bluuebunny/crossref_metadata_2025 huggingface dataset:
Acclimation processes in the light harvesting complex of the red alga Porphyridium purpureum (Bory) Drew et Ross, according to irradiance and nutrient availability
Authors: ALGARRA P. , RUDIGER W.
Published: April 2006

The time‐course of acclimation (0‐5h) of the red alga Porphyridium purpureum with respect to total proteins, phycoerythrin (PE) and phycobilisomes (PBS) has been studied at different N availability and different light regimes. After a high N input, acclimation takes place in two phases. The first one, which is photoindependent is characterized by simultaneous increase of proteins and PE. At low N input, this first phase is not detected. In the second phase the PE content increases only under low light together with an increase of the PBS size, followed probably by an increase in the number of PBS. The effectiveness of the energy transfer increases under these conditions. A rapid decrease in the PBS size correlated with a decrease of the energy transfer is observed at high irradiance. Free PE plays an important role in the organization‐disorganization of the PBS at low N concentration (inverse correlation between free PE and PE attached to PBS). Free PE is not accumulated in the cell after a high N input at high irradiance. Independently of photoacclimation, two species of PBS appear with different PE content and different capacities to aggregate with other compounds. A clear correlation appears between the level of coupling of the PBS and the fluorescence ‘in vivo’ of the whole cells. The comparison between dissociated and undissociated PBS as well as between PBS obtained after the different acclimation processes allows the determination of the presence of two linker polypeptides probably associated with B‐PE (37 and 32–5 kDa) and two associated with PC and APC (27 and 25 kDa). That suggests that acclimation of PBS requires a parallel stoichiometric response of biliproteins and the linker polypeptides involved in the efficiency of the energy transfer.
Root growth as a function of ammonium and nitrate in the root zone
Authors: BLOOM A. J. , JACKSON L. E. , SMART D. R.
Published: April 2006

We examined the effect of soil NH4+ and NO3− content upon the root systems of field‐grown tomatoes, and the influence of constant, low concentrations of NH4+ or NO3− upon root growth in solution culture. In two field experiments, few roots were present in soil zones with low extractable NH4+ or NO3−; they increased to a maximum in zones having 2μg‐N NO3− g−1 soil and 6 μg‐N NO3= g−1 soil, but decreased in zones having higher NH4+ or NO3− levels. Root branching was relatively insensitive to available mineral nitrogen. Plants maintained in solution culture at constant levels of NH4+ or NO3−, had similar shoot biomass, but all root parameters – biomass, length, branching and area – were greater under NH4 nutrition than under NO3−. These results suggest that the size of root system depends on a functional equilibrium between roots and shoots (Brouwer 1967) and on the balance between soil NH4+ and NO3−.
Modulation of photosynthesis and respiration in guard and mesophyll cell protoplasts by oxygen concentration
Authors: MAWSON B. T.
Published: April 2006

Stomatal movement is an energetic oxygen‐requiring process. In the present study, the effect of oxygen concentration on mitochondrial respiratory activity and red‐light‐dependent photosynthetic oxygen evolution by Vicia faba and Brassica napus guard cell protoplasts was examined. Comparative measurements were made with mesophyll cell protoplasts isolated from the same species. At air saturated levels of dissolved oxygen in the protoplast suspension media, respiration rates by mesophyll protoplasts ranged from 6 to 10μmoles O2 mg−1 chl h−1, while guard cell protoplasts respired at rates of 200–300 μmoles O2 mg chl−1 h−1, depending on the species. Lowering the oxygen concentration below 50–60 mmol m−3 resulted in a decrease in guard cell respiration rates, while rates by mesophyll cell protoplasts were reduced only at much lower concentrations of dissolved oxygen. Rates of photosynthesis in mesophyll cell protoplasts isolated from both species showed only a minor reduction in activity at low oxygen concentrations. In contrast, photosynthesis by guard cell protoplasts isolated from V. faba and B. napus decreased concomitantly with respiration. Oligomycin, an inhibitor of oxidative phos‐phorylation, reduced photosynthesis in mesophyll cell protoplasts by 27–46% and in guard cell protoplasts by 51–58%. The reduction in both guard cell photosynthesis and respiration following exposure to low oxygen concentrations suggest close metabolic coupling between the two activities, possibly mediated by the availability of substrate for respiration associated with photosynthetic electron transport activity and subsequent export of redox equivalents.
The effects of external NaCl on thylakoid stacking in lettuce plants
Authors: CARTER D. R. , CHEESEMAN J. M.
Published: April 2006

The average degree of thylakoid stacking was determined for loose‐leaf lettuce plants which were grown in complete nutrient solutions containing either 10 or 100mol m−3NaCl. Digitonin fractionation and differential centrifugation were used to assay the level of thylakoid stacking. Based on a comparison between 10mol m−3NaCl‐grown and 100mol m−3NaCl‐grown lettuce plants of equal ages, digitonin assays indicated that significantly less stacking occurred in 100mol m−3NaCl‐grown plants. Isolated thylakoid membranes from 100mol m−3NaCl‐grown plants were also characterized by a greater capacity to absorb divalent cations and by a higher chlorophylla/bratio. Since plants from both growth salinities were capable of a marked increase in thylakoid stacking upon a transition from high to low irradiance, the observed differences in thylakoid stacking were not attributed to a salinity‐related impairment of stacking mechanisms. Instead, the salinity‐induced differences in thylakoid stacking appear to represent a process of controlled adjustment.
Photoinhibition of isolated mesophyll cells from cold‐hardened and non‐hardened winter rye
Authors: LAPOINTE L. , HUNER N. P. A.
Published: April 2006

