bluuebunny/crossref_metadata_2025 huggingface dataset:
The relationship between leaf growth and ABA accumulation in the grass leaf elongation zone
Authors: DODD I. C. , DAVIES W. J.
Published: April 2006

Detached barley (Hordeum vulgare L.) shoots, maintained at different air temperatures and VPDs, were fed ABA via the sub‐crown internode in a leaf elongation assay. Analysis of variance of leaf elongation rate (LER) showed significant effects of temperature (T), fed [ABA] and the interaction T × [ABA]. However, the interaction became non‐significant when LER was modelled against the [ABA] of the elongation zone, [EZ‐ABA]

When detached barley shoots were fed sap from droughted maize (Zea mays L.) plants, sap [ABA] could not explain the growth inhibitory activity. Measurement of [EZ‐ABA] accounted for this ‘unexplained’ growth inhibition.

The detached shoot experiments indicated that [EZ‐ABA], and not xylem sap [ABA], was an appropriate explanatory variable to measure in droughted plants. However, ABA accumulation in the elongation zone could not explain a 35% growth reduction in intact droughted plants; thus we considered an interaction of water status and ABA.

Using a coleoptile growth assay, we applied mild osmotic stresses (ψ=0 to −0.06 MPa) and 10−4 mol m−3 ABA. Individually, these treatments did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants.
Changes in photon flux can induce stomatal patchiness
Authors: ECKSTEIN J. , BEYSCHLAG W. , MOTT K. A. , RYEL R. J.
Published: April 2006

Images of chlorophyll fluorescence were used to detect the occurrence of stomatal patchiness in leaves from eight species under variable photon flux conditions. Pronounced stomatal patchiness was induced within 5–10 min after PFD was changed from intermediate (∼450 μmol quanta m−2 s−1) to low (∼150 μmol quanta m−2 s−1) levels. This effect was completely reversible by returning PFD to intermediate levels. The pattern of heterogeneous fluorescence for each leaf was usually similar during repeated applications of medium and low PFD. In three species, stomatal patchiness could only be induced in slightly water‐stressed plants. Leaves of more severely water‐stressed Xanthium strumarium plants in low air humidity exhibited oscillations in fluorescence that corresponded with oscillatory changes in leaf diffusion conductance for water vapour. Stomatal patchiness was also induced by illuminating dark‐adapted leaves with low PFD (below 200–300 μmol quanta m−2 s−1). Infiltration of leaves with distilled water showed that heterogeneous chlorophyll fluorescence was caused by changes in stomatal apertures.
Complete mechanical impedance increases the turgor of cells in the apex of pea roots
Authors: CLARK L.J. , WHALLEY W.R. , DEXTER A.R. , BARRACLOUGH P.B. , LEIGH R.A.
Published: April 2006

In this paper we describe an experimental approach which allows turgor (p) in an impeded root to be measured without the need to remove the root from the impeding environment. The maximum axial growth pressure (σmax) generated by completely impeded pea (Pisum sativum L.) roots was measured using a novel apparatus incorporating a force transducer. The apparatus was designed so that it was possible to gain access to the impeded root with the microcapillary of a pressure probe and so obtain in situ measurements of P. Turgor in cells in the apical region of impeded roots was 0.78 MPa, compared with 0.55 MPa in unimpeded roots. In impeded roots, σmax was 0.52 MPa, showing that the pressure component resulting from cell wall tension (W, where W=P–σ) decreased from 0.55 to 0.26 MPa as the roots became impeded. When impeded roots were removed from the apparatus, there was no decrease in P over the following 90 min. Impedance did not cause P to change in the non‐elongating part of the roots further from the apex.
Acclimation to low water potential determines changes in membrane fatty acid composition and fluidity in potato cells
Authors: LEONE A. , COSTA A. , GRILLO S. , TUCCI M. , HORVATH I. , VIGH L.
Published: April 2006

Potato (Solanum tuberosum) cells were gradually acclimated to grow at low water potential created by addition of increasing concentrations of polyethylene glycol (PEG) 8000 to the nutrient medium. While growth of cells abruptly transferred into a medium containing 20% PEG was dramatically inhibited, under the same stress intensity gradually acclimated cells were able to sustain active growth similarly to control unstressed cells. Gradual acclimation allowed the maintenance of normal cellular and subcellular membrane structure at an osmotic potential of ‐2.3 MPa, which caused extended membrane disruption when applied to non‐acclimated control cells. Analysis of fatty acids (FAs) from control and acclimated cells revealed that the mol percentage composition of the major extra‐plastidial phospholipids (PLs) was basically unchanged, but both phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were more saturated in acclimated cells. The decrease in unsaturation occurred primarily at the expense of linoleic acid (18:2) in the PE fraction. A significant increase in the percentage content of palmitic acid (16:0) of the PE fraction was also detected in acclimated cells. Membrane microviscosity, as determined by diphenyl‐hexatriene (DPH) labelling, was higher in PEG‐acclimated cells, as expected from the lower degree of unsaturation of membrane FAs found in acclimated cells. The steady‐state transcript levels of a Δ9‐stearoyl‐acyl carrier protein‐desaturase and a Δ12‐oleoyl‐desaturase gene decreased in acclimated cells, suggesting that the variation in the level of FA unsaturation was at least partially associated with down‐regulation of the expression of these genes.
Responses to elevated CO2 of Flaveria linearis plants having a reduced activity of cytosolic fructose‐1,6‐bisphosphatase
Authors: MICALLEF B. J. , VANDERVEER P. J. , SHARKEY T. D.
Published: April 2006

