bluuebunny/crossref_metadata_2025 huggingface dataset:
Plant growth regulators and phloem exudates modulate root nodulation of soybean
Authors: Bano Asghari , Harper James E.
Published: March 2003

A time-course study was conducted on the effect of synthetic ABA and phloem exudate on root nodulation, nitrogenase activity, and dry matter content of pouch-grown soybean [Glycine max (L.) Merr.]. A hypernodulating mutant (NOD1-3) derived from the cultivar Williams was evaluated. The normally-nodulating cultivar Williams 82 served as the control. All phases of nodulation (nodule initiation, development and function) were inhibited by 1–50 μM synthetic ABA treatment of roots of pouch-grown soybean. The ABA effect was more pronounced for the treatment made 24 h prior to inoculation. The magnitude of inhibition of nodulation by ABA and by phloem exudate from inoculated Williams 82 was decreased when application was delayed to 48 h post-inoculation. Synthetic indoleacetic acid and benzyladenine (1 and 5 μM) also inhibited the number of nodules per plant, and the magnitude of inhibition was greater in the normally-nodulating Williams 82 cultivar than in the NOD1-3 hypernodulating mutant. Changes in endogenous free ABA in cotyledons, leaves, and xylem sap of seedlings were monitored at 24-h intervals after inoculation in both lines, and appeared to respond to inoculation. Addition of crude phloem exudate from Williams 82 or hypernodulating NOD1-3 was inhibitory to root nodulation of NOD1-3 — this effect was concentration dependent. Partially-purified (acidic aqueous fraction) phloem exudate from Williams 82 suppressed nodulation in NOD1-3 by 54% (averaged over a 7-d time-course), while partially-purified phloem exudate (acidic aqueous fraction) from NOD1-3 was much less inhibitory (23%). ABA levels in the phloem fractions applied were nearly identical (15.6 vs 16.0 ng mL–1), which indicates that ABA was not directly affecting nodulation control. Partially-purified extract (acidic aqueous fraction) from leaves of nodulated NOD1-3 was much less inhibitory to nodulation of NOD1-3 seedlings than was the comparable fraction from Williams 82. Although free ABA applied exogenously was capable of altering nodulation, it does not appear that endogenous ABA levels directly account for the autoregulatory control of nodulation in soybean. This implies that there may be factor(s) moving in the phloem other than ABA that possibly impact expression of root nodulation. Alternatively, it may be that plant development is responsive to interactions among plant growth regulators mediated by subtle changes in individual hormone concentrations. The possibility remains that normally-nodulating and hypernodulating lines have differential sensitivity to ABA, and that this explains altered nodulation control at similar levels of endogenous ABA.
High-throughput vectors for efficient gene silencing in plants
Authors: Helliwell Chris A. , Wesley S. Varsha , Wielopolska Anna J. , Waterhouse Peter M.
Published: March 2003

A major challenge in the post-genome era of plant biology is to determine the functions of all genes in the plant genome. A straightforward approach to this problem is to reduce or knockout expression of a gene with the hope of seeing a phenotype that is suggestive of its function. Insertional mutagenesis is a useful tool for this type of study but is limited by gene redundancy, lethal knockouts, non-tagged mutants, and the inability to target the inserted element to a specific gene. The efficacy of gene silencing in plants using inverted-repeat transgene constructs that encode a hairpin RNA (hpRNA) has been demonstrated by a number of groups, and has several advantages over insertional mutagenesis. In this paper we describe two improved pHellsgate vectors that facilitate rapid generation of hpRNA-encoding constructs. pHellsgate 4 allows the production of an hpRNA construct in a single step from a single polymerase chain reaction product, while pHellsgate 8 requires a two-step process via an intermediate vector. We show that these vectors are effective at silencing three endogenous genes in Arabidopsis, FLOWERING LOCUS C, PHYTOENE DESATURASE and ETHYLENE INSENSITIVE 2. We also show that a construct of sequences from two genes silences both genes.
The riddle of root pressure. II. Root exudation at extreme osmolalities
Authors: Pickard William F.
Published: March 2003

To test the predictions of a recent theory of root pressure and exudation, exudation from detopped tomato seedlings was studied. Experimental findings were generally in qualitative accord with the theory. Two confirmed predictions are of particular interest. First, when a normally exuding stump was challenged by having its roots exposed to concentrated osmolyte, the exudation rate dropped quickly to zero but later often recovered to a higher level. Second, when exudation from a plant challenged by concentrated osmolyte did not recover, flushing the soil solution away with tap water usually resulted in a sudden resumption of exudation followed by a rapid falloff.
Space and time dependence of temperature and freezing in evergreen leaves
Authors: Ball Marilyn C. , Wolfe Joe , Canny Martin , Hofmann Martin , Nicotra Adrienne B. , Hughes Dale
Published: March 2003