Cold‐hardened rye leaves have been shown to be more resistant to low temperature photoinhibition than non‐hardened rye leaves. Isolated mesophyll cells from winter rye (Secale cereale L. cv. Musketeer) were exposed to photoinhibitory light conditions to estimate the importance of leaf morphology and leaf optical properties in the resistance of cold‐hardened rye leaves to photoinhibition. Cold‐hardened rye cells showed more resistance to photoinhibition than non‐hardened rye cells when monitored with chlorophyll a variable to maximal fluorescence ratio (Fv/Fm). Thus, leaf morphology does not contribute to the resistance of cold‐hardened rye leaves to low temperature photoinhibition. However, cold‐hardened and non‐hardened rye cells showed a similar extent of photoinhibition when photsynthetic CO2 fixation rates were measured. They also showed the same capacity to recover from photoinhibition. During both photoinhibition and recovery, Fv/Fm and light limited CO2 fixation rates showed different kinetics. We propose that inactivation and subsequent reactivation during recovery of some light activated Calvin cycle enzymes explain the greater extent of photoinhibition of light limited CO2 fixation and its faster recovery compared to Fv/Fm kinetics during photoinhibition.
Extraction of water from Eucalyptus trees for analysis of deuterium and oxygen‐18: laboratory and field techniques
Authors: THORBURN P. J. , WALKER G. R. , BRUNEL J.‐P.
Published: April 2006

This study investigated a methodology for sampling and extracting water from Eucalyptus spp. for analysis of stable isotopes of water. Firstly, water of known isotopic composition was extracted from stems of seedlings of Eucalyptus spp. by azeotropic distillation using three solvents (hexane, toluene and kerosene) in laboratory and glasshouse experiments. Kerosene and toluene gave the most accurate extractions, with a small negative bias (∼‐2‰ for δ2H and ∼‐0.4‰ for δ18O) due to incomplete extraction. This bias was not evident in further experiments, using kerosene, where water was extracted from stems and roots that had been pre‐treated in a manner that more closely reflected field conditions. Secondly, variation in δ2H in trees was investigated in the field to determine an accurate and convenient place for sampling. There was no significant difference in δ2H between water extracted from trunk sapwood and whole twigs, even twigs close to leaves, in samples taken from four Eucalyptus spp. at two field sites. The results mean that the most convenient part of the tree can be sampled. Also, there was little difference in δ2H between sapwood and heart‐wood samples, although small increases (∼ 2‰) in δ2H were observed in bark of trunks and twigs. Tree roots (15–25 mm diameter) had similar δ2H values to the surrounding soil in many cases, especially surface (<0.2 m depth) roots, rather than having δ2H values similar to twigs of the trees. These roots were absorbing water from surrounding soil, but not contributing significant quantities of water to the tree canopy. Thus, the δ2H of root water is not a good indicator of the isotopic composition of water in the upper parts of Eucalyptus trees.
Limitation of transpiration by hydraulic conductance and xylem cavitation in Betula occidentalis
Authors: SPERRY J. S. , POCKMAN W. T.
Published: April 2006

The extent to which stomatal conductance (gs) was capable of responding to reduced hydraulic conductance (k)and preventing cavitation‐inducing xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air‐injection technique. From 1 to 18 d after shoot injection we measured midday transpiration rate (E), gs, and xylem pressure (Ψp‐xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2–3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and gs. The k measurement was expressed per unit leaf area (kl, leaf‐specific conductance). Leaves measured within 2 d of shoot injection showed reduced gs and E relative to non‐injected controls, and both parameters were strongly correlated with kl At this time, there was no difference in leaf Ψp‐xylem between injected shoots and controls, and leaf Ψp‐xylem was not significantly different from the highest cavitation‐inducing pressure (Ψp‐cav) in the branch xylem (‐1.43 ± 0.029 MPa, n=8). Leaves measured 7–18 d after shoots were injected exhibited a partial return of gs and E values to the control range. This was associated with a decrease in leaf Ψp‐xylem below Ψp‐cav and loss of foliage. The results suggest the stomata were incapable of long‐term regulation of E below control values and that reversion to higher E caused dieback via cavitation.
The effects of enhanced ozone and enhanced carbon dioxide concentrations on biomass, pigments and antioxidative enzymes in spruce needles (Picea abies L.)
Authors: POLLE A. , PFIRRMANN T. , CHAKRABARTI S. , RENNENBERG H.
Published: April 2006

