bluuebunny/crossref_metadata_2025 huggingface dataset:
Nitrogen relations of the sorghum‐Sfriga hermonthica hostparasite association: growth and photosynthesis
Authors: CECHIN I. , PRESS M. C.
Published: April 2006

The extent to which the parasitic angiosperm Striga hermonthica reduces the growth of its sorghum host is dependent on the concentration of nitrogen (as NH4NO3 in 40% Long Ashton Solution) supplied to the plants. The biomass of 0.5,1 and 2 mol m−3 N‐grown infected plants was 22,30 and 66%, respectively, of uninfected plants after 140d growth. The biomass of 3 and 4 mol m−3 N‐grown infected plants differed little from uninfected plants. No grain was set in 0.5 and 1 mol m−3 N‐grown infected plants, grain yield reached 42 and 73% of controls in 2 and 3 mol m−3 N‐grown plants, and was unaffected in 4 mol m−3 N‐grown plants. Striga hermonthica also altered the allometry and architecture of the host, at all but the highest N concentration. Higher N concentration (3 and 4 mol m −3 N) reduced the growth of S. hermonthica. Foliar N concentrations in sorghum ranged from 11 mg g−1 dwt. in 0.5 mol m−3 N‐grown plants, to 28 mg g−1 dwt. in 4 mol m−3 N‐grown plants, and were not affected by S. hermonthica. Higher N concentrations were measured in S. hermonthica, and ranged from 18 to 45 mg g−1 dwt. in 0.5 and 3 mol m−3 N‐grown plants, respectively. The relationship between photosynthesis (CO2 flux) and N concentration differed between uninfected and infected sorghum. This was most apparent in 0.5 mol m−3 N‐grown plants, with rates of 16 and 11 μmol m−2 s−1 in uninfected and infected plants, respectively (at 1500–1800 μmol m−2 s−1 photosynthetic photon flux density). At higher N concentrations, this difference was smaller, with both sets of plants reaching 26 μmol m−2 s−1 at 4 mol m−3 N. Varying the level of S. hermonthica infection showed that the effect of N on host photosynthesis cannot be explained by differences in the mass or number of parasites supported by the host. At low levels of infection in 1 mol m−3 N‐grown plants, the negative effect of the parasite was reversed, and photosynthesis in infected plants exceeded that in uninfected plants by 20%. Photosynthesis in S. hermonthica at 3 mol m−3 N (8 μmol m−2 s−1) was double that in 0.5 mol m−3 N‐grown plants. Stable carbon isotope and gas exchange measurements data demonstrated that this higher level of autotrophic carbon fixation was accompanied by a lower dependency on hetero trophic carbon. The latter ranged from 27 to 6% in 0 5 mol m−3 and 3 mol m−3 N‐grown plants, respectively.
The effects of temperature and pH on the apparent Michaelis constant of glutathione reductase from maize (Zea mays L.)
Authors: TURNER L. B. , POLLOCK C. J.
Published: April 2006

The interacting effects of temperature and pH on the kinetics of glutathione reductase from maize have been studied in detail. The apparent Km for oxidized glutathione (GSSG) measured with desalted crude extracts increased in an exponential manner with rising temperature as a single variable. Increasing pH as a single variable also resulted in higher values of apparent Km for GSSG. When pH was allowed to vary with temperature, a curve which combined the pH and temperature responses was observed. Temperature had the stronger influence and this combined curve was displaced from the temperature curve due to the effect of pH. The pH to which the assay buffer was adjusted at 30°C had an influence on the pattern of the results in this type of experiment. The response of apparent Km for NADPH, and of apparent Km for GSSG using partially‐purified extracts, were also examined. The variation with temperature, at constant pH, was again exponential. The pattern of change of apparent Km with temperature is strongly dependent on experimental conditions. Affinity/temperature relationships deduced from such data would only reflect enzyme function in vivo if the physiological environment could be reproduced exactly in the assay mixture.
The effect of ammonium ions on membrane potential and anion flux in roots of barley and tomato
Authors: AYLING S. M.
Published: April 2006

The effects of ammonium (0–5 mol m−3) on root hair membrane potential and on the influx of nitrate and phosphate were investigated in roots of intact barley and tomato plants. In both species, addition of ammonium to the medium bathing the roots caused an almost immediate depolarization of the membrane potential; the depolarization was greater at higher concentrations of ammonium. Influx of 13NC3− and 32Pi was inhibited over the same time scale and concentration range. In tomato roots, there was little further depolarization of the membrane potential or inhibition of anion influx at ammonium concentrations above 0.4 mol m−3. In barley roots, the inhibition of nitrate influx and the depolarization of the membrane potential did not saturate below 5 mol m−3 ammonium.
Prosopis chilensis is a plant highly tolerant to heat shock
Authors: MEDINA C. , CARDEMIL L.
Published: April 2006