Infrared video thermography was used to study space and time dependence of freezing in intact, attached leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings. Freezing initiated in the midvein and spread through the apoplast at 10 mm s–1. Freezing of apoplastic water was detected by a local, rapid increase in temperature, and was followed by a slower increase in leaf temperature to the equilibrium freezing temperature as symplastic water moved from cells to extracellular sites of ice formation. The duration of freezing varied with position, leaf thickness and water content. Most of the cellular water in the leaf tip and margins froze quickly, while freezing was slower near the petiole and midvein. Regions that had frozen more rapidly then began to cool more rapidly, producing steep gradients in leaf temperatures and hence also freeze-induced dehydration. Thus, spatial variation in physical properties of leaves could affect the distribution of minimum leaf temperatures, and hence, the distribution and extent of damage due to freeze-induced dehydration. These results are consistent with patterns of freezing damage in autumn when the duration of freezing may be insufficient for the whole leaf to freeze before sunrise, and may explain the general observation of increased leaf water content and thickness with altitude.
Sensitivity of ragweed (Ambrosia artemisiifolia) growth to urban ozone concentrations
Authors: Ziska Lewis H.
Published: March 2003

Although the sensitivity of growth and yield to ground-level ozone (O3) has been determined for a variety of agronomic crops and trees, little information is available for weedy species. Common ragweed (Ambrosia artemisiifolia L.) is recognized both as a common agricultural weed and the principle source of pollen for Fall (late August to November) allergies in North America. To quantify the extent to which ambient O3 limits growth and reproduction, ragweed was grown from germination to floral initiation at control [carbon filtered (CF), 15.8� nL� O3� L–1 air] and treatment levels (63.5 nL O3 L–1 air) of tropospheric O3. This 8-h treatment average is similar to the average 1-h O3 peak values from May through September in urban areas of Washington D.C. and Baltimore, MD. By 48 d after sowing, during floral initiation, no significant differences in total plant or floral biomass were observed as a function of O3 concentration, relative to the CF control. Analysis of leaf area ratio, relative growth rate and net assimilation rate at approximately 10-d intervals during early vegetative growth also did not demonstrate any significant effect of O3. Data from this experiment indicate that ragweed is insensitive to O3 levels up to four times that of the CF control, and suggests that O3 levels associated with urban environments may not limit the growth or reproductive development of ragweed.
Ozone-induced oxidation of Rubisco: from an ELISA quantification of carbonyls to putative pathways leading to oxidizing mechanisms
Authors: Kanoun Myriam , Goulas Philippe , Bassères Anne , Biolley Jean-Philippe
Published: March 2003

In an attempt to detect a possible relationship between protein oxidation and ozone (O3) atmospheric concentration, we used a sensitive enzyme-linked immunosorbent assay (ELISA) method for measuring carbonyl formation in amino acid residues that constitute Rubisco (EC 4.1.1.39) small subunit (Rubisco-SSU). Using open-top chamber technology, bean plants (Phaseolus vulgaris L. cv. Bergamo) were exposed for 21 d (from emergence) to four different atmospheres characterized by average daylight O3 concentrations of 12, 70, 89 and 109�nL L–1. Rubisco-SSU extracted from primary leaves was fixed specifically on wells coated with anti-SSU antibodies. Aldehydes and ketones, previously derivatized with 2,4-dinitrophenylhydrazine (DNP), were quantified with anti-DNP antibodies conjugated with alkaline phosphatase. A significant positive O3 effect on carbonyl formation was detected, and the number of carbonyls was found to be linearly increased (r2=0.82) when plotted against increasing external O3 dose expressed as accumulated exposure over a threshold of 40 nL L–1 h (AOT40). Furthermore, these O3-induced oxidative modifications were connected with a significant reduction in the amount of native SSU that linearly decreased (r2=0.95) as AOT40 increased from 0 to about 8295 nL L–1 h. A possible pathway leading to oxidation of Rubisco is proposed, with special reference to O3 reactivity.
Influence of rising atmospheric CO2 since 1900 on early growth and photosynthetic response of a noxious invasive weed, Canada thistle (Cirsium arvense)
Authors: Ziska Lewis H.
Published: March 2003

Using climate-controlled growth chambers, the growth, morphology and photosynthetic response of Canada thistle (Cirsium arvense L. Scop.), a recognized invasive weed, was determined at CO2 concentrations ([CO2]) of 285, 382 and 721 μmol mol–1 during the vegetative state. These concentrations correspond roughly to ambient [CO2] from 1900, 2001 and those projected for 2100, respectively. At the end of the vegetative stage, leaves grown at 721 μmol mol–1 still had significantly higher photosynthetic rates under the growth conditions, relative to the 285 or 382 μmol mol–1 CO2 treatment. Significant differences in leaf morphology, particularly the number and length of leaf spines, were also observed, with spine number and length increasing as a function of [CO2]. At the end of the vegetative stage, at 54 d after sowing (DAS), whole plant biomass had increased by 69% for the elevated (720 μmol mol–1) CO2 treatment relative to current ambient CO2. However, biomass had increased by 126% for current ambient (380 μmol mol–1) relative to the concentration that existed in 1900 (285 μmol mol–1). Data from this study indicate that rising [CO2] may have already had a substantial influence on the vegetative development of Canada thistle, by significantly stimulating growth and photosynthesis, as well as altering leaf defenses to potential herbivory. Overall, these changes could have important implications for growth and control of this noxious weed as atmospheric CO2 continues to increase.
The effect of drought on plant water use efficiency of nine NAD - ME and nine NADP - ME Australian C4 grasses
Authors: Ghannoum Oula , Caemmerer Susanne von , Conroy Jann P.
Published: March 2003

We investigated the response to drought of nine NAD–malic enzyme (NAD–ME) and nine NADP–malic enzyme (NADP–ME) C4 grasses. Species were grown from seeds in potted soil in a glasshouse. Seedlings were either watered regularly or exposed to two successive drying cycles of 8–10 d each, after which plants were harvested. Under well-watered conditions, average water use efficiency (WUE; dry mass gain per unit water transpired) was similar for NAD–ME and NADP–ME C4 grasses, and ranged between 6.0 and 8.7 g dry mass kg–1 H2O. Drought enhanced WUE of most species, but to a significantly greater extent in NAD–ME (1.20-fold) than NADP–ME (1.11-fold) grasses. Inhibition of dry matter accumulation (average of 12%) and shoot elongation under drought was similar among the C4 grasses. Leaf dry matter carbon (δ13C) and oxygen (δ18O) isotope compositions were significantly different between the two C4 subtypes. Leaf δ13C averaged –13.3 and –12.2, and leaf δ18O averaged 26.0 and 26.9 in well-watered NAD–ME and NADP–ME grasses, respectively. Drought significantly reduced leaf δ13C in most C4 grasses by an average 0.5. Leaf δ18O was not significantly affected by drought, indicating that leaf δ18O does not reflect drought-induced changes in leaf transpiration of C4 grasses. In the experiment reported here, NAD–ME grasses increased their WUE under drought to a greater extent than their NADP–ME counterparts. Increased WUE of the C4 grasses under drought was primarily related to control of water loss relative to carbon gain at the leaf, rather than the plant, level.
Non-photochemical quenching of chlorophyll a fluorescence: early history and characterization of two xanthophyll-cycle mutants of Chlamydomonas reinhardtii
Authors: Govindjee  , Seufferheld Manfredo J.
Published: March 2003

This paper deals first with the early, although incomplete, history of photoinhibition, of 'non-QA-related chlorophyll (Chl) a fluorescence changes', and the xanthophyll cycle that preceded the discovery of the correlation between non-photochemical quenching of Chl a fluorescence (NPQ) and conversion of violaxanthin to zeaxanthin. It includes the crucial observation that the fluorescence intensity quenching, when plants are exposed to excess light, is indeed due to a change in the quantum yield of fluorescence. The history ends with a novel turn in the direction of research — isolation and characterization of NPQ xanthophyll-cycle mutants of Chlamydomonas reinhardtii Dangeard and Arabidopsis thaliana (L.) Heynh., blocked in conversion of violaxanthin to zeaxanthin, and zeaxanthin to violaxanthin, respectively. In the second part of the paper, we extend the characterization of two of these mutants (npq1, which accumulates violaxanthin, and npq2, which accumulates zeaxanthin) through parallel measurements on growth, and several assays of PSII function: oxygen evolution, Chl a fluorescence transient (the Kautsky effect), the two-electron gate function of PSII, the back reactions around PSII, and measurements of NPQ by pulse-amplitude modulation (PAM 2000) fluorimeter. We show that, in the npq2 mutant, Chl a fluorescence is quenched both in the absence and presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). However, no differences are observed in functioning of the electron-acceptor side of PSII — both the two-electron gate and the back reactions are unchanged. In addition, the role of protons in fluorescence quenching during the 'P-to-S' fluorescence transient was confirmed by the effect of nigericin in decreasing this quenching effect. Also, the absence of zeaxanthin in the npq1 mutant leads to reduced oxygen evolution at high light intensity, suggesting another protective role of this carotenoid. The available data not only support the current model of NPQ that includes roles for both pH and the xanthophylls, but also are consistent with additional protective roles of zeaxanthin. However, this paper emphasizes that we still lack sufficient understanding of the different parts of NPQ, and that the precise mechanisms of photoprotection in the alga Chlamydomonas may not be the same as those in higher plants.
Allelopathic and autotoxic interactions in selected populations of Loliumperenne grown in monoculture and mixed culture
Authors: Kraus Edwin , Voeten Margje , Lambers Hans
Published: March 2003

Autotoxicity and allelopathy affect the respiration and yield of GL66 and GL72, two populations of perennial ryegrass (Lolium perenne L. cv. S23) that were originally selected for contrasting rates of mature-leaf dark respiration under conditions where allelopathic effects could not occur and autotoxic effects were minimal. The aim of this study was to further investigate growth and biomass allocation of these two populations in relation to their autotoxic and allelopathic properties. To this end, plants were subjected to two conditions (monoculture and mixed culture) and two treatments (growth in 'renewed' and 'replenished' nutrient solution, allowing for short- and long-term accumulation of allelochemicals, respectively). The fast-respiring population, GL66, showed a reduced total yield due to allelopathic effects only when long-term accumulation of allelochemicals was allowed (mixed culture, replenished). However, short-term accumulation (mixed culture, renewed) of allelochemicals was sufficient to affect allocation of biomass to leaf sheaths. The slow-respiring population, GL72, suffered from autotoxicity only when long-term accumulation was allowed (monoculture, replenished), and from allelopathy under both short- and long-term accumulation (mixed culture, either renewed or replenished). The predominant allelopathic and autotoxic effect was on dry matter percentage and dry weight of leaf sheaths. We conclude that the roots of both populations release one or more chemical compounds that primarily affect biomass allocation to leaf sheaths, both of the same and of the other population. Sensitivity to the putative inhibitor(s) released by the other population was greater than sensitivity to the inhibitor(s) released by a population's own roots.
Structure - function analysis of photosystem II subunit S (PsbS) in vivo
Authors: Li Xiao-Ping , Phippard Alba , Pasari Jae , Niyogi Krishna K.
Published: March 2003