During one growing period, 5‐year‐old spruce trees (Picea abies L., Karst.) were exposed in environmental chambers to elevated concentrations of carbon dioxide (750 cm3 m−3) and ozone (008 cm3 m−3) as single variables or in combination. Control concentrations of the gases were 350cm3 m−3CO2 and 0.02 cm3 m −3 ozone. To investigate whether an elevated CO2 concentration can prevent adverse ozone effects by reducing oxidative stress, the activities of the protective enzymes superoxide dismutase, catalase and peroxidase were determined. Furthermore, shoot biomass, pigment and protein contents of two needle age classes were investigated. Ozone caused pigment reduction and visible injury in the previous year's needles and growth reduction in the current year's shoots. In the presence of elevated concentrations of ozone and CO2, growth reduction in the current year's shoots was prevented, but emergence of visible damage in the previous year's needles was only delayed and pigment reduction was still found. Elevated concentrations of ozone or CO2 as single variables caused a significant reduction in the activities of superoxide dismutase and catalase in the current year's needles. Minimum activities of superoxide dismutase and catalase and decreased peroxidase activities were found in both needle age classes from spruce trees grown at enhanced concentrations of both CO2 and ozone. These results suggest a reduced tolerance to oxidative stress in spruce trees under conditions of elevated concentrations of both CO2 and ozone.
Effects of water velocity on photosynthesis and dark respiration in submerged stream macrophytes
Authors: MADSEN T. V. , ENEVOLDSEN H. O. , JØRGENSEN T. B.
Published: April 2006

The effects of flow velocities on dark respiration and net photosynthesis of eight submerged stream macrophytes were examined in a laboratory oxygen chamber. The shoots/leaves were exposed to saturating free‐CO2 concentrations and were attached basally so that they could move in the flowing water. Net photosynthesis declined by 34–61% as flow velocity increased from 1 to 8.6cm s−1, while dark respiration increased 2.4‐fold over the same range. The increase in dark respiration could only account for between 19 and 67% of the decrease in net photosynthesis. The relationship between flow velocity (U) and net photosynthesis (P) was described by: P=b×Ua. The exponent, a, varied from ‐0.20 to –0.48 and showed a negative correlation to the surface: volume (SA: V) ratio of the plants, i.e. species with high SA: V ratio were more sensitive to flow. In contrast, net photosynthesis of plants firmly attached to a supporting frame was not significantly affected by increasing flow velocity. This result indicates that the physical stress imposed on the plants by agitation or stretching in the flowing water is a key factor for the observed response.
Sodium relations in Chenopodiaceae: a comparative approach
Authors: REIMANN C. , BRECKLE S.‐W.
Published: April 2006

Sodium relations of 15 species of Chenopodiaceae were studied in seedlings grown on quartz sand at 10 mol m−3 of sodium and potassium. Uptake of sodium and potassium into whole plants and shoots was followed over 2 weeks. High alkali ion uptake rates were found in all species. The apparent selectivity of alkali ion uptake showed a continuous variation between species, from nearly perfect sodium exclusion to negligible cation selection. K/Na ratios above 6 were found in the shoots of eight species. For most of these plants above ground sodium concentrations were highest in the hypocotyls. However, in Chenopodium hybridum (shoot K/Na = 10) and C. urbicum (shoot K/Na = 17) above ground sodium concentrations were lowest in hypocotyls and highest in leaves, as in those species accumulating larger amounts of sodium. These differences are discussed with respect to the underlying mechanisms of ion regulation.
Plastid‐localized 3‐deoxy‐D‐arabino‐heptulosonate 7‐phosphate synthase (DS‐Mn): the early‐pathway target of sequential feedback inhibition in higher plants
Authors: DOONG R. L. , GANSON R. J. , JENSEN R. A.
Published: April 2006

A plastid‐localized isozytne of 3‐deoxy‐D‐arabino‐heptulosonate 7‐phosphate (DAHP) synthase, denoted DS‐Mn, has been identified in a number of higher‐plant species. Parallel characterizations were made of DS‐Mn from Spinacia oleracea leaf tissue, Solanum tuberosum tubers, and Nicotiana silvestris suspension culture as sources of enzyme from plant materials which vary in phytogeny, developmental and tissue state, and physiological state. A highly conserved property of DS‐Mn is a transition between inactive and active states, mediated by DTT as a hysteretic activator. A procedure for isolation of DS‐Mn in the labile, inactive state is given. The process of activation appears to exhibit a higher pH optimum than the catalytic optimum. DTT‐containing preparations are very stable. The enzyme characteristically exhibits stimulation by Mn++ in the range of 45–50%, relatively high affinity for erythrose‐4‐phosphate (E4P), dramatic substrate inhibition above about 0.5mol m−3 E4P, sigmoid substrate saturation curves for both E4P and phosphoenolpyruvate, and inhibition by L‐arogenate (competitive against E4P and non‐competitive against PEP). DS‐Mn has a relatively high temperature optimum in the range of 45–50°C. Enzyme activity was lost when bound metal was stripped away by EDTA treatment. Reconstitution of the native‐enzyme level of activity was obtained with Ca++, and additional stimulation was achieved with Mn++. DS‐Mn control by L‐arogenate in the chloroplast is proposed as one key circuit in an overall pattern of allosteric control for the entire pathway of aromatic amino acid biosynthesis. This pattern is called sequential feedback inhibition. The potential for modulation of this control system by environmental cues induced by light‐dark transitions is discussed.
Stomatal control by both [ABA] in the xylem sap and leaf water status: a test of a model for draughted or ABA‐fed field‐grown maize
Authors: TARDIEU F. , ZHANG J. , GOWING D. J. G.
Published: April 2006

