A Shoot:Root Partitioning Model

A model for partitioning newly-synthesized structural dry matterbetween shoot and root is developed. It is based on a postulatedpartitioning function, which depends upon the relative levelsof carbon and nitrogen substrates, with parameters determiningthe control point and also the degree of control. The modelis used to investigate the relationships between plant specificgrowth rate, shoot:root ratio, and the specific activities ofshoot and root (which depend upon environment), during steady-stateexponential growth; the transient behaviour of the model isalso explored and oscillations in these quantities are obtained. Shoot:root ratio, specific growth rate, mathematical model, partition of assimilates     

A Model of Shoot: Root Partitioning with Optimal Growth

A shoot: root partitioning model is presented, which is a developmentof previous approaches in the area. The model incorporates asa physiologically reasonable apparent ‘goal’ forthe plant, the assumption that the partitioning of growth betweenthe shoot and root maximizes the plant specific growth ratein balanced exponential growth. The analysis is concerned principallywith plant growth being a function of carbon and nitrogen only,although it is indicated how other nutrients, or growth factors,may be incorporated. Plant growth is driven by the environmentalconditions, and partitioning is defined entirely in terms ofthe shoot: root ratio and carbon and nitrogen status of theplant. In its basic form the model requires the definition ofa single plant growth parameter, along with the shoot and rootspecific activities and structural composition. Shoot: root partitioning, specific growth rate, vegetative phase     

Root System Architecture from Coupling Cell Shape to Auxin Transport

Lateral organ position along roots and shoots largely determines plant architecture, and depends on auxin distribution patterns. Determination of the underlying patterning mechanisms has hitherto been complicated because they operate during growth and division. Here, we show by experiments and computational modeling that curvature of the Arabidopsis root influences cell sizes, which, together with tissue properties that determine auxin transport, induces higher auxin levels in the pericycle cells on the outside of the curve. The abundance and position of the auxin transporters restricts this response to the zone competent for lateral root formation. The auxin import facilitator, AUX1, is up-regulated by auxin, resulting in additional local auxin import, thus creating a new auxin maximum that triggers organ formation. Longitudinal spacing of lateral roots is modulated by PIN proteins that promote auxin efflux, and pin2,3,7 triple mutants show impaired lateral inhibition. Thus, lateral root patterning combines a trigger, such as cell size difference due to bending, with a self-organizing system that mediates alterations in auxin transport.     

Carbon Partitioning in Split Root Systems of Barley: Relation to Metabolism

We tested four hypotheses for the control of partitioning ofphotoassimilated C-11 between the two halves of split root systemsof young barley plants. Our data supported the hypothesis thatphloem is unloaded without the use of metabolic energy, sinceseveral metabolic inhibitors applied to one half of a splitroot system reduced respiratory oxygen uptake without alteringimport of C-11. The hypothesis that rate of import C-11 is directlyrelated to metabolic activity in the root was rejected, since(a) certain inhibitors reduced respiration but not import and(b) exogenous sucrose reduced import into the root half to whichit was supplied. Our data were consistent with the hypothesisthat import is related to the total ability (metabolism plusstorage) of the sink to use sucrose. Treatments that would haveled to greatly decreased use of sucrose (iodoacetate inhibition)decreased import before those which would have led to a smallerdecrease in sucrose use (FCCP inhibition). These data, and thereduction in import to a root half supplied exogenously withsucrose, supported the hypothesis that the size of soluble sugarpools within the roots is, in the short-term, inversely proportionalto rate of import, the soluble sugar pools thus acting as amediator between rate of sucrose use and supply from the phloem.     

Carbon Partitioning in Split Root Systems of Barley: The Effect of Osmotica

Vegetative barley plants, grown with their root systems splitbetween two containers, were supplied with ¹¹CO₂ to the secondleaf. Both halves of the split root were monitored separatelywith scintillation detectors and a pre-treatment value for partitioningbetween them established. Mannitol or sorbitol was added tothe medium around one root-half to lower the solute potentialto –0?4 MPa, and partitioning of C-11 between the tworoot halves was followed continuously for 3 h. Alternatively,¹⁴CO₂ was supplied at, or 3 h or 24 h after, treatment withosmotica, and roots were harvested and counted 3 h later. Osmoticarapidly increased the import of isotope into the treated root-half,but this effect and a respiratory increase induced by the osmoticawas transient. Within 3 h of applying the osmoticum, partitioningbetween the root halves was little affected. Slow addition ofosmotica (0?12 MPa h^–1) did not affect partitioning betweenroot halves. We conclude that the solute potential of the apoplastcan have a major if transient effect on carbon partitioningbut that this is unlikely to be important in vivo as osmoregulationby the root can negate its effects. Key words: Carbon partitioning, barley, osmotica, split-root system     

