Modelling the effect of varying soil water on root growth dynamics of annual crops

We present a simple framework for modelling root growth and distribution with depth under varying soil water conditions. The framework considers the lateral growth of roots (proliferation) and the vertical extension of roots (root front velocity). The root front velocity is assumed to be constant when the roots descend into an initially wet soil profile. The lateral growth of roots is governed by two factors: (1) the current root mass or root length density at a given depth, and (2) soil water availability at that depth.Under non-limiting soil water conditions, the increase in root mass at any depth is governed by a logistic equation so that the root length density (R _v) cannot exceed the maximum value. The maximumR _v, is assumed to be the same for all depths. Additional dry matter partitioned to roots is initially distributed according to the current root mass at each depth. As the root mass approaches the maximum value, less dry matter is partitioned to that depth.When soil water is limiting, a water deficit factor is introduced to further modify the distribution of root dry matter. It is assumed that the plant is an energy minimiser so that more root mass is partitioned to the wetter regions of the soil where least energy will be expended for root growth. Hence, the model allows for enhanced root growth in areas where soil water is more easily available.Simulation results show that a variety of root distribution patterns can be reproduced due to varying soil water conditions. It has been demonstrated that broad patterns of root distribution reported in the literature can also be simulated by the model.     

Microbial abundance in the rhizosphere: A computer model

Summary A mathematical model is described which can predict the abundance of microorganisms in the rhizosphere (as g microbial dry weight/cm³ soil) in relation to distance from the root surface and time since the root started exuding substrate. The growth rate of the microorganisms at each point in the soil is assumed to be controlled by the concentration of soluble organic substrate. The concentration of substrate changes due to (1) its production by the root and diffusion through the soil, (2) its production in the soil by breakdown of insoluble organic matter, and (3) its use by the microorganisms. Values for all of the required input parameters have been obtained from the literature.The model predicts that a high population density will develop near the root surface, but the density will fall off steeply with increasing distance from the root. At the root surface microbial growth continues for many days, provided exudation by the root continues at a steady rate, but further away the population reaches a peak and then declines. This is because the amount of substrate reaching the outer soil is no longer adequate to support the maintenance requirement of the population. Starting with a microbial concentration of 2 g/cm³, and using what are considered to be average values for other input parameters, the microbial concentrations predicted after 10 days are 1509 g/cm³ at the root surface, and 2.2 g/cm³ at 1.8 mm from the root. The model also predicts the substrate concentrations in the soil: these reach a maximum within the first day and then decline, reaching by 10 days values not very different from those in root-free soil.The model is used to predict the effect on microbial and substrate concentrations of changes in soil water content, root density, root exudation rate, initial microbial concentration and microbial response to substrate concentration. Where the predictions of the model can be tested against observed data there is good agreement. re]19760308     

Root growth and nitrate utilization of maize cultivars under field conditions

In a 2-year field study conducted on a high fertilized Gleyic Luvisol in Stuttgart-Hohenheim significant differences among 10 maize cultivars were observed in soil nitrate depletion. The different capability of the cultivars to utilize nitrate particularly from the subsoil was positively correlated with (a) shoot N uptake at maturity, and (b) root length density (L_v) in the subsoil layers at silking. Critical root length densities for nitrate uptake were estimated by (a) calculating uptake rates per unit root length (U), (b) subsequent calculation of needed nitrate concentration in soil solution (C₁) to sustain calculated U according to the Baldwin formula, and (c) reducing measured L_v and proportionate increase of U until needed concentration equaled measured concentration. Uptake rate generally increased with soil depth. Critical root length densities for cultivar Brummi (high measured root length densities and soil nitrate depletion) at 60–90 cm depth ranged from 7 % (generative growth) to 28 % (vegetative growth) of measured L_v Measured root length density of each other cultivar was higher than critical root length density for Brummi indicating that the root system of each cultivar examined would have been able to ensure N uptake of Brummi. Positive relationships between root length density and nitrate utilization as indicated by correlation analysis therefore could not be explained by model calculations. This might be due to simplifying assumptions made in the model, which are in contrast to non-ideal uptake conditions in the field, namely irregular distribution of roots and nitrate in the soil, limited root/soil contact, and differences between root zones in uptake activity. It is concluded from the field experiment that growing of cultivars selected for high N uptake-capacity of the shoots combined with high root length densities in the subsoil may improve the utilization of a high soil nitrate supply.     