At temperatures between 25 and 35°C, 100% of Prosopis chilensis seeds germinated within 24 h. At higher temperatures, the germination rate was reduced; at 50°C, seeds did not germinate. After germination at 25°C, the optimal temperature for seedling growth was 35°C and the seedlings did not grow at a temperature of 50°C. However, when germination was at 35°C, the optimal temperature for seedling growth was 40°C and some seedlings grew at 50°C, suggesting that thermotolerance was induced during seed germination at 35°C. Further thermotolerance can be induced in seedlings germinated at 35°C, by exposing them to 40°C for 2h. Under these conditions, seedlings exhibited increased growth rate at 45 and 50°C. Fluorography of SDS‐polyacrylamide gel electrophoresis of the proteins synthesized and accumulated during 2 h at temperatures of 35, 40, 45 and 50°C in the presence of [35S]methionine revealed the expression of 11 proteins not detectable at 35°C. Most of the proteins present at 35°C also increased in expression. The temperature for maximal expression of these proteins was 45°C.
Purification of β1,4 glucanase from ethylene‐treated leaflet abscission zones of Sambucus nigra
Authors: WEBB S. T. J. , TAYLOR J. E. , COUPE S. A. , FERRARESE L. , ROBERTS J. A.
Published: April 2006

Leaflet abscission in Sambucus nigra is the result of cell wall breakdown at the site of separation. Associated with wall degradation is an increase in the activity of the enzyme β1,4 glucanase (E.C.3.1.2.4) in the cells that comprise the abscission zone. The enzyme has been extracted from abscission zone tissue and purified using a substrate affinity column. A qualitative enzyme assay procedure has been developed and this has facilitated the purification process. The β1,4 glucanase enzyme has a pH optimum of 7 and a molecular mass of 54kDa. Antibodies have been raised to the purified protein. The role of the enzyme in the abscission process is discussed.
Integration of hydraulic and chemical signalling in the control of stomatal conductance and water status of droughted plants
Authors: TARDIEU F. , DAVIES W. J.
Published: April 2006

We describe here an integration of hydraulic and chemical signals which control stomatal conductance of plants in drying soil, and suggest that such a system is more likely than control based on chemical signals or water relations alone. The determination of xylem [ABA] and the stomatal response to xylem [ABA] are likely to involve the water flux through the plant. (1) If, as seems likely, the production of a chemical message depends on the root water status (Ψr), it will not depend solely on the soil water potential (Ψs) but also on the flux of water through the soil‐plant‐atmosphere continuum, to which are linked the difference between Ψr and Ψs. (2) The water flux will also dilute the concentration of the message in the xylem sap. (3) The stomatal sensitivity to the message is increased as leaf water potential falls. Stomatal conductance, which controls the water flux, therefore would be controlled by a water‐flux‐dependent message, with a water‐flux‐dependent sensitivity. In such a system, we have to consider a common regulation for stomatal conductance, leaf and root water potentials, water flux and concentration of ABA in the xylem. In order to test this possibility, we have combined equations which describe the generation and effects of chemical signals and classical equations of water flux. When the simulation was run for a variety of conditions, the solution suggested that such common regulation can operate. Simulations suggest that, as well as providing control of stomatal conductance, integration of chemical and hydraulic signalling may also provide a control of leaf water potential and of xylem [ABA], features which are apparent from our experimental data. We conclude that the root message would provide the plant with a means to sense the conditions of water extraction (soil water status and resisance to water flux) on a daily timescale, while the short‐term plant response to this message would depend on the evaporative demand.
15N natural abundance of plant and soil components of a Banksia woodland ecosystem in relation to nitrate utilization, life form, mycorrhizal status and N2‐fixing abilities of component species
Authors: PATE J. S. , STEWART G. R. , UNKOVICH M.
Published: April 2006

Studies of the variation in δ15N values for plants from a fire‐prone Banksia woodland in South West Australia showed that pioneer herbaceous, non‐mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ15N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ15N values and moderately active in nitrate reduction and storage. Nitrogen‐fixing species had the lowest δ15N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ15N values which fell within the range of values obtained for the miscellaneous assemblage of N2‐fixing species. Consequently, 15N abundance values failed to distinguish N2 fixing from non‐fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N2 fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.
Effects of the ecto‐ and VA‐mycorrhizal fungi Hydnagium carneum and Glomus clarum on the δ15N and δ13C values of Eucalyptus globulus and Ricinus communis
Authors: HANDLEY L. L. , DAFT M. J. , WILSON J. , SCRIMGEOUR C. M. , INGLEBY K. , SATTAR M. A.
Published: April 2006

