Showing papers in "Functional Plant Biology in 2008"

OpenAlea: a visual programming and component-based software platform for plant modelling.

Abstract: As illustrated by the approaches presented during the 5th FSPM workshop (Prusinkiewicz and Hanan 2007, and this issue), the development of functional-structural plant models requires an increasing amount of computer modeling. All these models are developed by different teams in various contexts and with different goals. Efficient and flexible computational frameworks are required to augment the interaction between these models, their reusability, and the possibility to compare them on identical datasets. In this paper, we present an open-source platform, OpenAlea, that provides a user-friendly environment for modelers, and advanced deployment methods. OpenAlea allows researchers to build models using a visual programming interface and provides a set of tools and models dedicated to plant modeling. Models and algorithms are embedded in OpenAlea components with well defined input and output interfaces that can be easily interconnected to form more complex models and define more macroscopic components. The system architecture is based on the use of a general purpose, high-level, object-oriented script language, Python, widely used in other scientific areas. We briefly present the rationale that underlies the architectural design of this system and we illustrate the use of the platform to assemble several heterogeneous model components and to rapidly prototype a complex modeling scenario.

Plant-growth-promoting rhizobacteria and arbuscular mycorrhizal fungi modify alleviation biochemical mechanisms in water-stressed plants

Abstract: This study examined the effect of inoculation with the plant-growth-promoting rhizobacterium (PGPR) Pseudomonas mendocina Palleroni, alone or in combination with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith) or Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe, on antioxidant enzyme activities (superoxide dismutase, catalase and total peroxidase activities), phosphatase and nitrate reductase activities and solute accumulation in leaves of Lactuca sativa L. cv. Tafalla affected by three different levels of water stress. At moderate drought, bacterial inoculation and mycorrhizal inoculation with G. intraradices, alone or in combination, stimulated significantly nitrate reductase activity. At severe drought, fertilisation and P. mendocina inoculation, alone or in combination with either of the selected AM fungi, increased significantly phosphatase activity in lettuce roots and proline accumulation in leaves. Total peroxidase (POX) and catalase (CAT) activities increased in response to drought, whereas superoxide dismutase activity decreased. Inorganic fertilisation and both combined treatments of PGPR and AM fungus showed the highest values of leaf POX activity under severe drought. The highest CAT activity was recorded in the fertilised plants followed by the P. mendocina-inoculated plants grown under severe stress conditions. These results support the potential use of a PGPR as an inoculant to alleviate the oxidative damage produced under water stress.

Auxin: at the root of nodule development?

Abstract: Root nodules are formed as a result of an orchestrated exchange of chemical signals between symbiotic nitrogen fixing bacteria and certain plants. In plants that form nodules in symbiosis with actinorhizal bacteria, nodules are derived from lateral roots. In most legumes, nodules are formed de novo from pericycle and cortical cells that are re-stimulated for division and differentiation by rhizobia. The ability of plants to nodulate has only evolved recently and it has, therefore, been suggested that nodule development is likely to have co-opted existing mechanisms for development and differentiation from lateral root formation. Auxin is an important regulator of cell division and differentiation, and changes in auxin accumulation and transport are essential for lateral root development. There is growing evidence that rhizobia alter the root auxin balance as a prerequisite for nodule formation, and that nodule numbers are regulated by shoot-to-root auxin transport. Whereas auxin requirements appear to be similar for lateral root and nodule primordium activation and organ differentiation, the major difference between the two developmental programs lies in the specification of founder cells. It is suggested that differing ratios of auxin and cytokinin are likely to specify the precursors of the different root organs.

Why is plant-growth response to elevated CO2 amplified when water is limiting, but reduced when nitrogen is limiting? A growth-optimisation hypothesis

Abstract: Experimental evidence indicates that the stomatal conductance and nitrogen concentration ([N]) of foliage decline under CO2 enrichment, and that the percentage growth response to elevated CO2 is amplified under water limitation, but reduced under nitrogen limitation. We advance simple explanations for these responses based on an optimisation hypothesis applied to a simple model of the annual carbon-nitrogen-water economy of trees growing at a CO2-enrichment experiment at Oak Ridge, Tennessee, USA. The model is shown to have an optimum for leaf [N], stomatal conductance and leaf area index (LAI), where annual plant productivity is maximised. The optimisation is represented in terms of a trade-off between LAI and stomatal conductance, constrained by water supply, and between LAI and leaf [N], constrained by N supply. At elevated CO2 the optimum shifts to reduced stomatal conductance and leaf [N] and enhanced LAI. The model is applied to years with contrasting rainfall and N uptake. The predicted growth response to elevated CO2 is greatest in a dry, high-N year and is reduced in a wet, low-N year. The underlying physiological explanation for this contrast in the effects of water versus nitrogen limitation is that leaf photosynthesis is more sensitive to CO2 concentration ([CO2]) at lower stomatal conductance and is less sensitive to [CO2] at lower leaf [N].

