bluuebunny/crossref_metadata_2025 huggingface dataset:
A relationship between humidity response, growth form and photosynthetic operating point in C3 plants
Authors: Franks P. J. , Farquhar G. D.
Published: March 2003

Gas exchange experiments were performed with 13 plant species that differ from each other in growth‐form and natural habitat. These comprised three herbaceous species, two ferns, two temperate deciduous trees, five rainforest trees and one liana from wet tropical forest. The aims were to investigate whether plants of similar growth‐form and from similar habitats tended to respond similarly to a change in leaf‐to‐air vapour pressure difference (VPD), and to compare their ratio of intercellular to ambient partial pressures of CO2 for given conditions. Leaves were subjected to a step change in VPD and the initial and final steady rates of transpiration were used to calculate an index of sensitivity, φ, which enabled comparison of species. The results suggest that species of similar growth‐form and habitat respond similarly to increasing VPD, with the temperate deciduous trees undergoing a greater reduction in stomatal conductance than the herbaceous plants in well‐watered soil. Also, for these experimental conditions, the ratio of leaf internal to ambient CO2 partial pressure (pi/pa) was positively correlated with both CO2 assimilation rate and stomatal insensitivity to VPD, across the 13 species. The results are discussed in terms of growth strategies and possible advantages and limitations of hydraulic systems in different plants.
Ultraviolet‐B‐induced stress and changes in gene expression in Arabidopsis thaliana: role of signalling pathways controlled by jasmonic acid, ethylene and reactive oxygen species
Authors: A.‐H.‐Mackerness S. , Surplus S. L. , Blake P. , John C. F. , Buchanan‐Wollaston V. , Jordan B. R. , Thomas B.
Published: March 2003

Treatment with supplementary UV‐B resulted in decreases in transcripts of the photosynthetic genes Lhcb and psbA and concomitant increase in transcripts of two pathogen‐related genes, PR‐1 and PDF1·2, in Arabidopsis thaliana. UV‐B exposure caused increases in jasmonic acid (JA) levels and ethylene production. UV‐B treatment of jar1 and etr1‐1 mutants, which are insensitive to JA and ethylene, respectively, showed that the increase in PR‐1 transcripts was dependent on ethylene and PDF1·2 transcripts on both JA and ethylene. In contrast, the down‐regulation of photosynthetic transcripts was independent of both compounds. Previous studies have indicated a role for reactive oxygen species (ROS) in the UV‐B‐induced down‐regulation of the photosynthetic genes and up‐regulation of PR‐1 genes. Here we have shown that ROS are also required for the UV‐B‐induced up‐regulation of PDF1·2 genes. The results indicate that the effects of UV‐B on the three sets of genes are mediated through three distinct signal transduction pathways which are similar, but not identical, to pathways initiated in response to pathogen infection. In addition, the increased sensitivity of both jar1 and etr1‐1 mutants to UV‐B radiation, as compared with wild‐type plants, indicated that intact JA and ethylene signal pathways are required for defence against UV‐B damage.
Changes in carbon allocation and expression of carbon transporter genes inBetula pendulaRoth. colonized by the ectomycorrhizal fungusPaxillus involutus(Batsch) Fr.
Authors: Wright D. P. , Scholes J. D. , Read D. J. , Rolfe S. A.
Published: March 2003

Comparative analyses of aspects of the carbon (C) physiology and the expression of C transporter genes in birch (Betula pendulaRoth.) colonized by the ectomycorrhizal fungusPaxillus involutus(Batsch) Fr. were performed using mycorrhizal (M) and non‐mycorrhizal (NM) plants of similar foliar nutrient status. After six months of growth, the biomass of M plants was significantly lower than that of NM plants. Diurnal C budgets of both sets of plants revealed that M plants exhibited higher rates of photosynthesis and root respiration expressed per unit dry weight. However, the diurnal net C gain of M and NM plants remained similar. Ectomycorrhizal roots contained higher soluble carbohydrate pools and increased activity of cell wall invertase, suggesting that additional C was allocated to these roots and their ectomycorrhizal fungi consistent with an increased sink demand for C due to the presence of the mycobiont. In M roots, the expression of two hexose and one sucrose transporter genes of birch were reduced to less than one‐third of the expression level observed in NM roots. Analysis using a probe against the birch ribosomal internal transcribed spacer region revealed that M roots contained 22% less plant RNA than NM roots. As the expression of birch hexose and sucrose transporter genes was reduced to a much greater extent, this suggests that these specific genes were down‐regulated in response to alterations in C metabolism within M roots.
Stem water storage capacity and efficiency of water transport: their functional significance in a Hawaiian dry forest
Authors: Stratton L. , Goldstein G. , Meinzer F. C.
Published: March 2003