Glasshouse experiments with Ricinus communis showed that the presence/absence of a VA mycorrhizal fungus (Glomus clarum) changed the δ15N value of the host by as much as 2‰ when the plants were given urea (released as NH4+) as their only N‐source. This small change in Δ15N would create a large error in calculating sources of plant N. In particular, these results throw into doubt any models of N‐cycling which assume that soil N can be treated as a single source. The correct N‐source value for VAM‐infected NH4− ‐using plants may be the δ15N of soil NH4++ 2‰. Treatment effects were also found in the distribution of δ15N and % N among plant organs. Plants with VAM had a lower N:P atom ratio and were larger in total biomass. Carbon discrimination (δ13C) was greater in the VA‐infected plants. The measured effects of VAM infection suggest that for some plants the fungus may be the primary site of N assimilation. A parallel experiment with Eucalyptus globulus and the ectomycorrhizal fungus Hydnangium carneum resulted in no significant differences in any of the variables measured for this host‐fungus pair when the sole N‐sources were inorganic (NO3− and NH4+ released from urea). Ectomycorrhizal fungi are diverse in their physiological behaviour, and these data should not be taken as being representative of the whole group. More work is required with other types of mycorrhiza and more complex sources of N. Future work will include a water balance to partition the effects of water use and nutrient supply in determining δ13C. An on‐line combustion‐ANCA‐MS method is described for fully automated measurement of natural abundance levels of 15/14N and 13/12C for plant materials. This method achieves the required precision while dramatically increasing sample throughout.
Ear photosynthesis, carbon isotope discrimination and the contribution of respiratory CO2 to differences in grain mass in durum wheat
Authors: ARAUS J. L. , BROWN H. R. , FEBRERO A. , BORT J. , SERRET M. D.
Published: April 2006

The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO2 diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (Rd) was greater than the net photosynthesis rate (Pn) by a factor of 1.74 in the spike, whereas Rd was much smaller, only 22.1, 65.7 and 24.8% of Pn in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO2 exchange. Results of 13C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high Rd values of ears as well as their low diffusive conductance suggest that CO2 from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO2 as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear‐darkened plants, probably because of some degree of refixation (recycling) of respired CO2 in the grains.
Investigation of phosphoeno/pyruvate carboxylase (PEPCase) in Mesembryanthemum crystallinum L. in C3 and CAM photosynthetic states
Authors: SLOCOMBE S. P. , WHITELAM G. C. , COCKBURN W.
Published: April 2006

When exposed to osmotic stress, Mesembryanthemum crystallinum plants switch from C3 to CAM photosynthesis. Phosphoenolpyruvate carboxylase (PEPCase) is a key enzyme in CAM plants, being responsible for the initial fixation of CO2. In C3 plants the enzyme has been shown to be involved in the replenishing of TCA cycle intermediates and in the operation of stomatal guard cells. Multiple PEPCase isoforms were observed in C3‐performing leaves with four isoelectric points of 5.2, 5.5, 5.6 and 5.9 and four apparent subunit molecular masses of 105, 108, 113 and 116 kDa. In some instances, subunits of different size possessed exactly the same pI. The induction of CAM led to the predominance of a new isoform of pI 6.5 with subunit molecular mass of 108 kDa, but in addition, changes were observed in some of the isoforms present in the C3 plant. PEPCase subunits were purified from the C3 and CAM forms of M. crystallinum and subjected to pep‐tide mapping. Two distinct though similar sets of maps were obtained, one from the CAM isoform (pI 6.5) and C3‐associated subunits of pi 5.9 and another for C3 subunits of pI 5.2 and 5.5. It was inferred from these data that the C3 isoforms expressed in the leaf were derived from at least two genes. The C3 isoform (pI 5.9) showing greatest similarity to the CAM isoform in terms of peptide mapping also increased in response to salt stress. It is speculated that the CAM isoform may have evolved from this enzyme.
Degradation of proteins from thylakoid membranes in senescing wheat leaves at high temperature
Authors: FERGUSON* D. L. , AL‐KHATIB K. , GUIKEMA J. A. , PAULSEN G. M.
Published: April 2006

This investigation determined whether thylakoid proteins would be degraded more rapidly or not in senescing wheat (Triticum aestivum L. em. Thell.) leaves concurrently exposed to high temperatures. Excised leaves were pulse‐labelled with [35S]‐methionine for a 12 h period, and then incubated at 22,32 or 42°C for 0, 1, 2, or 3 d, before extracting a thylakoid enriched membrane sample. After electrophoretic separation, two prominent [35S]‐labelled protein bands were chosen for further analyses. Band A contained the D‐1 thylakoid protein and band B contained thylakoid proteins of the light harvesting complex (LHCII) associated with photosystem II (PSII). Total protein, [35S]‐labelled protein, band A protein, and band B protein within the thylakoid enriched membrane samples were measured. Unlabelled thylakoid enriched membrane samples, extracted from leaves given similar treatments, were used to measure uncoupled whole‐chain and photosystem II (PSII) electron transport and chlorophyll fluorescence. Accentuated decline in whole‐chain and PSII electron transport, increasing Fo values, and decreasing Fmax values were a result of high temperature injury in leaves treated at 42°C. None of the thylakoid enriched membrane protein fractions were degraded more rapidly in high‐temperature treated leaves. Degradation of the total [35S]‐labelled membrane proteins and band B was inhibited by the 42°C treatment. The results indicate that high temperature stress may disrupt some aspects of normal senescence.
Different vulnerabilities of Quercus ilex L. to freeze‐ and summer drought‐induced xylem embolism: an ecological interpretation
Authors: LO GULLO M. A. , SALLEO S.
Published: April 2006

Quercus ilex L. growing in the southern Mediterranean Basin region is exposed to xylem embolism induced by both winter freezing and summer drought. The distribution of the species in Sicily could be explained in terms of the different vulnerability to embolism of its xylem conduits. Naturally occurring climatic conditions were simulated by: (1) maintaining plants for 3h at ambient temperatures of 0, ‐1.5, ‐2.5, ‐5.0 and ‐11°C; and (2) allowing plants to dry out to ratios of their minimum diurnal leaf water potentials (Ψ1) to that at the turgor loss point (Ψtlp) of 0.6, 0.9, 1.05, 1.20 and 1.33. The loss of hydraulic conductivity of one‐year‐old twigs reached 40% at ‐1.5°C and at Ψ1/ΨtlP= 1.05. Recovery from these strains was almost complete 24 h after the release of thermal stress or after one irrigation, respectively. More severe stresses reduced recovery consistently. The percentages of xylem conduits embolized following application of the two stresses, were positively related to xylem conduit diameter. The capability of the xylem conduits to recover from stress was positively related to the conduit diameter in plants subjected to summer drought, but not in the plants subjected to winter freezing stress. The ecological significance of the different vulnerabilities to embolism of xylem conduits under naturally occurring climatic conditions is discussed.
Effects of increased CO2 concentration and temperature on growth and yield of winter wheat at two levels of nitrogen application
Authors: MITCHELL R. A. C. , MITCHELL V. J. , DRISCOLL S. P. , FRANKLIN J. , LAWLOR D. W.
Published: April 2006

Winter wheat (Triticum aestivum L., cv. Mercia) was grown in chambers under light and temperature conditions similar to the UK field environment for the 1990/1991 growing season at two levels each of atmospheric CO2 concentration (seasonal means: 361 and 692 μmol mol−1), temperature (tracking ambient and ambient +4°C) and nitrogen application (equivalent to 87 and 489 kg ha−1 total N applied). Total dry matter productivity through the season, the maximum number of shoots and final ear number were stimulated by CO2 enrichment at both levels of the temperature and N treatments. At high N, there was a CO2‐induced stimulation of grain yield (+15%) similar to that for total crop dry mass (+12%), and there was no significant interaction with temperature. This contrasts with other studies, where positive interactions between the effects of increases in temperature and CO2 have been found. Temperature had a direct, negative effect on yield at both levels of the N and CO2 treatments. This could be explained by the temperature‐dependent shortening of the phenological stages, and therefore, the time available for accumulating resources for grain formation. At high N, there was also a reduction in grain set at ambient +4°C temperature, but the overall negative effect of warmer temperature was greater on the number of grains (‐37%) than on yield (‐18%), due to a compensating increase in average grain mass. At low N, despite increasing total crop dry mass and the number of ears, elevated CO2 did not increase grain yield and caused a significant decrease under ambient temperature conditions. This can be explained in terms of a stimulation of early vegetative growth by CO2 enrichment leading to a reduction in the amount of N available later for the formation and filling of grain.
Growth and maintenance components of leaf respiration of cotton grown in elevated carbon dioxide partial pressure
Authors: THOMAS R. B. , REID C. D. , YBEMA R. , STRAIN B. R.
Published: April 2006

Elevated atmospheric carbon dioxide partial pressures have been shown to have variable direct and indirect effects on plant respiration rates. In this study, growth, leaf respiration, and leaf nitrogen and carbohydrate partitioning were measured in Gossypium hirsutum L. grown in 35 and 65 Pa CO2 for 30d. Growth and maintenance coefficients of leaf respiration were estimated using gas exchange techniques both at night and during the day. Elevated CO2 stimulated biomass production (107%) and net photo‐synthetic rates (35–50%). Total day‐time respiration (Rd) was not significantly affected by growth CO2 partial pressure. However, night respiration (Rn) of leaves grown in 65 Pa CO2 was significantly greater than that of plants grown in 35 Pa CO2. Correlation of Rd and Rn with leaf expansion rates indicated that plants in both CO2 treatments had equivalent growth respiration coefficients but maintenance respiration was significantly greater in elevated CO2. Increased maintenance coefficients in elevated CO2 appeared to be related to increased starch accumulation rather than to changes in leaf nitrogen.
Isoprene emission by plants is affected by transmissible wound signals
Authors: LORETO F. , SHARKEY T. D.
Published: April 2006

