Root elongation rate is correlated with the length of the bare root apex of maize and lupin roots despite contrasting responses of root growth to compact and dry soils

Background

We investigated interacting effects of matric potential and soil strength on root elongation of maize and lupin, and relations between root elongation rates and the length of bare (hairless) root apex.

Methods

Root elongation rates and the length of bare root apex were determined for maize and lupin seedlings in sandy loam soil of various matric potentials (?0.01 to ?1.6 MPa) and bulk densities (0.9 to 1.5 Mg m^?3).

Results

Root elongation rates slowed with both decreasing matric potential and increasing penetrometer resistance. Root elongation of maize slowed to 10 % of the unimpeded rate when penetrometer resistance increased to 2 MPa, whereas lupin elongated at about 40 % of the unimpeded rate. Maize root elongation rate was more sensitive to changes in matric potential in loosely packed soil (penetrometer resistances <1 MPa) than lupin. Despite these differing responses, root elongation rate of both species was linearly correlated with length of the bare root apex (r² 0.69 to 0.97).

Conclusion

Maize root elongation was more sensitive to changes in matric potential and mechanical impedance than lupin. Robust linear relationships between elongation rate and length of bare apex suggest good potential for estimating root elongation rates for excavated roots.     

Root cap removal increases root penetration resistance in maize (Zea mays L)

The root cap assists the passage of the root through soil by means of its slimy mucilage secretion and by the sloughing of its outer cells. The root penetration resistance of decapped primary roots of maize (Zea mays L. cv. Mephisto) was compared with that of intact roots in loose (dry bulk density 1.0 g cm-3; penetration resistance 0.06 MPa) and compact soil (1.4 g cm-3; penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the impedance to root growth. Root elongation rate and diameter were the same for decapped and intact roots when the plants were grown in loose soil. In compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots, whilst the diameter was 30% larger. Root penetration resistances of intact and decapped seminal axis were 0.31 and 0.52 MPa, respectively, when the roots were grown in compacted soil. These results indicated that the presence of a root cap alleviates much of the mechanical impedance to root penetration, and enables roots to grow faster in compacted soils.     

Water stress induced by PEG decreases the maximum growth pressure of the roots of pea seedlings

Roots of plants growing in dry soil often experience large mechanical impedance because the decreased soil water content is associated with increased in soil strength. The combined effect of mechanical impedance and water stress hinders the establishment of seedlings in many soils, but little is known about the interaction between these two stresses. A method has been designed that, for the first time, measured the maximum axial force exerted by a root growing under controlled water stress. Using this technique the axial force exerted by a pea radicle was measured using a shear beam, while the seedling was suspended in an aerate solution of polyethylene glycol 20 000 at osmotic potentials between 0 and -0.45 MPa. The maximum growth force was then divided by the cross-sectional area of the root to give the maximum axial growth pressure. The value of maximum axial growth pressure decreased linearly from 0.66 and 0.35 MPa as the osmotic potentials of the solution of PEG decreased from 0 to -0.45 MPa. In dry soil, therefore, the maximum strength of soil that a root can penetrate is decreased because of the decrease in maximum growth pressure. The elongation rates of unimpeded roots were similar whether the roots were subject to either a matric potential in soil or to an osmotic potential in a solution of PEG.Key words: Pisum sativum L, pea, mechanical impedance, axial growth pressure, water stress, PEG 20 000.      

Effect of soil strength on root growth under different water conditions

The objective of this study was to determine the effects of soil water and soil strength on root growth in situations where the individual effects of both of these factors were important. Three grain legumes were grown from pre-germinated seeds for five days on 50-mm compacted columns of two major soils of Sri Lanka. Four or five levels of bulk density (1.1 to 1.8 Mg.m^–3) and five or six levels of matric potential (–0.02 to–2.0 MPa) were used.Soil strength and matric potential effects on root growth were independently significant for most crop and soil combinations. Under high (wet) matric potential (>–0.77 MPa) soil conditions, the effect of soil water on root growth was evident only in its effect on soil strength. Bulk density had a significant effect on root growth independent of soil strength and matric potential in three cases.For all crops and soils, root penetration was 80% of the maximum or greater when the average soil strength (soil water not limiting) was 0.75 MPa or less, and when the average matric potential (soil strength not limiting) was –0.77 MPa or greater (wetter). Root penetration was 20% of the maximum or less when the soil strength was greater than 3.30 MPa (soil water not limiting), and when matric potential (soil strength not limiting) was less than –3.57 MPa. The use of pre-germinated seeds, which contained imbibed water, combined with a lack of water loss from the closed chambers containing the plants is the probable cause for the very low (–3.57 MPa) matric potential that allowed root growth at 20% of the maximum.     

Soil strength and macropore volume limit root elongation rates in many UK agricultural soils

Background and Aims

Simple indicators of crop and cultivar performance across a range of soil types and management are needed for designing and testing sustainable cropping practices. This paper determined the extent to which soil chemical and physical properties, particularly soil strength and pore-size distribution influences root elongation in a wide range of agricultural top soils, using a seedling-based indicator.

Methods

Intact soil cores were sampled from the topsoil of 59 agricultural fields in Scotland, representing a wide geographic spread, range of textures and management practices. Water release characteristics, dry bulk density and needle penetrometer resistance were measured on three cores from each field. Soil samples from the same locations were sieved, analysed for chemical characteristics, and packed to dry bulk density of 1·0 g cm⁻³ to minimize physical constraints. Root elongation rates were determined for barley seedlings planted in both intact field and packed soil cores at a water content close to field capacity (–20 kPa matric potential).

Key Results

Root elongation in field soil was typically less than half of that in packed soils. Penetrometer resistance was typically between 1 and 3 MPa for field soils, indicating the soils were relatively hard, despite their moderately wet condition (compared with <0·2 MPa for packed soil). Root elongation was strongly linked to differences in physical rather than chemical properties. In field soil root elongation was related most closely to the volume of soil pores between 60 µm and 300 µm equivalent diameter, as estimated from water-release characteristics, accounting for 65·7 % of the variation in the elongation rates.

Conclusions

Root elongation rate in the majority of field soils was slower than half of the unimpeded (packed) rate. Such major reductions in root elongation rates will decrease rooting volumes and limit crop growth in soils where nutrients and water are scarce.     

Root elongation is restricted by axial but not by radial pressures: so what happens in field soil?

Root distribution determines largely the zone of soil that roots have access to for water and nutrient uptake, and is of great importance especially if water and fertilizer input is restricted. Mechanical impedance is the major limitation to root elongation in many field soils. Until now, experiments have focused largely on the axial resistance to root growth. In a fascinating study of the radial forces exerted by the roots of chickpea, root extension, diameter change, and the radial forces that axially unimpeded roots exert are reported: Kolb et al. (this volume) record radial stresses of about 0.3?MPa that are broadly consistent with cell turgor pressures, but, interestingly, find no restriction to axial elongation. This result is in marked contrast to large decreases in elongation of pea radicles resulting from much smaller axial pressures reported elsewhere in the literature (e.g., an 85?% decrease in root elongation in response to axial pressures of?<?0.1?MPa). The situation is different also from that in homogeneous soil, where root penetration resistance pressures of 0.4-1.0?MPa are typically required to halt root elongation. Soil structure and strength properties will determine the balance of axial and radial pressures on an individual root tip, and hence the root elongation response. It appears that a degree of radial confinement may help roots to extend axially into hard soil. This result also complements recent findings that in strong field soils the availability of soil macropores has a large influence on regulating the root-elongation rates of seedlings.     

Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils

Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.     

Response of microbial activity and community structure to decreasing soil osmotic and matric potential

Low soil water content (low matric potential) and salinity (low osmotic potential) occur frequently in soils, particularly in arid and semi-arid regions. Although the effect of low matric or low osmotic potential on soil microorganisms have been studied before, this is the first report which compares the effect of the two stresses on microbial activity and community structure. A sand and a sandy loam, differing in pore size distribution, nutrient content and microbial biomass and community structure, were used. For the osmotic stress experiment, salt (NaCl) was added to achieve osmotic potentials from ?0.99 to ?13.13 MPa (sand) and from ?0.21 to 3.41 MPa (sandy loam) after which the soils were pre-incubated at optimal water content for 10d. For the matric stress experiment, soils were also pre-incubated at optimal water content for 10d, after which the water content was adjusted to give matric potentials from ?0.03 and ?1.68 MPa (sand) and from ?0.10 to 1.46 MPa (sandy loam). After amendment with 2% (w/w) pea straw (C/N 26), soil respiration was measured over 14d. Osmotic potential decreased with decreasing soil water content, particularly in the sand. Soil respiration decreased with decreasing water potential (osmotic?+?matric). At a given water potential, respiration decreased to a greater extent in the matric stress experiment than in the osmotic stress experiment. Decreasing osmotic and matric potential reduced microbial biomass (sum of phospholipid fatty acids measured after 14 days) and changed microbial community structure: fungi were less tolerant to decreasing osmotic potential than bacteria, but more tolerant to decreasing water content. It is concluded that low matric potential may be more detrimental than a corresponding low osmotic potential at optimal soil water content. This is likely to be a consequence of the restricted diffusion of substrates and thus a reduced ability of the microbes to synthesise osmolytes to help maintain cell water content. The study also highlighted that it needs to be considered that decreasing soil water content concentrates the salts, hence microorganisms in dry soils are exposed to two stressors.     

Soil penetration resistance, a suitable soil property to account for variations in root elongation and branching

Background and Aims

Root system development is affected by soil conditions. The effects of bulk density, water content and penetration resistance on root development processes were investigated in peach trees.

Methods

Peach tree rootstocks were grown in various soil conditions, combining two bulk densities (1.2 and 1.5?g soil.cm^-3) and three water contents (0.14, 0.17 and 0.20?g.g^-1soil). Root parameters (tip diameter, length of apical unbranched zone, branching density and diameters of main and lateral roots) and plant growth (leaves, branches, trunk, root dry mass) were measured. Root growth processes (elongation, branching) were studied using relationships between root parameters.

Results

The proportion of biomass allocated to each plant compartment was similar whatever the soil conditions. Variations in root development were best explained by the variation in penetration resistance, rather than other soil properties. Increased soil penetration resistance reduced the root elongation rate, especially for thick roots. In addition, the branching pattern was affected. In soil with a high penetration resistance, the root system shape differs from a typical herringbone pattern.

Conclusions

These results allow quantification of the root system plasticity, and improve our understanding of the interactions between root development and soil properties.     

Contribution of root cap mucilage and presence of an intact root cap in maize (Zea mays) to the reduction of soil mechanical impedance

BACKGROUND AND AIMS: The impedance to root growth imposed by soil can be decreased by both mucilage secretion and the sloughing of border cells from the root cap. The aim of this study is to quantify the contribution of these two factors for maize root growth in compact soil. METHODS: These effects were evaluated by assessing growth after removing both mucilage (treatment I -- intact) and the root cap (treatment D -- decapped) from the root tip, and then by adding back 2 micro L of mucilage to both intact (treatment IM -- intact plus mucilage) and decapped (treatment DM -- decapped plus mucilage) roots. Roots were grown in either loose (0.9 Mg m(-3)) or compact (1.5 Mg m(-3)) loamy sand soils. Also examined were the effects of decapping on root penetration resistance at three soil bulk densities (1.3, 1.4 and 1.5 Mg m(-3)). KEY RESULTS: In treatment I, mucilage was visible 12 h after transplanting to the compact soil. The decapping and mucilage treatments affected neither the root elongation nor the root widening rates when the plants were grown in loose soil for 12 h. Root growth pressures of seminal axes in D, DM, I and IM treatments were 0.328, 0.288, 0.272 and 0.222 MPa, respectively, when the roots were grown in compact soil (1.5 Mg m(-3) density; 1.59 MPa penetrometer resistance). CONCLUSIONS: The contributions of mucilage and presence of the intact root cap without mucilage to the lubricating effect of root cap (percentage decrease in root penetration resistance caused by decapping) were 43 % and 58 %, respectively. The lubricating effect of the root cap was about 30 % and unaffected by the degree of soil compaction (for penetrometer resistances of 0.52, 1.20 and 1.59 MPa).     

The gradient between pre-dawn rhizoplane and bulk soil matric potentials, and its relation to the pre-dawn root and leaf water potentials of four species

Uptake of soil water by plants may result in significant gradients between bulk soil and soil in the vicinity of roots. Few experimental studies of water potential gradients in close proximity to roots, and no studies on the relationship of water potential gradients to the root and leaf water potentials, have been conducted. The occurrence and importance of pre-dawn gradients in the soil and their relation to the pre-dawn root and leaf water potentials were investigated with seedlings of four species. Pre-germinated seeds were grown without watering for 7 and lid in a silt loam soil with initial soil matric potentials of -0.02, -0.1 and -0.22 MPa. Significant gradients, independent of the species, were observed only at pre-dawn soil matric potentials lower than -0.25 MPa; the initial soil matric potentials were -0.1 MPa. At an initial bulk soil matric potential of -0.22 MPa, a steep gradient between bulk and rhizoplane soil was observed after 7 d for maize (Zea mays L. cv. Issa) and sunflower (Helianthus annuus L. cv. Nanus), in contrast to barley (Hordeum vulgare L. cv. Athos) and wheat (Triticum aestivum L. cv. Kolibri). Pre-dawn root water potentials were usually about the same as the bulk soil matric potential and were higher than the rhizoplane soil matric potential. Pre-dawn root and leaf water potentials tended to be much higher than rhizoplane soil matric potentials when the latter were lower than -0.5 MPa. It is concluded that plants tend to become equilibrated overnight with the wetter bulk soil or with wetter zones in the bulk soil. Plants can thus circumvent negative effects of localized steep pre-dawn soil matric potential gradients. This may be of considerable importance for water uptake and growth in drying soil.     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

Growth of seedling roots in response to external osmotic stress by polyethylene glycol 20,000

Seedling roots of ten plant species were grown in siliceous sand wetted with solutions of polyethylene glycol (PEG) of MW=20,000 with osmotic potentials of 0.0, ? 0.25, ? 0.5 and ? 1.0 MPa. After 48 h growth under controlled lighting, root elongation and root diameter were measured. Root elongation of all species was reduced by increasing levels of external osmotic stress. Dicotyledonous species were affected more than monocotyledons at potentials of ? 0.25 and ? 0.5 MPa but less at ? 1.0 MPa. Root diameters of all the species were thicker than those of the unstressed at potentials of ? 0.25 and ? 0.5 MPa. At a potential of ? 1.0 MPa the dicotyledons were still thicker, though not by as much as they were at ? 0.25 and ? 0.5 MPa. The monocotyledons, in contrast, were thinner at ? 1.0 MPa. There was a significant positive correlation (r=0.81, p <0.01) between root diameter and root elongation at ? 1.0 MPa potential. Species were ranked according to the relative root elongation (RRE) and relative root thickness (RRT) at the highest level of stress (? 1.0 MPa). In both rankings dicotyledonous species were in the top ranks and monocotyledous species were in lower positions. The results are compared with those for the elongation and thickening of roots growing against external mechanical stress obtained in a previous study. There were good correlations between the responses observed for the two types of external stress. The implications of these findings are discussed.     

Modelling the effect of soil moisture on germination and emergence of wheat and sugar beet with the minimum number of parameters

Soil moisture and temperature, sowing depth and penetration resistance affect the time and percentage of seedling emergence, which are crucial for the simulation of drought‐limited crop production. The aim of this research was to measure the effect of soil water potential on germination and emergence, shoot and root elongation rates (SER and RER) of two different seed/crop types. Sugar beet and durum wheat seeds were sown into two soils (clay and loam), submitted to five matric potentials (?0.01, ?0.1, ?0.2, ?0.4 and ?0.8 MPa) and incubated at constant temperature (25°C) and humidity. Cumulative count analysis was used to estimate parameters of the distribution of germination or emergence times for each box of beet or wheat seeds and to derive estimates for base potentials (ψ_b), hydrothermal times (H) and numbers of viable units. In a second experiment, NaCl solution was used to mimic the soil matric potentials to estimate potential RER and SER. Germination of sugar beet was slightly more sensitive to matric potential than durum wheat (ψ_b of ?1.13 and ?1.23 MPa, respectively). H_(g) was longer for sugar beet than for durum wheat (67 vs 47 MPa °Cd). For emergence ψ_b was similar for both seed types and soils but hydrothermal times (H_(e)) were 40 MPa °Cd higher for sugar beet than for wheat. Emergence was about 20 MPa °Cd earlier in loam than in clay. SER measured in soils were similar for both crops and for durum wheat it agreed with those determined in NaCl solution. RER and SER fell with decreasing osmotic potential to approximately 20% of their maximum values (1.03 mm h^?1 and 0.57 mm h^?1, respectively). Seedling viability decreased with decreasing matric potential and more in clay than in loam soil and more for sugar beet than durum wheat. Seed and soil aggregate size are discussed with respect to the effects of water diffusion and soil–seed contact on germination and emergence modelling.     

Refining and unifying the upper limits of the least limiting water range using soil and plant properties

The least limiting water range (LLWR) was introduced as an integrated soil water content indicator, measuring the impact of mechanical impedance, oxygen and water availability on water uptake and crop growth. However, a rigorous definition of the upper limit of the LLWR using plant physiological and soil physical concepts was not given. We introduce in this study an upper limit of the LLWR, based on soil physical and plant physiological properties. We further evaluate the sensitivity of this boundary to different soil and crop variables, and compare the sensitivity of the upper limit of the LLWR to previous definitions of soil water content at field capacity. The current study confirms that the upper limit of the LLWR can be predicted from knowledge of the soil moisture characteristic curve, plant root depth and oxygen consumption rate. The sensitivity analysis shows further that the upper limit of the LLWR approaches the volumetric soil water content at saturation when the oxygen consumption rate by plants becomes less than 2 µmol m^?3 s^?1. When plants are susceptible to aeration (e.g. potato and avocado), there is a big difference between the upper limit of the LLWR and the soil water content at field capacity, in particular for sandy soils. Results also show that the soil water content at aeration porosity corresponding to 10% cannot be considered as an appropriate upper limit of LLWR because it does not appropriately reflect the crop water requirements. Similar poor results are obtained when considering the soil water content at matric potential ?0.033 MPa or when defining the soil water content at field capacity based on drainage flux rate. It is observed that the upper limit of the LLWR is higher than either soil water content at ?0.033 MPa matric potential or soil water content at field capacity as based on drainage flux rate, especially in sandy soils.     

Root elongation of seedling peas through layered soil of different penetration resistances

Field soils contain localized zones of larger penetration resistance within peds and compacted layers, while cracks and biopores offer low resistance pathways to roots. Root responses to such localized conditions have not been investigated in detail. This study examined what happens to the root elongation rate when roots grew through a layer of hard soil into a layer of looser soil for a 4 day period. The experiment was performed twice; firstly with the shoot in continuous darkness, and secondly with it exposed to a day-night cycle to prevent etiolation of the shoot. Pea seedlings were grown in columns of a sandy loam soil which was packed to bulk densities of 0.85, 1.1, 1.3 or 1.4 Mg/m³ in the top layer and 0.85 Mg/m³ in the bottom layer. The root elongation rate in the top layer of 1.4 Mg/m³ soil (penetrometer resistance=1.8 MPa) was only 55% of the elongation rate in the top layer of 0.85 Mg/m³ soil (penetrometer resistance=0.06 MPa). The elongation rate of roots that had grown through the top layer of 1.4 Mg/m³ soil into the bottom layer of loose soil was reduced by some residual effect of the mechanical impedance. The root elongation rate in the bottom layer of loose soil decreased as the penetrometer resistance of the top layer of soil increased. The daily elongation rate of the roots in the bottom layer that had grown through the 1.4 Mg/m³ soil averaged only about 65% of the elongation rate of the roots that had grown through the 0.85 Mg/m³ soil. This residual effect of mechanical impedance on root elongation persisted for at least 2 days and was more severe in the day-night cycle experiment than in the dark experiment. These results have important implications for modelling root elongation in any soil in which the soil strength changes with distance or with time.     

Soil physical behaviour and crop responses to tillage in lowland rice soils of varying clay content

The influence of various tillage methods on two wetland rice soils in the Philippines is reported. The soils differed principally in clay content, 38% for the clay loam (clayey, mixed isohyperthermic Entic Hapludoll) while 56% for the clay (clayey, mixed noncalcareous, isohyperthermic Andaqueptic Haplaquoll). This had a marked effect on their response to tillage and varying water regime. The clay soil, under field conditions, showed little change in pore size distribution or soil water behaviour with different tillage methods. Crop (Rice, Oryza sativa L., var. IR20) yields were unaffected by tillage.In contrast, tillage effects were very marked in the clay loam soil, which consisted of a greenhouse and a field trial. In the greenhouse, which experienced severe dry periods, wet tillage not only increased the moisture retentivity but also the soil impedance at soil matric potential ()<–0.01 MPa. Seasonal average was <–1 MPa. Root length density decreased by 39% with dry tillage and by 56% with wet tillage compared with zero tillage. Grain yield however, did not vary with soil treatment. In the field, which experienced moderate dry spells, varied between –0.13 and –0.48 MPa. Root length density was significantly reduced at soil impedance >0.75 MPa. Wet tillage increased soil moisture storage which minimized the soil impedance during the dry cycle more effectively than did dry tillage. The crop performed best under wet tillage and least under zero tillage. Wet tillage in this soil was more effective under moderate than under severe water stress conditions.     

Root Caps and Rhizosphere

In this paper we discuss recent work on the physiological, molecular, and mechanical mechanisms that underlie the capacity of root caps to modulate the properties of the rhizosphere and thereby foster plant growth and development. The root cap initially defines the rhizosphere by its direction of growth, which in turn occurs in response to gradients in soil conditions and gravity. The ability of the root cap to modulate its environment is largely a result of the release of exudates and border cells, and so provides a potential method to engineer the rhizosphere. Factors affecting the release of border cells from the outer surface of the root cap, and function of these cells and their exudates in the rhizosphere, are considered in detail. Release of border cells into the rhizosphere depends on soil matric potential and mechanical impedance, in addition to a host of other environmental conditions. There is good evidence of unidentified feedback signals between border cells and the root cap meristem, and some potential mechanisms are discussed. Root border cells play a significant mechanical role in decreasing frictional resistance to root penetration, and a conceptual model for this function is discussed. Root and border cell exudates influence specific interactions between plant hosts and soil organisms, including pathogenic fungi. The area of exudates and border cell function in soil is an exciting and developing one that awaits the production of appropriate mutant and transgenic lines for further study in the soil environment.     

Root Caps and Rhizosphere

In this paper we discuss recent work on the physiological, molecular, and mechanical mechanisms that underlie the capacity of root caps to modulate the properties of the rhizosphere and thereby foster plant growth and development. The root cap initially defines the rhizosphere by its direction of growth, which in turn occurs in response to gradients in soil conditions and gravity. The ability of the root cap to modulate its environment is largely a result of the release of exudates and border cells, and so provides a potential method to engineer the rhizosphere. Factors affecting the release of border cells from the outer surface of the root cap, and function of these cells and their exudates in the rhizosphere, are considered in detail. Release of border cells into the rhizosphere depends on soil matric potential and mechanical impedance, in addition to a host of other environmental conditions. There is good evidence of unidentified feedback signals between border cells and the root cap meristem, and some potential mechanisms are discussed. