bluuebunny/crossref_metadata_2025 huggingface dataset:
Interaction between atmospheric and pedospheric nitrogen nutrition in spruce (Picea abies L. Karst) seedlings
Authors: MULLER B. , TOURAINE B. , RENNENBERG H.
Published: April 2006

The effect of NO2 fumigation on root N uptake and metabolism was investigated in 3‐month‐old spruce (Picea abics L. Karst) seedlings. In a first experiment, the contribution of NO2 to the plant N budget was measured during a 48 h fumigation with 100mm3m−3 NO2. Plants were pre‐treated with various nutrient solutions containing NO2 and NH4+, NO3− only or no nitrogen source for 1 week prior to the beginning of fumigation. Absence of NH4+ in the solution for 6d led to an increased capacity for NO3− uptake, whereas the absence of both ions caused a decrease in the plant N concentration, with no change in NO3− uptake. In fumigated plants, NO2 uptake accounted for 20–40% of NO3− uptake. Root NO3− uptake in plants supplied with NH4+plus NO3− solutions was decreased by NO2 fumigation, whereas it was not significantly altered in the other treatments. In a second experiment, spruce seedlings were grown on a solution containing both NO2 and NH4+ and were fumigated or not with 100mm3m−3 NO2 for 7 weeks. Fumigated plants accumulated less dry matter, especially in the roots. Fluxes of the two N species were estimated from their accumulations in shoots and roots, xylem exudate analysis and 15N labelling. Root NH4+ uptake was approximately three times higher than NO3− uptake. Nitrogen dioxide uptake represented 10–15% of the total N budget of the plants. In control plants, N assimilation occurred mainly in the roots and organic nitrogen was the main form of N transported to the shoot. Phloem transport of organic nitrogen accounted for 17% of its xylem transport. In fumigated plants, neither NO3− nor NH4+ accumulated in the shoot, showing that all the absorbed NO2 was assimilated. Root NO3− reduction was reduced whereas organic nitrogen transport in the phloem increased by a factor of 3 in NO2‐fimugated as compared with control plants. The significance of the results for the regulation of whole‐plant N utilization is discussed.
Carbon isotope composition of C4 grasses is influenced by light and water supply
Authors: BUCHMANN N. , BROOKS J. R. , RAPP K. D. , EHLERINGER J. R.
Published: April 2006

The carbon isotope composition of C4 grasses has the potential to be used as an indicator of changes in the isotopic composition and concentration of atmospheric CO2, especially for climate reconstruction. The usefulness of C4 grasses for this purpose hinges on the assumption that their photosynthetic discrimination against 13C remains constant in a wide range of environmental conditions. We tested this assumption by examining the effects of light and water stress on the carbon isotope composition of C4 grasses using different biochemical subtypes (NADP‐ME, NAD‐ME, PCK) in glasshouse experiments. We grew 14 different C4 grass species in four treatments: sun‐watered, sun‐drought, shade‐watered and shade‐drought. Carbon isotope discrimination (Δ) rarely remained constant. In general, Δ values were lowest in sun‐watered grasses, greater for sun‐drought plants and even higher for plants of the shade‐watered treatment. The highest Δ values were generally found in the most stressed grasses, the shade‐drought plants. Grasses of the NADP‐ME subtype were the least influenced by a change in environmental variables, followed by PCK and NAD‐ME subtypes. Water availability affected the carbon isotope discrimination less than light limitation in PCK and NAD‐ME subtypes, but similarly in NADP‐ME subtypes.

In another experiment, we studied the effect of increasing light levels (150 to 1500 μmol photons m−2 s−1) on the Δ values of 18 well‐watered C4 grass species. Carbon isotope discrimination remained constant until photon flux density (PFD) was less than 700 μmol photons m−2 s−1. Below this light level, Δ values increased with decreasing irradiance for all biochemical subtypes. The change in A was less pronounced in NADP‐ME and PCK than in NAD‐ME grasses. Grasses grown in the field and in the glasshouse showed a similar pattern. Thus, caution should be exercised when using C4 plants under varying environmental conditions to monitor the concentration or carbon isotopic composition of atmospheric CO2 in field/glasshouse studies or climate reconstruction.
Microclimate and fluxes of water vapour, sensible heat and carbon dioxide in structurally differing subalpine plant communities in the Central Caucasus
Authors: Tappeiner U. , Cernusca A.
Published: April 2006

Effects of canopy structure on microclimate, energy budget and CO2 exchange were analysed in a pasture, two hay meadows, a tall herb community and a dwarf shrub community in the subalpine belt of the Central Caucasus. The results show that canopy structure exerts a marked influence on the distribution of photon flux density, temperature and canopy photosynthesis Ac. Three canopy types were distinguished. Type 1 (pasture) has a small LAI (leaf area index) and more than two‐thirds of the phytomass is concentrated in the lowest few cm of the canopy, mainly as planophile leaves. This results in (1) a low degree of utilization of photosynthetic photon flux density (PPFD) by assimilatory plant components, (2) high leaf temperatures and a high soil heat flux during the phase of incoming radiation, and (3) a relatively low Ac/ LAI ratio. Type 2 (meadows), in spite of its erect leaves, which at high solar elevations permit light to penetrate to the lower canopy layers, is characterized by (1) marked effects of mutual shading in the lower canopy layers for most of the day, and thus (2) only slight variations in air and leaf temperatures and (3) a comparatively low Ac/LAI ratio. In canopies of type 3 (tall herb and dwarf shrub communities), there is a concentration of flat leaves in the upper layers. This results in (1) very good utilization of PPFD; (2) no strong fluctuations in canopy temperature as the flat leaves act as a buffer, reducing the amounts of incoming and outgoing radiation in lower canopy layers, and (3) high values of the Ac/LAI ratio. The energy budgets of the canopies investigated are governed not so much by their spatial structure, but rather indirectly by LAI and the degree of coupling of the canopy with the atmosphere.
Spatial variation in carbon isotope discrimination across the thalli of several lichen species
Authors: MÁGUAS C. , BRUGNOLI E.
Published: April 2006