Isoprene (2‐methyl 1,3‐butadiene) is emitted from many plants, but the signals regulating isoprene emission are unknown. Mounting leaves in a gas exchange chamber or taking small leaf punches for biochemical analysis was found to reduce the rate of isoprene emission (Loreto & Sharkey 1993). This phenomenon was investigated by putting terminal leaflets of velvet bean (Mucuna deeringeniana L.) and kudzu [Pueraria lobaia (Willd) Ohwi.] into a gas exchange chamber and monitoring isoprene emission and photosynthesis. Lateral leaflets or remote leaves were then wounded or mechanically stimulated. The rate of isoprene emission was reduced after 1 min by up to 75% by burning a lateral leaflet with a match. Even a 7 ms−1 (25km h−1) wind imposed on a lateral leaflet reduced isoprene emission from the terminal leaflet by 18%. Photosynthesis rates were either unaffected by these treatments or reduced more slowly than isoprene emission rates, indicating that the effect of isoprene emission rates was not a consequence of changes in photosynthetic activity. Isoprene emission from a terminal leaflet was reduced by burning leaves above and below the monitored leaflet when on the same stem. The effect was much reduced if the burned leaf (all three leaflets) was on a different stem from the monitored leaflet. Reduction of the rate of isoprene emission was observed even when the burned leaf was 52 cm distant from the measured leaflet. Increasing the distance between the stressed leaf and the monitored leaf caused the effect to be slower and smaller. It is speculated that a signal is generated by wounding which propagates through the plant at 1.3 mm s−1. This velocity was consistent throughout the measurements and is similar to the rate of propagation of electrical signals such as action potentials and variation potentials. The effect of the environmental stress, and particularly the wind effect, can be frequent in nature and should be considered when estimating local and regional emission of isoprene for modelling atmospheric chemistry. If leaf samples used for isoprene determination are exposed to the type of stress we investigated, isoprene emission inventories based on leaf level measurements will be underestimated.
Facility for studying the effects of elevated carbon dioxide concentration and increased temperature on crops
Authors: LAWLOR D. W. , MITCHELL R. A. C. , FRANKLIN J. , MITCHELL V. J. , DRISCOLL S. P. , DELGADO E.
Published: April 2006

The requirements for the experimental study of the effects of global climate change conditions on plants are outlined. A semi‐controlled plant growth facility is described which allows the study of elevated CO2 and temperature, and their interaction on the growth of plants under radiation and temperature conditions similar to the field. During an experiment on winter wheat (cv. Mercia), which ran from December 1990 through to August 1991, the facility maintained mean daytime CO2 concentrations of 363 and 692 cm3 m−3 for targets of 350 and 700 cm3 m−3 respectively. Temperatures were set to follow outside ambient or outside ambient +4°C, and hourly means were within 0.5°C of the target for 92% of the time for target temperatures greater than 6°C. Total photosynthetically active radiation incident on the crop (solar radiation supplemented by artifieal light with natural photoperiod) was 2% greater than the total measured outside over the same period.
Genetic approaches to auxin action
Authors: HOBBIE L. , ESTELLE M.
Published: April 2006

Answers to long‐standing questions concerning the molecular mechanism of auxin action and auxin's exact functions in plant growth and development are beginning to be uncovered through studies using mutant and transgenic plants. We review recent work in this area in vascular plants. A number of conclusions can be drawn from these studies. First, auxin appears essential for cell division and viability, as auxin auxotrophs isolated in tissue culture are dependent on auxin for growth and cannot be regenerated into plants even when auxin is supplied exogenously. Secondly, plants with transgenes that alter auxin levels are able to regulate cellular auxin concentrations by synthesis and conjugation; wild‐type plants are probably also capable of such regulation. Thirdly, the phenotypes of transgenic plants with altered auxin levels and of mutant plants with altered sensitivity to auxin confirm earlier physiological studies which indicated a role for auxin in regulation of apical dominance, in development of roots and vascular tissue, and in the gravitropic response. Finally, the cloning of a mutationally identified gene important for auxin action, along with accumulating biochemical evidence, hints at a major role for protein degradation in the auxin response pathway.
Metabolic control analysis of plant metabolism
Authors: Ap REES T. , HILL S. A.
Published: April 2006

Metabolic control analysis and its major coefficients are introduced. The importance of measuring both elasticity and concentration‐control coefficients as well as flux‐control coefficients is stressed. The conditions that need to be met before control analysis can be applied experimentally are emphasized. It is argued that successful application of this approach requires methods for the measurement of flux, maximum catalytic activities of enzymes and substrate contents. The measurement of flux by consumption of substrate, production of product, and the distribution of isotope after metabolism of labelled substrates is discussed. The need to ensure that measurements of enzymes and substrates are reliable and authenticated is stressed. Particular emphasis is placed on the ease with which such measurements can be invalidated in plant tissues, and ways for countering these difficulties are discussed.
Use of transgenic plants and mutants to study the regulation and function of lipid composition
Authors: GIBSON S. , FALCONE D. L. , BROWSE J. , SOMERVILLE C.
Published: April 2006