Genetic variation in tolerance to the osmotic stress component of salinity stress in durum wheat

Abstract: Salinity affects plant growth by the osmotic stress of the salt around the roots as well as by toxicity caused by excessive accumulation of salt in leaves. The aim of this study was to determine whether there is significant genetic variation in tolerance to osmotic stress that can be useful in improving the salinity tolerance of crop plants. Durum wheat is a salt-sensitive crop whose yield is reduced by moderately saline soils. Genetic variation in tolerance to osmotic stress in durum wheat was examined in 50 international durum varieties and landraces by measuring the response of stomatal conductance to salt stress before salts built up in the leaf. Stomatal conductance is a sensitive indicator of the osmotic stress because it is reduced immediately with the onset of salinity, and is the initial and most profound cause of a decline in CO2 assimilation rate. Genetic differences of 2-3-fold were found in the magnitude of the response of stomatal conductance to salt-induced osmotic stress. Higher stomatal conductance in salt related to higher CO2 assimilation rate. There was a positive relationship between stomatal conductance and relative growth rate in salt. This study shows the potential for new genetic gains in salt tolerance in durum wheat.

Nitrogen metabolism in durum wheat under salinity : accumulation of proline and glycine betaine

Abstract: We studied the effect of salinity on amino acid, proline and glycine betaine accumulation in leaves of different stages of development in durum wheat under high and low nitrogen supply. Our results suggest that protective compounds against salt stress are accumulated in all leaves. The major metabolites are glycine betaine, which preferentially accumulates in younger tissues, and proline, which is found predominantly in older tissues. Proline tended to accumulate early, at the onset of the stress, while glycine betaine accumulation was observed during prolonged stress. Nitrate reductase (NR) and glutamate synthase (GOGAT) are positively correlated with these compatible solutes: proline is associated with NR in the oldest leaves of high-nitrate plants and glycine betaine is associated with GOGAT in the youngest leaves of both low- and high-nitrate plants. In high-nitrate conditions proline accounts for more than 39% of the osmotic adjustment in the cytoplasmic compartments of old leaves. Its nitrogen-dependent accumulation may offer an important advantage in that it can be metabolised to allow reallocation of energy, carbon and nitrogen from the older leaves to the younger tissues. The contribution of glycine betaine is higher in young leaves and is independent of nitrogen nutrition.

Alternative oxidase: an inter-kingdom perspective on the function and regulation of this broadly distributed 'cyanide-resistant' terminal oxidase

Abstract: Alternative oxidase (AOX) is a terminal quinol oxidase located in the respiratory electron transport chain that catalyses the oxidation of quinol and the reduction of oxygen to water. However, unlike the cytochrome c oxidase respiratory pathway, the AOX pathway moves fewer protons across the inner mitochondrial membrane to generate a proton motive force that can be used to synthesise ATP. The energy passed to AOX is dissipated as heat. This appears to be very wasteful from an energetic perspective and it is likely that AOX fulfils some physiological function(s) that makes up for its apparent energetic shortcomings. An examination of the known taxonomic distribution of AOX and the specific organisms in which AOX has been studied has been used to explore themes pertaining to AOX function and regulation. A comparative approach was used to examine AOX function as it relates to the biochemical function of the enzyme as a quinol oxidase and associated topics, such as enzyme structure, catalysis and transcriptional expression and post-translational regulation. Hypotheses that have been put forward about the physiological function(s) of AOX were explored in light of some recent discoveries made with regard to species that contain AOX. Fruitful areas of research for the AOX community in the future have been highlighted.

Evidence from near-isogenic lines that root penetration increases with root diameter and bending stiffness in rice.

Abstract: Deep rooting can be inhibited by strong layers, although there is evidence for species and cultivar (cv.) differences in their penetration ability. Here, the availability of near-isogenic lines (NILs) in rice (Oryza sativa L.) was exploited to test the hypothesis that increased root diameter is associated with greater root bending stiffness, which leads to greater root penetration of strong layers. Wax/petrolatum discs (80% strong wax) were used as the strong layer, so that strength can be manipulated independently of water status. It was found that good root penetration was consistently associated with greater root diameter and bending stiffness, whether comparisons were made between cvs or between NILs. With NILs, this effect was seen with 'research' lines bred from recombinant inbred lines of a cross between cvs Bala and Azucena and also in improved lines developed from cv. Kalinga III by introgression of parts of the genome from Azucena. Much of the bending behaviour of roots could be explained by treating them as a simple cylinder of material. In both wax disc and sand culture systems, roots that had encountered a strong layer had lower bending stiffness than roots that had not encountered a strong layer which is a novel result and not previously reported.

The rule-based language XL and the modelling environment GroIMP illustrated with simulated tree competition.