Variations in stable carbon isotope discrimination (δ) were investigated across the thalli of several lichen species possessing different photobiont associations. Lichens containing (i) green algae (phycobiont), (ii) green algae in association with cyanobacteria confined in cephalodia, or (iii) cyanobacteria (cyanobiont) as the photobiont partner were studied. Carbon isotope discrimination was analysed in different thallus sections, which varied in distance from the margin and in age. The marginal thallus region is considered to be youngest, while the central region is thought to be oldest. This analysis showed a clear variation in δ across the thallus related to distance from the growing margin. In most of the species examined, the highest δ values were found in marginal regions (younger), while the central and basal regions (older) showed significantly lower δ. To investigate the effects of the historical increase in atmospheric CO2 concentration and the concurrent decrease in the 13C content of atmospheric CO2 on the δ of lichens, experiments were carried out on herbarium samples of Lobaria pulmonaria collected from the mid 19th Century to 1953. The results obtained showed a pattern of variation of δ consistent with that of freshly collected samples; δ decreased substantially with increasing distance from the thallus margin, irrespective of the collection date. Moreover, no consistent variation of discrimination was found among different collection dates. These results demonstrate that the observed variation in δ is caused by age‐related changes in the physiological behaviour of different thallus sections, and that the past 150 years of increasing CO2 concentration have not had significant effects on A in L. pulmonaria. Photosynthesis measurements, chlorophyll analysis and observations using optical microscopy, performed on freshly collected lichens, showed significant changes in morphological and physiological characteristics across the thallus. Particularly, remarkable variations in thickness were found across the thallus. These anatomical changes may be responsible for the variation in δ, through variations in CO2 transfer resistance and, consequently, in CO2 availability across the thallus. The lack of age‐dependent variation in δ in cyanobiont lichens is possibly attributable to the operation of a CO2‐coneentrating mechanism and, therefore, to a more constant CO2 environment across the thallus in this lichen group.
Toxicity symptoms caused by high expression of Tet represser in tomato (Lycopersicon esculentum Mill. L.) are alleviated by tetracycline
Authors: CORLETT J. E. , MYATT S. C. , THOMPSON A. J.
Published: April 2006

As a first step towards transferring a tetracycline (Tc)‐inducible gene expression system to tomato, we have transformed tomato plants with the Tn10‐encoded tet repressor gene (tetR). Homozygous transformed plants with high expression of tetR mRNA show a deleterious phenotype, having reduced shoot dry weights and leaf chlorophyll content, an even more marked reduction in root dry weight and leaf size, and altered photosynthetic physiology. It appears that TetR protein exerts its toxicity only when expressed beyond a threshold level and by interacting with a process that is non‐limiting under slow growth conditions. The deleterious phenotype was almost completely reversed by the application of 1 mg dm−3 Tc to plants grown in sand. The possiblity is discussed that TetR causes these symptoms by binding to a specific DNA sequence functioning as a Tet operator.

The effect of Tc on growth and physiology in wild‐type plants grown in sand or rockwool is described. Tc at 0.1 mg cm−3 had no effect. Tc at 1 mg dm−3 caused a small reduction in root growth, while 5 and 20 mg dm−3 Tc caused large reductions in growth and photosynthetic parameters.
Wetting of the upper needle surface of Abies grandis: influence of pH, wax chemistry and epiphyllic microflora on contact angles
Authors: SCHREIBER L.
Published: April 2006

Wetting of the upper needle surface of Abies grandis Lindl. by aqueous solutions of different pH values was investigated. With increasing needle age, contact angles decreased significantly from about 75° on current‐year needles to values lower than 30° on 4‐year‐old needles. On older needles, contact angles were significantly lower, by more than 10°, when aqueous solutions of pH9‐0 were used compared with those of pH3‐0. On the surfaces of older needles, contact angle titrations were carried out, contact angles being measured with aqueous solutions covering a pH range from 3.0 to 11.0. Measured titration curves showed clear inflection points around pH 7.0, indicating the existence of ionizable carboxylie groups in the interface between needle surface and atmosphere. The evidence seems convincing that the pronounced pH dependence of wetting is mainly due to the presence and/or activity of epiphyllic micro‐organisms, whereas the cuticular wax composition of Abies grandis needles does not appear to contribute significantly to this phenomenon. Thus, the results presented here allow the general conclusion that changes of contact angles measured on leaf surfaces may not always be due to changes in the leaf surface chemistry and/or the fine structure of leaf surface waxes, but may also be due to increased amounts of epiphyllic micro‐organisms significantly altering the leaf surface wetting properties.
Phloem transport of abscisic acid in Ricinus communis L. seedlings
Authors: ZHONG W. , HARTUNG W. , KOMOR E. , SCHOBERT C.
Published: April 2006

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m−3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3‐fold and that in the sieve tube sap increasing by up to 50‐fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase.