Mutants and transgenic plants with altered expression of genes implicated in lipid metabolism are providing fresh insights into the regulation and function of lipid composition. To date, several genes encoding fatty acid desaturases, acyltransferases, a thioesterase, a lipid transfer protein and an isoform of acyl‐carrier protein have been introduced into transgenic plants. Despite the fact that some of these transgenic plants had large alterations in lipid composition, they showed surprisingly little phenotypic variation from wild‐type plants. Although detailed analyses of these plants are just beginning, several theories regarding the roles of particular genes in various plant processes, such as cold tolerance and transfer of lipids between membranes, have been either substantiated or discarded on the basis of the data already obtained. In addition, constructs that contain the promoter regions of genes implicated in lipid metabolism fused to reporter genes have been introduced into transgenic plants and are providing some clues as to how lipid composition is regulated.
Mutational analysis of blue‐light sensing in Arabidopsis
Authors: LISCUM E. , HANGARTER R. P.
Published: April 2006

Blue light regulates many physiological and developmental processes in higher plants through the action of multiple photosensory systems. The analysis of photomorphogenic mutants is leading to a better understanding of how the different photosensory systems mediate the wide range of responses observed in blue light. A review of the current literature on photomorphogenic mutants makes it apparent that redundancies exist in photoreceptor function. For example, many blue‐light responses that have been shown to be regulated by a blue‐light photosensory system are also under phytochrome control. The study of various light‐response mutants suggests that a complex sensory network regulates light‐mediated responses. This article attempts to piece together information regarding the sensory systems responsible for blue‐light‐regulated responses.
Manipulation of sink‐source relations in transgenic plants
Authors: SONNEWALD U. , LERCHL J. , ZRENNER R. , FROMMER W.
Published: April 2006

Since 1980, the use of transgenic plants in modern plant science has become a powerful tool to study whole plant physiology. In this review, we try to summarize the data obtained in the field of photoassimilate partitioning. Attempts to study sink‐source interactions concern factors which might limit sink strength and source capacity. Transgenic plants have been used to manipulate the sucrose to starch ratio in order to produce plants with higher sucrose levels in their source leaves. Alterations in partitioning were achieved by manipulating Calvin cycle enzymes, transport proteins and sucrose biosynthetic enzymes. The ability of sink tissues to attract photoassimilates has been altered by either increasing or decreasing sucrose hydrolytic activities. The increase of sucrose hydrolysis was achieved by creating transgenic potato plants with tuber specific yeast‐derived invertase. Decreased sucrose utilization was achieved by antisense inhibition of sucrose synthase in potato tubers.
Insights into root development from Arabidopsis root mutants
Authors: BENFEY P. N. , SCHIEFELBEIN J. W.
Published: April 2006

The root provides a useful system for the analysis of plant organ formation. Mutations that affect root development and physiology have been identified in Arabidopsis thaliana. Affected processes include embryonic root formation, cell expansion, cell differentiation and response to environmental stimuli. Analysis of these mutations is providing insight into fundamental questions of plant development.
Phenotypic plasticity of stem elongation in two ecotypes of Stellaria longipes: the role of ethylene and response to wind
Authors: EMERY R. J. N. , REID D. M. , CHINNAPPA C. C.
Published: April 2006

Using two ecotypes of Stellaria longipes an alpine form with low plasticity and a prairie form with high plasticity, we investigated whether ethylene was involved in the response to wind stress and might be important in controlling plasticity of stem elongation. Stem growth inhibition was positively correlated with concentration of ethephon application and elevation in ambient ethylene in alpine ecotypes, whereas stem growth in prairie plants was stimulated by low ethephon concentrations. When treated with high AVG, the effects were reversed: alpine plant growth was promoted and prairie plant growth was inhibited. Prairie plants exhibited a daily rhythm in ethylene evolution which increased and peaked at 1500 h, and which was absent in alpine plants. Ethylene evolution did not change significantly during the first 2 weeks of growth in alpine plants, whereas ethylene in prairie plants increased significantly during periods of rapid stem elongation. Wind treatment inhibited growth in both ecotypes, but only alpine plants showed a recovery of growth to control levels when wind stressed plants were pretreated with STS. In addition, only alpine plants showed an increase in ethylene evolution in response to wind simulation, whereas prairie plant ethylene evolution did not deviate from rhythms observed in unstressed plants. We concluded that ethylene dwarfs stems in alpine S. longipes in response to wind stress. However, low levels of ethylene may stimulate growth in prairie ecotypes and act independently of wind stress intensity. The contrasting ability to synthesize and respond to ethylene can account for part of the difference in plasticity documented between the two ecotypes.
How much sodium accumulation is necessary for salt tolerance in subspecies of the halophyte Atriplex canescens?
Authors: GLENN E. P. , OLSEN M. , FRYE R. , MOORE D. , MIYAMOTO S.
Published: April 2006

