Physics of Root Growth

THE elongation of a plant cell involves the yielding of the cell wall under the action of tensile stresses in the wall¹. The rate of elongation, R, can be expressed simply as R=mW, where m is the extensibility of thé cell wall material and W is the wall pressure. Changes in either m or W have been used to explain the effect of biochemical factors on the growth rate of plant cells^2,3. Cell growth is also affected by the physical environment and this becomes particularly important in the case of plant roots where soil water potential and the mechanical resistance of the soil to deformation can become rate controlling. Working with 3-5 day old radicles of Pisum sativum, growing in soil cores, we have obtained values of wall pressure in terms of these two properties and we find that the rate of root elongation can be described by a simple extensibility equation.     

The Dynamics of Root Growth: A Geometric Model

A new model for macroscopic root growth based on a dynamical Riemannian geometry is presented. Assuming that the thickness of the root is much less than its length, the model is restricted to growth in one dimension (1D). We treat 1D tissues as continuous, deformable, growing geometries for sizes larger than 1 mm. The dynamics of the growing root are described by a set of coupled tensor equations for the metric of the tissue and velocity field of material transport in non-Euclidean space. These coupled equations represent a novel feedback mechanism between growth and geometry. We compare 1D numerical simulations of these tissue growth equations to two measures of root growth. First, sectional growth along the simulated root shows an elongation zone common to many species of plant roots. Second, the relative elemental growth rate calculated in silico exhibits spatio-temporal dynamics recently characterized in high-resolution root growth studies but which thus far lack a biological hypothesis to explain them. In our model, these dynamics are a direct consequence of considering growth as both a geometric reaction–diffusion process and expansion due to a distributed source of new materials.     

Analysis of Root Growth by Impedance Spectroscopy (EIS)

Electrical impedance spectroscopy (EIS) is investigated as a non-destructive method for monitoring root growth of tomato. This paper aims to (i) review the basic principles of EIS applied to the characterisation of the different parts of the soil–root–stem-electrode continuum, (ii) experiment the validity of the relationship between root weight and root capacitance taking into account the influence of the soil and plant electrodes position, (iii) describe an EIS analysis of the root growth of tomato plants. All experiments were carried out in 50 dm³ containers either in hydroponics at 930 μS for the test of root fresh or dry weight and root capacitance relationships, or in a potting mix (oxisol) for electrode placement tests and EIS estimation of root growth. Electrical measurements of the soil–root–stem-electrode continuum were done with a two-electrode measuring system using unpolarisable Ag–AgCl electrodes. A ‘root cutting’ and a ‘progressively immersed root system’ experiments were carried out in order to validate the relationship between root capacitance and root mass at 1 kHz. The effects of soil electrode and plant electrode placement were also tested, pointing out the sensitivity of the method to the insertion height of the “plant electrode” into the stem. For the root growth experiment, Impedance Spectra (IS) measurements were made just before harvesting the roots for dry weight and length determination. Measurements were made 14, 22, 26 and 39 days after planting, until flowering. The IS of the soil–root–stem-electrode continuum was modelled by a lumped electric circuit consisting of a series resistor R ₀ for the soil and of four parallel resistance (R _i )-capacitance (C _i ) circuits for the other components of the circuit. The model had nine parameters whose values were estimated by Complex Nonlinear Least Squares curve fitting. A stepwise ascendant regression was used to identify the electrical parameters that better correlated with root dry mass or length increment: C ₃ and C ₄ were well correlated with root dry mass with a r ² of 0.975, whereas root length was explained by the combination of 1/R ₃, C ₃, 1/R ₂ and 1/R ₁ with a r ² of 0.986. This work may be considered as a new methodological contribution to the understanding of root electrical properties in the non-destructive diagnosis of root systems.     

Dependence of Root Cell Growth and Division on Root Diameter

Primary roots of 98 species from different families of monocotyledonous and dicotyledonous plants and adventitious roots obtained from bulbs and rhizomes of 24 monocot species were studied. Root growth rate, root diameter, length of the meristem and elongation zones, number of meristematic cells in a file of cortical cells, and length of fully elongated cells were evaluated in each species after the onset of steady growth. The mitotic cycle duration and relative cell elongation rate were calculated. In all species, the meristem length was approximately equal to two root diameters. When comparing different species, the rate of root growth increased with a larger root diameter. This was due to an increase in the number of meristematic cells in a row and, to a lesser degree, to a greater length of fully elongated cells. The duration of the mitotic cycle and the relative cell elongation rate did not correlate with the root diameter. It is suggested that the meristem size depends on the level of nutrient inflow from upper tissues, and is thereby controlled during further growth.     