Incubation of the cotyledons of endosperm‐free seedlings in various ABA concentrations (up to 100 mmol m−3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K+ and Mg2+ within the experimental period, i.e. up to 10h.

It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem‐mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.
The effects of pH and dissolved inorganic carbon on external carbonic anhydrase activity in Chlorella saccharophila
Authors: WILLIAMS T. G. , COLMAN B.
Published: April 2006

External carbonic anhydrase (CA) activity in Chlorella saccharophila is suppressed by growth at high dissolved inorganic carbon and at acid pH. External CA activity was shown to be suppressed by growth at pHs below 7.0, with total repression at pH5.0. Growth in the presence of the buffer 3‐[N‐Morpholino]propane‐sulphonic acid (MOPS) between pH 7 and 8 suppressed CA activity. Cells grown at pH8.0 aerated at 6 dm3 h−1 exhibited external CA activity of 5 units mg−1 Chl once the dissolved inorganic carbon (DIC) was reduced to 300 mmol m−3, and this increased to 30 units mg−1 Chl over a period of 3d while the DIC dropped to 30mmol m−3. Cells aerated at 180 dm3 h−1 showed a similar trend in CA activity, although the onset was delayed by 1 d and the DIC did not drop below 300 mmol m−3. Cells grown at pH 7.8 near an air equilibrium DIC of 300 mmol m−3had no detectable external CA activity. It is probable that it is the CO2 supply to the cell, and not total DIC or HCO−3 which controls external CA activity. Cells grown at pH 5.0 had no detectable activity, although they reduced the CO2 concentration to 0.6 mmol m−3. The loss of CA upon transfer of air‐grown cells to 10 mmol mol−1 CO2 took place over 48 h and was light dependent, while the loss upon transfer from alkaline pH to acid pH look place over 12 h and was independent of light. The effects of pH are independent of the response to CO2.
Evidence for a role for L‐proline as a salinity protectant in the cyanobacterium Nostoc muscorum
Authors: SINGH A. K. , CHAKRAVARTHY D. , SINGH T. P. K. , SINGH H. N.
Published: April 2006

Nostoc muscorum required an active proline oxidase in order to assimilate exogenous proline as a source of fixed nitrogen. A mutant strain (Acr) resistant to growth inhibition by L‐azetidine‐2‐carboxylate (AC) was found to be deficient in proline oxidase activity, and to be a proline overaccumulator. Proline overaccumulation, resulting either from mutational acquisition of the Acr phenotype or from salinity‐inducible uptake of exogenous proline, conferred enhanced salinity tolerance in this cyanobacterium.
Effect of cytokinins supplied via the xylem at multiples of endogenous concentrations on transpiration and senescence in derooted seedlings of oat and wheat
Authors: BADENOCH‐JONES J. , PARKER C.W. , LETHAM D.S. , SINGH S.
Published: April 2006

This study was conducted lo determine whether naturally occurring xylem cytokinins, when supplied to leaves via the xylem at approximately endogenous concentrations, increase transpiration and delay senescence in selected monocot species (oat and wheat). The concentrations of some of the major cytokinins (zeatin, dihydrozeatin, ciszeatin and their ribosides, the O‐glucosides and nucleotides) were determined in the xylem exudate of oat and wheat seedlings by radioimmunoassay. Evidence is presented that the small volume of exudate (4–5 mm3) collected per plant was xylem sap in transit at the time of shoot excision. Using the data on cytokinin levels, the individual bases and ribosides (and a base/riboside mixture), at multiples of concentrations determined in xylem sap, were tested in transpiration and senescence bioassays. The individual O‐glucosides (and mixtures of the O‐glucosides) were similarly tested at (i) multiples of the molar concentrations of the corresponding bases and ribosides, and/or at (ii) multiples of the endogenous concentrations. Similarly, zeatin and dihydrozeatin nucleotides were tested at multiples of the molar concentration of zeatin riboside and, in some instances, at multiples of endogenous concentrations.

Our results suggest that, at least in oat and possibly in wheat, zeatin‐type bases, ribosides and O‐glucosides supplied to the leaf in xylem sap are likely to play a role in regulating transpiration in vivo. O‐glucosides in oat xylem sap may be important regulators of leaf senescence in the intact plant. The nucleotides were present in xylem sap at lower concentrations than most of the bases, ribosides and O‐glucosides. The nucleotides appear likely to play a lesser role than the bases, riboside and O‐glucosidcs in controlling transpiration and senescence in the intact plant.
Aphototropic mutants of the moss Ceratodon purpureus with spectrally normal and with spectrally dysfunctional phytochrome
Authors: LAMPARTER T. , ESCH H. , COVE D. , HUGHES J. , HARTMANN E.
Published: April 2006

Following UV mutagenesis of protonemal tissue of the moss Ceratodon purpureus we have isolated different aphototropic mutant lines that can be divided into two distinct classes. One class, represented by the line ptr1, shows characteristic features of phytochrome chromophore deficiency. ptrl shows negligible photoreversibility (<5% of wild type), whereas immunoblots show normal apoprotein levels. The aphototropic phenotype could be partially restored with biliverdin, a precursor of the phytochrome chromophore. It was found that, whereas in wild type formation of Pfr leads to suppression of gravitropism, there is no such suppression ptrl. In addition, ptr1 shows lower chlorophyll levels than the wild type. These findings indicate that, as expected for a chromophore‐deficient mutant, multiple phytochrome effects are lost. The other class of mutants, represented by the line ptr103, shows more specific effects. In this mutant, only phototropism is affected. Suppression of gravitropism, the content of chlorophyll and photoreversibility of phytochrome were similar to those of the wild type.
Short‐term regulation of cytosolic Ca2+, cytosolic pH and vacuolar pH under NaCl stress in the charophyte alga Nitellopsis obtusa
Authors: OKAZAKI Y. , KIKUYAMA M. , HIRAMOTO Y. , IWASAKI N.
Published: April 2006