Two sympatric subspecies of the xerohalophyte Atriplex canescens Pursh. (Nutt.) were compared for 84 d in outdoor salinity trials in their native coastal desert environment in Sonora, Mexico. Subspecies linearis grows naturally on sea water in the high intertidal zone of estuaries while subspecies canescens grows on dunes. In lysimeter pot experiments, ssp. linearis exhibited 50% growth reduction when the mean root zone salinity reached 1160 mol m−3 NaCl compared to just 760 mol m−3 for ssp. canescens. When irrigated with sea water in a flood plot, ssp. linearis had 50% higher growth rates than ssp. canescens. The specialization of ssp. linearis for a saline environment was associated with greater net transport of Na+ from root to shoot, greater Na+ accumulation in the leaves and a higher Na:K ratio in the leaves compared to ssp. canescens. On the other hand, the two subspecies achieved approximately the same degree of osmotic adjustment in the leaves, equal to two to three times the external salinity, and had similar water use efficiencies. Even at relatively low salinities, both subspecies accumulated larger quantities of Na+ for osmotic adjustment than K+. The results suggest that breeding for Na+ accumulation rather than exclusion might be the more effective strategy for improving salt tolerance of conventional crop plants.
Sites of action of sulphite and bisulphite in the photosynthetic apparatus of sugar maple leaves as studied by photo‐acoustic and modulated fluorescence methods
Authors: N'SOUKPOÉ‐KOSSI C. N. , VEERANJANEYULU K. , LEBLANC R. M.
Published: April 2006

The photosynthetic activity of intact sugar maple leaves has been assessed in the presence of exogenous sulphite, bisulphite and sulphate under varying light conditions using photo‐acoustic and modulated fluorescence methods. In the light, bisulphite was found to be more toxic than the other two, sulphate being the least toxic of all. Interestingly, the vitality index, Rrd, measured as the ratio of the modulated fluorescence decrease (Fd) to the steady‐state fluorescence (Fs), which indicates the efficiency of the whole photosynthetic activity, was more affected than the total photosynthetic energy storage (PES) of PSII and PSI during linear and cyclic electron transport, and Fv/Fm(PSII activity). The severity of the damage appeared to be a function of light intensity. Bisulphite treatment in darkness resulted in a dramatic decrease in Rrd, a moderate decrease in PES and a marginal decrease in Fv/Fm. As for sulphite, the effect was negligible with a tendency for enhanced activity. It is inferred that the Calvin cycle is a good candidate for the primary site of bisulphite and sulphite action. Recovery of activity, especially at the Rrdlevel, obtained in the presence of ascorbate, glutathione and L‐cysteine, indicated a contribution of O2free radicals to the observed inhibition of the photosynthetic activity in the light.
Circadian rhythms and the induction of flowering in the long‐day grass Lolium temulentum L.
Authors: PÉRILLEUX C. , BERNIER G. , KINET J.‐M.
Published: April 2006

Plants of Lolium temulentum L. strain Ceres were grown in 8‐h short day (SD) for 45 d before being exposed either to a single long day (LD) or to a single 8‐h SD given during an extended dark period. For LD induction, the critical photoperiod was between 12 and 14 h, and more than 16 h were needed for a maximal flowering response. During exposure to a single 24‐h LD, the translocation of the floral stimulus began between the fourteenth and the sixteenth hours after the start of the light period, and was completed by the twenty‐fourth hour. Full flowering was also induced by one 8‐h SD beginning 4 or 28 h after the start of a 40‐h dark period, i.e. by shifting 12 h forward or beyond the usual SD. The effectiveness of a so‐called ‘displaced short day’ (DSD) was not affected by light quality and light intensity. With a mixture of incandescent and fluorescent lights at a total photosynthetic photon flux density of 400 μmol m−2 s−1, a 4‐h light exposure beginning 4 h after the start of a 40‐h dark period was sufficient to induce 100% flowering. The flower‐inducing effect of a single 8‐h DSD was also assessed during a 64‐h dark period. Results revealed two maxima at a 20‐h interval. This fluctuation in light sensitivity suggests that a circadian rhythm is involved in the control of flowering of L. temulentum.
Chloroplast ultrastructure changes in Pisum sativum associated with supplementary ultraviolet (UV‐B) radiation
Authors: HE J. , HUANG L.‐K. , WHITECROSS M.I.
Published: April 2006

Pea plants (Pisum sativum L. cv. Greenfeast) were grown and exposed to supplementary UV‐B radiation from day 17 after planting under growth cabinet conditions. The effects of this exposure on the ultrastructure of chloroplasts and the total soluble sugar and starch concentrations were estimated. Supplementary UV‐B radiation was shown to damage the structure of chloroplasts, as manifested by dilation of thylakoid membranes, a progressive disruption of the thylakoid structure and disintegration of the double membrane envelope surrounding the chloroplast, accompanied by the accumulation of large starch grains. Diurnal changes observed in starch concentration suggest that the higher concentration of starch in supplementary UV‐B‐treated leaves is due to its immobilization, rather than to any increase in starch synthesis: soluble sugars accumulated and remained at a higher level and then later declined.
Plant defence systems induced by ozone
Authors: KANGASJÄRVI J. , TALVINEN J. , UTRIAINEN M. , KARJALAINEN R.
Published: April 2006