Abstract: The programming language XL ('eXtended L-system language') is an extension of Java, which supports the specification and execution of relational growth grammars, a variant of parallel graph grammars. XL is a powerful generalisation of the well-known L-system approach to functional-structural plant modelling. Some features of XL are discussed that are particularly useful for combining structure and function and for querying plant architectural data, and an exemplary functional-structural plant model of young beech trees is presented that is implemented in XL and includes PAR distribution, assimilate allocation and morphological plasticity. Together with a simpler model of spruce trees, this beech model is included in a virtual landscape with a mixed-species forest stand where competition for light occurs. The open-source platform GroIMP was used for the complete model development process and for visualising the results.

Preliminary use of ground-penetrating radar and electrical resistivity tomography to study tree roots in pine forests and poplar plantations.

Abstract: In this study, we assess the possibility of using ground penetrating radar (GPR) and electrical resistivity tomography (ERT) as indirect non-destructive techniques for root detection. Two experimental sites were investigated: a poplar plantation [mean height of plants 25.7 m, diameter at breast height (dbh) 33 cm] and a pinewood forest mainly composed of Pinus pinea L. and Pinus pinaster Ait. (mean height 17 m, dbh 29 cm). GPR measures were taken using antennas of 900 and 1500 MHz applied in square and circular grids. ERT was previously tested along 2-D lines, compared with GPR sections and direct observation of the roots, and then using a complete 3-D acquisition technique. Three-dimensional reconstructions using grids of electrodes centred and evenly spaced around the tree were used in all cases (poplar and pine), and repeated in different periods in the pine forest (April, June and September) to investigate the influence of water saturation on the results obtainable. The investigated roots systems were entirely excavated using AIR-SPADE Series 2000. In order to acquire morphological information on the root system, to be compared with the GPR and ERT, poplar and pine roots were scanned using a portable on ground scanning LIDAR. In test sections analysed around the poplar trees, GPR with a high frequency antenna proved to be able to detect roots with very small diameters and different angles, with the geometry of survey lines ruling the intensity of individual reflectors. The comparison between 3-D images of the extracted roots obtained with a laser scan data point cloud and the GPR profile proved the potential of high density 3-D GPR in mapping the entire system in unsaturated soil, with a preference for sandy and silty terrain, with problems arising when clay is predominant. Clutter produced by gravel and pebbles, mixed with the presence of roots, can also be sources of noise for the GPR signals. The work performed on the pine trees shows that the shape, distribution and volume of roots system, can be coupled to the 3-D electrical resistivity variation of the soil model map. Geophysical surveys can be a useful approach to root investigation in describing both the shape and behaviour of the roots in the subsoil.

MAppleT: simulation of apple tree development using mixed stochastic and biomechanical models

Abstract: Construction of tree architectural databases over years is time consuming and cannot easily capture event dynamics, especially when both tree topology and geometry are considered. The present project aimed to bring together models of topology and geometry in a single simulation such that the architecture of an apple tree may emerge from process interactions. This integration was performed using L-systems. A mixed approach was developed based on stochastic models to simulate plant topology and mechanistic model for the geometry. The succession of growth units (GUs) along axes and their branching structure were jointly modelled by a hierarchical hidden Markov model. A biomechanical model, derived from previous studies, was used to calculate stem form at the metamer scale, taking into account the intra-year dynamics of primary, secondary and fruit growth. Outputs consist of 3-D mock-ups - geometric models representing the progression of tree form over time. To asses these models, a sensitivity analysis was performed and descriptors were compared between simulated and digitised trees, including the total number of GUs in the entire tree, descriptors of shoot geometry (basal diameter, length), and descriptors of axis geometry (inclination, curvature). In conclusion, despite some limitations, MAppleT constitutes a useful tool for simulating development of apple trees in interaction with gravity.

Arabidopsis thaliana MYB75/PAP1 transcription factor induces anthocyanin production in transgenic tomato plants.

Abstract: Tomato (Solanum lycopersicum L.) cv. Micro-Tom plants were transformed with the Arabidopsis thaliana (L.)Heyhn. MYB75/PAP1 (PRODUCTION OF ANTHOCYANIN PIGMENT 1) gene. This gene encodes for a well known transcription factor, which is involved in anthocyanin production and is modulated by light and sucrose. Transgenic tomato plants expressing AtMYB75 were characterised by a significantly higher anthocyanin production in leaves, stems, roots and flowers under normal growth conditions. Further, they also exhibited anthocyanins in fruits. Anthocyanin accumulation was not widespread but took place in specific groups of cells located in epidermal or cortical regions or in proximity of vascular bundles. In all the organs of the transgenic plants, where AtMYB75 overexpression was determined, a clear increase in the accumulation of DFR (DIHYDROFLAVONOL 4-REDUCTASE) transcript was also detected. The expression of the tomato MYB-gene ANT1 (ANTHOCYANIN1), which had previously been identified as a transcriptional endogenous regulator of anthocyanin biosynthesis, was not altered. The higher basal content of anthocyanins in the leaves of the transgenic plants could be further increased in the presence of high light conditions and contributed to mitigate photobleaching damages under high irradiance.