Isolated characean internodal cells of Nitellopsis obtusa can be stored in artificial pond water for many days, but they cannot survive in 100mol m−3 NaCl solution unless more than several mol m−3 Ca2+ is added. Short‐term effects of NaCl stress on the cytosolic concentration of Ca2+ ([Ca2+]c), cytosolic pH (pHc) and vacuolar pH (pHv) were studied in relation to the external concentration of Ca2+ ([Ca2+]e). Changes in [Ca2+]c were measured with light emission from a Ca2+‐sensitive photoprotein, semisynthetic fch‐aequorin which had been injected into the cytosol. Both pHc and pHv were measured with double‐barrelled pH‐sensitive microelectrodes. When internodal cells were treated with 100 mol m−3 NaCl (0–1 mol m−3 NaCl (0.1 mol m−3 [Ca2+]e), [Ca2+]c increased and then recovered to the original level within 60 min. The time course of the transient change in [Ca2+]c was not influenced by the level of [Ca2+]c (0.1 and 10 mol m−3). In some cases, the transient increase in [Ca2+]c was induced only by increasing external osmotic pressure with sorbitol. In response to treatment with 100 mol m−3 NaCl (0.1 mol m−3 [Ca2+]c), pHc decreased by 0.1–0.2 units after 10min but recovered after 30–60 min, while pHv increased by 0.4–0.5 units after 2–50 min and tended to recover after 60 min. The initial changes in both pHc and pHv were suppressed when [Ca2+]e was raised from 0.1 to 10mol m−3. These results show that the charophyte alga Nitellopsis can regulate [Ca2+]c, pHc and pHv under NaCl stress in the short term and that the protective effect of Ca2+ on salinity stress is apparently unrelated to perturbation of Ca2+ and pH homeostasis.
Immunogold localization of nodule‐enhanced phosphoenolpyruvate carboxylase in alfalfa
Authors: Robinson D. L. , Pathiran S. M. , Gantt J. S. , Vance C. P.
Published: April 2006

Phosphoenolpyruvate carboxylase (PEPC; EC 4‐1‐1‐31) plays a paramount role in providing carbon for synthesis of malate and aspartate in alfalfa (Medicago sativa L.) root nodules. PEPC protein and activity levels are highly enhanced in N2‐fixing alfalfa nodules. To ascertain the relationship between the cellular location of PEPC and root nodule metabolism, enzyme localization was evaluated by immunogold cytochemistry using alfalfa nodule PEPC antibodies. Gold labelling patterns in effective nodules showed that PEPC is a cytosolic enzyme and is distributed relatively equally in infected and uninfected cells of the nodule symbiotic zone. A high amount of labelling was also observed in pericycle cells of the nodule vascular system. Labelling was also detected within inner cortical cells, but the density was reduced by 60%. When Lotus corniculatus was transformed with a chimeric gene consisting of the 5′‐upstream region of the PEPC gene fused to β‐glucuronidase (GUS), GUS staining in nodules was consistent with immunogold localization patterns. The occurrence of PEPC in both infected and uninfected cells of the symbiotic zone of effective nodules coupled to the reduced amounts in ineffective nodules suggests a direct role for this enzyme in supporting N2‐fixation. PEPC localization in the uninfected, interstitial cells of the symbiotic zone indicates that these cells may also have a role in nodule carbon metabolism. Moreover, the association of PEPC with the nodule vascular system implies a role for the enzyme in the transport of assimilates to and from the shoot.
From epiphyte to tree: differences in leaf structure and leaf water relations associated with the transition in growth form in eight species of hemiepiphytes
Authors: HOLBROOK N. M. , Putz F. E.
Published: April 2006

Stranglers must undergo a transformation in growth form from epiphyte to tree to become reproductive mature and thus require developmental and/or physiological plasticity to cope with radical changes in their rooting environment. Differences in leaf structure and water relations between epiphytic‐phase and free‐standing individuals were marked in the five Ficus species examined. Epiphytic Ficus had several‐fold higher specific leaf area (cm2 g−1) and 2‐ to 4‐fold lower stomatal densities than conspecific trees. Osmotic potentials at full saturation were, on average, 0‐6 MPa higher (less negative) and the bulk modulus of elasticity approximately 50% lower in epiphytic plants than in conspecific trees. This resulted in leaves of epiphytic and tree individuals losing turgor at approximately the same relative water content, hut at a substantially higher leaf water potential in the epiphytic plants. In contrast, differences in leaf structure and water relations between epiphytes and trees of Clusia minor and Coussapoa villosa were small. In greenhouse experiments, alteration of the water and nutrient supply to epiphytic F. tuerckheimmii plants did not lead to significant changes in leaf structure.
Long‐term exposure to elevated [CO2] in a natural Quercus ilex L. community: net photosynthesis and photochemical efficiency of PSII at different levels of water stress
Authors: SCARASCIA‐MUGNOZZA G. , DE ANGELIS P. , MATTEUCCI G. , VALENTINI R.
Published: April 2006