Vertical distribution and diurnal migrations of krill — Euphausia superba Dana — from hydroacoustical observations,SIBEX, December 1983/January 1984

Summary The depth distributions of krill density with the resolution of 0.435 m has been obtained on the basis of SIBEX hydroacoustic data. Krill was observed to be mainly in a layer from 20 to 100 m with the maximum of biomass migrating from the near surface area at night to greater depths during day. The krill migration pattern can be described by the function: H(t)=A+Bcos (2t/T+), where H is depth of the mass center of biomass, t-time, A-mean depth of krill occurrence, B-amplitude of diurnal changes, T=24 h and -phase of migration process. The parameters of migration pattern: A, B, T and depend on the body length of krill.     

Length densities,occupancies and weights of apple root systems

The root systems of apple trees from five orchards ranging in age from 1.5-y to 14-y were sampled to depths of between one and two metres using soil cores. Although trees came from orchards which differed in soil-type, tree spacings and management, consistent patterns were found in root systems. In orchards of 4-y and older, roots of adjacent trees met so that soil volumes within the planting grids (i.e, tree spacings of approximately 5 m inter-row×4 m intra-row distances) were completely explored, although not completely occupied by roots. Mean root-length densities declined with depth for these orchards. In the 1.5-y orchard, roots from adjacent trees did not meet and root-length densities declined with radial distance from the stem as well as with depth.Root-length densities in the top 1 m ranged from zero to about 1.0 cm.cm^–3 in all orchards and were highly variable. The proportions of core samples having zero values for root-length density were used to subdivide the root zone into volumes in which all samples contained roots, and volumes in which some samples had no roots.Results suggest that roots in an average tree penetrate to at least one metre depth in all but very young orchards so that soil in this volume is fully explored. Volumes filled by roots and volumes occupied at any particular root-length density appear to reach a maximum at about 4 years. Volumes of soil occupied at any particular root-length density were equal in all orchards older than 4 years. This suggests that root growth was balanced by root death. In contrast woody roots continue to accumulate with time.     

Stage-specificity and the synchronisation of life-cycles to periodic environmental variations

Previous work has shown that life-histories consisting of a contiguous series of stages all with density independent development rates exhibiting the same dependence on time cannot synchronise to a periodic environmental variation. This work also examined models representing both dormancy and quiescence at specific points in the life cycle and showed that both could produce strong synchronising effects. In this paper we examine a very general strategic model of an organism with a two-stage life-cycle each stage having a density independent development rate with a characteristic (periodic) time-dependence. We develop a compact representation of this model in terms of a circle map composed from two simple rotations and the interphase map representing the relationship between the physiological times for the two life-history stages. We derive a series of analytic results relating the behaviour of systems whose interphase maps are interrelated and give analytic conditions for a broad class of two-stage circle maps to have a fixed point (that is for the systems they describe to reach the critical life-history stage at the same point in each environmental cycle). Finally we report the results of a numerical investigation of the relationship between the biological characteristics of the development functions and the fine-scale details of the locking behaviour of the systems they define.     

Seasonal variations in the spatial distribution of root tips in teak (Tectonia grandis Linn. f.) plantations in the Varanasi Forest Division,India

Summary Seasonal variations in the spatial distribution of root tips were studied in 19 and 29 year old teak plantations, located on red and alluvial soils respectively. The pattern was essentially similar at both sites, but generally the alluvial soil site exhibited a greater number of root tips. Root tips decreased with increasing distance from the tree base. Through-out most of the year the relative distribution of root tips decreased with depth; the difference between 0–10 and 10–20 cm depths was marginal, but 20–40 cm depth contained distinctly fewer root tips. At all distances a similar seasonal trend was noticed, a mid rainy season peak being followed by a steady decline until the dry summer except for an abrupt rise to a smaller peak in February after the winter rains.The root tip density was positively correlated with the 2 mm root biomass and both showed a similar bimodal annual cycle. Of three environmental variables studied, soil moisture and rainfall were significantly positively correlated with root tip densityl the relationship between soil temperature and root tip density was negative and non-significant. The combined effect of soil moisture and temperature on root tip density, evaluated by a multiple regression model, accounted for 80–95% of the variation in root tip density.     