In land plants, photosystem II subunit S (PsbS) plays a key role in xanthophyll- and pH-dependent non-photochemical quenching (qE) of excess absorbed light energy. Arabidopsis thaliana (L.) Heynh. npq4 mutants are defective in the psbS gene and have impaired qE. Exactly how the PsbS protein is involved in qE is unclear, but it has been proposed that PsbS binds H+ and/or de-epoxidized xanthophylls in excess light as part of the qE mechanism. To identify amino acid residues that are important for PsbS function, we sequenced the psbS gene from eight npq4 point mutant alleles isolated by forward genetics screening, including two new alleles. In the four transmembrane helices of PsbS, several amino acid residues were found to affect the stability and/or function of the protein. By comparing the predicted amino acid sequences of PsbS from several plant species and studying the proposed topological structure of PsbS, eight possible H+-binding amino acid residues on the lumenal side of the protein were identified and then altered by site-directed mutagenesis in vitro. The mutant psbS genes were transformed into npq4-1, a psbS deletion mutant, to test the stability and function of the mutant PsbS proteins in�vivo. The results demonstrate that two conserved, protonatable amino acids, E122 and E226, are especially critical for the function of PsbS.
Morphological plasticity of spring and winter wheats in response to changing temperatures
Authors: Equiza María A. , Tognetti Jorge A.
Published: March 2003

Temperature change induces morpho-physiological responses, whose significance for plant function under this modified environmental condition is mostly unclear. Plastic responses to temperature changes from optimal (25°C) to chilling (5°C) conditions or vice versa, as compared with constant temperature treatments, were studied in spring and winter wheat. Plants grown at 5°C had smaller leaves, less stomata and larger root systems than their counterparts grown at 25°C. Plants at 5°C therefore had larger effective root : shoot ratio, defined as the ratio between total area of roots and stomata. Spring and winter cultivars differed in root growth, leaf growth and stomatal frequency following a shift in temperature. Nevertheless, both cultivar types reached similar effective root : shoot ratios, which were typical for each temperature. We also found changes in leaf thickness, transverse vascular bundle area and xylem : phloem ratios. Shifting temperature triggered rapid changes in leaf and root carbohydrate content and osmotic potential. Our results suggest that temperature-induced plasticity may play a role in ameliorating possible water deficits caused by large soil–air temperature differentials in natural environments.
Factors affecting CO2 assimilation, leaf injury and growth in salt-stressed durum wheat
Authors: James Richard A. , Rivelli Anna Rita , Munns Rana , Caemmerer Susanne von
Published: March 2003

To examine the factors that affect tolerance to high internal salt concentrations, two tetraploid wheat genotypes that differ in the degree of salt-induced leaf injury (Wollaroi and Line 455) were grown in 150 mM NaCl for 4 weeks. Shoot biomass of both genotypes was substantially reduced by salinity, but genotypic differences appeared only after 3 weeks, when durum cultivar Wollaroi showed greater leaf injury and a greater reduction in biomass than Line 455. Ion accumulation, water relations, chlorophyll fluorescence and gas exchange were followed on one leaf (leaf 3) throughout its life. Salinity caused a large decrease in stomatal conductance (gs) of both genotypes. This was not due to poor water relations, as leaf turgor of both genotypes was higher in the salt treatment than in controls, so chemical signals were likely to have caused the decrease in gs. Reductions in assimilation rate were initially due to gs and, with time, were due to a combination of stomatal and non-stomatal limitations. The non-stomatal limitations were associated with a build up of Na+ above 250 mM. The efficiency of PSII photochemistry in Line 455 was unaffected throughout. However, in Wollaroi, the potential and actual quantum yield of PSII photochemistry began to decline as the leaf aged and the thermal energy dissipation of excess light energy (NPQ) increased. This coincided with high Na+ and Cl– concentrations in the leaf and with chlorophyll degradation, indicating that these later reductions in CO2 assimilation in Wollaroi were a consequence of a direct toxic ion effect. The earlier reduction in CO2 assimilation and greater leaf injury explain why growth of Wollaroi was less than Line 455. The most sensitive indicator of salinity stress was gs, followed by CO2 assimilation, with fluorescence parameters other than NPQ being no more sensitive than chlorophyll itself.
Effect of cadmium stress on nitrogen metabolism in nodules and roots of soybean plants
Authors: Balestrasse Karina B. , Benavides María Patricia , Gallego Susana M. , Tomaro María L.
Published: March 2003