Naturally grown trees of Mediterranean evergreen oak (Quercus ilex L.), representing the climax species of the region, were enclosed in six large open‐top chambers and exposed to ambient and elevated CO2 concentrations during a 3 year period. Maximum daily net photosynthetic rates measured at the two different CO2 concentrations were from 30 to 100% higher in elevated than in ambient [CO2] throughout the experimental period. The increase in maximum daily photosynthesis was also accompanied by a 93% rise in the apparent quantum yield of CO2 assimilation, measured during periods of optimum soil moisture conditions. Hence, no clear evidence of down‐regulation of net photosynthetic activity was found. Interactions between atmospheric CO2 concentration and plant water stress were studied by following the natural evolution of drought in different seasons and years. At each level of water stress, the maximum rate of carbon assimilation was higher in elevated than in ambient [CO2] by up to 100%.

Analysis of in vivo chlorophyll fluorescence parameters in normal (21%) and low (2%) oxygen concentrations provided useful insights into the functioning and stability of the photosynthetic processes. The photochemical efficiency of PSII (Fv/Fm) progressively decreased as drought conditions became more evident; this trend was accentuated under elevated [CO2]. Thermal de‐excitation processes were possibly more significant under elevated than under ambient [CO2], in a combination of environmental stresses.

This research suggests two possible conclusions: (i) a ‘positive’ interaction between elevated [CO2] and carbon metabolism can be obtained through relief of water stress limitation in the summer months, and (ii) elevated [CO2], under drought conditions, may also enhance the significance of slow‐relaxing quenching.
CAM induction in Clusia minor L. during the transition from wet to dry season in Trinidad: the role of organic acid speciation and decarboxylation
Authors: BORLAND A. M. , GRIFFITHS H. , MAXWELL C. , FORDHAM M. C. , BROADMEADOW M. S. J.
Published: April 2006

The interrelationships between the induction of CAM and the turnover of malate and citrate in the dicotyledenous tree Clusia minor were compared with seasonal changes in rainfall, leaf water status, PFD and photoinhibitory responses during the transition from wet to dry season in Trinidad. Over a period of 8 weeks, as rainfall declined from a maximum observed around week 3, leaf xylem tensions measured at dusk and dawn reflected the concurrent reduction in day‐time carbon gain and an increase in the diel turnover of malate (exposed leaves) and citrate (shaded leaves). Clear seasonal trends were observed in the turnover of malate and citrate during the transition from wet to dry season. In contrast to the declining back‐ground concentrations of citrate during the wet‐dry season transition, malate accumulation was markedly enhanced and the ratio of malalc:citrate accumulated overnight increased as the dry season advanced. Photo‐inhibitory responses, assessed by chlorophyll fluorescence, indicated that photochemistry was largely determined by the diurnal course of PFD incident on leaves, regardless of the magnitude of internal CO2 release from malate and citrate decarboxylation. In the long term, photochemical efficiency in both shaded and exposed leaves appeared to decline as the dry season progressed. Although there was a clear linear relationship between integrated PFD and overnight accumulation of malate, no such correlation was found for citrate. However, citrate breakdown during the day showed a much closer correlation with PFD as compared to malate, with levels of citrate measured at dusk declining in response to higher daily light intensities. Moreover, enhanced citrate decarboxylation during the day was strongly correlated with increased CAM and overnight accumulation of both malate and citrate.
Marcescence and senescence in a submediterranean oak (Quercus subpyrenaica E.H. del Villar): photosynthetic characteristics and nutrient composition
Authors: ABADíA A. , GIL E. , MORALES F. , MONTAÑÉS L. , MONTSERRAT G. , ABADÍA J.
Published: April 2006

We have studied some characteristics of marcescent leaves (withered retained leaves) and senescent leaves in Quercus subpyrenaica, a tree species that plays a major role in the climatic transition forests between temperate and mediterranean environments in north‐eastern Spain. Leaves were taken from the upper and lower halves of the tree crown, both in the south‐ and north‐exposed parts of the tree. Leaves receiving low photosynthetic photon flux density (PPFD) undergo autumnal senescence, which is associated with decreases in photosynthetic pigments and decreases in the chlorophyll a to chlorophyll b ratio. Leaves receiving higher PPFD underwent senescence at a later date. Leaves situated in the upper, south‐exposed part of the tree, which receive the highest PPFD, showed no signs of senescence and remained photosynthetically active for a longer period of time, marcescence occurring suddenly at the end of the season. Marcescence is unlikely to cause an improvement in nutrient recycling, since the removal of nutrients proceeded similarly in marcescent and senescent leaves. Marcescence may increase the time‐span of the assimilation capacity for a significant period of time during September and October, when high light intensities and mild temperatures occur. The phenomena triggering marcescence may be related to low temperatures.
Control of sodium influx by calcium and turgor in two charophytes differing in salinity tolerance
Authors: DAVENPORT R. , REID R. J. , SMITH F. A.
Published: April 2006