RootScape: A Landmark-Based System for Rapid Screening of Root Architecture in Arabidopsis

The architecture of plant roots affects essential functions including nutrient and water uptake, soil anchorage, and symbiotic interactions. Root architecture comprises many features that arise from the growth of the primary and lateral roots. These root features are dictated by the genetic background but are also highly responsive to the environment. Thus, root system architecture (RSA) represents an important and complex trait that is highly variable, affected by genotype × environment interactions, and relevant to survival/performance. Quantification of RSA in Arabidopsis (Arabidopsis thaliana) using plate-based tissue culture is a very common and relatively rapid assay, but quantifying RSA represents an experimental bottleneck when it comes to medium- or high-throughput approaches used in mutant or genotype screens. Here, we present RootScape, a landmark-based allometric method for rapid phenotyping of RSA using Arabidopsis as a case study. Using the software AAMToolbox, we created a 20-point landmark model that captures RSA as one integrated trait and used this model to quantify changes in the RSA of Arabidopsis (Columbia) wild-type plants grown under different hormone treatments. Principal component analysis was used to compare RootScape with conventional methods designed to measure root architecture. This analysis showed that RootScape efficiently captured nearly all the variation in root architecture detected by measuring individual root traits and is 5 to 10 times faster than conventional scoring. We validated RootScape by quantifying the plasticity of RSA in several mutant lines affected in hormone signaling. The RootScape analysis recapitulated previous results that described complex phenotypes in the mutants and identified novel gene × environment interactions.Roots have a crucial impact on plant survival because of their major functions: anchorage of the plant in the soil, water and nutrient acquisition, and symbiotic interaction with other organisms (Den Herder et al., 2010). One important characteristic of root systems is the manner in which the primary and lateral roots comprise the superstructure or root architecture. Root architecture is an ideal system for studying developmental plasticity, as it continually integrates intrinsic and environmental responses (Malamy, 2005), which represents a vital and dynamic component of agricultural productivity (Lynch, 1995).Root system architecture (RSA) is defined as the spatial configuration of the roots in their environment (Lynch, 1995). The complexity of RSA was initially appreciated several decades ago, and terms like morphology, topology, distribution, and architecture were often used to describe the nature of RSA (Fitter, 1987; Fitter and Stickland, 1991; Lynch, 1995). These early reports argued that simple traits like root mass are insufficient to describe roots, because they do not capture the spatial configuration of roots in the soil, which is critical to plant performance (Fitter and Stickland, 1991). Root systems are integrated organs that adopt specific architectures to maximal foraging of the heterogeneous soil environment in different ways (Fitter, 1987; Fitter and Stickland, 1991; Lynch, 1995). More recently, new approaches have incorporated the measurement of many individual developmental traits that together comprise RSA (De Smet et al., 2012; Dubrovsky and Forde, 2012). For example, one recent report identified three fundamental components of RSA in generating complex topologies, including the contribution of lateral axes to branching, the rate and path of growth of the axis, and the increase in root surface area (Topp and Benfey, 2012). Thus, RSA is an important and complex trait that requires convenient measurement methods for rapid screening of diverse plant mutants and genotypes.With increasing research in RSA in the genetically tractable model plant Arabidopsis (Arabidopsis thaliana), the need for high-throughput methods of root phenotyping has dramatically increased over the years. Consequently, different methods and approaches have been developed in order to address this demand. Currently, three major approaches for phenotyping RSA are used (for review, see Zhu et al., 2011; De Smet et al., 2012). The first group of methods uses classical measures of RSA, which involve measurements of individual root traits. These methods often use software to manually draw the RSA onto digital two-dimensional images to quantify root length and number (Abramoff et al., 2004; http://www.machinevision.nl). These traditional methods provide the most