Callus induction and plant regeneration from different explant types of Miscanthus x ogiformis Honda ‘Giganteus’

Different explants of Miscanthus x ogiformis Honda Giganteus were tested in order to develop an efficient tissue culture system. Shoot apices, leaf and root sections from in vitro-propagated plants, and leaf and immature inflorescence sections from 6-month-old greenhouse-grown plants were used. The explants were cultured on Murashige and Skoog medium supplemented with 4.5, 13.6, 22.6 or 31.7 M 2,4-dichlorophenoxyacetic acid. Three types of callus were formed but only one was embryogenic and regenerated plants. Callus induction and formation of embryogenic callus depended on the type and developmental stage of the explants. Shoot apices formed the highest percentage of embryogenic callus. There was a difference in the formation of embryogenic callus between leaf explants from in vitro-propagated shoots and greenhouse-grown plants. The best results were obtained from newly formed leaves of in vitro-propagated shoots and older leaves of greenhouse-grown plants. Immature inflorescences smaller than 2.5 cm produced a higher percentage of embryogenic callus than larger more mature inflorescences. Embryogenic callus derived from immature inflorescences had the highest regeneration capacity. Differences in 2,4-dichlorophenoxyacetic acid concentrations had no significant effect on callus induction, embryogenic callus formation and plant regeneration.Abbreviations MS Murashige & Skoog - 2,4-d 2,4-dichlorophenoxyacetic acid - BA benzyladenine - NAA 1-naphthaleneacetic acid - PPFD photosynthetic photon flux density     

A negative image of the quiescent centre in regenerating root apices of Zea mays

Usually the presence of the quiescent centre in roots is demonstrated by the absence of labelled nuclei following treatment of the root with appropriate radioactive markers. By modification of the pulselabelling technique, a negative image of the quiescent center, showing more intense labelling from [³H]thymidine than the surrounding area, was obtained in regenerating root apices of Zea mays L.     

Root apical meristems ofAllium porrum L. as affected by arbuscular mycorrhizae and phosphorus

Summary Arbuscular mycorrhizal (AM) fungi significantly improve plant growth in soils with low phosphorus availability and cause many changes in root morphology, similar to those produced by increased P nutrition, mainly depending on root apex size and activity. The aim of this work was to discriminate between the morphogenetic role of AM fungi and P in leek (Allium porrum L.) by feeding mycorrhizal and nonmycorrhizal plants with two nutrient solutions containing 3.2 or 96 M P and examining specific parameters related to adventitious root apices (apex size, mitotic cycle, and RNA synthesis). The results showed that AM fungi blocked meristem activity as indicated by the higher percentages of inactive apices and metaphases in the apical meristem of mycorrhizal plants, whereas the high P supply lengthened the mitotic cycle without blocking the apices, resulting in steady, slow root growth. The possible involvement of abscisic acid in the regulation of root apex activity is discussed.Abbreviations ABA abscisic acid - AM arbuscular mycorrhizae - CI and CII nonmycorrhizal control plants grown with low or high phosphorus concentration - MI and MII mycorrhizal plants grown with low or high phosphorus concentration - PGR plant growth regulator     

Genetic differentiation amongst populations of the coral Pocillopora damicornis off southwestern Australia

An electrophoretic study of genetic differentiation amongst local populations of the reef-coral Pocillopora damicornis was used to group coral heads into units defined as the area of effective dispersal of a clone, and termed colonies. For reefs off southwestern Australia, colonies were usually under a few hundred metres in extent. Although most new recruits within a colony were derived asexually, sexually produced propagules acted to connect populations outside the boundaries of a colony. Such connections were weak, and allelic frequencies varied considerably over a few kilometres. The primary agent of genetic differentiation was suggested to be the small effective population size resulting from the asexual proliferation of a few genotypes at any site. The effective number of genotypes per colony was approximately six. Asexual reproduction appears also to limit gene flow and accentuate selection in this species.     