The effects of Ca2+ and cell turgor on Na+ influx were examined in two charophytes, lamprothamnium papulo‐SUM (salt‐tolerant) and Chara corallina (salt‐sensitive), to try to identify causes of salinity toxicity. Mortality was associated with Na+ influx, with the two species showing similar sensitivities to high Na+ influx. In Lamprothamnium, toxic influxes of Na+ occurred at much higher external Na+ concentrations than in Chara. The differences in Na+ influx at the same Na+ concentration were not due to different responses to external Ca2+. Lamprothamnium adjusts its turgor in response to increasing NaCl whereas Chara cannot. In solutions of KC1 up to at least 200 mol m‐3, however, Chara regulated turgor, and when KC1 was subsequently replaced with NaCl, Na+ influx was low and similar to that in Lamprothamnium at the same Na* concentration. Chara cells which were not turgor‐adjusted in KCI had Na+ influxes 2‐5‐fold higher than the turgid cells. Thus, it appears that turgor is a major determinant of Na+ influx, and therefore of cell survival. We found no evidence that the mechanism of Na+ influx in Chara is different from that in Lamprothamnium. Higher susceptibility of Chara to NaCl seems to result from inability to regulate turgor, in turn leading to toxic Na+ influx.
Carbon isotope fractionation in tree ring early and late wood in relation to intra‐growing season water balance
Authors: LIVINGSTON N. J. , SPITTLEHOUSE D. L.
Published: April 2006

We determined the stable carbon isotope composition (δ1.3C) of cellulose extracted from early and late wood in Douglas fir [Pseudotsuga menziexii (Mirb.) Franco] tree rings. Data were obtained for the period 1962 to 1981, at the start of which the trees were 20 years old. A water balance model was used to calculate daily stand transpiration and water deficit. The model incorporates site factors (soil water availability, slope and aspect) and environmental variables (solar radiation, air temperature and rainfall). There was far greater variability in late wood than in early wood δ1.3C. In wet years, late wood δ1.3C was significantly lighter (by as much as 2δ) than early wood δ1.3C but in dry years this difference was reversed. Differences between spring and summer cumulative transpiration accounted for almost 60δ of the variability in differences between early and late wood δ1.3C. We found excellent correspondence between summer cumulative transpiration and late wood δ1.3C, with estimates of transpiration accounting for up to 93% of the variability in δ1.3C. Correlations between early wood δ1.3C and spring transpiration were generally poor (r2<0.4), but we were able to identify those exceptional years in which there had been a very dry spring. Our results indicate that, while tree ring δ1.3C correlates reasonably well with basal area increment, it is a far better indicator of inter‐ and intra‐annual variability in water availability than radial growth.
Hierarchy establishment among potentially similar buds
Authors: NOVOPLANSKY A.
Published: April 2006

A plant has a great excess of buds each with the potential of developing an entire shoot system. The general question tackled was to what extent shoot size and time of bud development are important for bud hierarchy. Pea seedlings with two shoots, which were either equal or unequal in size, were obtained by the early removal of the seminal shoot. When these two shoots were also removed, one of the two cotyledon buds next to the bases of the shoots developed into the new shoot system. The determination of which of the buds became dominant was studied as a function of the relative sizes of the two primary cotyledonary shoots, of differences in the timing of the removal of these shoots and of the size of the buds. The bud that became dominant was not necessarily the larger one, nor did it always emerge from the axil of the larger shoot. Instead, it was usually the bud that was inhibited for a shorter period by the shoot next to it. It is suggested that the fate of a bud is predominantly determined by developmental parameters, for example lime of release, which are correlated with its developmental status and not necessarily with its physical size or with the past development of its shoot.
Photoinhibition of photosynthesis on a coral reef
Authors: FRANKLIN L. A. , SEATON G. G. R. , LOVELOCK C. E. , LARKUM A. W. D.
Published: April 2006

Photoinhibition of macroalgae in the epilithic algal community (KAC) of coral reefs was studied using chlorophyll fluorescence techniques at One Tree Island, Great Barrier Reef, Australia. Fv/Fm (variable to maximum fluorescence, darkened samples) of shallow macroalgae declined by 50% on fine summer and winter days, recovering in late afternoon. Within a species, thalli from low‐light habitats were more photoinhibited (2h at 1400μimol m−2 s−1) than those from high‐light habitats. The sensitivity of Lobophora variegata (Phacophyta) and Chlorodesmis fastigiata (Chlorophyta) increased with depth (1 versus 20 m). However, shallow Halimeda tuna (Chlorophyta) plants growing between corals were more photoinhibited than those from deep, open areas.

Photoinhibition and recovery were depth‐ and species‐specific. Shallow Lobophora and Chlorodesmis maintained a greater degree of QA oxidation during photoinhibition. In deep thalli, reduced effective quantum yield of open photosystem II centres reflected lower proportions and excitation capture efficiencies of open centres. In Lobophora, zeaxanthin formation accompanied non‐photochemical fluorescence quenching (NPQ), but in Chlorodesmis NPQ was limited and no zeaxanthin or antherxanthin formed. Higher photosynthetic efficiency in the lower storey of the EAC may compensate for photoinhibition in the upper storey, thereby reconciling photoinhibition of individual thalli with previous observations of no net inhibition of community productivity.
Nitrate and ammonium influxes in soybean (Glycine max) roots: direct comparison of 13N and 15N tracing
Authors: CLARKSON D.T. , GOJON A. , SAKER L.R. , WIERSEMA P.K. , PURVES J.V. , TILLARD P. , ARNOLD G.M. , PAANS A.J.M. , VAALBURG W. , STULEN I.
Published: April 2006