accurate measurements of the root system but have a major disadvantage in being extremely time consuming.The second group of methods utilizes advanced semiautomated software for RSA measurements like EZ-Rhizo (Armengaud et al., 2009). EZ-Rhizo also uses digital two-dimensional images of plants grown on vertical plates (similar to the classical methods above) but is faster and produces different traits and basic statistics. The method works best when root features do not physically overlap, but we have found root overlap to be common when working with Arabidopsis plants older than 10 d. Other recent programs also provide semiautomated analysis of RSA, including RootReader2D (http://www.plantmineralnutrition.net/rootreader.htm) and SmartRoot (Lobet et al., 2011). However, while completely automated detection is potentially the highest throughput, we found that the root surface detection step is frequently prone to failure when using both of these programs, even after considerable adjustment by the user, where root features are missed or background noise is incorrectly labeled as roots.Finally, in a third group, recent developments include three-dimensional analysis of RSA of plants grown on transparent gel cylinders or in soil. The three-dimensional gel-based imaging approach is reported to be suitable for high-throughput phenotyping (Iyer-Pascuzzi et al., 2010). However, this approach requires special equipment, and imaging the root system of single plants can take 10 min (Iyer-Pascuzzi et al., 2010). X-ray computed tomography (Perret et al., 2007; Tracy et al., 2010) and magnetic resonance imaging (Van As, 2007) also provide highly detailed three-dimensional RSA analysis, but they require long scanning times and are extremely expensive and inaccessible. Most laboratories still utilize relatively convenient, inexpensive, and rapid two-dimensional phenotypic characterization of RSA, at least for initial screening purposes.The aim of this work is to address the need for a simple method to measure many different aspects of root architecture for high-throughput laboratory screening of mutants and genotypes in Arabidopsis. Here, we describe a landmark-based allometric (size and shape) approach called RootScape, a user-friendly software platform that enables rapid, comprehensive, and integrative phenotyping of the RSA in Arabidopsis. Unlike recent methods that collect information on different root traits to describe the RSA, RootScape places user-defined root landmarks on a two-dimensional grid to measure root architecture as a single integrated root system. The method employs rapid manual placement of root system landmarks. This manual step avoids one of the most problematic steps in automated image analysis (recognition of the root surface), providing a simple tool that does not require image processing. This method uses simple, two-dimensional digital images of the root system and a 20-point landmark model created in AAMToolbox, a freely available MATLAB plugin. While in-depth developmental analysis of root systems will often require knowing the contribution of individual traits, RootScape is a rapid method to access the holistic contribution of many individual root traits to RSA and to capture the overall property of the spatial configuration of roots in the soil (Fitter and Stickland, 1991). To demonstrate its utility, we used RootScape to quantify the root plasticity of Arabidopsis plants (Columbia [Col-0]) grown on four different media and compared the RootScape results with conventional measurements of individual root traits captured using the Optimas6 image-analysis software or Image J (Abramoff et al., 2004). This analysis showed that by measuring integrative root traits using RootScape, we could capture the vast majority of the individual trait variation, as verified by multiple regression analysis. Additionally, we tested the ability of RootScape to quantify the plasticity response in Arabidopsis mutants defective in hormone signaling. For this analysis, wild-type Col-0 and three hormone signaling mutants (auxin-resistant4 [axr4], abscisic acid insensitive4 [abi4], and cytokinin response1 [cre1]) were treated with auxin, cytokinin, or abscisic acid (ABA) versus controls. Statistical analyses (ANOVA/multivariate ANOVA [MANOVA]) allowed us to confirm most of the previously known interactions of genotype with these distinct environments and to potentially identify novel ones. Thus, we demonstrate that RootScape can be used as a rapid and efficient approach for quantifying the plasticity of the RSA in mutant (or ecotype) backgrounds of Arabidopsis and can identify new conditional root phenotypes.     