Gibberellins in apical shoot meristems of flowering and vegetative sugarcane

Gibberellins A₁, A₃, iso-A₃, A₄, A₁₉, A₂₀, and A₃₆ were identified by gas chromatography-selected ion monitoring in apices of sugarcane (Saccharum spp. hybrids). Flowering apices (i.e., 2–4 cm panicle) contained 8–9 times more (estimated by bioassay) endogenous gibberellins A and iso-GA₃ (ratio of 1:6:8, respectively; in total 51 ng g^–1 fresh weight) than vegetative apices (6.4 ng g^–1 fresh weight). Vegetative apices contained small but significant levels of GA₁₉, which could not be detected in flowering apices; vegetative apices also contained approximately four times more of a GA₃₆-like substance than flowering apices. Since the two apex types developed under the same photoperiod, the increased levels of GA and iso-GA₃ and the reduced levels of GA₁₉ and GA₃₆-like substances are correlated with the flowering state rather than with photoperiod or photoperiod changes per se. Since there were relatively high levels of C₁₉ GAs along with low levels of C₂₀ GAs in flowering apices, and since the converse is true in vegetative apices, metabolism of C₂₀ to C₁₉ GAs may be enhanced in flowering apices.Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by U.S. Department of Agriculture and does not imply its approval to the exclusion of other products or vendors that may also be suitable.     

Effect of calmodulin antagonists on the growth and graviresponsiveness of primary roots of maize

We examined the effect of calmodulin (CaM) antagonists applied at the root tip on root growth, gravity-induced root curvature, and the movement of calcium across the root tip and auxin (IAA) across the elongation zone of gravistimulated roots. All of the CaM antagonists used in these studies delayed gravity-induced curvature at a concentration (1 M) that did not affect root growth. Calmodulin antagonists ( 1M) inhibited downward transport of label from ⁴⁵Ca²⁺ across the caps of gravistimulated roots relative to the downward transport of ⁴⁵Ca²⁺ in gravistimulated roots which were not treated with CaM antagonists. Application of CaM antagonists at the root tip ( 1M) also decreased the relative downward movement of label from ³H-IAA applied to the upper side of the elongation zone of gravistimulated roots. In general, tip application of antagonists inhibited neither the upward transport of ⁴⁵Ca²⁺ in the root tip nor the upward movement of label from ³H-IAA in the elongation zone of gravistimulated roots. Thus, roots treated with CaM antagonists ( 1 M) become less graviresponsive and exhibit reduced or even a reversal of downward polarity of calcium transport across the root tip and IAA transport across the elongation zone. The results indicate that calmodulin-regulated events play a role in root gravitropism.     

A graphical method of determining the Michaelis-Menten constant free of external mass transfer resistance for immobilized enzymes in a packed bed reactor

Summary A graphical method of determining the Michaelis-Menten constant free of the external mass transfer resistance for a packed bed immobilized enzyme system was illustrated with examples from 3 different enzyme reactions. The intercept at the ordinate obtained by the straight line extrapolation of data points in the plot of apparent K_m value vs. the reciprocal of superficial velocity in column allowed an easy calculation of K_m free of external mass transfer resistance. An asymptotic value of apparent K_m value at infinite zero superficial velocity was ascribed to the fact that the mass transfer coefficient k_L, approached a definite value at this condition.Nomenclature K_m Michaelis-Menten constant, M/L³ - K_m' K_m free of external mass transfer resistance in a given ionic strength, M/L³ - Km" apparent K_m with external mass transfer resistance, M/L³ - S substrate concentration, M/L³ - S_o initial substrate concentration, M/L³ - k₂ rate constant, t^-1 - E enzyme concentration in support, M/L³ - void volume per unit volume of reactor, dimensionless - u superficial velocity of substrate, L/t - K_L mass transfer coefficient in liquid film, L/t - a external surface area of support per unit volume of reactor, L^-1 - ratio of average channeling length to particle diameter, dimensionless - d_p diameter of support particle, L - X fractional conversion of substrate, dimensionless - H partition coefficient, dimensionless - k a constant, 3 k₂E(1-)d_p/4 - T space time, t - N molecular flux, M/L²t - r radius of immobilized enzyme particle, L     

Evaluating the role of root citrate exudation as a mechanism of aluminium resistance in maize genotypes

Organic anion exudation by roots as a mechanism of aluminium (Al) resistance has been intensively studied lately. In the present study, we evaluated qualitative and quantitative aspects of root exudation of organic anions in maize genotypes of distinct sensitivity to Al in response to Al exposure. Maize seedlings were grown axenically in nutrient solution and root exudates were collected along the whole seminal root axis for a short period (4 h) using a divided-root-chamber technique. In root exudates collected from 10-mm long root apices, citrate accounted for 67% of the total organic anions found, followed by malate (29%), trans-aconitate (3%), fumarate (<1%), and cis-aconitate (1%). Rates of citrate exudation from root apices of two genotypes with differential resistance to Al were consistently higher in the Al resistant one, differing by a factor of 1.7 – 3.0 across a range of external Al concentrations. Furthermore, relative Al resistance of eight maize genotypes correlated significantly well with their citrate exudation rate measured at 40 M Al. Higher exudation rates were accompanied by a less inhibited root elongation. The exudation of citrate along the longitudinal axis of fully developed seminal roots showed a particular pattern: citrate was exuded mainly in the regions of root apices, either belonging to the main root or to the lateral roots in the most basal part of the main root. The involvement of citrate in a mechanism of Al resistance is evaluated in terms of protection of the root from the effects of excess Al on root elongation and on nutrient uptake along a root axis showing distinct sites of citrate exudation.     

Deterministic cellular descendance and its relationship to the branching of plant organ axes

Summary. A double-wall map L-system, designated as S_5-5, was developed to simulate the cellular pattern found at the summit of shoot apices of Psilotum nudum. Commencing from a 3-sided autoreproductive founder cell, fives steps of simulation established a basic set of ten different cell types. Continuing the simulation beyond the fifth step revealed that, in addition to the regular production of new 3-sided cells, a group of autoreproductive 5-sided cells came into being. A close correspondence exists between the cells of the two-dimensional simulation and the two-dimensional cellular patterns found on the epidermis of the apices of Psilotum species. The 3-sided cells produced during the simulation correspond to the potentially organogenetic 3-sided cells that can be seen upon the apical surfaces. Successive generations of these 3-sided apical cells (which are actually 4-sided tetrahedral cells when viewed in three dimensions) and their immediate descendants are thought to be selected to organise the successive pairs of apices that bring about the repeated bifurcation of the Psilotum shoots. The 5-sided cells contribute to the cellular pavements which separate these pairs of organogenetic centres, each with their 3-sided apical cells. The cellular patterns simulated by the S_5-5 system may also correspond to the cellular patterns found on the surfaces of some other pteridophyte apices, including that of the rhizophores of Selaginella species. Tritiated-thymidine labelling of rhizophore apices revealed a group of nonproliferating cells that was associated with rhizophore bifurcation and which may correspond to a group of pavement cells. Nonproliferating cells, by regulating the siting of new organogenetic centres, may have evolved as an accompaniment to branching events such as the bifurcation of root and organ axes.Correspondence and reprints: School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, United Kingdom.     

Conditions for open-air outdoor culture of Dunaliella salina in southern Spain

The optimization of the operation, under the climatic conditions of southern Spain, of an experimental plant for -carotene production by Dunaliella has been pursued. The effects of mixing, culture depth, cell density and dilution cycles on -carotene and biomass productivity were studied under a semicontinuous culture regime in open tanks outdoors. Using 3 m²-surface containers, the highest productivity values, for both -carotene and biomass, were recorded with a flow rate of 0.55 m s^–1; 10 cm depth; 0.7 – 0.9 × 10⁶cell ml^–1, population density; and dilution cycles of two days. An average annual productivity of 1.65 g (dry wt) m^–2 d^–1 was estimated for Dunaliella biomass, being that for -carotene of about 0.1 g m^–2 d^–1. Under these optimized conditions, experiments have been carried out at the Cadiz Bay with 20 m²-surface tanks during a whole-year cycle. The results obtained have validated this location and the operating conditions established as being most appropriate for efficient mass production of -carotene rich D. salina.     