Recent advances in the understanding of the molecular basis of plant response to ozone attack are reviewed. Plants grown in elevated atmospheric ozone are known to undergo several biochemical changes before any actual damage can be detected. These reactions include increases in the activities of enzymes associated with general plant defence mechanisms. Ozone exposure often causes a surge in the production of the plant hormone ethylene, as well as changes in polyamine metabolism and increases in the activities of several phenylpropanoid and flavonoid pathway enzymes. The activities of superoxide dismutase and peroxidases that protect cells from the oxidative damage caused by hydroxyl radicals, H2O2 and superoxides also increase. However, ozone‐induced changes in plant cells at the gene level are almost unknown. The limited data available suggest close similarities between ozone‐induced and pathogen‐induced defence responses in plants. Several general defence genes that have been cloned in other studies will soon be applied to studies of gene expression in ozone‐exposed plants. The use of molecular biological tools in ozone research should enable the development of highly specific and sensitive molecular markers for biomonitoring ozone‐induced injuries in plants.
Within‐season variability in mycorrhizal benefit to reproduction in Abutilon theophrasti Medic.
Authors: SHUMWAY D. L. , KOIDE R. T.
Published: April 2006

Mycorrhizal infection of Abutilon theophrasti Medic, increased seed quality and resultant offspring performance, but the improvement varied significantly with the timing of seed production. In general, offspring from mycorrhizal plants were superior to offspring from non‐mycorrhizal plants, but the difference was greatest for the early cohort of seeds. Because increased seed size and quality can affect subsequent offspring competitive ability, increased variability in seed size and quality due to mycorrhizal infection may have an important impact on population dynamics of A. theophrasti.
An osmotic hypothesis for the regulation of oxygen permeability in soybean nodules
Authors: PURCELL L. C. , SINCLAIR T. R.
Published: April 2006

Nodule permeability (P) controls the amount of O2 entering the nodule, and thereby the rates of both nodule respiration and N2 fixation. P may be regulated by changes in the effective thickness of a water‐filled diffusion barrier in the nodule cortex. Regulation of diffusion barrier thickness was hypothesized to result from changes in the water content of intercellular spaces. Modulation of intercellular water would be a response to osmotic potential gradients in the tissue. To test this hypothesis, preliminary experiments examined three classes of solutes (soluble sugars, free amino acids, and ureides) in nodules of intact plants exposed to 10 or 21 kPa O2 for 24 h. Neither soluble sugars nor free amino acids in nodules were responsive to O2 treatments. However, nodule ureides accumulated after exposure to 10kPa O2 for 24 h. A symplastic increase in nodule ureides under the 10kPa O2 treatment compared to the 21 kPa O2 treatment may have removed water from intercellular spaces in the nodule cortex and increased P. In addition, the nodule cortex of intact plants was infiltrated with water, polyethylene glycol (PEG), KC1, or Na‐succinate solutions to determine the effect of intercellular water and osmoticants on dinitrogenase activity and P. Results from infiltrating the apoplast of the nodule cortex with osmotic solutions indicated that both increases in intercellular water and decreases in the apoplastic water potential decrease dinitrogenase activity and P. Furthermore, the inability to recover dinitrogenase activity and P following the infiltration of the cortex with PEG compared to either KCl or Na‐succinate treatments may indicate that recovery was dependent upon removal of the solute from the apoplast.
Abscisic acid in apoplastic sap can account for the restriction in leaf conductance of white lupins during moderate soil drying and after rewatering
Authors: CORREIA M. J. , PEREIRA J. S.
Published: April 2006

We studied the effects of drought on leaf conductance (g) and on the concentration of abscisic acid (ABA) in the apoplastic sap of Lupinus albus L. leaves. Withholding watering for 5d resulted in complete stomatal closure and in severe leaf water deficit. Leaf water potential fully recovered immediately after rewatering, but the aftereffect of drought on stomata persisted for 2d. ABA and sucrose were quantified in pressurized leaf xylem extrudates. We assumed that the xylem sucrose concentration is negligible and hence that the presence of sucrose in leaf extrudates indicated that they were contaminated by phloem. To eliminate this interference, the concentration of ABA in leaf apoplast was estimated by extrapolation to zero sucrose concentration, using the regression between ABA and sucrose concentrations. The estimated apoplastic ABA concentration increased by 100‐fold with soil drying and did not return to pre‐stress values immediately following rewatering. g was closely related to the concentration of ABA in leaf apoplast. Furthermore, the feeding of exogenous ABA to leaves detached from well‐watered plants brought about the same degree of depression in g as resulted from the drought‐induced increase in ABA concentration. We therefore conclude that the observed changes in the concentration of ABA in leaf apoplast were quantitatively adequate to explain drought‐induced stomatal closure and the delay in stomatal reopening following rewatering.