We compared influxes and internal transport in soybean plants (Glycine max cv. Kingsoy) of labelled N from external solutions where either ammonium or nitrate was labelled with the stable isotope15N and the radioactive isotope13N. The objective was to see whether mass spectrometric determinations of tissue 15N content were sufficiently sensitive to measure influxes accurately over short time periods. Our findings were as follows. (1) There was a close quantitative correspondence between estimates of N influx of individual plants using 15N or 13N measurements with either NO3/− or NH4+ at 4 or 2 mol−3, respectively in the external solution. (2) Transport to the shoot of N from NO3 absorbed over a 5–15 min period could be monitored when the external NO3− concentration ranged from 0–05 to 4 mol m−3. NH4+ as the N source labelled shoot tissue more slowly, and estimates of the transport between root and shoot could be made only with 13N. (3) Influx of NO3− into root tissue could be measured by 15N enrichment after 5–10 min at concentrations approaching the probable KM of the high‐affinity transport system. (4) There was some indication of isotope discrimination, especially with respect to the movement of labelled N to the shoot, when NO3− is the N source.

For many purposes, 15N tracing can be used satisfactorily to estimate influxes of both NO3− and NH4+ in soybean roots. Use of the short‐lived radio nuclide 13N remains the method of choice for more refined measurements of internal distribution and assimilation.
Where does all the calcium go? Evidence of an important regulatory role for trichomes in two calcicoles
Authors: DE SILVA D.L.R. , HETHERINGTON A.M. , MANSFIELD T.A.
Published: April 2006

In previous studies of the calcicoles Centaurea scabiosa and Leontodon hispidus, the stomata on isolated epidermis closed partially when the concentration of calcium in the medium was above 1 mol m−3. This is a much smaller concentration than that believed to be delivered into the leaves in xylem sap when the plants are growing in a calcium‐rich medium, and hence the mechanism for ‘protecting’ stomata from excessive exposure to free calcium is thought to be of great physiological significance. It is shown here that, in the leaves of both species, a substantial amount of the calcium they contain is located within meso‐phyll cells, and virtually all of that which does enter the epidermis is contained within trichomes, probably as calcium oxalate. The amounts of calcium in the vicinity of the stomata thus remain small despite high concentrations elsewhere, ensuring that the essential role of Ca2+ in intra‐cellular signalling in guard cells can continue to be performed without disturbance.
Stable carbon isotopes as indicators of increased water use efficiency and productivity in white spruce (Picea glauca (Moench) Voss) seedlings
Authors: SUN Z. J. , LIVINGSTON N. J. , GUY R. D. , ETHIER G. J.
Published: April 2006

The relationship among water use efficiency (WUE), productivity and carbon isotopic composition (δ13C) in white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings representing 10 controlled crosses were planted in the field in individual buried sand‐filled cylinders. The soil water content in the cylinders was measured using time domain reflectometry over two growing seasons and seedling water use determined by water balance. Two watering treatments were imposed: irrigation and dry land. There was significant (1.6–2.0%c) genetic variation in needle δ13C. Ranking of crosses in terms of δ13C was generally maintained over watering treatments and there was not a significant genetic versus environmental interaction. There was a positive correlation between δ13C and both intrinsic and long‐term WUE (more positive δ13C with increased WUE) and between δ13C and productivity, suggesting a correlation due to variation in photosynthetic capacity. Root to shoot ratios did not increase in water‐stressed plants, indicating that responses to drought were primarily at the level of gas exchange, rather than through morphological changes. Our results indicate that it should be possible to use δ13C as a surrogate for WUE and to select white spruce genotypes for high WUE without compromising yield.
Antisense inhibition of the sucrose transporter in potato: effects on amount and activity
Authors: LEMOINE R. , KÜHN C. , THIELE N. , DELROT S. , FROMMER W.B.
Published: April 2006

The sucrose proton‐cotransporter gene from potato (StSUT1) is mainly expressed in the phloem of mature, exporting leaves. To study the in vivo role of the protein, potato plants were transformed with antisense constructs of the sucrose transporter cDNA under control of the CaMV35S and the rolC promoters, respectively. Both types of transgenic plant develop symptoms characteristic of an inhibition of phloem loading. To determine the level of inhibition, immunological and transport studies were performed. Purified antibodies directed against a peptide from the central loop of SUT1 recognized a transporter with an apparent molecular mass of 47 kDa in leaf plasma membrane vesicles. Antisense repression under control of the non‐specific CaMV35S promoter led to a strong reduction in SUT1 protein, whereas no such reduction could be detected when the companion cell‐specific rolC promoter was used. Similarily. sucrose uptake in plasma membrane vesicles was reduced by 50–75% in CaMV35S but not in rolC plants. These data suggest that, unlike the rolC promoter, the sucrose transporter is expressed not only in the companion cells but also in other leaf cells. However, inhibition of the transporter by rolC‐controlled antisense repression is sufficient to impair phloem loading.
Water availability – a physiological constraint on nitrate utilization in plants of Australian semi‐arid muiga woodlands
Authors: ERSKINE P.D. , STEWART G.R. , SCHMIDT S. , TURNBULL M.H. , UNKOVICH M. , PATE J.S.
Published: April 2006

Nitrate was found to be the predominant form of available nitrogen in mulga soils. Nitrate reductase activities on a fresh mass basis of a range of plants from eastern (Queensland) mulga ecosystems 2 weeks after partial relief from drought were uniformly low for both herbaceous species (165 ± 25 pkat g−1) and woody perennials (77 ± 14 pkat g−1). Supply of nitrate for 24 h to cut transpiring shoots of woody species or application of nitrate solution to the rooting zone of herbaceous species promoted little further increase in mean shoot nitrate reductase activities. Most species exhibited high tissue nitrate concentrations during water stress and soluble organic N profiles were in many cases dominated by the osmoprotective compounds, proline or glycine betaine. Species with low levels of proline or glycine betaine showed high foliar concentrations of other compatible osmotica such as polyols or sugars. Effects of relieving water stress on nitrate reductase activity, proline, glycine betaine and nitrate levels were followed over, 3d of irrigation. Available soil nitrate rose 10‐fold immediately and, following rapid restoration of leaf water status of the eight study species, a 4‐fold increase occurred in mean nitrate reductase activity together with progressive decreases in mean tissue concentrations of nitrate, proline and glycine betaine over the 3 d period. Similar changes in soil nitrate, nitrate reductase activity, proline and tissue nitrate were observed in the same ecosystem following a natural rainfall event and in western (S.W. Australia) mulga following irrigation. It is concluded that, although nitrate nitrogen is present at high concentrations and is the predominant inorganic nitrogen source in soils of the mulga biogeographic region, its assimilation by perennial and ephemeral vegetation is limited primarily by water availability. A scheme is presented depicting interrelated physiological and biochemical events in typical mulga species following a rain event and subsequent drying out of the habitat.
Buder revisited: cell and organ polarity during phototropism*
Authors: NICK P. , FURUYA M.
Published: April 2006

The induction of a radial polarity by environmental stimuli was studied at the cellular and organ levels, with phototropism chosen as a model. The light gradient acting on the whole coleoptile was opposed to the light direction acting upon individual cells in the classical Buder experiment, irradiating from the inside out. Alternatively, the stimulus was administered to the coleoptile tip with a microbeam‐irradiation device. Tropistic curvature was assayed as a marker for the response of the whole organ, whereas cell elongation and the orientation of cortical microtubules were taken as markers for the responses of individual cells. Upon tip irradiation, signals much faster than basipetal auxin transport migrate towards the base. The data are discussed in terms of an organ polarity that is the primary result of the asymmetric light signal and affects, in a second step, an endogenous radial polarity of epidermal cells.
Long‐ and short‐term effects of decreased sink demand on carbohydrate levels and photosynthesis in wheat leaves
Authors: MORCUENDE R. , PÉEREZ P. , MARTÍNEZ‐CARRASCO R. , MARTIN DEL MOLING I. , SANCHEZ DE LA PUENTE L.
Published: April 2006

Wheat (Triticum aestivum L.) ears were removed to investigate long‐term regulation of photosynthesis by sink demand at ambient CO2 and 22 °C. The CO2 level was also increased to 660 μmol mol−1 and temperature was lowered to 5 °C to examine short‐term responses of photosynthesis to low sink demand. Sink removal inhibited photosynthesis and increased leaf levels of glucose, fructose and ribulose‐1, 5‐bisphosphate (RuBP), and the glucose‐6‐phosphate (G6P)/fructose‐6‐phosphate (F6P) and RuBP/3‐phosphoglycerate (PGA) ratios under growth conditions, but had no effect on the activity and activation state of ribulose‐1, 5‐bisphosphate carboxylase oxygenase (Rubisco) either under growth or short‐term conditions, suggesting an inhibition of photosynthesis by decreased in vivo catalysis of Rubisco. Photosynthesis increased similarly in eared and earless shoots after a rise in CO2 concentration, and the ratio of triose‐phosphates (glyceraldehyde 3‐phosphate and dihydroxyacetone phosphate, TP) to PGA was similar or higher for removed than intact ears, suggesting that feedback inhibition of photosynthesis was not caused by a limitation of ATP synthesis in chloroplasts. Under short‐term conditions (660 μmol mol−1 CO2, 5 °C), TP and RuBP levels and the TP/PGA and TP/RuBP ratios were increased by sink removal, indicating an additional limitation of photosynthesis by the rate of RuBP regeneration.
EcoCELLs: tools for mesocosm scale measurements of gas exchange
Authors: GRIFFIN K. L. , ROSS P. D. , SIMS D.A. , LUO Y. , SEEMANN J. R. , FOX C. A. , BALL J. T.
Published: April 2006

We describe the use of a unique plant growth facility, which has as its centerpiece four ‘EcoCELLs’, or 5x7 m mesocosms designed as open‐flow, mass‐balance systems for the measurement of carbon, water and trace gas fluxes. This system is unique in that it was conceived specifically to bridge the gap between measurement scales during long‐term experiments examining the function and development of model ecosystems. There are several advantages to using EcoCELLs, including (i) the same theory of operation as leaf level gas exchange systems, but with continuous operation at a much larger scale: (ii) the ability to independently evaluate canopy‐level and ecosystem models; (iii) simultaneous manipulation of environmental factors and measurement of system‐level responses, and (iv) maximum access to, and manipulation of, a large rooting volume.

In addition to discussing the theory, construction and relative merits of EcoCELLs, we describe the calibration and use of the EcoCELLs during a ‘proof of concept’ experiment. This experiment involved growing soybeans under two ambient CO2 concentrations (−360 and 710μmol mol−1. During this experiment, we asked ‘How accurate is the simplest model that can be used to scale from leaf‐level to canopy‐level responses?’ in order to illustrate the utility of the EcoCELLs in validating canopy‐scale models.