External Nitrogen and Carbon Source-Mediated Response on Modulation of Root System Architecture and Nitrate Uptake in Wheat Seedlings

Nitrogen uptake efficiency is an important component trait that could be targeted for improving nitrogen use efficiency of crop plants. To understand the responses of different nitrate transport systems and the influence of root system architecture on nitrate uptake under limited nitrate conditions in wheat (Triticum aestivum L.) at the seedling stage, we studied nitrate uptake, root system architecture, and expression of different nitrate transporter genes in induced and non-induced wheat seedlings. Further, effects of inclusion of sucrose and two amino acids (glutamine and asparagine) in induction medium on these parameters were also studied. We observed that the induced wheat root system took up more nitrate as compared to non-induced root system in a dose-dependent manner. Gene expression of both high- and low-affinity nitrate transporter gene showed differential expression in the induced root tissues, as compared to non-induced tissues, depending on the concentration of nitrate present in induction medium. External nutrient media containing sucrose, glutamine, and asparagine reduce nitrate concentration in both root and shoot tissues and also influence the gene expression of these transporters. Our observations indicate that upon induction with milder external nitrate concentrations, the root architecture is modulated by changing overall lateral root size and 1st order lateral root numbers along with activation of nitrate transporters which acquire and transport nitrate in roots and shoots, respectively, depending on the carbon and nitrogen source available to seedlings.

     

Transport and Partitioning of CO(2) Fixed by Root Nodules of Ureide and Amide Producing Legumes

Nodulated and denodulated roots of adzuki bean (Vigna angularis), soybean (Glycine max), and alfalfa (Medicago sativa) were exposed to ¹⁴CO₂ to investigate the contribution of nodule CO₂ fixation to assimilation and transport of fixed nitrogen. The distribution of radioactivity in xylem sap and partitioning of carbon fixed by nodules to the whole plant were measured. Radioactivity in the xylem sap of nodulated soybean and adzuki bean was located primarily (70 to 87%) in the acid fraction while the basic (amino acid) fraction contained 10 to 22%. In contrast, radioactivity in the xylem sap of nodulated alfalfa was primarily in amino acids with about 20% in organic acids. Total ureide concentration was 8.1, 4.7, and 0.0 micromoles per milliliter xylem sap for soybean, adzuki bean, and alfalfa, respectively. While the major nitrogen transport products in soybeans and adzuki beans are ureides, this class of metabolites contained less than 20% of the total radioactivity. When nodules of plants were removed, radioactivity in xylem sap decreased by 90% or more. Pulse-chase experiments indicated that CO₂ fixed by nodules was rapidly transported to shoots and incorporated into acid stable constituents. The data are consistent with a role for nodule CO₂ fixation providing carbon for the assimilation and transport of fixed nitrogen in amide-based legumes. In contrast, CO₂ fixation by nodules of ureide transporting legumes appears to contribute little to assimilation and transport of fixed nitrogen.     

Leaf Phosphate Status, Photosynthesis, and Carbon Partitioning in Sugar Beet: III. Diurnal Changes in Carbon Partitioning and Carbon Export

The effect of low phosphate supply (low P) was determined on the diurnal changes in the rate of carbon export, and on the contents of starch, sucrose, glucose, and fructose 2,6-bisphosphate (F2,6BP) in leaves. Low-P effects on the activities of a number of enzymes involved in starch and sucrose metabolism were also measured. Sugar beets (Beta vulgaris L. cv. F58-554H1) were cultured hydroponically in growth chambers and the low-P treatment induced nutritionally. Low-P treatment decreased carbon export from the leaf much more than it decreased photosynthesis. At growth chamber photon flux density, low P decreased carbon export by 34% in light; in darkness, export rates fell but more so in the control so that the average rate in darkness was higher in low-P leaves. Low P increased starch, sucrose, and glucose contents per leaf area, and decreased F2, 6BP. The total extractable activities of enzymes involved in starch and sucrose synthesis were increased markedly by low P, e.g. adenosine 5-diphosphoglucose pyrophosphorylase, cytoplasmic fructose-1,6-bisphosphatase, uridine 5-diphosphoglucose pyrophosphorylase, and sucrose-phosphate synthase. The activities of some enzymes involved in starch and sucrose breakdown were also increased by low P. We propose that plants adapt to low-P environments by increasing the total activities of several phosphatases and by increasing the concentrations of phosphate-free carbon compounds at the expense of sugar phosphates, thereby conserving Pi. The partitioning of carbon among the various carbon pools in low-P adapted leaves appears to be determined in part by the relative capacities of the enzymes for starch and sucrose metabolism.     

Carbon Transport and Root Respiration of Split Root Systems of Phaseolus vulgaris Subjected to Short Term Localized Anoxia

The influence of anoxia on carbon transport and root respiration was evaluated by applying [U-¹⁴C]sucrose to the foliage. Translocation patterns to the root systems of two dry edible bean genotypes (Phaseolus vulgaris L.) were examined after a 3-day exposure to aerated and nonaerated environments. Localized anoxia of root systems was simulated by growing roots in split configurations and exposing half of the system to anoxic conditions. Anoxia of the root system for 72 hours reduced the movement of ¹⁴C label into the roots with concurrent accumulations in the hypocotyl region. The translocation of ¹⁴C label to anoxic roots was less than 50% of the aerated controls of both genotypes. Most of the ¹⁴C label translocated to anoxic root systems was excluded from respiratory metabolism during the 3-hour pulse/chase period and was an order of magnitude less than the aerated controls. These observations suggest that the bulk of ¹⁴C label which entered the root during the anoxic period was unavailable for metabolism by the enzymes of glycolysis and/or was diluted by a relatively large metabolite pool. A higher percentage of ¹⁴C label was translocated to the aerated half of the localized anoxia treatment relative to the half of the aerated controls. The proportion of ¹⁴C label translocated to the root system in the aerated control was 20 and 16% compared to 28 and 25% in the aerated localized anoxia treatment for the genotypes Seafarer and line 31908, respectively. Line 31908 partitioned a greater percentage of ¹⁴C-labeled compounds to the actively growing fraction of the root system in the localized anoxia treatment than did Seafarer. This suggests a greater reliance on previously stored carbohydrate for immediate root growth in Seafarer than in line 31908.     

A Simulation Model for Dry Matter Partitioning in Cucumber

A dynamic model is developed for the simulation of the dailydry matter distribution between the generative and vegetativeplant parts and the distribution among individual fruits ingreenhouse cucumber. The model is based on the hypothesis thatdry matter partitioning is regulated by the sink strengths ofthe plant organs. The sink strength of an organ is defined hereas its potential growth rate, i.e. the growth rate at non-limitingassimilate supply. The sink strength of each individual fruitis described as a function of its temperature sum after anthesisand the actual temperature, that of the vegetative plant partsas a function of actual temperature only. The formation rateof non-aborting fruits is essentially a function of the source/sinkratio. Model results agreed well with the measured fluctuating distributionof dry matter between fruits and vegetative parts. The measuredeffects of three intensities of fruit removal were also simulatedsatisfactorily. When simulating the partitioning among individualfruits the final fruit size was simulated quite well. However,the growth rate of young fruits was usually overestimated andthat of old fruits underestimated, because of dominance amongfruits. This phenomenon could be accounted for by incorporatingpriority functions into the model. Finally, a sensitivity analysisof the model was performed to investigate the effects of someclimatic factors, manipulations of the number of fruits on aplant and model parameters on dry matter distribution. Strategiesto manipulate the dry matter distribution are discussed.Copyright1994, 1999 Academic Press Cucumber, Cucumis sativus (L.), dry matter distribution, fruit growth, partitioning, simulation model, source-sink     

A Transport-resistance Model of Forest Growth and Partitioning

The transport-resistance approach to dry-matter partitioningis used to construct a model of forest growth The model is atthe stand level for a monoculture of identical trees of thesame age There are five major organ compartments in the modelfoliage, branches, stem, coarse roots, and fine roots and mycorrhizasThe matter in each compartment is further subdivided into menstem,structure, carbon substrate, and nitrogen substrate The modelis driven by daily radiation including day length, ambient CO₂concentration, and daily means of air and soil temperature Thefine roots are provided with constant values of soil mineralnitrogen pools (ammonium and nitrate) from which uptake occursGrowth over about 100 years is simulated for various environmentalconditions and soil mineral nitrogen levels, thinning is alsosimulated Natural tree death occurs within the model Particularattention is paid to dry matter partitioning patterns, and tothe dry matter per stem when death occurs The model is robustand responsive, and provides a framework for further developmentand application to many ecological and environmental scenarios,as well as to some forest management problems Model, forest, growth, partitioning     

高等植物光合同化物的运输与分配

高等植物光合同化物的运输受维管束发育状况影响较大,有时会限制产量。而同化物在各库器官间的分配主要决定于库本身的特性,它常用库强度和优先权来描述。库强度是库容量和库活力的乘积,库容量用细胞数目来度量,而库活力常用相对生长速度来度量。近年来人们也用酶少戌一来度量库活力或库强度。而库的优先权描述的是各库器官需求同化物的优先次序,种子被认为是优等权最高的库。同化物的运输分配不仅决定于植物本身源、流、库的特