Wide variation exists in the growth responses of C3 plants to elevated CO2 levels. To investigate the role of photosynthetic feedback in this phenomenon, photosynthetic parameters and growth were measured for lines of Flaveria linearis with low, intermediate or high cytosolic fructose‐1,6‐bisphosphatase (cytFBPase) activity when grown at either 35 or 65 Pa CO2. The effects of pot size on the responses of these lines to elevated CO2 were also examined. Photosynthesis and growth of plants with low cytFBPase activity were less responsive to elevated CO2, and these plants had a reduced maximum potential for photosynthesis and growth. Plants with intermediate cytFBPase activity also showed a lower relative growth enhancement when grown at 65 Pa CO2. There was a significant pot size effect on photosynthesis and growth for line 85‐1 (high cytFBPase). This effect was greatest for line 85‐1 when grown at 35 Pa CO2, since these plants showed the greatest downward acclimation of photosynthesis when grown in small pots. There was a minimal pot size effect for line 84‐9 (low cytFBPase), and this could be partly attributed to the reduced CO2 sensitivity of this line. It is proposed that the capacity for sucrose synthesis in C3, plants is partly responsible for their wide variation in CO2 responsiveness.
Photosynthetic performance and resistance to photoinhibition of Zea mays L. leaves grown at sub‐optimal temperature
Authors: HALDIMANN P. , FRACHEBOUD Y. , STAMP P.
Published: April 2006

The performance of the photosynthetic apparatus was examined in the third leaves of Zea mays L. seedlings grown at near‐optimal (25 °C) or at suboptimal (15 °C) temperature by measuring chlorophyll (ChI) a fluorescence parameters and oxygen evolution in different temperature and light conditions. In leaf tissue grown at 25 and 15 °C, the quantum yield of PSII electron transport (ψPSII) and the rate of O2 evolution decreased with decreasing temperature (from 25 to 4 °C) at a photon flux density of 125 μmol m−2 s−1. In leaves grown at 25 °C, the decrease of ψPSII correlated with a decrease of photochemical ChI fluorescence quenching (qp), whereas in leaves crown at 15 °C qp was largely insensitive to the temperature decrease. Compared with leaves grown at 25 °C, leaves grown at 15 °C were also able to maintain a higher fraction of oxidized to reduced QA (greater qp) at high photon flux densities (up to 2000 μmol m−2 s−1), particularly when the measurements were performed at high temperature (25 °C). With decreasing temperature and/or increasing light intensity, leaves grown at 15 °C exhibited a substantial quenching of the dark level of fluorescence F0 (q0) whereas this type of quenching was virtually absent in leaves grown at 25 °C. Furthermore, leaves grown at 15 °C were able to recover faster from photo inhibition of photosynthesis after a photoinhibitory treatment (1200 μmol m−2 s−1 at 25, 15 or 6 °C for 8 h) than leaves grown at 25 °C. The results suggest that, in spite of having a low photosynthetic capacity, Z. mays leaves grown at sub optimal temperature possess efficient mechanisms of energy dissipation which enable them to cope better with photoinhibition than leaves grown at near‐optimal temperature. It is suggested that the resistance of Z. mays leaves grown at 15 °C to photoinhibition is related to the higher content of carotenoids of the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) measured in these leaves than in leaves grown at 25 °C.
Stomatal conductance in relation to xylem sap abscisic acid concentrations in two tropical trees, Acacia confusa and Litsea glutinosa
Authors: LIANG J. , ZHANG J. , WONG M.H.
Published: April 2006

Two tropical trees, Acacia confusa and Litsea glutinosa, were grown under controlled conditions with their roots subjected to soil drying and soil compaction treatments. In both species, a decline in stomatal conductance resulting from soil drying took place much earlier than the decline of leaf water potential. Soil compaction treatment also resulted in a substantial decrease in stomatal conductance but had little effect on leaf water potential. A rapid and substantial increase in xylem abscisic acid (ABA) concenation ([ABA]), rather than hulk leaf ABA, was closely related to soil drying and soil compaction. A significant relationship between stomatal conductance (gs) and xylem [ABA] was observed in both species. Artificially feeding ABA solutions to excised leaves of both species showed that the relationship bet ween gs and [ABA] was very similar to that obtained from the whole plant, i.e. the relationship between gs and xylem [ABA]. These results suggest that xylem ABA may act as a stress signal in the control of stomatal conductance.
Adaptation of pea to elevated atmospheric CO2: Rubisco, phosphoenolpyruvate carboxylase and chloroplast phosphate translocator at different levels of nitrogen and phosphorus nutrition
Authors: RIVIERE‐ROLLAND H. , CONTARD P. , BETSCHE T.
Published: April 2006

Resource allocation in high CO2 was studied with respect to plant nutrition. Pea (Pisum sativum) was grown in CO2‐enriched air (1000 cm 3 m3 CO2) during the entire vegetative phase, or grown in ambient air (340 cm3 m3 CO2), with different levels of nitrogen or phosphorus supply. Rubisco specific activity, abundance and small subunit transcript levels were unaltered at high N but declined at reduced N depending upon the degree of N deprivation. It is proposed that (a) a threshold value for the N status occurs in pea above which Rubisco is not down‐regulated by high CO2 and (b) a high leaf level of soluble carbohydrates is not a sufficient condition to downregulate Rubisco in high CO2. Phosphoenolpyruvate (PEP) carboxylase decreased, and chloroplast phosphate (P)‐translocator increased, in high CO2. In contrast to Rubisco, down‐regulation of PEP carboxylase was alleviated by low N and enhanced by low P. The increase in the P‐translocator was little affected by N but was accentuated by low P. The increase in the P‐translocator is considered to be one way of alleviating low P conditions in the chloroplast and thus rebalancing carbon partitioning between starch and soluble carbohydrates and amino acids. It is proposed that acclimation of PEP carboxylase and P‐translocator reflects adaptation to metabolic perturbations caused by high CO2.
The influence of water content and leaf anatomy on carbon isotope discrimination and photosynthesis in Sphagnum
Authors: RICE S. K. , GILES L.
Published: April 2006

The relative effect of diffusional resistance due to water films (rwf) and leaf anatomy (rp) on rates of net photosynthesis and on‐line measures of carbon isotope discrimination (Δ=Δδ13C) was investigated in Sphagnum. Sphagnum species differ in the exposure of photosynthetic cells at the leaf surface. In S. affine, photosynthetic cells are widely exposed at the surface, whereas in S. magellanicum, photo‐synthetic cells are enclosed within water‐filled hyaline cells. This difference is expected to lead to variation in diffusive resistance within leaves (rp). Net photosynthesis and on‐line Δ were measured at two water contents: greenhouse water content (wet) and blotted dry (dry). Without correcting for respiration, on‐line Δ values differed significantly between wet (23.7%o) and dry (30.9%o) plants. However, there was no significant difference between species means and no species × water content interaction. Corrections for respiration lowered Δ values by approximately 8.1%o and reduced the mean difference to 3.1%o, but did not alter the rank order of treatments. Net photosynthesis also decreased by 16% in wet plants, but there was no significant difference between the two species. In addition, five populations of S. affine and S. magellanicum grown in a common garden were analysed for their organic matter carbon isotope composition (δ13C). These values varied more within each species (0.9–1.2%o) than between the two species (0.6%o). Therefore, we conclude that variation in surface water films leads to a greater difference in resistance to CO2 uptake and carbon isotope discrimination than that due to variation in leaf anatomical properties in Sphagnum.
Antioxidant composition of selected high alpine plant species from different altitudes
Authors: WILDI B. , LÜTZ C.
Published: April 2006

Selected high alpine plant species were collected from different elevations in the Obergurgl/Ötztal subnival and nival regions in Austria to investigate the content of antioxidants in plants growing under the particular in vivo conditions experienced in this area (e.g. chilling stress, short vegetation period and high irradiation). The contents of antioxidants (ascorbic acid, tocopherol and glutathionc) and photosynthetic pigments were measured throughout the day. The contents of most compounds were found to follow a diurnal rhythm, with the maximum occurring at midday and the minimum during the night. It was not clear whether these fluctuations were temperature‐dependent or light‐dependent.

Analyses of the antioxidant spectrum in the same plant species at different altitudes (and thus under different environmental conditions: as altitude increases, for example, day temperature decreases and light intensity increases) revealed that the total amount of antioxidants increases as altitude increases. This enhancement was mainly due to ascorbic acid contents. Each plant species displayed a specific reaction to the increase in stress that accompanies an increase in altitude, resulting in a broad adaptation spectrum for these plants. The present study suggests that the combined effect of lower temperature and higher light intensity induces higher antioxidant contents.
Ultraviolet‐B‐induced DNA lesions and their removal in wheat (Triticum aestivum L.) leaves
Authors: TAYLOR R. M. , NIKAIDO O. , JORDAN B. R. , ROSAMOND J. , BRAY C. M. , TOBIN A. K.
Published: April 2006

Monoclonal antibodies were used in an enzyme‐linked immunosorbent assay (ELISA) to detect the induction and removal of cyclobutane pyrimidine dimers (CPDs) and (6‐4) photoproducts in DNA isolated from ultraviolet B (UV‐B)‐exposed primary wheat (Triticum aestivum L. cv. Chinese Spring) leaf tissue. The accumulation of lesions in the primary leaves of 6‐d‐old wheat seedlings was followed during the exposure of the leaf to an approximate dose of 3.6×10−1 W m−2 UV‐B (Caldwell weighting). Significant increases in the levels of both CPDs and (6‐4) photoproducts were detected in wheat leaves exposed to UV‐B in the absence of other light However, only an increase in (6‐4) photoproduct levels could be measured in wheat leaves exposed to the same UV‐B source in the presence of supplemental white light. The removal of CPD antibody binding sites in the DNA after irradiation was rapid under conditions of high light intensity in contrast to the removal of (6‐4) photoproduct antibody binding sites, which was significantly slower. The removal of CPDs appeared to be light dependent, this rate of removal decreasing with decreasing light fluences. The removal of (6‐4) photoproducts also appeared light dependent, but to a lesser extent than the removal of CPDs, under the conditions studied here. Gene expression in the primary wheat leaf was measured and showed an up‐regulation of chalcone synthase expression and a reduction in expression of chlorophyll a/b‐binding protein (cab) in response to supplementary UV‐B. No effect was seen on the expression of the other photosynthetic genes studied (the genes coding for the enzymes sedoheptu‐lose 1,7‐bisphosphatase and fructose 1,6‐bisphosphatase). Measurement of the levels of DNA lesions in this same tissue showed that the observed changes in gene expression accompanied the appearance of UV‐B induced lesions in the form of (6‐4) photoproducts in the wheat leaf genome.
Leaf gas exchange and carbon isotope composition responses to drought in a drought‐avoiding (Pinus pinaster) and a drought‐tolerant (Quercus petraea) species under present and elevated atmospheric CO2 concentrations
Authors: PICON C. , GUEHL J. M. , FERHI A.
Published: April 2006

The responses of predawn leaf water potential (φwp), leaf conductance to water vapour diffusion (g), CO2 assimilation rate (A) and carbon isotope competition (δ13C) to a soil drying cycle were assessed in Pinus pinaster, a drought‐avoiding species with high stomatal sensitivity to drought, and Quercus petraea, a drought‐tolerant species with lower stomatal sensitivity to drought, under present (350 μmol−1) and elevated (700 μmol−1) atmospheric CO2 concentrations ([CO2]). In P. pinaster, decreasing A in response to drought was associated with increasing plant intrinsic water use efficiency (A/g) and with decreasing calculated intercellular [CO2] (C1), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C1 increased during the drying cycle, which suggests a non‐stomatal origin for the decrease in A. In P. pinaster, a negative relationship was observed between the gas exchange‐derived values of Ci/Ca and δ13C, which conforms to the classical two‐step carbon isotope discrimination model. In Q. petraea, the relationship between C1/Ca and δ13C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO2] than under present [CO2] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO2] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO2]. Whether the differences in the g response to elevated [CO2] found here can be generalized to other drought‐avoiding and non‐avoiding species remains to be assessed.
Differential effects of turgor on sucrose and potassium transport at the tonoplast and plasma membrane of sugar beet storage root tissue
Authors: BELL C. I. , LEIGH R. A.
Published: April 2006

When turgor was increased, by decreasing the concentration of mannitol bathing discs of sugar beet storage root tissue, the rates of sucrose and potassium uptake into the vacuole were decreased. At all external mannitol concentrations the rate of sucrose and potassium uptake across the plasma membrane was an order of magnitude greater than the rate of quasi‐steady uptake into the vacuole, implying a very large efflux. Efflux of both sucrose and potassium was increased at high turgor. However, while increasing turgor decreased the rate of K+ uptake, the rate of sucrose uptake at the plasma membrane increased with time. Compartmental analysis of tracer exchange kinetics was used to determine unidirectional K+ fluxes. From these results, it was estimated that the increase in K+ efflux accompanying a 1.5 MPa increase in turgor could lead to a net increase of 140mol−3h−1 in the external potassium concentration. It is suggested that the turgor‐imposed increase in solute efflux is a means of regulating intracellular osmotic pressure and/or turgor in sugar beet storage roots, but that sucrose is preferentially retrieved from the apoplast, even under conditions of excessively high turgor. However, much of this sucrose is probably lost from the cell, implying a ‘futile’ sucrose transport cycle at the plasma membrane. The turgor‐stimulated leak of potassium could play a major role in the regulation of turgor pressure in sugar beet storage root tissue.
The effects of long‐term elevation of air temperature and CO on the frost hardiness of Scots pine
Authors: REPO T. , HÄNNINEN H. , KELLOMÄKI S.
Published: April 2006

The frost hardiness of 20 to 25‐year‐old Scots pine (Pinus sylvestris L.) saplings was followed for 2 years in an experiment that attempted to simulate the predicted climatic conditions of the future, i.e. increased atmospheric CO2 concentration and/or elevated air temperature. Frost hardiness was determined by an electrolyte leakage method and visual damage scoring on needles. Elevated temperatures caused needles to harden later and deharden earlier than the controls. In the first year, elevated CO2 enhanced hardening at elevated temperatures, but this effect disappeared the next year. Dehardening was hastened by elevating CO2 in both springs. The frost hardiness was high (<−40 °C), even at elevated temperatures, in midwinter, at which time the electrolyte leakage method underestimated the frost hardiness compared with the visual scoring. In addition to the significant differences between treatments, there was also significant variation between trees in frost hardiness within treatments. These results suggest that the risks of frost damage are marked in the predicted climatic conditions in Finland, and, more specifically, they depend on how the occurrence of the frost episodes changes with respect to climatic warming during the annual cycle, especially in the autumn and spring. We also conclude that the conditions in midwinter are not critical for frost injury to trees in the future.
The effects of elevated temperature on the photosynthetic efficiency of zooxanthellae in hospite from four different species of reef coral: a novel approach
Authors: WARNER M. E. , FITT W. K. , SCHMIDT G. W.
Published: April 2006

Bleaching of reef corals is a phenomenon linked to temperature stress which involves loss of the symbiotic algae of the coral, which are known as zooxanthellae, and/or loss of algal pigments. The photosynthetic efficiency of zooxanthellae within the corals Montastrea annularis, Agaricia lamarki, Agaricia agaricites and Siderastrea radians was examined by pulse‐amplitude modulation fluorometry (PAM) during exposure to elevated temperatures (30–36°C). Zooxanthellae within M. annularis and A. lamarki were found to be more sensitive to elevated temperature, virtually complete disruption of photosynthesis being noted during exposure to temperatures of 32 and 34°C. The photosynthetic efficiency of zooxanthellae within S. radians and A. agaricites decreased to a lesser extent. Differences in the loss of algal cells on an aerial basis and in the cellular chlorophyll concentration were also found between these species. By combining the non‐invasive PAM technique with whole‐cell fluorescence of freshly isolated zooxanthellae, we have identified fundamental differences in the physiology of the symbionts within different species of coral. Zooxanthellae within M. annularis appear to be more susceptible to heat‐induced damage at or near the reaction centre of Photosystem II, while zooxanthellae living in S. radians remain capable of dissipating excess excitation energy through non‐photochemical pathways, thereby protecting the photosystem from damage during heat exposure.
Light and stage of development influence the expression of desiccation‐induced genes in the resurrection plant Craterostigma plantagineum
Authors: ALAMILLO J. M. , BARTELS D.
Published: April 2006

Craterostigma plantagineum is a representative of the resurrection plants, which are able to withstand complete dryness. During the dehydration process, many characteristic transcripts and proteins are induced; these have been isolated by molecular cloning. The expression of most of these gene products can also be triggered by ABA. Five representative desiccation‐related cDNA clones were selected. The effect of light and developmental stage on the expression of the transcripts and corresponding proteins was analysed during dehydration and ABA treatment. Desiccation and ABA treatment in the presence of light induced a marked increase in several of the transcripts, whereas light had the reverse effect on the levels of some proteins (relative to mRNA levels) localized in the chloroplast. Although very young plants have the capacity to resume full physiological activity after dehydration, some of the desiccation‐related gene products are still expressed at low levels.
Studies on the distribution, re‐translocation and homeostasis of inorganic phosphate in barley leaves
Authors: MIMURA T. , SAKANO K. , SHIMMEN T.
Published: April 2006

Changes in inorganic phosphate (Pi) concentrations in barley leaves during growth of plants with sufficient or deficient supplies of Pi were studied. Measurements of the Pi distribution from subcellular levels to the leaf tissue level under the same experimental conditions allowed us to analyse the relationship between the Pi homeostasis of various compartments and Pi re‐translocation in the whole plant. Under Pi deficiency, the finding of growth‐dependent changes in the Pi concentrations of whole leaves established that Pi was re‐translocated from the older leaves to the young leaves. Translocation of 32Pi was also confirmed with an ‘imaging plate’ system, which made it possible to follow Pi movement in the same plantlet. To analyse the mechanism of Pi re‐translocation, the Pi distribution amongst various compartments of the leaves was measured. Under Pi deficiency, the cytoplasmic Pi concentration of the first leaf remained constant until 16d after sowing, while vacuolar Pi was completely exhausted after 8 to 10d. Exhaustion of vacuolar Pi in the first leaf coincided with the appearance of the second leaf. The Pi concentration in the apoplast changed similarly to that of the whole leaf. However, the apoplastic Pi concentration was affected to some extent by the vacuolar Pi concentration and the growth of the younger leaf, because the main change in apoplastic Pi concentration coincided with the time of the disappearance of the vacuolar Pi and the appearance of the younger leaf. The Pi concentration in the apoplast was about 0.1 to I molm−3, even in the absence of Pi, which was much higher than that in the usual soil environment (a few mmolin−3). This suggests that the Pi absorbed by root cells is concentrated in the transport process from the root to the leaf apoplast. The content of Pi in the xylem exudate was constant irrespective of growth culture conditions. The root may be functioning as the constant Pi supplier to the above tissues.
Functional and molecular changes in the photosynthetic apparatus during senescence of flag leaves from field‐grown barley plants
Authors: HUMBECK K. , QUAST S. , KRUPINSKA K.
Published: April 2006

Changes in the function and composition of the photosynthetic apparatus were analysed during maturation and senescence of barley (Hordeum vulgare L.) flag leaves under field conditions. At about 8 d post‐antbesis. photo‐synthetic capacity, measured as oxygen evolution under light‐saturating conditions, and the D1 protein level started to decrease. At about 14 d post‐anthesis, the pigment content, photosystem II efficiency (Fv/Fm) and levels of cytochrome f and the large subunit of ribulose‐1,5‐bis‐phosphate carboxylase started to decline. The levels of cytochrome b559 and the small subunit of ribulose‐1,5‐bis‐phosphate carboxylase decreased later, at about 22 d post‐anthesis. High levels of the light‐harvesting complex of pliotosyslem II were still detectable at an even later stage of senescence, at 26 d post‐anthesis.

There were also differences in the kinetics of the declines in the levels of transcripts coding for components of the photosynthetic apparatus. rbcS mKNA abundance had already decreased when flag leaves reached their final lengths, at about 4 d post‐unthesis, whereas levels of psbA and petC transcripts did not decrease until 16 d post‐anthesis. In contrast, a senescence‐related transcript, pHvS40 (Becker & Apel 1993), accumulated in the flag leaves 14 d post‐anthesis.

Several senescence‐related processes which differ in their kinetics are described. The interrelationships among these processes and their regulation are discussed.
A mutant ofArabidopsisthaliana partially resistant to fusicoccin has reduced plasma membrane H+‐ATPase
Authors: DE MICHELIS M. I. , PUGLIARELLO M. C. , RASI‐CALDOGNO F. , SOAVE C.
Published: April 2006

5‐2is a mutant ofArabidopsis thalianawhich is partially resistant to fusicoccinin vivo. We have analysed fusicoccin binding and the activity and amount of H+‐ATPase in plasma membrane isolated from mature leaves of the wild type and of mutant5‐2. Fusicoccin binding was similar in plasma membrane from the two genotypes, while H+‐ATPase activity was markedly (c. 50%) lower in plasma membrane from mutant5‐2than in that from the wild type. The H+‐ATPase of mutant5‐2was activated by fusicoccin as much as that of the wild type. In plasma membrane from mutant5‐2, the amount of immunodetectable H+‐ATPase, quantified by densitometry of Western blots, was about half that in the wild type. These results indicate that the major defect of mutant5‐2detectable at the plasma membrane level is a reduction in the amount of H+‐ATPase.
Influence of carbon supply on the circadian rhythmicity of photosynthesis and its stimulation by blue light in Ectocarpus siliculosus: clues to the mechanism of inorganic carbon acquisition in lower brown algae
Authors: SCHMID R. , DRING M. J.
Published: April 2006

Stimulation or light‐saturated rates of photosynthesis in Ectocarpus siliculosus (Dillwyn) Lyngb. by blue light was eliminated by increasing dissolved inorganic carbon (DIC) or by lowering pH in natural seawater. The amplitude of the circadian rhythm of photosynthesis was also diminished under these conditions, and the pH compensation points in a closed system were higher in the presence of blue light and during the circadian day. These observations suggest that blue light and the circadian clock regulate the activity of a carbon acquisition system in these plants. The inhibitor of external carbonic anhydrase, acetazolamide, reduced overall rates of photosynthesis by only about 30%, but ethoxyzolamide suppressed the circadian rhythm of photosynthesis almost completely and markedly reduced the duration of responses to blue light pulses. Similar patterns were obtained when photosynthesis was measured in strongly limiting DIC concentrations (0–0.5 mol m−3). Since blue light stimulated photosynthesis under these conditions of strong carbon limitation, we suggest that blue light activates the release of CO2 from an internal CO2 store. We propose a metabolic pathway with similarities to that of CAM plants. Non‐photosynthetic fixation leads to the accumulation of a storage metabolite. The circadian clock and blue light control the mobilization of CO2 at the site of decarboxylation of this metabolite. In the presence of continuous blue light the pathway is proposed to cycle and act as a pump for CO2 into the chloroplasts. This hypothesis helps to explain a number of previously reported peculiarities of brown algal photosynthesis.
Influence of carbon supply on the stimulation of light‐saturated photosynthesis by blue light in Laminaria saccharina: implications for the mechanism of carbon acquisition in higher brown algae
Authors: SCHMID R. , MILLS J. A. , DRING M. J.
Published: April 2006

In saturating irradiances of red light, photosynthesis of Laminaria saccharina (L.) Lamouroux was stimulated by low irradiances of continuous blue light only when the supply of dissolved inorganic carbon (DIC) was limiting. The degree of this stimulation was inversely proportional to the logarithm of the concentration of free CO2, whether this was adjusted by varying the total DIC or the pH at a given DIC concentration. The final pH reached in a closed system was higher in blue light than in red light. Both acetazolamide and ethoxyzolamide suppressed the responses to blue light almost completely, but reduced photosynthesis in red light by only 30%. Buffering the pH of the seawater also suppressed the stimulation of photosynthesis by blue light without affecting the photosynthetic rate in red light. The transient stimulation of O2 evolution by a blue light pulse was not accompanied by a corresponding increase in CO2 consumption. These observations could be explained if, in analogy to the mechanism proposed for Ectocarpus (Schmid, Mills & Dring 1996, Plant Cell and Environment 19,373–382, this issue, accompanying paper), photosynthesis was supported by a blue‐light‐activated release of CO2 from an internal store. We suggest that the store is located in the vacuoles of the cortical tissue of the blades. The main photosynthetic tissue, however, is in the overlying meristoderm, and blue‐light‐activated mobilization of the store could stimulate O2 evolution only if periplasmic carbonic anhydrase was available to facilitate CO2 uptake from the cortex.
Interactive effects of mycorrhization and elevated atmospheric CO2 on sulphur nutrition of young pedunculate oak (Quercus robur L.) trees
Authors: SEEGMÜLLER S. , SCHULTE M. , HERSCHBACH C. , RENNENBERG H.
Published: April 2006

Pedunculate oak (Quercus robur L.) was germinated and grown at ambient CO2 concentration and 650 μmol mol−1 CO2 in the presence and absence of the ectomycorrhizal fungus Laccaria laccata for a total of 22 weeks under nonlimiting nutrient conditions. Sulphate uptake, xylem loading and exudation were analysed in excised roots. Despite a relatively high affinity for sulphate (KM= 1.6 mmol m−3), the rates of sulphate uptake by excised lateral roots of mycorrhizal oak trees were low as compared to herbaceous plants. Rates of sulphate uptake were similar in mycorrhizal and non‐mycorrhizal roots and were not affected by growth of the trees at elevated CO2. However, the total uptake of sulphate per plant was enhanced by elevated CO2 and further enhanced by elevated CO2 and mycorrhization. Sulphate uptake seemed to be closely correlated with biomass accumulation under the conditions applied.

The percentage of the sulphate taken up by mycorrhizal oak roots that was loaded into the xylem was an order of magnitude lower than previously observed for herbaceous plants. The rate of xylem loading was enhanced by mycorrhization and, in roots of mycorrhizal trees only, by growth at elevated CO2. On a whole‐plant basis this increase in xylem loading could only partially be explained by the increased growth of the trees. Elevated CO2 and mycorrhization appeared to increase greatly the sulphate supply of the shoot at the level of xylem loading. For all treatments, calculated rates of sulphate exudation were significantly lower than the corresponding rates of xylem loading of sulphate. Radiolabelled sulphate loaded into the xylem therefore seems to be readily diluted by unlabelled sulphate during xylem transport.

Allocation of reduced sulphur from oak leaves was studied by flap‐feeding radiolabelled GSH to mature oak leaves. The rate of export of radioactivity from the fed leaves was 4–5 times higher in mycorrhizal oak trees grown at elevated CO2 than in those grown at ambient CO2. Export of radiolabel proceeded almost exclusively in a basipetal direction to the roots. From these experiments it can be concluded that, in mycorrhizal oak trees grown at elevated CO2, the transport of sulphate to the shoot is increased at the level of xylem loading to enable increased sulphate reduction in the leaves. Increased sulphate reduction seems to be required for the enhanced allocation of reduced sulphur to the roots which is observed in trees grown at elevated CO2. These changes in sulphate and reduced sulphur allocation may be a prerequisite for the positive effect of elevated CO2 on growth of oak trees previously observed.
New evidence for large negative xylem pressures and their measurement by the pressure chamber method
Authors: SPERRY J. S. , SALIENDRA N. Z. , POCKMAN W. T. , COCHARD H. , CRUIZIAT P. , DAVIS S. D. , EWERS F. W. , TYREE M. T.
Published: April 2006

Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water‐stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air‐seeding). Deduced xylem pressure showed a one‐to‐one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air‐ and water‐filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air‐seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.
Systemin transiently depolarizes the tomato mesophyll cell membrane and antagonizes fusicoccin‐induced extracellular acidification of mesophyll tissue
Authors: MOYEN C. , JOHANNES E.
Published: April 2006

The plant polypeptide signal systemin induces proteinase inhibitor synthesis in tomato leaves. We show here that systemin elicits a transient depolarization of the tomato mesophyll cell membrane. Furthermore it triggers a transient decrease in the external pH of the mesophyll tissue which is followed by a sustained pH increase. In the presence of fusicoccin (which has been shown to antagonize the synthesis of proteinase inhibitors) the depolarization and transient H+ efflux are attenuated whereas the slower phase of the sustained electroneutral H+ influx persists. These results suggest that systemin‐induced changes in ion transport play a role in the early phases of systemin signal transduction.
The utilization of bicarbonate ions by the marine microalga Nannochloropsis oculata (Droop) Hibberd
Authors: MERRETT M. J. , NIMER N. A. , DONG L. F.
Published: April 2006

HCO3− utilization by the marine microalga Nannochloropsis oculata was investigated using a pH drift technique in a closed system. Light‐dependent alkalization of the medium resulted in a final pH of 10.5, confirming substantial HCO3− use by this alga. Alkalinity remained constant throughout the pH drift. Measurement of dissolved inorganic carbon (DIC) or the uptake of H14CO3− showed that nearly 50% of the total DIC remained external to the plasma membrane on completion of a pH drift. The rate of light‐driven alkalization was inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) and thus was dependent on photosynthesis. Light‐driven alkalization was not inhibited by a membrane‐impermeable inhibitor of carbonic anhydrase (CA), dcxtran‐bound sulphonamide (DBS), indicating that external CA was not involved in HCO3− utilization. The anion‐cxchangc inhibitor 4′,4′‐diisothiocyanostilbene‐2,2‐disulphonic acid (DIDS) completely inhibited light‐driven alkalization of the medium and H14CO3− uptake, providing unequivocal support for a direct uptake of H14CO3−. Chloride ions were essential for DIC‐dependent photosynthetic oxygen evolution, suggesting that bicarbonate transport occurs by HCO3−/CI− exchange.
Free Air Temperature Increase (FATI): a new tool to study global warming effects on plants in the field
Authors: NIJS I. , KOCKELBERGH F. , TEUGHELS H. , BLUM H. , HENDREY G. , IMPENS I.
Published: April 2006

A new technique, called Free Air Temperature Increase (FATI), was developed to artificially induce increased canopy temperature in field conditions without the use of enclosures. This acronym was chosen in analogy with FACE (Free Air CO2 Enrichment), a technique which produces elevated CO2 concentrations [CO2] in open field conditions. The FATI system simulates global warming in small ecosystems of limited height, using infrared heaters from which all radiation below 800 nm is removed by selective cut‐off filters to avoid undesirable photomorpho‐genetic effects. An electronic control circuit tracks the ambient canopy temperature in an unheated reference plot with thermocouples, and modulates the radiant energy from the lamps to produce a 2.5°C increment in the canopy temperature of an associated heated plot (continuously day and night). This pre‐set target differential is relatively‐constant over time due to the fast response of the lamps and the use of a proportional action controller (the standard deviation of this increment was <1°C in a 3 week field study with 1007 measurements). Furthermore, the increase in leaf temperature does not depend on the vertical position within the canopy or on the height of the stand. Possible applications and alternative designs are discussed.
Effects of parasitism by a mistletoe on the structure and functioning of branches of its host
Authors: TENNAKOON K. U. , PATE J. S.
Published: April 2006

Infestation of Acacia acuminata by the xylem‐tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and eventual death of host branch parts distal to the mistletoe. Branch sectional areas proximal (P) and distal (D) to mistletoes are used to classify stages of parasitism, with P:D area ratios of 5–6 invariably associated with distal branch senescence. As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining host foliage area, and the specific hydraulic conductivity of distal host wood declines sharply relative to that of proximal wood, mineral composition and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal wood. After the demise of the distal branch parts, the transectional area of the host branch stump increases linearly with increasing mistletoe leaf area, the branch area supporting a unit of mistletoe leaf area always being about 3 times greater than that supporting a unit of host foliage area on unparasitized branches. This differential, compounded with high transpiration rates and selective uptake of host xylem solutes by the haustorium, fosters substantial mineral enrichment of the mistletoe relative to its host. The study provides a background for future investigation of possible cellular mechanisms continuously driving structural and functional changes in favour of the mistletoe.
Stimulation of root hair elongation in Arabidopsis thaliana by low phosphorus availability
Authors: BATES T. R. , LYNCH J. P.
Published: April 2006

Low phosphorus availability stimulates root hair elongation in many plants, which may have adaptive significance in soil phosphorus acquisition. We investigated the effect of low phosphorus on the elongation of Arabidopsis thaliana root hairs. Arabidopsis thaliana plants were grown in plant culture containing high (1000 mmol m−3) or low (1 mmol m−3) phosphorus concentrations, and root hair elongation was analysed by image analysis. After 15d of growth, low‐phosphorus plants developed root hairs averaging 0.9 mm in length while high‐phosphorus plants of the same age developed root hairs averaging 0.3 mm in length. Increased root hair length in low‐phosphorus plants was a result of both increased growth duration and increased growth rate. Root hair length decreased logarithmically in response to increasing phosphorus concentration. Local changes in phosphorus availability influenced root hair growth regardless of the phosphorus status of the plant. Low phosphorus stimulated root hair elongation in the hairless axr2 mutant, exogenously applied IAA stimulated root hair elongation in wild‐type high‐phosphorus plants and the auxin antagonist CM PA inhibited root hair elongation in low‐phosphorus plants. These results indicate that auxin may be involved in the low‐phosphorus response in root hairs.
Proteolytic analysis of polymerized maize tubulin: regulation of microtubule stability to low temperature and Ca2+ by the carboxyl terminus of β‐tubulin
Authors: BOKROS C. L. , HUGDAHL J. D. , BLUMENTHAL S. S. D. , MOREJOHN L. C.
Published: April 2006

To obtain information on plant microtubule stability to low temperature and Ca2+, the regulatory domain of polymerized tubulin from maize (Zea mays ev. Black Mexican Sweet) was dissected by limited proteolysis with subtilisin. Tubulin in taxol‐stabilized microtubules was cleaved in a subtilisin concentration‐ and time‐dependent manner. Immunoblotting of microtubules with antibodies having mapped epitopes on α‐ and β‐tubulins revealed that cleavage initially removed ≤15 residues from the β‐tubulin carboxyl terminus to produce αβs‐microtubules. Subsequent cleavage occurred at an extreme site and an internal site within the α‐tubulin carboxyl terminus. Electron microscopy revealed that αβs‐microtubules were ultra structurally indistinguishable from uncleaved control αβ‐micro‐tubules. Quantitative polymer sedimentation showed that low temperature treatment (0°C) caused significant depolymerization of αβ‐microtubules, but little depolymerization of αβs‐microtubules. Ca2+ enhanced the cold‐induced depolymerization of both αβ‐ and αβs‐microtubules. However, αβs‐microtubules were significantly more stable to depolymerization by cold and Ca2+ than were αβ‐micro‐tubules. The results showed that maize microtubules containing shortened β‐tubulin carboxyl termini are relatively resistant to the combined depolymerizing effects of cold and Ca2+. Thus, the extreme carboxyl terminus of β‐tubulin is a crucial element of the plant tubulin regulatory domain and may be involved in the modulation of microtubule stability during the chilling response in plants.