Silicon-mediated improvement in the salt resistance of wheat (Triticum aestivum) results from increased sodium exclusion and resistance to oxidative stress.

Abstract: Silicon (Si) is reported to reduce the effect of salinity on wheat (Triticum aestivum L.) and other crops. In the present study, Si decreased plant Na+ uptake and shoot : root Na+ distribution of a salt-resistant as well as a salt-sensitive wheat genotype. Reduced shoot Na+ concentration and increased shoot K+ : Na+ ratio led to improved plant growth. Silicon increased cell-wall Na+ binding from 49% in SARC-1 and 37% in 7-Cerros under salinity to 87% in SARC-1 and 79% in 7-Cerros under salinity + silicon. It may also have resulted in decreased potentially toxic leaf sap Na+ concentration. The concentration of glutathione, an important antioxidant in plants, was increased due to the addition of Si under saline conditions. The salt-resistant wheat genotype SARC-1 was less Si-responsive in terms of shoot fresh weight, having a 39% increase compared with a 49% increase in 7-Cerros, as well as root fresh weight, having a 12% increase compared with a 22% in 7-Cerros. It is concluded that Si may have improved shoot growth of the salt-resistant as well as the salt-sensitive wheat genotype by decreasing plant Na+ uptake and shoot : root Na+ distribution as well as by increasing glutathione concentration. Silicon may have also improved in-plant Na+ detoxification by increasing cell-wall Na+ binding.

Expanding roles of plant aquaporins in plasma membranes and cell organelles

Abstract: Aquaporins facilitate water transport across biomembranes in a manner dependent on osmotic pressure and water-potential gradient. The discovery of aquaporins has facilitated research on intracellular and whole-plant water transport at the molecular level. Aquaporins belong to a ubiquitous family of membrane intrinsic proteins (MIP). Plants have four subfamilies: plasma-membrane intrinsic protein (PIP), tonoplast intrinsic protein (TIP), nodulin 26-like intrinsic protein (NIP), and small basic intrinsic protein (SIP). Recent research has revealed a diversity of plant aquaporins, especially their physiological functions and intracellular localisation. A few PIP members have been reported to be involved in carbon dioxide permeability of cells. Newly identified transport substrates for NIP members of rice and Arabidopsis thaliana have been demonstrated to transport silicon and boron, respectively. Ammonia, glycerol, and hydrogen peroxide have been identified as substrates for plant aquaporins. The intracellular localisation of plant aquaporins is diverse; for example, SIP members are localised on the ER membrane. There has been much progress in the research on the functional regulation of water channel activity of PIP members including phosphorylation, formation of hetero-oligomer, and protonation of histidine residues under acidic condition. This review provides a broad overview of the range of potential aquaporins, which are now believed to participate in the transport of several small molecules in various membrane systems in model plants, crops, flowers and fruits.

Osmotic adjustment leads to anomalously low estimates of relative water content in wheat and barley.

Abstract: Relative water content (RWC) is used extensively to determine the water status of plants relative to their fully turgid condition. However, plants often adjust osmotically to salinity or water deficit, which maintains turgor pressure and obscures the definition of ‘full turgidity’. To explore this problem, turgor was measured by isopiestic psychrometry in mature leaf blades of barley (Hordeum vulgare) and durum wheat (Triticum turgidum ssp. durum) salinised to 150 mm NaCl, or bread wheat (Triticum aestivum) grown in soil dehydrated to varying degrees. Osmotic adjustment maintained turgor in all the plants but despite full maintenance in some of the salinised plants, their leaf RWC decreased substantially. This occurred because excess water was absorbed while the samples were floated on water as part of the RWC measurement. The absorption falsely increased the weight of the ‘fully turgid’ condition, causing RWC to be anomalously low by 10–15%. Cell solution was secreted into intercellular spaces and was seen under a microscope, which is a test encouraged for all RWC measurements. Several alternate methods are suggested for rehydrating tissues while minimising excess water absorption, but no simple definition of ‘full turgidity’ seems possible. In general, direct measurements of osmotic adjustment and turgor are preferred.

Identification of grapevine MLO gene candidates involved in susceptibility to powdery mildew

Abstract: The European cultivated grapevine, Vitis vinifera L., is a host for the powdery mildew pathogen Erisyphe necator, which is the most economically important fungal disease of viticulture. MLO proteins mediate powdery mildew susceptibility in the model plant species Arabidopsis and the crop plants barley and tomato. Seven VvMLO cDNA sequences were isolated from grapevine and were subsequently identified as part of a 17 member VvMLO gene family within the V. vinifera genome. Phylogenetic analysis of the 17 VvMLO genes in the grape genome indicated that the proteins they encode fall into six distinct clades. The expression of representative VvMLOs from each clade were analysed in a range of grape tissues, as well as in response to a range of biotic and abiotic factors. The VvMLOs investigated have unique, but overlapping tissue expression patterns. Expression analysis of VvMLO genes following E. necator infection identified four upregulated VvMLOs which are orthologous to the Arabidopsis AtMLO2, AtMLO6 and AtMLO12 and tomato SlMLO1 genes required for powdery mildew susceptibility. This suggests a degree of functional redundancy between the proteins encoded by these genes in terms of susceptibility to powdery mildew, and, as such, represent potential targets for modification to generate powdery mildew resistant grapevines.

Modelling phloem and xylem transport within a complex architecture

Abstract: The function of the plant's vasculature, incorporating both phloem and xylem, is of fundamental importance to the survival of all higher plants. Although the physiological mechanism involved in these two transport pathways has been known for some time, quantitative modelling of this has been slow to develop. 1-D continuous models have shown that the proposed mechanisms are quantitatively plausible (Thompson and Holbrook 2003) but more complex geometries (architectures) have remained out of reach because of mathematical difficulties. In this work, we extend the alternative modular approach by Daudet et al. (2002) using recently developed numerical tools which allow us to model complex architectures. After a full description of the extended model, we first show that it efficiently reproduces the results of the continuous approach when applied to the same simple configurations. The model is then applied to a more complex configuration with two sinks, confirming that sink priority is an emergent property of the Munch flow as earlier found with a minimalist model (Minchin et al. 1993). It is further shown how source leaf transpiration can change the relative carbon allocation rates among sinks.

Integrating simulation of architectural development and source-sink behaviour of peach trees by incorporating Markov chains and physiological organ function submodels into L-PEACH

Abstract: L-PEACH is an L-system-based functional-structural model for simulating architectural growth and carbohydrate partitioning among individual organs in peach (Prunus persica (L.) Batsch) trees. The original model provided a prototype for how tree architecture and carbon economy could be integrated, but did not simulate peach tree architecture realistically. Moreover, evaluation of the functional characteristics of the individual organs and the whole tree remained a largely open issue. In the present study, we incorporated Markovian models into L-PEACH to improve the architecture of the simulated trees. The model was also calibrated to grams of carbohydrate, and tools for systematically displayingquantitativeoutputsandevaluatingthebehaviourofthemodelweredeveloped.TheuseoftheMarkovianmodel concept to model tree architecture in L-PEACH reproduced tree behaviour and responses to management practices visually similar to trees in commercial orchards. The new architectural model along with several improvements in the carbohydrate- partitioning algorithms derived from the model evaluation significantly improved the results related to carbon allocation, suchas organgrowth, carbohydrateassimilation, reserve dynamicsandmaintenance respiration. Themodelresultsare now consistent within the modelled tree structure and are in general agreement with observations of peach trees growing under field conditions.

Medicago truncatula as a model for understanding plant interactions with other organisms, plant development and stress biology: past, present and future

Abstract: Medicago truncatula Gaertn. cv. Jemalong, a pasture species used in Australian agriculture, was first proposed as a model legume in 1990. Since that time M. truncatula, along with Lotus japonicus (Regal) Larsen, has contributed to major advances in understanding rhizobia Nod factor perception and the signalling pathway involved in nodule formation. Research using M. truncatula as a model has expanded beyond nodulation and the allied mycorrhizal research to investigate interactions with insect pests, plant pathogens and nematodes. In addition to biotic stresses the genetic mechanisms to ameliorate abiotic stresses such as salinity and drought are being investigated. Furthermore, M. truncatula is being used to increase understanding of plant development and cellular differentiation, with nodule differentiation providing a different perspective to organogenesis and meristem biology. This legume plant represents one of the major evolutionary success stories of plant adaptation to its environment, and it is particularly in understanding the capacity to integrate biotic and abiotic plant responses with plant growth and development that M. truncatula has an important role to play. The expanding genomic and genetic toolkit available with M. truncatula provides many opportunities for integrative biological research with a plant which is both a model for functional genomics and important in agricultural sustainability.

Hydraulic and chemical signalling in the regulation of stomatal conductance and plant water use in field grapevines growing under deficit irrigation

Abstract: Effects of irrigation strategies on stomata and plant water use were studied in field-grown grapevines (Vitis viniferaL)Weassessedtheimportanceofroot-derivedchemicalsignalsvshydraulicsignallinginstomatalregulationThe experimentincludedtwotreatments withthesamewateraddedtothesoil(50%ETc)appliedeithertothewholerootsystem (DI) or to half of the roots, alternating irrigation side every 15 days (PRD) Well-watered plants (FI) (100% ETc) and non- irrigatedgrapevines(NI)werealsostudiedPartialstomataclosureoccurredinbothPRDandDIplants(ABA)ofxylemsap remainedconstantduringthedayandwasmaintainedthroughouttheseason,withhighervaluesinNIplantsXylemsappH was not affected by soil water availability A positive correlation between ypd and maximum gs was found, indicating that grapevine stomata strongly respond to plant water status In contrast, ABA did not explain stomatal control at veraison At mid-ripening gswas significantly correlated with ABA, apparently interacting with the rise in xylem sap pH Therefore, our data suggest that hydraulic feedback and feed-forward root-to-shoot chemical signalling mechanisms might be involved in the control of stomata in response to decreased soil water availability, hydraulic signals playing the dominant role

Cell death in grape berries: varietal differences linked to xylem pressure and berry weight loss

Abstract: Some varieties of Vitis vinifera L can undergo berry weight loss during later stages of ripening This defines a third phase of development in addition to berry formation and berry expansion Berry weight loss is due to net water loss, but the component water flows through different pathways have remained obscure Because of the very negative osmotic potential of the cell sap, the maintenance of semipermeable membranes in the berry is required for the berry to counter xylem and apoplast tensions that may be transferred from the vine The transfer of tension is determined by the hydraulic connection through the xylem from the berry to the vine, which changes during development Here we assess the membrane integrity of three varieties of V vinifera berries (cvv Shiraz, Chardonnay and Thompson seedless) throughout development using the vitality stains, fluorescein diacetate and propidium iodide, on fresh longitudinal sections of whole berries We also measured the xylem pressure using a pressure probe connected to the pedicel of detached berries The wine grapes, Chardonnay and Shiraz, maintained fully vital cells after veraison and during berry expansion, but began to show cell death in the mesocarp and endocarp at or near the time that the berries attain maximum weight This corresponded to a change in rate of accumulation of solutes in the berry and the beginning of weight loss in Shiraz, but not in Chardonnay Continuous decline in mesocarp and endocarp cell vitality occurred for both varieties until normal harvest dates Shiraz grapes classified as high quality and sourced from a different vineyard also showed the same death response at the same time after anthesis, but they displayed a more consistent pattern of pericarp cell death The table grape, Thompson seedless, showed near to 100% vitality for all cells throughout development and well past normal harvest date, except for berries with noticeable berry collapse that were treated with giberellic acid The high cell vitality in Thompson seedless berries corresponded to negative xylem pressures that contrasted to the slightly positive pressures for Shiraz and Chardonnay We hypothesise that two variety dependent strategies exist for grapevine berries late in development: (1) programmed cell death in the pericarp and loss of osmotically competent membranes that requires concomitant reduction in the hydraulic conductance via the xylem to the vine; (2) continued cell vitality and osmotically competent membranes that can allow high hydraulic conductance to the vine

An analysis of the sensitivity of sap flux to soil and plant variables assessed for an Australian woodland using a soil–plant–atmosphere model

Abstract: Daily and seasonal patterns of tree water use were measured for the two dominant tree species, Angophora bakeri E.C.Hall (narrow-leaved apple) and Eucalyptus sclerophylla (Blakely) L.A.S. Johnson & Blaxell (scribbly gum), in a temperate, open, evergreen woodland using sap flow sensors, along with information about soil, leaf, tree and micro-climatological variables. The aims of this work were to: (a) validate a soil-plant-atmosphere (SPA) model for the specific site; (b) determine the total depth from which water uptake must occur to achieve the observed rates of tree sap flow; (c) examine whether the water content of the upper soil profile was a significant determinant of daily rates of sap flow; and (d) examine the sensitivity of sap flow to several biotic factors. It was found that: (a) the SPA model was able to accurately replicate the hourly, daily and seasonal patterns of sap flow; (b) water uptake must have occurred from depths of up to 3 m; (c) sap flow was independent of the water content of the top 80 cm of the soil profile; and (d) sap flow was very sensitive to the leaf area of the stand, whole tree hydraulic conductance and the critical water potential of the leaves, but insensitive to stem capacitance and increases in root biomass. These results are important to future studies of the regulation of vegetation water use, landscape-scale behaviour of vegetation, and to water resource managers, because they allow testing of large-scale management options without the need for large-scale manipulations of vegetation cover.

Comparison of architecture among different cultivars of hybrid rice using a spatial light model based on 3-D digitising.

Abstract: Modification of plant types (i.e. plant architecture) is an important strategy to enhance the yield potential of crops. The aims of this study were to specify rice plant types using 3-D modelling methodology. The architecture of three typical hybrid rice cultivars were measured in situ in a paddy field using a 3-D digitiser at four development stages from the panicle initiation to the filling stage. The structural parameters of the rice canopies were calculated and their light capture and potential carbon gain were simulated based on a 3-D light model. The results confirmed that a plant type with steeper leaf angles let light penetrate more deeply with relatively uniform light distribution in the canopy at higher sun elevation angles, although this result was related to leaf area index. The variations of plant types, however, did not convert into differences of light distribution across rice varieties at lower sun elevation angles. Light use efficiency at the higher leaf area index could be enhanced by reducing mutual-shading. These results indicate that a promising approach to quantify the rice architecture in situ is to combine 3-D digitising and a 3-D light model to evaluate light interception and photosynthesis of rice plant types.

Coupling a 3D virtual wheat (Triticum aestivum) plant model with a Septoria tritici epidemic model (Septo3D): a new approach to investigate plant–pathogen interactions linked to canopy architecture

Abstract: This work initiates a modelling approach that allows us to investigate the effects of canopy architecture on foliar epidemics development. It combines a virtual plant model of wheat (Triticum aestivum L.) with an epidemic model of Septoria tritici which is caused by Mycosphaerella graminicola, a hemi-biotrophic, splashed-dispersed fungus. Our model simulates the development of the lesions from the infected lower leaves to the healthy upper leaves in the growing canopy. Epidemics result from the repeated successions of lesion development (during which spores are produced) and spores dispersal. In the model, canopy development influences epidemic development through the amount of tissue available for lesion development and through its effects on rain penetration and droplets interception during spore dispersal. Simulations show that the impact of canopy architecture on epidemic development differs between canopy traits and depends on climate. Phyllochron has the strongest effect, followed by leaf size and stem elongation rate.

Deactivation of aquaporins decreases internal conductance to CO2 diffusion in tobacco leaves grown under long-term drought

Abstract: We compared the diffusion conductance to CO2 from the intercellular air space to the chloroplasts (internal conductance (g i)) between tobacco leaves acclimated to long-term drought (drought-acclimated (DA)) and those grown under sufficient irrigation (well-watered (WW)), and analysed the changes in g i in relation to the leaf anatomical characteristics and a possible CO2 transporter, aquaporin. The g i, which was estimated by combined analyses of CO2 gas exchange with chlorophyll fluorescence, in the DA plants was approximately half of that in the WW plants. The mesophyll and chloroplast surface areas exposing the intercellular air space, which potentially affect g i, were not significantly different between the WW and DA plants. The amounts of plasma membrane aquaporins (PIP), immunochemically determined using radish PIP antibodies, were unrelated to g i. After treatment with HgCl2, an aquaporin inhibitor, the water permeability of the leaf tissues (measured as the weight loss of fully-turgid leaf disks without the abaxial epidermis in 1 m sorbitol) in WW plants decreased with an increase in HgCl2 concentration. The g i in the WW plants decreased to similar levels to the DA plants when the detached leaflets were fed with 0.5 mm HgCl2. In contrast, both water permeability and g i were insensitive to HgCl2 treatments in DA plants. These results suggest that deactivation of aquaporins is responsible for the significant reduction in g i observed in plants growing under long-term drought.

Relating leaf photosynthetic rate to whole-plant growth: drought and shade effects on seedlings of four Quercus species

Abstract: Understanding the impacts of combined resource supplies on seedlings is critical to enable prediction of establishment growth, and forest dynamics. We investigated the effects of irradiance and water treatments on absolute growth,andrelativegrowth rate(RGR) anditscomponents,forseedlings offourQuercusspeciesdiffering inleafhabitand withawidevariationinseedmass.Plantsweregrownfor6.5monthsatthreelevelsofirradiance(100,27,and3%daylight), and treated during the last 2.5 months with two watering treatments (frequent watering v. suspended watering). Both shade and drought reduced seedling growth rates, with a significant interaction: under full irradiance the drought treatment had a stronger impact on RGR and final biomass than under deep shade. For three species, seed mass was positively related to absolutegrowth,withstrongercorrelationsatlowerirradiance.Theevergreenspeciesgrewfasterthanthedeciduousspecies, though leaf habit accounted for a minor part of the interspecific variation in absolute growth. Seedling biomass was determinedpositivelyeitherbyRGRorseedmass;RGRwaspositivelylinkedwithnetassimilationrate(NAR)andleafmass fraction (LMF), and seed mass was negatively linked with RGR and LMF, but positively linked with NAR. Seedling RGR was not correlated with light-saturated net photosynthetic rate, but was strongly correlated with the net carbon balance estimated, from photosynthetic light-response curves, considering daily variation in irradiance. These findings suggest an approach to applying short-term physiological measurements to predict the RGR and absolute growth rate of seedlings in a wide range of combinations of irradiance and water supplies.