The nitrogen metabolism of soybean (Glycine max L.) nodules and roots was studied in plants subjected to two different concentrations (50 and 200 μM) of CdCl2. Nitrogenase activity was decreased in nodules treated with 200 μM Cd2+. In 50 μM Cd2+-treated plants, NH4+ content showed similar values to controls in nodules, but increased by 55% in roots. However, after treatment with 200 μM Cd2+, NH4+ levels increased in both tissues. Glutamate (Glu) and protein contents remained unaltered in nodules treated with 50 μM Cd2+, while at the higher Cd2+ concentration both were decreased. Nevertheless, polyamine content was increased at the two Cd2+ concentrations. In roots, Glu, polyamine and protein levels were significantly diminished at 50 and 200 μM CdCl2. For nitrogen-assimilation enzymes, glutamate dehydrogenase activity was moderately increased in nodules and roots following the lower Cd2+ treatment, though at the higher Cd2+ concentration root enzyme activity returned to control levels. An impressive increase in enzyme activity was found in nodules. In roots, the glutamine synthetase / glutamate synthase pathway was decreased at the two Cd2+ concentrations, though in nodules it was diminished only at 200 μM Cd2+. No changes in protease activity were found in the two tissues treated with 50�μM�Cd2+. However, at 200 μM Cd2+, nodule and root protease activities decreased and increased, respectively. These results suggest that, in general, treatment with Cd2+ affects nitrogen assimilation and metabolism to a greater extent in soybean roots than in nodules.
Elevated CO2 increases the leaf temperature of two glasshouse-grown C4 grasses
Authors: Siebke Katharina , Ghannoum Oula , Conroy Jann P. , Caemmerer Susanne von
Published: March 2003

This study investigates the effect of elevated CO2 partial pressure (pCO2)-induced stomatal closure on leaf temperature and gas exchange of C4 grasses. Two native Australian C4 grasses, Astrebla lappacea (Lindl.) Domin and Bothriochloa bladhii Kuntze, were grown at three different pCO2 (35, 70 and 120 Pa) in three matched, temperature-controlled glasshouse compartments. The difference between leaf and air temperature (ΔT) was monitored diurnally with thermocouples. ΔT increased with both step-increases of ambient pCO2. Average noon leaf temperature increased by 0.4 and 0.3°C for A. lappacea with the 35–70 and 70–120 Pa steps of pCO2 elevation, respectively. For B. bladhii, the increases were 0.5°C for both pCO2 steps. ΔT was strongly dependent on irradiance, pCO2 and air humidity. Leaf gas exchange was measured at constant temperature and high irradiance at the three growth pCO2. Under these conditions, CO2 assimilation saturated at 70 Pa, while stomatal conductance decreased by the same extent (0.58-fold) with both step-increases in pCO2, suggesting that whole-plant water use efficiency of C4 grasses would increase beyond a doubling of ambient pCO2. The ratio of intercellular to ambient pCO2 was not affected by short- or long-term doubling or near-tripling of pCO2, in either C4 species when measured under standard conditions.
Whole-plant hydraulic conductance and root-to-shoot flow of abscisic acid are independently affected by water stress in grapevines
Authors: Lovisolo Claudio , Hartung Wolfram , Schubert Andrea
Published: March 2003

In order to investigate whether plant hydraulic conductance (gplant) is reduced under drought conditions via an ABA-related mechanism, a water-stress experiment was carried out using split-rooted grapevines. In addition, inversion of shoot growth orientation was imposed to reduce gplant independently of soil water availability, and thus of the putative ABA root-generated stress message. As expected, water stress imposed on split-roots affected ABA accumulation. ABA drought-stress message negatively affected stomatal conductance (gs) and transpiration (E), but modified neither leaf or stem water potentials (Ψleaf and Ψstem, respectively), nor gplant. When gplant was reduced in split-rooted, shoot-inverted (s-r/s-i) grapevines, Ψleaf and Ψstem decreased, without changes in ABA accumulation, gs and E. The ABA drought-stress message did not modify gplant, nor did gplant (impaired by shoot-growth inversion) decrease ABA delivery to the leaves. However, leaf growth was depressed in s-r/s-i grapevines. The fact that no interaction between ABA stress messages (caused by split-root technique) and hydraulic constraints to sap flow (caused by shoot inversion) was necessary to impair leaf growth suggests that the targets of ABA and hydraulic-limitation effects on leaf expansion are not the same.
Spatial patterns of radial oxygen loss and nitrate net flux along adventitious roots of rice raised in aerated or stagnant solution
Authors: Rubinigg Michael , Stulen Ineke , Elzenga J. Theo M. , Colmer Timothy D.
Published: March 2003

Roots of rice (Oryza sativa L.) grown in stagnant de-oxygenated solution contain a 'tight' barrier to radial oxygen loss (ROL) in basal zones, whereas roots of plants grown in aerated solution do not. It is generally accepted that the barrier to ROL involves anatomical modifications in the apoplast of cell layers exterior to the aerenchyma. A possible drawback of this adaptation is a reduced capacity for nutrient uptake. Whether or not induction of a barrier to ROL influences the capacity of adventitious roots of rice to take up NO3– was determined in the present study, using NO3–-selective microelectrodes. When transferred into O2-free root medium, ROL from positions at 30–50 mm behind the tip of adventitious roots of plants raised in stagnant solution was only 4–6% of the rate from roots of plants raised in aerated solution, indicating the barrier to ROL was induced by growth in stagnant solution. For plants transferred into aerobic nutrient solution containing 0.1 mM NO3–, net NO3– uptake by these root zones, with or without a barrier to ROL, was the same. It is concluded that induction of a barrier to ROL had no effect on the capacity of adventitious roots of rice to take up NO3– from aerobic solution.
Plant respiration in productivity models: conceptualisation, representation and issues for global terrestrial carbon-cycle research
Authors: Gifford Roger M.
Published: March 2003