We investigated the contribution of internal water storage and efficiency of water transport to the maintenance of water balance in six evergreen tree species in a Hawaiian dry forest. Wood‐saturated water content, a surrogate for relative water storage capacity, ranged from 70 to 105%, and was inversely related to its morphological correlate, wood density, which ranged between 0·51 and 0·65 g cm−3. Leaf‐specific conductivity (kL) measured in stem segments from terminal branches ranged from 3 to 18 mmol m−1 s−1 MPa−1, and whole‐plant hydraulic efficiency calculated as stomatal conductance (g) divided by the difference between predawn and midday leaf water potential (ΨL), ranged from 70 to 150 mmol m−2 s−1 MPa−1. Hydraulic efficiency was positively correlated with kL (r2 = 0·86). Minimum annual ΨL ranged from − 1·5 to − 4·1 MPa among the six species. Seasonal and diurnal variation in ΨL were associated with differences among species in wood‐saturated water content, wood density and kL. The species with higher wood‐saturated water content were more efficient in terms of long‐distance water transport, exhibited smaller diurnal variation in ΨL and higher maximum photosynthetic rates. Smaller diurnal variation in ΨL in species with higher wood‐saturated water content, kL and hydraulic efficiency was not associated with stomatal restriction of transpiration when soil water deficit was moderate, but avoidance of low minimum seasonal ΨL in these species was associated with a substantial seasonal decline in g. Low seasonal minimum ΨL in species with low kL, hydraulic efficiency, and wood‐saturated water content was associated with higher leaf solute content and corresponding lower leaf turgor loss point. Despite the species‐specific differences in leaf water relations characteristics, all six evergreen tree species shared a common functional relationship defined primarily by kL and stem water storage capacity.
ABA‐deficient (aba1) and ABA‐insensitive (abi1‐1, abi2‐1) mutants of Arabidopsis have a wild‐type stomatal response to humidity
Authors: Assmann Sarah M. , Snyder Jo Ann , Lee Yuh‐Ru Julie
Published: March 2003

In most plant species, a decrease in atmospheric humidity at the leaf surface triggers a decrease in stomatal conductance. While guard cells appear to respond to humidity‐induced changes in transpiration rate, as opposed to relative humidity or vapour pressure difference, the underlying cellular mechanisms for this response remain unknown. In the present set of experiments, abscisic acid (ABA)‐deficient (aba1) and ABA‐insensitive (abi1‐1 and abi2‐1) mutants of Arabidopsis thaliana were used to test the hypothesis that the humidity signal is transduced by changes in the flux or concentration of ABA delivered to the stomatal complex in the transpiration stream. In gas exchange experiments, stomatal conductance was as sensitive to changes in vapour pressure difference in aba1, abi1‐1 and abi2‐1 mutant plants as in wild‐type plants. These experiments appear to rule out an obligate role for either the concentration or flux of ABA or ABA conjugates as mediators of the guard cell response to atmospheric water potential. The results stand in contrast to the well‐established role of ABA in mediating guard cell responses to decreases in soil water potential.
Regulation of nitrate reductase expression in leaves by nitrate and nitrogen metabolism is completely overridden when sugars fall below a critical level
Authors: Klein D. , Morcuende R. , Stitt M. , Krapp A.
Published: March 2003

These experiments use Nia30(145), a tobacco nia1nia2 double null mutant transformed with a NIA2 construct, to define when sugar supply plays the dominating role in the regulation of nitrate reductase (NIA) expression. The null alleles of Nia30(145) are transcribed and translated to produce non‐functional NIA transcript and NIA protein, providing an endogenous reporter system to track NIA expression at the transcript and protein level. The re‐introduced NIA2 construct is expressed at low efficiency, providing a background in which the response to changes in sugar status is not complicated by simultaneous changes in the rate of nitrate assimilation and the levels of nitrate and glutamine. In an alternating light–dark regime, Nia30(145) contained high levels of nitrate and low levels of glutamine and other amino acids. This drives constitutive overexpression of NIA. After transfer of Nia30(145) to continuous darkness, nitrate remains high and glutamine low, but the NIA transcript level and NIA protein decreased significantly within 24 h and were undetectable from 48 h onwards. The decrease of the NIA transcript level was fully reversed and the decrease of NIA protein was partly reversed when leaves were detached from the pre‐darkened plants and supplied with sucrose in the dark. The decrease was not reversed by nitrate or cytokinin. The NIA transcript disappeared when the leaf sugar content fell below 4 μmol hexose equivalents g−1 FW, and recovered when sugars rose above 8 μmol hexose equivalents g−1 FW. It is concluded that low sugar represses NIA, completely overriding signals derived from nitrate and nitrogen metabolism.
Effect of mycorrhizal‐enhanced leaf phosphate status on carbon partitioning, translocation and photosynthesis in cucumber
Authors: Black K. G. , Mitchell D. T. , Osborne B. A.
Published: March 2003

An assessment of the effects of arbuscular mycorrhizal (AM) infection on photosynthesis, carbon (C) allocation, translocation and biomass production of cucumber, grown in sand culture, was made using a previously determined phosphorus (P) supply (0·13 mol m−3 P) which had a significant impact on AM infection. Separation of a direct effect of AM infection from an indirect one due to an enhanced leaf P status was achieved using a comparable non‐mycorrhizal treatment (NAM + P) supplemented with extra P (0·19 mol m−3 P). Total leaf P concentration, specific leaf mass, photosynthetic capacity, and incorporation of 14C into non‐structural carbohydrate pools were dependent on leaf age. Both maximum and ambient photosynthetic rates were significantly higher in the youngest fully expanded leaves from AM and NAM + P plants which also had the higher leaf P concentrations. There were no differences in the total concentrations of starch, sucrose, raffinose or stachyose in young or old leaves among AM, non‐mycorrhizal (NAM) and NAM + P treatments. However, younger leaves of NAM plants showed a shift in 14C‐partitioning from stachyose and raffinose synthesis to starch accumulation. Determination of ADP‐glucose pyrophosphorylase (AGPase), sucrose synthase and sucrose phosphate synthase enzyme activities revealed that only AGPase activity was correlated with the increased incorporation rate of 14C into starch in young leaves of NAM plants. Although there were significant AM‐specific effects on C translocation to the root system, AM plants had similar rate of photosynthesis to NAM + P plants. These results suggest that the increase in photosynthetic rate in leaves of AM‐infected cucumber was due to an increased P status, rather than a consequence of a mycorrhizal ‘sink’ for assimilates.
Blue light inhibits stomatal development in soybean isolines containing kaempferol‐3‐O‐2G‐glycosyl‐gentiobioside (K9), a unique flavonoid glycoside
Authors: Liu‐Gitz L. , Britz S. J. , Wergin W. P.
Published: March 2003

Stomata have a fundamental role in controlling plant photosynthesis and transpiration, but very little is known about factors controlling stomatal differentiation and development. Lines of soybean that contain a specific flavonol glycoside, kaempferol‐3‐O‐2‐glycosyl‐gentiobioside (K9), as well as greatly reduced stomatal density, especially on the adaxial epidermis, have been identified. The specific effects of blue light photoreceptors on stomatal development in K9 lines and their isoline pairs containing no K9 were studied. Low irradiances of blue light (7% of total photosynthetically active radiation) added to high irradiances from low‐pressure sodium lamps strongly inhibited stomatal development on the adaxial epidermis of K9 lines, but not in isoline pairs differing putatively in only one gene and lacking K9. Overall, blue light slightly increased stomatal density on the abaxial epidermis in all isolines, demonstrating differential regulation of stomatal development in the upper and lower epidermis. Blue light also caused an increase in leaf area in all isolines, indicating that changes in stomatal density were not the non‐specific result of alterations in leaf area. Morphological studies revealed that the blue light‐induced reduction in stomatal density in K9 lines was due to reduced stomatal initiation as well as aborted or abnormal stomatal development. As the phytochrome photostationary state was kept constant, the results indicate that one or more blue light receptors are involved in the control of stomatal development. This system should be useful for the study of mechanisms controlling stomatal development, even if the photo‐inhibitory response is unique to K9 lines.
Identification of a novel multiple environmental factor‐responsive 1‐aminocyclopropane‐1‐carboxylate synthase gene, NT‐ACS2, from tobacco
Authors: Ge L. , Liu J. Z. , Wong W. S. , Hsiao W. L. W. , Chong K. , Xu Z. K. , Yang S. F. , Kung S. D. , Li N.
Published: March 2003

Many abiotic environmental factors elicit the production of stress‐ethylene in higher plants. To elucidate the molecular mechanisms underlying the regulation of stress‐ethylene production and the physiological roles played by stress‐ethylene in stress responses of plants, we studied the gene expression of ACC synthase in tobacco plants that had been subjected to environmental stresses. Four new tobacco ACC synthase cDNA fragments, NT‐ACS2, NT‐ACS3, NT‐ACS4 and NT‐ACS5, were identified and sequenced. It was found that NT‐ACS2 could be induced by wounding, cold temperature and, especially, sunlight. NT‐ACS4 was induced at a faster kinetics by wounding. The multiple environmental stress‐responsive (MESR) NT‐ACS2 gene was found to contain three introns and four exons and encode a polypeptide of 484 amino acids, 54·6 kDa and pI 6·87. Computer analysis of the 3·4 kb 5′ flanking region upstream of the ACS coding region revealed the existence of a group of putative cis‐acting regulatory elements potentially conferring wounding, chilling, and UV light inducibility. Phylogenetic analysis of ACC synthase genes of different plant origins indicated that the chill‐inducible NT‐ACS2 gene is closely related to a chilling‐inducible citrus ACS gene.
The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance
Authors: Bungard R. A. , Scholes J. D. , Press M. C.
Published: March 2003

Dipterocarps dominate the canopy of lowland tropical rain forest in South‐east Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth depends largely upon the increased irradiance that can occur with the creation of canopy gaps. However, the response of dipterocarp seedlings to increased irradiance and their subsequent establishment in the canopy may be influenced by the availability of other resources, such as nutrient availability. We investigated the influence of nitrogen supply on aspects of the photosynthetic physiology and growth of seedlings of four dipterocarp species (Shorea leprosula,Shorea johorensis,Shorea oleosaandDryobalanops lanceolata) growing under low irradiance, during transfer from low to high irradiance, and during subsequent growth at high irradiance. All four species increased growth and photosynthetic capacity in response to N‐supply at high irradiances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N‐supply were exposed to a large increase in irradiance, all species showed some degree of initial photodamage (measured through chlorophyll fluorescence), the extent of which was similar between species but differed markedly depending on the pre‐exposure growth irradiance and N‐supply. Greater photodamage occurred in seedlings grown at lower compared with higher N‐supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to recover from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N‐supply. Under high N‐supply all species apart fromS. oleosaincreased photosynthetic capacity per unit chlorophyll following exposure to high irradiance by increasing photosynthetic capacity per unit leaf area while, under low N‐supply, an increase in photosynthetic capacity per unit leaf only occurred inD. lanceolata. Our results suggest that variations in N‐availability may have a much greater impact on the relative competitiveness of dipterocarp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N‐availability in the regeneration dynamics and distribution of canopy‐dominating dipterocarp species.
Changes in Rubisco and Rubisco activase gene expression and polypeptide content in Pinus halepensis M. subjected to ozone and drought
Authors: Pelloux J. , Jolivet Y. , Fontaine V. , Banvoy J. , Dizengremel P.
Published: March 2003

The regulation of ribulose‐1,5‐biphosphate carboxlase/oxygenase (Rubisco) and Rubisco activase was followed for 3 months in an experiment studying the effects of ozone and water stress on Aleppo pine. Rubisco activity was shown to be reduced by 30% in the presence of ozone, whereas no significant effect of water stress was noticed. The effect of combined stresses on Rubisco activity was similar to the effect of ozone. The changes in protein quantity of Rubisco large subunit (LSU) and Rubisco activase (RCA), compared with control plants, were similar to that of the Rubisco activity. Using homologous probes obtained by reverse transcription (RT)‐polymerase chain reaction (PCR), rbcL and rca transcript quantities were quantified during the course of the experiment. RbcL and rca mRNA quantities decreased in ozone and after drought. Changes in rbcL transcript quantity in needles subjected to the combination of ozone and drought were similar to the ones detected when drought was applied alone. On the contrary, the pattern of rca changes under the combination of the two stresses was similar to that of ozone applied alone. A positive correlation existed between the effects of ozone on Rubisco activase and Rubisco LSU protein quantities, which was not so obvious by comparing transcript quantities. This could suggest a potential post‐transcriptional coordinated regulation of the two proteins under stress‐imposed conditions.
Low‐temperature resistance in Polylepis tarapacana, a tree growing at the highest altitudes in the world
Authors: Rada F. , García‐Núñez C. , Boero C. , Gallardo M. , Hilal M. , González J. , Prado F. , Liberman‐Cruz M. , Azócar A.
Published: March 2003

The Polylepis tarapacana forests found in Bolivia are unique with respect to their altitudinal distribution (4200–5200 m). Given the extreme environmental conditions that characterize these altitudes, this species has to rely on distinct mechanisms to survive stressful temperatures. The purpose of this study was to determine low‐temperature resistance mechanisms in P. tarapacana. Tissue was sampled for carbohydrate and proline contents and micro‐climatic measurements were made at two altitudes, 4300 and 4850 m, during both the dry cold and wet warm seasons. Supercooling capacity (−3 to −6 °C for the cold dry and −7 to −9 °C for the wet warm season) and injury temperatures (−18 to −23 °C for both seasons), determined in the laboratory, indicate that P. tarapacana is a frost‐tolerant species. On the other hand, an increase in supercooling capacity, as the result of significant increase in total soluble sugar and proline contents, occurs during the wet warm season as a consequence of higher metabolic activity. Hence, P. tarapacana, a frost‐tolerant species during the colder unfavourable season, is able to avoid freezing during the more favourable season when minimum night‐time temperatures are not as extreme.
Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain
Authors: Evans J. R. , Poorter H.
Published: March 2003

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C3 species grown in photon irradiances of 200 and 1000 µmol m−2 s−1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.
Chilling‐induced Ca2+ overload enhances production of active oxygen species in maize (Zea mays L.) cultured cells: the effect of abscisic acid treatment
Authors: Chen W. P. , Li P. H.
Published: March 2003

Chilling (4 °C) induced a prolonged high level of intracellular Ca2+ (Ca2+ overload) and lipid peroxidation in maize (Zea mays L. cv Black Mexican Sweet) cultured cells. However, such Ca2+ overload and enhanced lipid peroxidation were not seen in abscisic acid (ABA)‐treated cells, which had an improved chilling tolerance. A Ca2+ ionophore, A23187, caused Ca2+ overload in both ABA‐treated maize cells and the untreated control, whereas an enhanced lipid peroxidation was detected only in the control. The high level of active oxygen species (AOS) in the control during chilling at 4 °C could be reduced by the presence of lanthanum (La3+), a Ca2+ channel blocker, in the medium. Moreover, both the A23187‐induced lipid peroxidation and AOS production in the control could be reduced by extracellular EGTA, a Ca2+ chelator. Laser‐scanning confocal microscopy revealed that mitochondria were one of the major AOS sources under chilling and during A23187 treatment. In vitro assays showed that superoxide production in isolated maize mitochondria was enhanced by the presence of Ca2+. Findings suggest that chilling‐induced Ca2+ influx in the control triggers a marked generation of AOS, which in turn results in the enhanced lipid peroxidation. The ability of ABA‐treated cells to avoid the chilling‐induced Ca2+ influx may serve as a mechanism that prevents the chilling‐induced oxidative stress and thus results in less chilling injury.
Stress‐related RNase PR‐10c is post‐translationally modified by glutathione in birch
Authors: Koistinen K. M. , Kokko H. I. , Hassinen V. H. , Tervahauta A. I. , Auriola S. , Kärenlampi S. O.
Published: March 2003

The PR‐10c (previously termed as Bet v 1‐Sc3) protein of birch belongs to the family of intracellular pathogenesis‐related proteins. The high‐performance liquid chromatography electrospray ionization ion trap mass spectrometry (HPLC‐ESI‐MS) analysis of PR‐10c‐His fusion protein, produced in Escherichia coli, revealed three major peaks and masses. Enzymatic digestions and HPLC‐ESI‐MS and matrix assisted laser desorption/ionization – time of flight mass spectrometry (MALDI‐TOF‐MS) analyses of each fraction indicated that PR‐10c‐His protein is post‐translationally modified by carbamylation and S‐glutathiolation. Carbamylation was localized into the N‐terminal end of PR‐10c‐His and does not represent a biologically significant modification. The possible nuclease activity of PR‐10c was analysed with S‐glutathiolated and reduced fractions of PR‐10c‐His fusion protein. Both forms of PR‐10c‐His as well as the dimeric form of the protein possess RNase activity which is capable of digesting different RNA substrates. None of the fractions showed activity against single‐ or double‐stranded DNA. The MALDI‐TOF‐MS analysis of PR‐10c polypeptide extracted from zinc‐exposed birch roots showed that the protein is post‐translationally modified by glutathione (γ‐Glu‐Cys‐Gly) also in vivo. The S‐glutathiolated cysteine residue of PR‐10c is not conserved among Bet v 1 homologous proteins and is also unique in the PR‐10 family. As far as we know this is the first observation of S‐glutathiolation in plants, or any post‐translational modification in the PR‐10 family of proteins.
How does photorespiration modulate leaf amino acid contents? A dual approach through modelling and metabolite analysis
Authors: Novitskaya L. , Trevanion S. J. , Driscoll S. , Foyer C. H. , Noctor G.
Published: March 2003

The aim of this work was to establish the quantitative impact of photorespiration on leaf amino acid contents. Attached leaves of wheat and potato were incubated for 30–40 min under defined conditions in which net CO2uptake (A) was manipulated by irradiance, ambient CO2or ambient O2. The incubated portion of the leaf was sampled by a rapid‐quench method and photorespiratory flux (vo) was modelled from the measured rate of net CO2uptake. In both wheat and potato, the ratio between glycine and serine showed a strong positive correlation withvo. Aspartate and alanine correlated negatively withvobut glutamate and glutamine showed less clear relationships. In potato, glutamate and glutamine did not correlate clearly with eitherAorvo. In wheat, glutamine showed a general increase withAbut no relationship withvo, whereas 2‐oxoglutarate contents correlated positively withvoand negatively withA. As a result, glutamine : glutamate and glutamine : 2‐oxoglutarate increased with net CO2uptake in wheat, observations that are attributed primarily to imperfect and variable coupling between the supply of NH3in primary nitrogen assimilation and the associated delivery of 2‐oxoglutarate to the chloroplast. A simple theoretical analysis is used to illustrate the potentially marked impact of primary nitrogen assimilation on leaf glutamine, even against a background of high rates of photorespiratory ammonia recycling.
The effect of growth irradiance on leaf anatomy and photosynthesis in Acer species differing in light demand
Authors: Hanba Y. T. , Kogami H. , Terashima I.
Published: March 2003

Variation in light demand is a major factor in determining the growth and survival of trees in a forest. There is strong relation between the light‐demand and the effect of growth irradiance on leaf morphology and photosynthesis in three Acer species: A. rufinerve (light‐demanding), A. mono (intermediate) and A. palmatum (shade‐tolerant). The increase in mesophyll thickness and surface area of chloroplasts facing the intercellular airspaces (Sc) with growth irradiance was highest in A. rufinerve. Although the increase in light‐saturated photosynthesis (Amax) was similar among the species, the increase in water use efficiency (WUE) was much higher in A. rufinerve than that in the other species, indicating that the response to water limitation plays an important role in leaf photosynthetic acclimation to high light in A. rufinerve. The low CO2 partial pressure at the carboxylation site (Cc) in A. rufinerve (130 µmol mol−1) at high irradiance was caused by low stomatal and internal conductance to CO2 diffusion, which minimized the increase in Amax in A. rufinerve despite its high Rubisco content. Under shade conditions, interspecific differences in leaf features were relatively small. Thus, difference in light demand related to leaf acclimation to high light rather than that to low light in the Acer species.
Contrasted responses to carbohydrate limitation in tomato fruit at two stages of development
Authors: BALDET P. , DEVAUX C. , CHEVALIER C. , BROUQUISSE R. , JUST D. , RAYMOND P.
Published: March 2003

The metabolic consequences of long‐term carbohydrate depletion have been well documented in many sink organs but not extensively in fruit. Therefore, in the present study the response to sugar limitation in tomato fruit (Lycopersicon esculentum Mill.) was investigated at two developmental stages; during the cell division and cell expansion phases. First, the response in excised fruit cultured in vitro was characterized. Sugar depletion caused an arrest of growth and an exhaustion of carbon reserves. The proteins that were degraded and the nitrogen released was transiently stored as asparagine and glutamine in both developmental stages and also as γ ‐aminobutyric acid (GABA) in expanding fruit. Fruit at the cell division stage appeared to be more sensitive to sugar limitation. The response to sugar depletion was then characterized in fruit from plants submitted to extended darkness. In planta, the effects of sugar‐limitation were similar to those described in vitro but much more attenuated, especially in expanding fruit, which still accumulated dry matter. The expression of cell cycle genes, sugar‐ and nitrogen‐related genes was reduced by darkness. Only asparagine synthetase gene expression was induced in both dark‐treated fruit. Together the present data revealed that the effects of the carbon limitation are more pronounced in the youngest fruits as it is probably controlled by the relative sink strength of the fruit.
The case for cytosolic NO3– heterostasis: a critique of a recently proposed model
Authors: BRITTO D. T. , KRONZUCKER H. J.
Published: March 2003

A model recently proposed by Siddiqi & Glass (Plant, Cell, and Environment 25, 1211–1217, 2002) attempts to reconcile discrepancies in the measurement of cytosolic nitrate concentrations ([NO3–]cyt) in plant root cells, specifically between low (∼ 4 mm) homeostatic values reported in studies using ion‐specific microelectrodes on the one hand, and wide fluctuations in [NO3–]cyt reported in other studies, especially those using compartmental analysis by tracer efflux (CATE). Although Siddiqi & Glass concede that cytosolic NO3– homeostasis, as determined by microelectrodes, is at odds with certain experimental observations, they nevertheless promote a model that takes microelectrode readings at face value, and assert that the variations seen using CATE methodology are artefacts attributable to contributions from substantial, rapidly exchanging, and highly variable NO3– pools putatively residing in organelles such as plastids and the endoplasmic reticulum. We show here that such a model is not tenable, drawing upon experimental evidence from previous studies, and from a more comprehensive model that examines the characteristics and consequences of subcompartmented cytoplasmic exchange in root cells.
Susceptibility in oats to stem rust induced by co‐infection with leaf rust
Authors: Adhikari K. N. , McIntosh R. A.
Published: March 2003

Induction of susceptibility in oats to a normally avirulent pathotype of Puccinia graminis f.sp. avenae was studied in the presence of different pathotypes of P. coronata f.sp. avenae. Induction occurred on seedlings only in the presence of a virulent culture of P. coronata avenae and was not dependent on time or order of inoculation of either pathogen. This phenomenon was restricted to seedlings of lines possessing the Pg‐a source of oat stem rust resistance. The specificity of induced susceptibility can be used as a valuable bioassay for screening and identifying Pg‐a. Induced susceptibility occurred only at the seedling stage, and apparently provides no obstacle to the use of Pg‐a as a source of stem rust resistance in oats.
Hyaline mutants from Verticillium dahliae, an example of selection and characterization of strains for host–parasite interaction studies
Authors: Daayf F. , Nicole M. , Bélanger R. R. , Geiger J.‐P.
Published: March 2003

The objective of the present work was to select stable well‐characterized strains of Verticillium dahliae that could be used as biological tools in genetic and plant–microbe interaction studies. Hyaline mutants, known for their stability in pathogenicity were chosen for the study. Diversity in pathogenicity was found among hyaline subclones obtained from a defoliating wild‐type clone, but not within those from nondefoliating ones. Most subclones from the defoliating clone had parental pathotypes, but one (V7‐2) exhibited weak pathogenicity. This subclone (V7‐2), together with a highly virulent one (V7‐7) deriving from the same defoliating parent clone (7), were selected for further characterization, because of their differences in pathogenicity. When studied on the basis of their growth requirements, the two subclones expressed marked differences. V7‐7 grew better than V7‐2 over a wider range of temperature conditions. Both subclones grew similarly in media supplemented with NH4 as nitrogen source, but in those with NO3, V7‐7 grew more vigorously than V7‐2 and only the former could grow when NO2 was used. In spite of these differences, the two subclones were found to belong to the same vegetative compatibility group, confirming their genetic proximity. These results highlight the physiological and genetic complexity inherent in V. dahliae. In view of their characteristics, the clones obtained in this study should prove to be valuable tools in furthering the understanding of genetic and host–V. dahliae interactions.
Effects of fungicide spray timing on winter wheat disease control
Authors: Cook  , Hims  , Vaughan
Published: March 2003

A field experiment of novel systematic design was established on 17 sites in the three years 1988–90 to compare the effect of spray timing on the disease control provided by fungicides. At each site one set of plots received applications of a mixture of fenpropimorph and prochloraz or triadimenol, tridemorph and chlorothalonil fungicides, applied at successively later start dates to produce a series of temporally and spatially related, but discrete, epidemics curtailed at consecutively later stages of development between GS32 and GS75. A second set of plots was treated with a single spray of one of a set of the nine candidate fungicides between GS32 and GS71. Disease developed at each site, with moderate septoria leaf spot (Septoria tritici) at five of the eight sites in 1988 and four of the seven sites in 1989. Erysiphe graminis f.sp. tritici was present at two sites in 1988, four sites in 1989 and one site only in 1990. Puccinia striiformis (four sites only in 1989 and 1990) and brown rust (one site in each year) were less common. The systematic treatments showed consistent relationships between green leaf retention during grain filling and yield. They also provided clear indications that epidemics of foliar disease initiated before flag leaf emergence had the greatest impact on yield. After this stage, yield loss averaged 27.2 kg ha−1 for each day that elapsed before fungicide was applied. The sequential single spray treatments showed marked differences between the protectant and eradicant activity of the different fungicides used. Each disease was most effectively controlled on the final leaf by treatment at or immediately after its emergence. Chlorothalonil was as effective as any of the fungicides tested against septoria, when applied before flag leaf emergence, but was inferior when applied after this stage. The fungicides were equally effective against yellow rust when applied just before symptoms were detected, when spray timing appeared to be more important than choice of fungicide. The response of brown rust to fungicides was similar, except that chlorothalonil was the least effective. For mildew, timing seemed to be less important, although sprays applied immediately after leaf emergence provided the best control on each individual leaf layer. Yield was well related to disease and green leaf area late in the season, these factors being more important than which disease was present, which fungicide, and which cultivar were used.
Reduced persistence of metalaxyl in soil associated with its failure to control cavity spot of carrots
Authors: Davison  , McKay
Published: March 2003

Metalaxyl was used to control Pythium diseases of carrots in experiments on farms with a history of cavity spot. The first experiment compared the method of application (sprayed, banded or broadcast) and rate (0, 1.5, 3 or 6 kg a.i. ha−1) one week after sowing. Three additional experiments compared the rate (0, 0.75, 1.5 or 3 kg a.i. ha−1) and time (sowing, 1‐ to 2‐ or 4‐ to 5‐true‐leaf stage) of application. In expt 1, the application of metalaxyl, but not the method by which it was applied, increased yield by 20% and significantly reduced the incidence of cavity spot, forking and misshapen carrots. In expts 2, 3 and 4, neither the rate nor time of application affected yield or reduced the incidence of Pythium diseases. Comparison of the sites showed that they differed in past metalaxyl usage. Metalaxyl had not been used on the site of expt 1, but had been used previously at sites 2, 3 and 4. Laboratory experiments were conducted to determine whether these differences in efficacy resulted from reduced sensitivity of Pythium isolates to metalaxyl, or reduced persistence of metalaxyl in soil. ED50 values showed that there was no reduction in metalaxyl sensitivity. The half‐life of metalaxyl was 82 days in soil from expt 1, but was 10 days or fewer in soils from expts 2, 3 and 4. Thus the failure of metalaxyl to control Pythium diseases was associated with reduced persistence in soil, not reduced sensitivity of the target fungi.
Disease response of carrot and carrot somaclones to Alternaria dauci
Authors: Dugdale  , Mortimer  , Isaac  , Collin
Published: March 2003

Leaf spot of carrot, caused by the fungus Alternaria dauci, is a common disease of carrot throughout the world. To assess the disease response in different genotypes an in vitro method of assay has been developed. The terminal portion of excised mature leaves from carrot are incubated under lights at 25–26°C for 6 days with spores of A. dauci. After 6 days, chlorophyll and total soluble polyphenol contents of the leaves decline to a low level, whereas in the uninoculated control leaves levels of these compounds are largely unchanged. Electron microscope studies of the effect of infection on the ultrastructure of leaves from both cv. Fancy and wild plants show disintegration of the cell wall, plasmalemma and chloroplast membranes in the region close to the invading fungus. The response to A. dauci shows characteristics compatible with the presence of a phytotoxin.  Seedling hypocotyls of the carrot cv. Fancy are used as a source of explants to initiate callus tissue and regenerate embryos. Regenerant somaclone plants show variation in morphology (crown number, height, leaf number, leaf shape and flowering time) and susceptibility to the pathogen as measured by the loss in chlorophyll from infected detached leaves after a 6‐day period. Potentially susceptible and resistant regenerant plants are identified by the low and high chlorophyll levels, respectively, in excised leaves 6 days after infection. Selected high‐ and low‐chlorophyll regenerant plants and seedling‐derived cv. Fancy are self‐pollinated, then the seeds are used to produce a progeny generation, which is assessed for morphological changes and disease resistance as before. The response of leaves of progeny plants to infection shows that the plants derived from high‐chlorophyll regenerants still show a higher chlorophyll level after infection compared with leaves derived from low‐chlorophyll and control plants. High chlorophyll levels in the presence of infection in the progeny of high‐chlorophyll regenerants suggest that the capacity to resist infection in the selected somaclones is inherited.
The effect of temperature and water potential on the production of conidia by sclerotia of Botrytis squamosa
Authors: Clarkson  , Kennedy  , Phelps
Published: March 2003

The rate of conidiogenic germination of Botrytis squamosa was highest at 16°C and the greatest numbers of conidia per sclerotium (up to 5 × 104) were produced at temperatures of 5–10°C. At temperatures above 20°C, the percentage of sclerotia producing conidia declined rapidly. Decreasing water potential reduced the rate at which conidia were produced and also resulted in fewer conidia produced per sclerotium. However, conidia were produced at water potentials as low as −2 MPa, at which sclerotial germination was at least 60%. A simulation model that included effects of both temperature and water potential was developed from laboratory and field data obtained for conidial production in sclerotia exposed for periods of 1, 2, 3 or 4 weeks during an entire year. There was good agreement between conidiogenic germination predicted by the model and conidial production observed in onion plots artificially inoculated with sclerotia. Temperature and water potential were therefore considered to be the principal microclimatic factors affecting conidial production by B. squamosa. The role of sclerotia in the context of UK onion production is discussed.
Differences in cultivar response and complete sequence analysis of two isolates of wheat yellow mosaic bymovirus in China
Authors: Chen J. , Chen J.‐P. , Yang J.‐P. , Cheng Y. , Diao A. , Adams M. J. , Du J.
Published: March 2003

Seeds of selected European and Japanese winter wheat cultivars were grown at two experimental sites in China, namely Yaan, Sichuan province (YA), and Yangzhou, Jiangsu province (YZ), where wheat yellow mosaic bymovirus (WYMV) was severe. There were some differential responses of the cultivars to the virus isolates present at the two sites. The complete nucleotide sequence of both RNAs of both virus isolates was determined. Their genome organization was identical to that reported for a Japanese isolate and the sizes were very similar. Nucleotide comparisons demonstrated that parts of the CI and NIa coding regions on RNA1 and the N‐terminal part of the P2 coding region on RNA2 were particularly variable, while substantially conserved regions occurred in the 3′ UTR of RNA2, the 7K, one part of the CI and parts of the NIb and coat protein. It seems unlikely that differences in the 7K and NIa‐VPg proteins are responsible for virulence differences and the CI and NIb regions were considered the most promising for further study.
Systemic induction of phytoalexins in cucumber in response to treatments with fluorescent pseudomonads
Authors: Ongena M. , Daayf F. , Jacques P. , Thonart P. , Benhamou N. , Paulitz T. C. , Bélanger R. R.
Published: March 2003

The Pseudomonas putida isolate BTP1 and its sid− mutant M3 were recently reported to protect cucumber against Pythium aphanidermatum root rot. This protection was mainly associated with an accumulation of antifungal phenolics in the treated roots. In this study, analyses of root extracts from split‐root experiments showed that these phytoalexins were produced systemically. Indeed, several antifungal molecules accumulated similarly in both treated and nontreated root parts of plants protected against P. aphanidermatum with BTP1 or M3. In addition, analyses of leaf samples also revealed increased amounts of fungitoxic molecules in PGPR‐treated plants, although the nature of these molecules appeared to be different from those detected in roots. The antifungal compounds isolated both from roots and leaves were mainly detected in acid‐hydrolyzed extracts containing aglycones. These results suggest that PGPR can elicit phytoalexins systemically in cucumber and that the overall defence response is not based on a single phytoalexin but is chemically complex and organ‐specific.
Host adaptation to potato and tomato within the US−1 clonal lineage of Phytophthora infestans in Uganda and Kenya
Authors: Vega‐Sánchez M. E. , Erselius L. J. , Rodriguez A. M. , Bastidas O. , Hohl H. R. , Ojiambo P. S. , Mukalazi J. , Vermeulen T. , Fry W. E. , Forbes G. A.
Published: March 2003

Twenty isolates of Phytophthora infestans from potato and twenty‐two from tomato, collected in Uganda and Kenya in 1995, were compared for dilocus allozyme genotype, mitochondrial DNA (mtDNA) haplotype, mating type and restriction fragment length polymorphism (RFLP) fingerprint using probe RG57. Based on RFLP fingerprint and mtDNA haplotype, all isolates were classified in the US−1 clonal lineage. Nonetheless, isolates from potato differed from isolates from tomato in several characteristics. Isolates from potato had the 86/100 glucose‐6‐phosphate isomerase (Gpi) genotype, while those from tomato were 100/100, which represents a variant of US−1 that had been identified previously as US−1.7. Furthermore, while pure cultures of the pathogen were acquired from infected potato leaflets by first growing the isolates on potato tuber slices, this approach failed with infected tomato tissue because the isolates grew poorly on this medium. Tomato isolates were eventually purified using a selective medium. Six isolates from each host were compared for the diameter of lesions they produced on three tomato and three potato cultivars in one or two detached‐leaf assays (four isolates from the first test were repeated in the second). On potato leaflets, isolates from potato caused larger lesions than isolates from tomato. On tomato leaflets, isolates from that host caused larger lesions than did isolates from potato, but the difference was significant in only one test. The interaction between source of inoculum (potato or tomato) and inoculated host (potato or tomato) was significant in both tests. Isolates from tomato were highly biotrophic on tomato leaflets, producing little or no necrosis during the seven days following infection, even though abundant sporulation could be seen. In contrast, isolates from potato sporulated less abundantly on tomato leaflets and produced darkly pigmented lesions that were most visible on the adaxial side of the leaflets. Nonetheless, all isolates infected and sporulated on both hosts, indicating that host adaptation is not determined by an ability to cause disease but rather by quantitative differences in pathogenic fitness. Assessment of Gpi banding patterns, mtDNA haplotype and RFLP fingerprint of 39 isolates from potato collected in Uganda and Kenya in 1997 indicated that the population had not changed on this host. The population of P. infestans from Kenya and Uganda provides an interesting model for the study of quantitative host adaptation.