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     

Shoot and Root Growth of Lettuce Seedlings Following Root Pruning

Hydroponically-grown lettuce seedlings with 13 to 18 primarylateral roots were root pruned in one of four ways; the rootapices were removed from the main root only (1) or from allthe root membranes (2), or half the total root system was removedwith the remaining apices left intact (3) or removed (4). Duringthe following 8 d the rate of lateral root production on prunedplants increased, decreased, and then increased again relativeto the unpruned control. Conversely, the rate of increase intotal root length decreased, then increased, and if all theroot apices were removed, declined again, prior to increasingon day 8. These changes in the rates of lateral root productionand growth resulted in similar, but less pronounced, patternsof change in the total root length and the total number of lateralroots with time. The changes in total lateral root productionwere related to differences in the rates of primary, secondaryand tertiary root emergence. The shoot d. wt of the most severely root pruned seedlings (treatment4) fell below that of the control 4 d after pruning and remainedlower than the control on day 14, whereas the root d. wt hadrecovered to the control level by day 6. The root: shoot d.wt ratio, which was reduced by root pruning, rose above thatof the control on days 6 and 8. Lactuca sativa L., lettuce, root pruning, root growth, lateral root, nutrient solution     

The Effect of Coumarin on Root Growth and Root Histology

The effect of coumarin on the root growth was studied on roots from intact plants, isolated roots and isolated elongating zones. All material was cultivated aseptically. A new method was developed for sterile culture of intact plants in flowing nutrient medium. The effects on cell division and cell elongation were studied separately. An effect on both these processes can be established at all concentrations that affect the root growth. The concentration-growth curve has an “all-or-none” appearance. Coumarin inhibits the transverse divisions in all cell layers; the perivascular layers seem to be more sensitive. Also the mitotic activity that is involved in the initiation of laterals is inhibited. The longitudinal divisions within the stele are enhanced. Coumarin decreases the cell length in all cell layers, most likely with greater relative sensitivity in the perivascular layers. Studies on the time course of cell elongation in both attached corn roots and isolated elongating zones reveal that the decrease in cell length is caused exclusively by a decrease in the maximal rate of elongation, whereas the duration of the elongation is unchanged. With each decrease of the cell length, the cell diameter is increased. The two changes are intimately connected within the greater part of the active region of concentration. Studies on the time course of the radial expansion in isolated elongating zones show a strict connection in time between cell elongation and radial expansion. The radial expansion leads to unchanged or increased cell volume at most concentrations and for most cell types. Coumarin causes an inhibition of the longitudinally directed processes and a stimulation of the radially directed ones. This is interpreted as indicating that the formative system is disengaged or reorientated, i.e., the polarity of the cells is changed. Through experiments partly with isolated elongating zones and partly by disruption of the linear phase by means of mannitol, the inhibitory effect of coumarin could be localized to the first non-linear phase of the elongation. The results were compared with earlier findings in the literature. The microtubuli are proposed as a conceivable main Component in the formative system common to both cell division and cell elongation. These are assumed to be affected by changes in the SH/SS balance produced by coumarin.     

Root Growth Inhibitors from Root Cap and Root Meristem of Zea mays L.

A micro-assay based on the growth inhibition of root segmentsof the seminal roots of Zea mays has been used to investigatethe root-growth-inhibiting substances in root caps and meristemsrespectively of the roots of Zea mays. This micro-assay is sensitiveto 50 pg of IAA or less. Paper chromatography of the acid fractionof methanolic extracts shows the presence of one main inhibitorin root caps and a different main inhibitor in root meristems.Neither is IAA, whose presence in meristems is sometimes indicatedby small inhibitions (or stimulations) at the characteristicRf of IAA. A Commelina leaf-epidermis assay shows the presenceof one stomata-closing ABA-like substance in root caps and onein meristems, one corresponding in Rf to the main root-growthinhibitor from the root cap. The implications of these findingsfor the geotropic responses of roots is briefly discussed.     

Root Growth of Bulbous Species during Winter

Bulbs of Allium moly, Scilla siberica, Triteleia uniflora, Tulipakaufmanniana and Narcissus lobularis were planted in a rhizotronand root growth was monitored weekly for an 18 week period.All the genera produced roots until the soil temperature droppedto 3 °C. Root outgrowth from the bulbs and growth of emergedroots then ceased or was strongly inhibited in N. lobularis,S. siberica and T. uniflora but not in the other genera. Mostbulbs resumed root elongation when the soil temperature hadwarmed to 4 °C. Root emergence and continued growth occurredfor a longer period during the winter than has generally beenassumed. Allium moly, Scilla siberica, Triteleia uniflora, Tulipa kaufmanniana, Narcissus lobularis, rhizotron, bulb, root growth     

不同供水条件下水稻幼苗根系形成的遗传分析

利用分子标记图谱对溶液培养与旱作培养（纸培养）下的水稻（Oryza sativa L.)幼苗的种子根与最长不定根长,不定根数,总根干重,根冠比等性状进行了基因定位与遗传分析。4种参数共检测到6个数量性状位点（quantitative trait loci,QTLs)与22对上位性互作位点,其中溶液培养中的最长不定根长,总根干重和旱作培养中的总根干重检测到的QTLs位点对总变异的贡献率分别为20％,23％和13％左右;旱作培养中的最长不定根长,不定根数,根冠比和溶液培养中的根冠比仅检测到上位性位点,对表型变异的贡献率在12％－61％之间,溶液培养与旱作条件下没有一个或一对检测到的QTL或互作位点完全相同,提示溶液培养和旱作条件下影响幼苗根系生长的遗传机制差异,上位性作用对旱作培养条件下的根生长具重要影响。     

Primary Root Growth and the Pattern of Root Apical Meristem Organization are Coupled

Root apical meristems (RAMs) in dicotyledonous plants have two organizational schemes; closed (with highly organized tiers) and open (tiers lacking or disorganized). These schemes are commonly believed to remain unchanged during the growth of the root axis. Individual roots are commonly thought to have indeterminate growth. We challenge these two generalizations through the study of five species with closed apical organization: Clarkia unguiculata L., Oxalis corniculata L., Dianthus caryophyllus L., Blumenbachia hieronymi Urb., and Salvia farinaceae Benth. cv. “Strata”. These roots have phased growth patterns where early growth is followed by deceleration, after which the initial cells stop dividing, elongation ceases, and the root reaches its determinate length. At or before reaching determinacy, the root apical meristem stops maintaining its closed organization and becomes less organized. These observations will be placed in context with observations from the literature to suggest two new generalizations, namely, that apical organization does change over the growth phases of roots, and that roots are determinate.     

A Periscope Method for Observing Root Growth and Distribution in Field Soil

This paper describes a technique for observing root growth inthe field using glass tubes and a periscope. Roots of two crops(winter wheat and millet) were studied in situ with the periscopeand the results compared with those obtained from washed soilsamples. Generally, both techniques gave similar patterns ofgrowth and distributions of roots although the periscope measurements,when compared with washed soil samples, tended to underestimateroot density close to the soil surface and overestimate densitieslower in the profile. Both methods allowed differences in theroot systems of irrigated and unirrigated crops to be distinguishedbut the periscope method was considerably faster     

Root Growth, Secondary Root Formation and Root Gravitropism in Carotenoid-deficient Seedlings of Zea mays L.

The effect of ABA on root growth, secondary-root formation androot gravitropism in seedlings of Zea mays was investigatedby using Fluridone-treated seedlings and a viviparous mutant,both of which lack carotenoids and ABA. Primary roots of seedlingsgrown in the presence of Fluridone grew significantly slowerthan those of control (i.e. untreated) roots. Elongation ofFluridone-treated roots was inhibited significantly by the exogenousapplication of 1 mM ABA. Exogenous application of 1 µMand 1 nM ABA had either no effect or only a slight stimulatoryeffect on root elongation, depending on the method of application.The absence of ABA in Fluridone-treated plants was not an importantfactor in secondary-root formation in seedlings less than 9–10d old. However, ABA may suppress secondary-root formation inolder seedlings, since 11-d-old control seedlings had significantlyfewer secondary roots than Fluridone-treated seedlings. Rootsof Fluridone-treated and control seedlings were graviresponsive.Similar data were obtained for vp-9 mutants of Z. mays, whichare phenotypically identical to Fluridone-treated seedlings.These results indicate that ABA is necessary for neither secondary-rootformation nor for positive gravitropism by primary roots. Zea mays, gravitropism, carotenoid-deficient, Fluridone, root growth, vp-9 mutant     

Root Growth at the Cellular Level in Plants of Different Species: Comparative Analysis

Russian Journal of Plant Physiology - Analysis of root growth parameters was carried out on the seedlings of 53 monocotyledonous and 78 dicotyledonous plant species. Daily increases of the...