Plant respiratory regulation is too complex for a mechanistic representation in current terrestrial productivity models for carbon accounting and global change research. Accordingly, simpler approaches that attempt to capture the essence of respiration are commonly adopted. Several approaches have been used in the literature: respiration may be embedded implicitly in growth algorithms; assumed values for specific respiration rates may be adopted; respiration may be calculated in terms of growth and maintenance components; conservatism in the ratio of respiration to photosynthesis (R : P) may be assumed; or a more complex process or residual approach may be adopted. Review of this literature suggests that the assumption of conservative R : P ratio is an effective and practicable approach in the context of C-cycle modelling for global change research and documentation, requiring minimal ecosystem-specific data on respiration.Some long-standing controversies in respiration are now becoming resolved. The apparently wasteful process of cyanide-resistant respiration by the alternative oxidase may not be wasteful, as it is thought to be involved in protecting the plant from 'reactive oxygen species'. It is now clear that short-term respiratory response coefficients of plants (e.g. the Q10) do not predict their long-term temperature response. A new experimental approach suggests that leaf respiration is not suppressed by light as previously thought. Careful experiments, taking account of several proposed measurement artefacts, indicate that plant respiration is not suppressed by elevated CO2 concentration in a short-term reversible way.
How does ecosystem water balance affect net primary productivity of woody ecosystems?
Authors: Eamus Derek
Published: March 2003

Australia is a very dry continent, with low levels of rainfall and high temperatures and evaporative demand for much of the continent for much of the year. Net primary productivity (NPP) of Australia is also low. These two observations are presumably related, and, at a catchment scale, correlations between ecosystem water balance and NPP surely exist. This paper shows that these assumptions are probably correct, and then reviews the mechanisms by which ecosystem water balance may determine NPP.The measurement and quantification of ecosystem water balance is not always easy, and several measures, involving various ratios of rainfall, potential evapotranspiration, soil water content and related parameters, have been used. This topic is briefly discussed.Ecosystem water balance influences NPP through effects at several scales. Ecosystem water balance influences several leaf attributes, especially specific leaf area and foliar nitrogen content. In addition, carbon and nitrogen partitioning between photosynthetic and non-photosynthetic tissues, rates of photosynthesis and stomatal conductance are affected. At the larger scale, whole-tree hydraulic conductance and allocation of biomass to root, stem and leaf mass are affected by ecosystem water balance and influence NPP. Respiration rates and the relative contributions of root, stem and leaf respiration vary with ecosystem water balance and therefore influence NPP. Finally, attributes at the canopy and stand scales also respond to catchment water balance and influence NPP. These mechanisms are discussed in relation to woody ecosystems. In addition, I propose that stands of young trees on xeric sites behave functionally and mechanistically the same as old stands on mesic sites.
Importance of mechanisms and processes of the stabilisation of soil organic matter for modelling carbon turnover
Authors: Krull Evelyn S. , Baldock Jeffrey A. , Skjemstad Jan O.
Published: March 2003

This paper reviews current knowledge of soil organic carbon (SOC) dynamics with respect to physical protection, soil moisture and temperature, and recalcitrant carbon fractions (such as charcoal) in predominantly agricultural soils. These factors are discussed within the framework of current soil organic matter models. The importance of soil structure in the stabilisation of organic residues through physical protection has been documented previously in various studies. In addition, changes in soil structure associated with tillage can significantly affect soil organic matter decomposition rates. The concept of physical protection has been incorporated into several soil carbon models as a function of soil texture. While soil texture can affect the soil's capacity for aggregation and adsorption, factors such as soil moisture and temperature may further enhance or reduce the extent of physical protection. While adsorption and aggregation can slow decomposition processes, it is unlikely that these processes are solely responsible for the high mean residence times measured in biologically active surface soils. Accordingly, chemical recalcitrance appears to be the only mechanism by which soil organic carbon can be protected for long periods of time.
Interactive effects of nitrogen and irradiance on growth and partitioning of dry mass and nitrogen in young tomato plants
Authors: Groot Corine C. de , Marcelis Leo F. M. , Boogaard Riki van den , Lambers Hans
Published: March 2003

The interactive effects of irradiance and N on growth of young tomato plants (Lycopersicon esculentum Mill.) were studied. Plants were grown at 70 or 300 μmol photons m–2 s–1, hereafter referred to as 'low' and 'high' irradiance, and at a range of exponential N supply rates (70–370 mg g–1 d–1) or at a constant concentration in the nutrient solution of 12 mM NO3–. At both irradiance levels, leaf area ratio was more important than net assimilation rate (NAR) in explaining effects of N on growth at mild N limitation. However, at severe N limitation, NAR became the most important parameter, as indicated by calculated growth response coefficients. Furthermore, this study shows that N supply and growth irradiance interacted strongly. The decrease of specific leaf area with increasing N limitation and increasing growth irradiance correlated with increasing leaf dry mass percentage and starch concentration. Furthermore, at low irradiance, plants partitioned more dry mass to the stem. Dry mass partitioning to roots increased with decreasing plant N concentration, and this relation appeared to be independent of irradiance. Shading increased plant N concentration and decreased dry mass partitioning to roots. Also, the relationship between plant N concentration and N partitioning to different plant organs was largely independent of growth irradiance.
Conversion of canopy intercepted radiation to photosynthate: review of modelling approaches for regional scales
Authors: Medlyn Belinda , Barrett Damian , Landsberg Joe , Sands Peter , Clement Robert
Published: March 2003

A fundamental component of most models of terrestrial carbon balance is an estimate of plant canopy photosynthetic uptake driven by radiation interception by the canopy. In this article, we review approaches used to model the conversion of radiation into photosynthate. As this process is well understood at the leaf-scale, the modelling problem is essentially one of up-scaling, to canopy, regional or global scale. Our review therefore focuses on issues of scaling, including model identification, parameterisation and validation at large scales. Four different approaches are commonly taken to modelling photosynthate production at large scales: the maximum productivity, resource-use efficiency, big-leaf, and sun-shade models. Models representing each of these approaches are discussed and model predictions compared with estimates of gross primary productivity derived from eddy covariance data measured above a Sitka spruce forest. The sun-shade model was found to perform best at all time scales considered. However, other models had significant advantages including simplicity of implementation and the ability to combine the model with remotely-sensed information on vegetation radiation interception. We conclude that all four approaches can be successfully used to model photosynthetic uptake and that the best approach in a given situation will depend on model objectives and data availability.
Effects of Cd and Pb in sugar beet plants grown in nutrient solution: induced Fe deficiency and growth inhibition
Authors: Larbi Ajmi , Morales Fermín , Abadía Anunciación , Gogorcena Yolanda , Lucena Juan J. , Abadía Javier
Published: March 2003

Effects of Cd and Pb toxicity were investigated in sugar beet (Beta vulgaris L.) grown in hydroponics under growth-chamber-controlled conditions. Chemical speciation calculations were used to estimate the chemical species in equilibrium. Cd, used as chloride salt or chelated to EDTA, decreased fresh and dry mass of both root and shoot, and increased root / shoot ratios. Plants developed few brownish roots with short laterals. Cd decreased N, P, Mg, K, Mn, Cu and Zn uptake, and facilitated Ca uptake. Leaves of plants treated with 10 or 50 μM Cd–EDTA and 10 μM CdCl2 developed symptoms of Fe deficiency. These symptoms included decreased leaf chlorophyll (Chl) and carotenoid concentrations, increased carotenoid / Chl and Chl a/b ratios, de-epoxidation of violaxanthin cycle pigments, and decreased photosynthetic rates and PSII efficiency. Plants treated with 50 μM CdCl2, however, had decreased growth but did not show marked leaf Fe-deficiency symptoms. All Cd treatments increased Fe(III)-chelate reductase activity in root tips, although Fe concentrations in shoots were similar to those found in control plants. Pb chelated with EDTA induced visual symptoms only at concentrations of 2 mM. Leaves of Pb-treated plants remained green and their edges were rolled inwards. Pb increased root fresh and dry mass with no changes in shoot mass, therefore increasing the root / shoot ratio. Changes in plant nutrient concentrations with Pb were only minor, although leaf Cu levels approached critical deficiency levels. No symptoms of Fe deficiency were apparent in leaves. Root tips of Pb-treated plants, however, had increased Fe(III)-chelate reductase activities.
Wheat dehydrin accumulation in response to drought stress during anthesis
Authors: Lopez Cesar G. , Banowetz Gary M. , Peterson C. James , Kronstad Warren E.
Published: March 2003

The winter wheat (Triticum aestivum L.) producing region of the US Pacific Northwest (PNW) is often subject to water deficits at sowing and during grain filling. Improved genetic adaptation of wheat cultivars to drought stress is one objective of breeding efforts in the region. Consequently, there is interest in identifying molecular markers associated with drought tolerance. Dehydrins, a family of proteins that accumulate in response to dehydrative stress, may provide a suitable marker for use in breeding programs. Seven cultivars (Connie, Gene, TAM105, Rod, Hiller, Rhode and Stephens) were evaluated in two experiments in which dehydrin accumulation and their association to stress tolerance during grain filling were characterized during progressive drought stress. A�24-kDa dehydrin was present in leaves at each sampling date in all seven cultivars. Quantitative differences in accumulation of this protein were observed between cultivars on the third sampling date (4 d of stress). This differential accumulation was associated with stress tolerance characterized by a lower yield reduction and a lowered rate of decrease in leaf water potential in Connie, TAM105 and Gene. In contrast to leaves, an increased number of dehydrins were observed in grains under stress and non-stress treatments. Despite the number of dehydrins detected, there was no apparent association between drought stress and dehydrin expression in grains. Although the specific role of these proteins remains unknown, their association with stress tolerance suggests that dehydrins have utility in improving adaptation to drought and as markers for drought tolerance.
Antioxidant capacities of green and cyanic leaves in the sun species, Quintinia serrata
Authors: Neill Samuel O. , Gould Kevin S. , Kilmartin Paul A. , Mitchell Kevin A. , Markham Kenneth R.
Published: March 2003