Multiple traits associated with salt tolerance in lucerne: revealing the underlying cellular mechanisms

Abstract: Salinity tolerance is a complex trait inferring the orchestrated regulation of a large number of physiological and biochemical processes at various levels of plant structural organisation. It remains to be answered which mechanisms and processes are crucial for salt tolerance in lucerne (Medicago sativa L.). In this study, salinity effects on plant growth characteristics, pigment and nutrient composition, PSII photochemistry, leaf sap osmolality, changes in anatomical and electrophysiological characteristics of leaf mesophyll, and net ion fluxes in roots of several lucerne genotypes were analysed. Salinity levels ranged from 40 to ~200 mm NaCl, and were applied to either 2-month-old plants or to germinating seedlings for a period of between 4 and 12 weeks in a series of hydroponic, pot and field experiments. Overall, the results suggest that different lucerne genotypes employ at least two different mechanisms for salt tolerance. Sodium exclusion appeared to be the mechanism employed by at least one of the tolerant genotypes (Ameristand 801S). This cultivar had the lowest leaf thickness, as well as the lowest concentration of Na+ in the leaf tissue. The other tolerant genotype, L33, had much thicker leaves and almost twice the leaf Na+ concentration of Ameristand. Both cultivars showed much less depolarisation of leaf membrane potential than the sensitive cultivars and, thus, had better K+ retention ability in both root and leaf tissues. The implications of the above measurements for screening lucerne germplasm for salt tolerance are discussed.

Quasi-Monte Carlo simulation of the light environment of plants

Abstract: The distribution of light in the canopy is a major factor regulating the growth and development of a plant. The main variables of interest are the amount of photosynthetically active radiation (PAR) reaching different elements of the plant canopy, and the quality (spectral composition) of light reaching these elements. A light environment model based on Monte Carlo (MC) path tracing of photons, capable of computing both PAR and the spectral composition of light, was developed by Měch (1997), and can be conveniently interfaced with virtual plants expressed using the open L-system formalism. To improve the efficiency of the light distribution calculations provided by Měch’s MonteCarlo program, we have implemented a similar program QuasiMC, which supports a more efficient randomised quasi-Monte Carlo sampling method (RQMC). We have validated QuasiMC by comparing it with MonteCarlo and with the radiosity-based CARIBU software (Chelle et al. 2004), and we show that these two programs produce consistent results. We also assessed the performance of the RQMC path tracing algorithm by comparing it with Monte Carlo path tracing and confirmed that RQMC offers a speed and/or accuracy improvement over MC.

Parametric identification of a functional-structural tree growth model and application to beech trees (Fagus sylvatica)

Abstract: Functional-structural models provide detailed representations of tree growth and their application to forestry seems full of prospects. However, owing to the complexity of tree architecture, parametric identification of such models remains a critical issue. We present the GreenLab approach for modelling tree growth. It simulates tree growth plasticity in response to changes of their internal level of trophic competition, especially topological development and cambial growth. The model includes a simplified representation of tree architecture, based on a species-specific description of branching patterns. We study whether those simplifications allow enough flexibility to reproduce with the same set of parameters the growth of two observed understorey beech trees (Fagus sylvatica L.) of different ages in different environmental conditions. The parametric identification of the model is global, i.e. all parameters are estimated simultaneously, potentially providing a better description of interactions between sub-processes. As a result, the source-sink dynamics throughout tree development is retrieved. Simulated and measured trees were compared for their trunk profiles (fresh masses and dimensions of every growth units, ring diameters at different heights) and compartment masses of their order 2 branches. Possible improvements of this method by including topological criteria are discussed.

An automated procedure for estimating the leaf area index (LAI) of woodland ecosystems using digital imagery, MATLAB programming and its application to an examination of the relationship between remotely sensed and field measurements of LAI

Abstract: Leaf area index (LAI) is one of the most important variables required for modelling growth and water use of forests. Functional–structural plant models use these models to represent physiological processes in 3-D tree representations. Accuracy of these models depends on accurate estimation of LAI at tree and stand scales for validation purposes. A recent method to estimate LAI from digital images (LAID) uses digital image capture and gap fraction analysis (Macfarlane et al. 2007b) of upward-looking digital photographs to capture canopy LAID (cover photography). After implementing this technique in Australian evergreen Eucalyptus woodland, we have improved the method of image analysis and replaced the time consuming manual technique with an automated procedure using a script written in MATLAB 7.4 (LAIM). Furthermore, we used this method to compare MODIS LAI values with LAID values for a range of woodlands in Australia to obtain LAI at the forest scale. Results showed that the MATLAB script developed was able to successfully automate gap analysis to obtain LAIM. Good relationships were achieved when comparing averaged LAID and LAIM (LAIM = 1.009 – 0.0066 LAID; R2 = 0.90) and at the forest scale, MODIS LAI compared well with LAID (MODIS LAI = 0.9591 LAID – 0.2371; R2 = 0.89). This comparison improved when correcting LAID with the clumping index to obtain effective LAI (MODIS LAI = 1.0296 LAIe + 0.3468; R2 = 0.91). Furthermore, the script developed incorporates a function to connect directly a digital camera, or high resolution webcam, from a laptop to obtain cover photographs and LAI analysis in real time. The later is a novel feature which is not available on commercial LAI analysis softwares for cover photography. This script is available for interested researchers.