Kinetic and fluorometric behavior of a phenol fermentation

Summary The kinetic and fluorometric behavior ofPseudomonas putida (ATCC 17484) growing on phenol as the carbon limiting substrate was investigated. A model was developed to describe the behavior under dynamic conditions of flow or substrate concentration shift-up. Parameters contained in the model were estimated. It was observed that the model described the kinetic behavior reasonably well. Factors which affected the fluorescence output of the whole broth culture were also studied.     

Die Primärwurzel von Zea mays L.

Zusammenfassung An Gewächshauskulturen kann gezeigt werden, daß die Primärwurzel von Zea mays L. während der ganzen Lebensperiode der Pflanze perenniert. Da auch bei Phoenix dactylifera L. eine solche Tendenz zu erkennen ist, ist es wahrscheinlich, daß weitere Monocotylen diese Art der Bewurzelung zeigen. Es ließe sich daraus ein Maiswurzeltyp der heterogenen Radication bei den Monocotylen ableiten, bei dem primäres und sproßbürtiges Wurzelsystem dauernd nebeneinander bestehen.

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The primary root of Zea mays L.

Summary Until now it has been presumed that the primary root of Zea mays L. dies very soon after the formation of the seedling. On the basis of this presumption maize root-system development has been used as an example for monocotyledons (z.B. Troll, 1937; Rauh, 1950).After passing the seedling stage, monocotyledons are said to have shoot-born roots only. There are a few hints in the literature of exceptions to this principle known in the case of some palms (Falkenberg, 1876; Fitting, 1954; Kausch, unpublished). In this paper it is shown that the primary root of maize does not die off soon, but remains living during the entire vegetation period. Investigations were made within the greenhouse, where in the stage of flowering of the plants the primary root reached downwards into the soil as far as 1.60 m (see also Fig. 2). There are also some observations in the open field showing that here too the primary root remains living until the plant dies in autumn.Nevertheless Zea mays has sekundäre Homorhizie and heterogene Radication (Troll, 1949). However, there is surely a large group within the monocotyledons which is capable of keeping the primary root system along with shoot-born roots. Of this group it may be said that it is of the Mais-Wurzel-Typ.

     

Root characteristics of selected field crops: Data from the Wageningen Rhizolab (1990–2002)

Since being built in 1990, the rhizotron facility in Wageningen, the Wageningen Rhizolab, has been used for experiments on crops (e.g. Alfalfa, Brussels sprouts, common velvet grass, field bean, fodder radish, leeks, lupins, maize, potato, beetroot, ryegrass, spinach, spring wheat, winter rye and winter wheat). In the experiments, horizontal glass minirhizotron tubes combined with auger sampling were used to assess rooting characteristics. For this paper we took the root data from these experiments and looked for a general relationship between thermal time/time after planting and rooting depth, the velocity of the root front and root proliferation. For certain depths (fixed by the depth at which the horizontal minirhizotrons were installed) a simple linear regression was established between the average root number per cm² minirhizotron surface area and thermal time after planting. The compartments selected for each crop were those in which there had been a control treatment and/or in which conditions for rooting were considered to be optimal. We performed regression analyses per compartment and per depth, but only for the period after planting in which a linear increase of root numbers vs. thermal time was observed. After averaging the results, the regression procedure yielded two parameters of rooting for each crop: (a) the actual or thermal time at which the first root appeared at a certain depth and (b) the root proliferation rate after the first root had appeared. In this way, inherent crop differences in rooting behaviour (rooting depth and root proliferation) became apparent. For each crop, the velocity of the root front after planting could be established (calculated in cm(°C day)^–1). This parameter differed greatly between crops. Some crops (such as leeks and common velvet grass) explored the soil profile slowly: the root front moved at a velocity of only 0.07cm(°C day)^–1. Among the crops whose roots grew down much faster (0.18–0.26cm (°C day)^–1) were cereals and fodder radish. For a day with an average temperature of 15°C these rates would have corresponded with the root front travelling approximately 1–4cm per day. In the crops studied the root front velocity did not correlate with the root proliferation rate.