uncomplexed state. Changes in the concentrations of uncomplexed Cd and Zn with time gave the best correlations with changes in plant uptake of these metals over time, supporting the hypothesis that plants mainly absorb the free metal ion from soil solution.
Effects of nitrogen limitation on water relations of jack pine (Pinus banksiana Lamb.) seedlings
Authors: TAN W. , HOGAN G. D.
Published: April 2006

The water relations of shoots of young jack pine (Pinus banksiana Lamb.) seedlings were examined 6 and 15 weeks after the initiation of four different dynamic nitrogen (N) treatments using a pressure‐volume analysis. The N treatments produced a wide range of needle N concentrations from 12 to 32 mg g−1 dry mass and a 10‐fold difference in total dry mass at 15 weeks. Osmotic potential at full turgor did not change over the range of needle N concentrations observed. Osmotic potential at turgor‐loss point, however, declined as N concentrations decreased, indicating an increased ability of N‐deficient jack pine plants to maintain turgor. The increase could be attributed largely to an increase in cell wall elasticity, suggesting that elasticity changes may be a common, significant adaptation of plants to environmental stresses. Dry mass per unit saturated water almost doubled as needle N level dropped from 32 to 12 mg g−1 and was inversely correlated to the bulk modulus of elasticity. This suggests that cell wall elasticity is determined more by the nature of its cross‐linking matrix than by the total amount of cell wall material present. Developmental change was evident in the response of some water relation variables to N limitation.
mRNA levels of four tomato (Lycopersicon esculentum Mill. L.) genes related to fluctuating plant and soil water status
Authors: THOMPSON A. J. , CORLETT J. E.
Published: April 2006

Expression of the genes le20, ni3212, le25 and lcyp2 was quantified in relation to soil and plant water status for pot‐grown tomato (Lycopersicon esculentum Mill. cv. Ailsa Craig) in a replicated trial in a partially environmentally controlled glasshouse. Leaf water potential, relative water content, stomatal conductance and gravimetric soil water content were measured at 0800,1200 and 1600 h on three days during the imposition of three different watering regimes. Paired leaf samples were analysed for mRNA. Plant‐to‐plant variability was quantified and partitioned. le20 mRNA was found in all leaves and responded both to short‐term diurnal fluctuations in water status and to more severe stress, though with reduced sensitivity to the greatest stresses. An overnight decline in le20 mRNA was independent of soil or plant water status. The pattern of expression for ni3212 was similar though ni3212 mRNA was much more abundant and the overnight drop in expression was not significant. le25 mRNA was only detected where leaf water potential was below −0.9 MPa and increased linearly with further decline in leaf water potential. lcyp2 was constitutively expressed with a diurnal pattern characterized by a sharp depression at 1600 h but with expression being enhanced by increasing water deficit at all times. The control and significance of the four genes are discussed in relation to the proposed function of their respective gene products.
Enhancement of seed germination in high salinity by engineering mannitol expression in Arabidopsis thaliana
Authors: THOMAS J. C. , SEPAHI M. , ARENDALL B. , BOHNERT H. J.
Published: April 2006

The bacterial gene mtlD, which encodes mannitol 1‐phosphate dehydrogenase (E. C. 1. 1. 1. 17), was transformed into Arabidopsis thaliana and expressed under control of the CaMV 35S promoter. MtlD‐transformants accumulated mannitol, a sugar alcohol that is not normally found in Arabidopsis. Amounts of soluble carbohydrates, sucrose, glucose, fructose, myo‐inositol and mannitol were determined in different tissues of wild‐type and transgenic plants. We estimated that less than 1& of the carbon assimilated was converted into mannitol by the transgenic plants. The establishment of individual transformed lines (after self‐crossing three times) resulted in high and low mannitol‐producing lines which were stably maintained. The presence of mannitol did not alter plant appearance or growth habit. When MtlD‐expressing seeds and control seeds (T3 generation) were imbibed with solutions containing NaCl (range 0 to 400 mol m−3), transgenic seeds containing mannitol germinated in medium supplemented with up to 400 mol m−3 NaCl, while control seeds ceased germination at 100 mol m−3 NaCl. It is doubtful whether the ability to germinate in high salt was a result of an osmotic effect exerted by elevated levels of mannitol, considering that mannitol concentrations were in the mol m−3 range in seeds. A specific effect of polyols, for example on the integrity of subcellular membranes or enzymes, cannot be excluded.
Guard cell zeaxanthin tracks photosynthetically active radiation and stomatal apertures in Vicia faba leaves*
Authors: SRIVASTAVA A. , ZEIGER E.
Published: April 2006

Zeaxanthin, antheraxanthin and violaxanthin concentrations in guard cells from sonicated abaxial epidermal peels of Vicia faba were measured from dawn to dusk, and compared with concentrations in mesophyll tissue of the same leaves. Measured changes in guard cell zeaxanthin and violaxanthin concentrations indicate that guard cells operate the xanthophyll cycle throughout the day. Mesophyll tissue had no detectable zeaxanthin at dawn, whereas guard cells had 30–50 mmol mol−1 chlorophyll a+b. On a chlorophyll basis, maximal zeaxanthin levels were 3–4 fold higher in guard cells than in mesophyll cells. Zeaxanthin concentrations tracked levels of photosynthetically active radiation (PAR) in both mesophyll and guard cells. In the mesophyll, most of the zeaxanthin changes occurred in mid‐morning and mid‐afternoon. In guard cells, zeaxanthin concentrations changed nearly linearly with PAR in the early morning and late afternoon, and closely tracked PAR levels throughout the day. Guard cell zeaxanthin concentrations were also closely correlated with stomatal apertures. The close relationship between zeaxanthin concentrations and PAR levels in guard cells indicates that zeaxanthin is well suited to function as a molecular photosensor in stomatal movements.
Nastic curvatures of wheat coleoptiles that develop in true microgravity
Authors: HEATHCOTE D. G. , CHAPMAN D. K. , BROWN A. H.
Published: April 2006

Dark‐grown wheat coleoptiles developed strong curvatures within 5 h of being transferred in orbit from a 1 g centrifuge to microgravity during an experiment flown on the IML‐1 shuttle mission. The curving tendency was strongest in seedlings that were immature, with coleoptiles shorter than 10 mm at the time of transfer. The curvature direction was non‐random, and directed away from the caryopsis (the coleoptile face adjacent to the caryopsis becoming convex). The curvatures were most marked in the basal third of the coleoptiles, contrasting with pho‐totropic responses, which occur in the apical third. We interpret these curvatures as being nastic, and related to the curvatures commonly reported to occur during clinostat rotation treatments.