Anthocyanins scavenge harmful reactive oxygen species (ROS), and may provide antioxidative protection within plant cells. For the shade species Elatostema rugosum A. Cunn., it was recently established that red-leaved morphs held a significant antioxidant advantage over green morphs. It is not known whether anthocyanins confer the same protection in sun species that are exposed to elevated levels of irradiation and thus greater photooxidative stress. Quintinia serrata A. Cunn., a canopy tree species, is polymorphic for anthocyanin expression. Levels of antioxidants in leaves of red and green morphs of Q. serrata were identified, and activities quantified using enzymatic and α,α-diphenyl-β-picrylhydrazyl (DPPH) assays and cyclic voltammetry. Both morphs contained similar levels of superoxide dismutase, catalase, ascorbic peroxidase and hydroxycinnamic acids, and showed similar ranges in DPPH scavenging activity and voltammetric profiles. Anthocyanin concentrations increased with leaf age in red morphs and were positively correlated with the extent of DPPH scavenging. However, hydroxycinnamic acids were the most active methanol-extractable fraction in both morphs, contributing 27% more to the overall low-molecular-weight antioxidant pool in the green morph than in the red. We conclude that anthocyanin biosynthesis can enhance, but is not a prerequisite for, oxidative protection in Q. serrata leaves. The possibility that anthocyanins provide a versatile mechanism for photoprotection is discussed.
Developmental regulation of the gibberellin pathway in pea shoots
Authors: Ross John J. , Davidson Sandra E. , Wolbang Carla M. , Bayly-Stark Emma , Smith Jennifer J. , Reid James B.
Published: March 2003

To investigate gibberellin (GA) biosynthesis in mature tissue of pea (Pisum sativum L.) in the absence of potentially GA-producing meristematic tissue we grafted wild-type scions to rootstocks of the GA-deficient ls-1 mutant and later decapitated the shoot. After 2 d, decapitated shoots contained as much GA19 (a precursor of the bioactive GA1) as comparable tissue from intact plants, even though applied [14C]GA19 was metabolised rapidly during this time. These results show that the pool size of endogenous GA19 was maintained, probably by de novo GA19 synthesis. We also found that the LS gene, which catalyses an early step in GA biosynthesis, is expressed in mature tissue, as are the shoot-expressed GA 20-oxidase and GA 3-oxidase genes. Nevertheless, mature tissue contained very low levels of GA1 and GA20 compared with immature tissue. Levels of GA19, GA29 and GA8 were less affected by tissue age. Metabolism studies using 14C-labelled GAs indicated that mature tissue rapidly converted GA19 to GA20 and GA20 to GA1; the latter step was promoted by IAA. However, the 2-oxidation steps GA1 to GA8, GA20 to GA29 and GA29 to GA29-catabolite appear to proceed very rapidly in mature tissue (regardless of IAA content), and we suggest this is the reason why GA1 and GA20 do not accumulate. This is supported by the high level of expression of a key GA 2-oxidase gene in mature tissue.
A comparison of three different canopy radiation models commonly used in plant modelling
Authors: Wang Ying Ping
Published: March 2003

Three radiation models are compared. They are the two-stream approximation, Goudriaan's radiation model and Beer's law. If direct beam and diffuse radiation are considered separately with the appropriate extinction coefficients, Beer's law can be used to estimate the fraction of absorbed visible radiation quite accurately, as compared with the other two models. However, Beer's law always overestimates the fraction of absorbed near-infrared radiation because of the significant loss of scattered radiation in the near-infrared radiation waveband; the error can be up to 50%, compared with the two-stream approximation. The error is systematic and is not reduced by integrating over a day or longer. It can be even larger if absorption of direct beam and diffuse radiation are not considered separately. Compared with the two-stream approximation, Goudriaan's model underestimates the amount of absorbed visible and near-infrared radiation, and consequently net photosynthesis, and overestimates sensible heat fluxes of the canopy when Goudriaan's model is used to estimate the amount of radiation absorbed by sunlit and shaded leaves within a canopy in the CSIRO Biosphere Model. However, the mean differences in the calculated hourly fluxes of net canopy photosynthesis, latent and sensible heat are less than 5% for a wheat crop during the whole growing season.
Research note: Maternally-controlled ovule abortion results from cosuppression of dihydroflavonol-4-reductase or flavonoid-3′,5′-hydroxylase genes in Petunia hybrida
Authors: Jorgensen Richard A. , Que Qiudeng , Napoli Carolyn A.
Published: March 2003

Transgenes designed to overexpress anthocyanin genes An6 (encoding dihydroflavonol-4-reductase) or Hf1 (encoding flavonoid-3′,5′-hydroxylase) in Petunia hybrida L. produced flower colour phenotypes similar to those caused by sense cosuppression of chalcone synthase (Chs) genes. However, unlike Chs, sense cosuppression of An6 and Hf1 resulted in female infertility in transgenotes exhibiting complete phenotypic suppression of anthocyanins. Female sterility appeared to be due to embryo abortion, with discolouration of ovules first appearing about 4 d post-fertilization, followed by gradual collapse of the ovule. Pollen from cosuppressed, female-sterile transgenotes placed on wild-type stigmas produced normal seed set, indicating that sterility of cosuppressed plants was maternally controlled. We suggest an hypothesis that cosuppression of An6 and Hf1 leads to accumulation of dihydroflavonols in the seed coat, a maternal tissue, and that this accumulation inhibits embryo growth, either directly or indirectly. In this hypothesis, direct inhibition of embryo growth would require that dihydroflavonols diffuse from the seed coat into the embryo and act there, whereas indirect inhibition would require that dihydroflavonols interfere with some capacity of the seed coat to promote embryo growth.