A model of maize stomatal behaviour has been developed, in which stomatal conductance is linked to the concentration of abscisic acid ([ABA]) in the xylem sap, with a sensitivity dependent upon the leaf water potential (Ψ1). It was tested against two alternative hypotheses, namely that stomatal sensitivity to xylem [ABA] would be linked to the leaf‐to‐air vapour pressure difference (VPD), or to the flux of ABA into the leaf. Stomatal conductance (gs) was studied: (1) in field‐grown plants whose xylem [ABA] and Ψ1 depended on soil water status and evaporative demand; (2) in field‐grown plants fed with ABA solutions such that xylem [ABA] was artificially raised, thereby decreasing gs and increasing Ψ1 and leaf‐to‐air VPD; and (3) in ABA‐fed detached leaves exposed to varying evaporative demands, but with a constant and high Ψ1. The same relationships between gs, xylem [ABA] and Ψ1, showing lower stomatal sensitivity to [ABA] at high Ψ1, applied whether variations in xylem [ABA] were due to natural increase or to feeding, and whether variations in Ψ1, were due to changes in evaporative demand or to the increased Ψ1 observed in ABA‐fed plants. Conversely, neither the leaf‐to‐air VPD nor the ABA flux into the leaf accounted for the observed changes in stomatal sensitivity to xylem [ABA]. The model, using parameters calculated from previous field data and the detached‐leaf data, was tested against the observations of both ABA‐fed and droughted plants in the field. It accounted with reasonable accuracy for changes in gs (r2 ranging from 0.77 to 0.81). These results support the view that modelling of stomatal behaviour requires consideration of both chemical and hydraulic aspects of root‐to‐shoot communication.
Stomatal and environmental control of transpiration in a lowland tropical forest tree
Authors: MEINZER F. C. , GOLDSTEIN G. , HOLBROOK N. M. , JACKSON P. , CAVELIER J.
Published: April 2006

Stomatal control of crown transpiration was studied in Anacardium excelsum, a large‐leaved, emergent canopy species common in the moist forests of Central and northern South America. A construction crane equipped with a gondola was used to gain access to the uppermost level in the crown of a 35‐m‐tall individual. Stomatal conductance at the single leaf scale, and transpiration and total vapour phase conductance (stomatal and boundary layer) at the branch scale were measured simultaneously using the independent techniques of porometry and stem heat balance, respectively. This permitted the sensitivity of transpiration to a marginal change in stomatal conductance to be evaluated using a dimensionless coupling coefficient (1‐ω) ranging from zero to 1, with 1 representing maximal stomatal control of transpiration. Average stomatal conductance varied from 0.09 mol m−2 s−1 during the dry season to 0.3 mol m−2 s−1 during the wet season. Since boundary layer conductance was relatively low (0.4 mol m−2 s−1), 1‐ω ranged from 0.46 during the dry season to only 0.25 during the wet season. A pronounced stomatal response to humidity was observed, which strongly limited transpiration as evaporative demand increased. The stomatal response to humidity was apparent only when the leaf surface was used as the reference point for measurement of external vapour pressure. Average transpiration was predicted to be nearly the same during the dry and wet seasons despite a 1 kPa difference in the prevailing leaf‐to‐air vapour pressure difference. The patterns of stomatal behaviour and transpiration observed were consistent with recent proposals that stomatal responses to humidity are based on sensing the transpiration rate itself.
Stress perception and response in a facultative halophyte: the regulation of salinity‐induced genes in Mesembryanthemum crystallinum
Authors: VERNON D. M. , OSTREM J. A. , BOHNERT H. J.
Published: April 2006

The facultative halophyte Mesembryanthemum crystallinum (ice plant) shifts from C3 to CAM photosynthesis in response to water stress. The switch to CAM can also be induced by high salinity, presumably because osmotic stress causes water loss from leaf and stem tissues. A number of mRNAs (Ppc1, Imt1, B5 and Gpd1) that encode proteins involved in different biochemical pathways accumulate in M. crystallinum leaf tissues when plants are salt‐stressed. We hypothesized that environmental challenges that result in water stress invoke a common mechanism that triggers the coordinated induction of mRNAs involved in different aspects of the ice plant's adaptive stress response. Nuclear run‐on experiments indicated that Ppc1, Imt1, B5 and Gpd1 are all transcriptional I y activated in salt‐stressed leaf tissue. However, a comparison of Ppc1, Imt1, and B5 transcript levels after exposure of plants to growth regulators (6‐benzylamino purine and abscisic acid) and to different environmental stress treatments that affect plant water status (drought, low temperature and salinity) indicated that, despite the coordinated transcriptional activation of these genes during salt stress, their repsonses to other stimuli that also upset water balance are not the same. Our results are consistent with a model involving multiple control mechanisms governing stress perception and molecular response in the ice plant.
Influence of electrolytes on growth, phototropism, nutation and surface potential in etiolated cucumber seedlings
Authors: SPALDING E. P. , COSGROVE D. J.
Published: April 2006

A variety of electrolytes (10–30 mol m−3) increased the relative growth rate of etiolated cucumber (Cucumis sativus L. cv. Burpee's Pickler) hypocotyls by 20–50% relative to water‐only controls. The nonelectrolyte mannitol inhibited growth by 10%. All salts tested were effective, regardless of chemical composition or valence. Measurements of cell‐sap osmolality ruled out an osmotic mechanism for the growth stimulation by electrolytes. This, and the nonspecificity of the response, indicate that an electrical property of the solutions was responsible for their growth‐stimulating activity. Measurements of surface electrical potential supported this reasoning. Treatment with electrolytes also enhanced nutation and altered the pattern of phototropic curvature development. A novel analytical method for quantitating these effects on growth was developed. The evidence indicates that electrolytes influence an electrophysiological parameter that is involved in the control of cell expansion and the coordination of growth underlying tropisms and nutations.
Xylem‐transported abscisic acid: the relative importance of its mass and its concentration in the control of stomatal aperture
Authors: GOWING D. J. G. , JONES H. G. , DAVIES W. J.
Published: April 2006

Abscisic acid (ABA) fed in pulses to the petioles of detached cherry leaves in enclosed leaf chambers, caused a reduction in leaf conductance. The degree of inhibition was analysed with respect to the amount of ABA fed and to concentration of ABA in the feeding solution. Regression analysis of the data showed both variables to have a significant effect on leaf conductance. A hypothetical maximum ABA concentration occurring in the leaf apoplast was calculated for each pulse from a simple model. This variable explained more of the variance within the data than either the amount or the applied concentration variable. A value for the rate at which ABA is removed from the apoplast is derived from the experimental data using the model. A second experiment attempted to evaluate this rate directly, by measuring the rate of catabolism of labelled ABA within the leaf. The results suggested a half‐life of 36 min for the initial rate of decay. This figure is similar to that derived from the model, the importance of ABA‐metabolism for the control of leaf conductance is discussed in the context of root‐to‐shoot communication by ABA in the xylem stream.
Rapid photosynthetic adaptation to heat stress triggered in potato leaves by moderately elevated temperatures
Authors: HAVAUX M.
Published: April 2006

Photosystem II (PSII) is considered to be one of the most thermolabile aspects of photosynthesis.In vivomeasurements of chlorophyll fluorescence and photosynthetic oxygen evolution in 25°C‐grown potato leaves (cv. Haig) indicated that the threshold temperature Tcabove which PSII denatures was indeed rather low–about 38°C–with temperatures higher than Tc causing a rapid and irreversible loss of PSII activity. The present study demonstrates the existence of adaptive processes which rapidly adjust the invivothermal stability of PSII in response to temperature increase. Transfer of potato leaves from 25°C to temperatures slightly lower than Tc(between 30 and 35°C) was observed to cause an upward shift of the Tc value without any appreciable loss of PSII activity. This increase in PSII thermotolerance was substantial (around +5°C in the Haig cultivar), rapid (with a half‐time of ∼20 min) and slowly reversible at 25°C (>24h). As a consequence, high temperatures (e.g. 40°C) which caused a complete and irreversible inhibition of the PSII function had very little effect in 35°C‐treated leaves, thus suggesting that the above‐described PSII changes could be of prime importance for the plant's behaviour in the field. Accordingly, the rise in Tc at 35°C was much larger (+8°C) in Sahel, a stress‐resistant potato variety, than in the heat‐sensitive Haig cultivar.
Role of abscisic acid in chilling tolerance of rice (Oryza sativa L.) seedlings. I. Endogenous abscisic acid levels
Authors: LEE T.‐M. , LUR H.‐S. , CHU C.
Published: April 2006

Changes of ABA levels in chilled rice (Oryza sativa L.) seedlings of two varieties were determined. On exposure to chilling, ABA concentration rapidly increased in the chilling‐tolerant cultivar (cv. Tainung 67, TNG.67) but not in the chilling‐sensitive cultivar (cv. Taichung Native 1, TN.1). Both detached shoots and roots of TNG.67 seedlings showed a significant ABA increase after exposure to chilling. TN.1 seedlings could not accumulate ABA under low temperature but well‐watered status. Exogenous application of the ABA biosynthetic inhibitor, fluridone, reduced ABA accumulation, as well as survival ratio of chilled TNG.67 seedlings. Electrolyte leakage and leaf conductance were also increased by the inhibitor and the effects could be reversed by exogenously applied ABA. ABA concentrations in xylem sap of TNG.67 seedlings increased within 4 h after chilling, and this was temporally coincident with the reduction of leaf conductance. The roles of endogenous ABA in the tolerance of rice seedlings to chilling on a whole plant basis are discussed and suggested.
Carbonic anhydrase in Ranunculus penicillatus spp. pseudofluitans: activity, location and implications for carbon assimilation
Authors: NEWMAN J. R. , RAVEN J. A.
Published: April 2006

Levels of carbonic anhydrase activity were determined on a total (60 EUmg−1 protein), external (7.36 EU), internal (50.14 EU) and protoplast (15.63 EU) basis for Ranunculus penicillatus (Dumort.) Bab ssp. pseudofluitans (Syme) S. Webster, a freshwater aquatic macrophyte, by conventional electrometric methods. The site of activity of ‘external’ carbonic anhydrase (CA) has been visualized using 5‐Dimethylaminonapthalene‐1‐ sulphonamide (DNSA)‐CA fluorescent complex formation, and is postulated to be closely associated with the epidermal cell wall. The photosynthetic rate of R. penicillatus ssp. pseudofluitans at pH 9.0 is in excess of the uncatalysed rate of production of CO2 from HCO−3, and this plant is therefore using HCO−3 for photosynthesis. The possible contribution of CA activity to inorganic carbon assimilation, and specifically to transport of HCO−3, in submerged aquatic plants is discussed.
Starch‐degrading enzymes during the induction of CAM in Mesembryanthemum crystallinum
Authors: PAUL M. J. , LOOS K. , STITT M. , ZIEGLER P.
Published: April 2006

Mesembryanthemum crystallinum plants were irrigated with 400 mol m−3 NaCl to induce CAM and levels of leaf starch, and activities of starch‐degrading enzymes were measured. During Crassulacean acid metabolism (CAM) induction, daily starch turnover gradually became more pronounced and was three‐ to four‐fold greater than in leaves of C3 plants after 3 weeks. Activities of α‐ and β‐amylase, D‐enzyme and starch phosphorylase all increased 10‐ to 20‐fold within 3 weeks of the start of salt treatment. Activities of α‐ and β‐amylase increased more than fourfold within the first 24 h of salt treatment, which is the fastest increase in enzyme activities so far measured during the induction of CAM with salt solution in intact plants of this species. Most enzyme activities were partially chloroplastic; however, the principal starch‐degrading activity was constituted by an extra‐chloroplastic β‐amylase. CAM starch‐phosphorylase activity, which was mainly chloroplastic, exhibited a two‐ to three‐fold diurnal change in parallel with starch content. CAM induction in M. crystallinum is clearly associated with greater starch turnover and enhanced starch‐degrading enzyme activities, which as catalysts of the initial reaction to release carbon for synthesis of phosphoenolpyruvate (PEP) appear highly significant for the functioning of the CAM pathway. The diurnal rhythm of phosphorylase activity may be of particular significance.
Effects of salinity and turgor on calcium influx in Chara
Authors: REID R. J. , TESTER M. , SMITH F. A.
Published: April 2006

Measurements were made of the influx of 45Ca into internodal cells of Chara corallina in solutions containing high concentrations of NaCl. Increasing salinity in the range 4–100mol m−3 NaCl resulted in a doubling of Ca2+ influx at the plasmalemma. A time‐course of Ca2+ influx in 50 mol m−3 NaCl, 0.5mol m−3 CaCl2 showed that while influx at the plasmalemma increased only 1.5‐fold, influx to the vacuole increased by up to 15‐fold. This was interpreted as being due to inhibition of active Ca2+ efflux from the cell. The stimulation of Ca2+ influx by increasing salinity appeared to be principally a response to reduced turgor since similar stimulations were obtained when turgor was reduced by NaCl, Na2SO4 or mannitol. When cells were plasmolysed Ca2+ influx increased by 10–20‐fold. The increased permeability was relatively specific for Ca2+ and was inhibitable by La3+. Survival of cells in high salt conditions was increased by 30 mmol m−3 La3+, which inhibited Ca2+ influx. Paradoxically, survival can also be extended by increasing external Ca2+ which leads to a higher influx. Therefore, it seems unlikely that the ameliorative effect of Ca2+ on the sensitivity of plants to high NaCl is mediated by Ca2+ entry across the plasmalemma. It seems more likely that the principal role of Ca2+ under these conditions is exerted externally through the control of membrane voltage and permeability.
Modulation of leaf elongation, tiller appearance and tiller senescence in spring barley by far‐red light
Authors: SKINNER R. H. , SIMMONS S. R.
Published: April 2006

Supplemental far‐red (FR) illumination of light‐grown grass seedlings inhibits tiller production while enhancing leaf elongation. Although much is known about FR enhancement of internode elongation in dicots, relatively little research has been conducted to determine the effects of FR on monocot development. In growth chamber experiments, fibre optics were used to direct supplemental FR to elongating leaf blades, main stem bases and mature leaf blades of light‐grown barley (Hordeum vulgare L.) seedlings. Our objective was to identify specific sites of perception for FR enhancement of leaf elongation and inhibition of tiller production, and to assess potential FR effects on tiller senescence. Far‐red illumination of elongating leaves or of the main stem base reduced the total number of tillers per plant, primarily by reducing secondary and tertiary tiller production, and enhanced leaf elongation. However, leaf elongation was less sensitive to stem base treatments than to illumination of the elongating blade. Increased leaf length resulted from increased leaf elongation rate, while the duration of leaf elongation was unaffected. Exposure of mature leaf blades to FR had no effect on tillering or leaf elongation. None of the FR treatments led to tiller senescence. Localization of FR perception in vertically oriented tissues such as elongating blades and stem bases permits early detection of reflected light from neighbouring plants, allowing rapid response to impending competition.
The effect of flutter on the temperature of poplar leaves and its implications for carbon gain
Authors: RODEN J. S. , PEARCY R. W.
Published: April 2006

The leaf temperatures of two poplar species (Populus tremuloides Michx. and P. fremontii Wats.) were characterized by attaching thermocouples to leaves that were either constrained to a fixed position or allowed to flutter naturally. There were no observed temperature differences between fluttering and constrained leaves in the lower canopy, but fluttering leaves at the top of the canopy were as much as 2–4°C cooler than constrained leaves. An increase in heat transfer, a decrease in light interception or both could account for these observed differences in the temperature of fluttering versus constrained leaves. Fluttering can increase the boundary‐layer conductance to convective heat exchange by as much as 50 and 20% for laminar and turbulent flow, respectively. The benefit that these leaf temperature differences may provide to the carbon economy of a poplar canopy was dependent on the ambient temperature. Populus fremontii, which is frequently exposed to daytime temperatures exceeding 35°C during summer months in the central valley of California, USA, could show an increase in carbon gain as a result of lower upper canopy leaf temperatures. For aspen, the benefit would be much smaller and often negative because of much lower air temperatures. Lower leaf temperatures may also increase the water use efficiency of poplars. However, the maintenance of lower leaf temperatures may not be the primary adaptive significance of leaf flutter.
Evidence for direct utilization of a siderophore, ferrioxamine B, in axenically grown cucumber
Authors: WANG Y. , BROWN H. N. , CROWLEY D. E. , SZANISZLO P. J.
Published: April 2006

To eliminate the confounding effects of microorganisms and to examine the direct utilization of microbial siderophores as iron sources by higher plants, a hydroponic cultural system and methodology was developed to grow plants with axenic roots. This report presents a description of this system, and also its use to determine the efficacy of the microbial siderophore ferrioxamine B (FOB), compared to the synthetic iron chelate FeEDTA, and the phytosiderophores (PS) of barley as an iron source for alleviating iron stress in the model dicot cucumber. It was observed that FOB gave superior plant biomass and was preferentially utilized to restore chlorophyll synthesis in long‐term experiments when chelates were supplied at 5mmol m−3 concentrations and nutrient solution was buffered against pH change at 7.4 with solid phase CaCO3. In addition, autoradiograms indicated that 59Fe from FOB was rapidly translocated to shoots through vascular tissues and was specifically distributed to regions of rapid growth and to iron‐stressed, but still expanding young leaves. The siderophore itself could be detected within 2h in xylem exudates, regardless of whether or not plants were exposed to metabolic inhibitors. It was concluded that the FOB and iron were taken up by the axenic roots of cucumber in a highly efficient manner, most likely as the iron‐siderophore complex, and at rates that could be significant to dicot nutrition. The results also suggested that cucumber may transport FOB through the transpiration stream to upper parts of plants, where the iron would be reductively released from the siderophore for shoot nutrition.
Heat balance sap flow gauge for small diameter stems
Authors: SENOCK R. S. , HAM J. M.
Published: April 2006

Applying heat balance sap flow gauges to plant stems <10 mm in diameter has been difficult because a miniature design is needed that can be attached to a range of stem geometries. This report presents a modified gauge design for use on small plant stems of irregular geometry and shows results from Glycine max with stem diameters of 3–4 mm. The gauge was evaluated on container‐grown plants by comparing gauge measurements of flow to gravimetric estimates of transpiration. Experiments were conducted in the laboratory and greenhouse, using artificial and natural lighting, respectively. Laboratory comparisons of gauge versus gravimetric water loss measurements indicated that the instrument was accurate to within ±5% when soil water was not limiting. Similar results were obtained from greenhouse tests except when soil water availability was low and cumulative gauge estimates became 30–45% less than gravimetric measurements. Differences may have reflected reduced plant water uptake or errors in sap flow estimates associated with low flow rates. Gauge accuracy was not improved by including the rate change in heat storage (S) in the flow calculations because S was always less then 3% of the total heat balance. Relationships between system temperature and sap flow rate suggested a diagnostic test for determining optimum power input. A time constant of 15 s indicated potential application in many agronomic and physiological studies.
Does Rubisco control the rate of photosynthesis and plant growth? An exercise in molecular ecophysiology
Authors: STITT M. , SCHULZE D.
Published: April 2006

Experiments are described in which tobacco (Nicotiana tabacum L.) transformed with antisense rbcS to decrease expression of ribulose‐1,5‐bisphosphate carboxylase‐oxygenase (Rubisco) was used to evaluate the contribution of Rubisco to the control of photosynthetic rate, and the impact of a changed rate of photosynthesis on whole plant composition, allocation and growth. (1) The concept of flux control coefficients is introduced. It is discussed how, with adequate precautions, a set of wild‐type and transgenic plants with varying expression of an enzyme can be used to obtain experimental values for its flux control coefficient. (2) The flux control coefficient of Rubisco for photosynthesis depends on the short‐term conditions. It increases in high light, or low CO2. (3) When plants are grown under constant irradiance, the flux control coefficient in the growth conditions is low (<0.2) at irradiances of up to 1000μmol quanta m−2 s−1. In a natural irradiance regime exceeding 1500μmol quanta m−2 s−2 over several hours the flux coefficient rose to 0.8–0.9. It is concluded that plants are able to adjust the balance between Rubisco and the remainder of the photosynthetic machinery, and thereby avoid a one‐sided limitation of photosynthesis by Rubisco over a wide range of ambient growth irradiance regimes. (4) When the plants were grown on limiting inorganic nitrogen, Rubisco had a higher flux control coefficient (0.5). It is proposed that, in many growth conditions, part of the investment in Rubisco may be viewed as a nitrogen store, albeit bringing additional marginal advantages with respect to photosynthetic rate and water use efficiency. (5) A change in the rate of photosynthesis did not automatically translate into a change in growth rate. Several factors are identified which contribute to this buffering of growth against a changed photosynthetic rate. (6) There is an alteration in whole plant allocation, resulting in an increase in the leaf area ratio. The increase is mainly due to a higher leaf water content, and not to changes in shoot/root allocation. This increased investment in whole plant leaf area partly counteracts the decreased efficiency of photosynthesis at the biochemical level. (7) Plants with decreased Rubisco have a lower intrinsic water use efficiency and contain high levels of inorganic cations and anions. It is proposed that these are a consequence of the increased rate of transpiration, and that the resulting osmotic potential might be a contributory factor to the increased water content and expansion of the leaves. (8) Starch accumulation in source leaves is decreased when unit leaf photosynthesis is reduced, allowing a more efficient use of the fixed carbon. (9) Decreased availability of carbohydrates leads to a down‐regulation of nitrate assimilation, acting via a decrease in nitrate reductase activity.
The use of mutants and transgenic plants to study nitrate assimilation
Authors: HOFF T. , TRUONG H.‐N. , CABOCHE M.
Published: April 2006

The nitrate assimilatory pathway has been the matter of intensive genetic and molecular analysis over the past decade. Mutants impaired in the expression of nitrate reductase have been characterized in a number of plant species. Molecular analysis of the Nia gene coding for nitrate reductase has been the basis for a three‐domain model of the structure of the enzyme, in agreement with biochemical and genetic data. Mutagenesis and antisense strategies have led to the description of nitrite reductase deficiencies. The molecular analysis of the corresponding Nii genes has provided invaluable information on the structure of nitrite reductase. Recently, a gene involved in nitrate uptake has also been identified. The regulation of the nitrate assimilatory pathway has been investigated. Analysis of the regulation of the pathway at the molecular level has shown evidence for the involvement of nitrate, light and/or sucrose, and reduced nitrogen in the regulation. Surprisingly, no bona fide regulatory mutant specific to this pathway has been identified so far in higher plants. This may reflect the redundancy of regulatory genes. The deregulated expression of one or the other step of the pathway obtained by ectopic expression of the corresponding genes is a new approach to study the physiological role of these regulations. Elements of the pathway have also been successfully used as transposon traps, or negatively selectable markers for other purposes. Finally, the identification at the molecular level of regulatory genes and structural elements involved in transport and storage of nitrate, or in the biosynthesis of cofactors of nitrate and nitrite reductases, will be the goal of the next decade.
Protection against oxygen radicals: an important defence mechanism studied in transgenic plants
Authors: FOYER C. H. , DESCOURVIÈRES P. , KUNERT K. J.
Published: April 2006

Free radicals and other active derivatives of oxygen are inevitable by‐products of biological redox reactions. Reduced oxygen species, such as hydrogen peroxide, the superoxide radical anion and hydroxyl radicals, inactivate enzymes and damage important cellular components. In addition, singlet oxygen, produced via formation of triplet state chlorophyll, is highly destructive. This oxygen species initiates lipid peroxidation, and produces lipid peroxy radicals and lipid hydroperoxides that are also very reactive. The increased production of toxic oxygen derivatives is considered to be a universal or common feature of stress conditions. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem. The antioxidant defence system of the plant comprises a variety of antioxidant molecules and enzymes. Considerable interest has been focused on the ascorbate‐glutathione cycle because it has a central role in protecting the chloroplasts and other cellular compartments from oxidative damage. It is clear that the capacity and activity of the antioxidative defence systems are important in limiting photo‐oxidative damage and in destroying active oxygen species that are produced in excess of those normally required for signal transduction or metabolism. In our studies on this system, we became aware that the answers to many unresolved questions concerning the nature and regulation of the antioxidative defence system could not be obtained easily by either a purely physiological or purely biochemical approach. Transgenic plants offered us a means by which to achieve a more complete understanding of the roles of the enzymes involved in protection against stress of many types: environmental and man‐made. The ability to engineer plants which express introduced genes at high levels provides an opportunity to manipulate the levels of these enzymes, and hence metabolism in vivo. Studies on transformed plants expressing increased activities of single enzymes of the antioxidative defence system indicate that it is possible to confer a degree of tolerence to stress by this means. However, attempts to increase stress resistance by simply increasing the activity of one of the antioxidant enzymes have not always been successful presumably because of the need for a balanced interaction of protective enzymes. The study of these transformed plants has allowed a more complete understanding of the roles of individual enzymes in metabolism. Protection against oxidative stress has become a feasible objective through the application of molecular genetic techniques in conjunction with a biochemical and physiological approach.
The use of mutants and transgenic plants to study amino acid metabolism
Authors: LEA P. J. , FORDE B. G.
Published: April 2006

Mutants of higher plants with alterations in amino acid metabolism have now been available for 20 years. Following the realization that at least four distinct classes of herbicides (phosphinothricins, glyphosates, imidazolinones and sulphonylureas) act by the inhibition of amino acid biosynthesis, mutants resistant to the herbicides have also been obtained. More recently, transgenic plants containing altered levels of enzymes of amino acid biosynthesis have been constructed. In this article, we have attempted to review several areas of amino acid biosynthesis including ammonia assimilation, the aspartate pathway, branched chain amino acids, aromatic amino acids and proline.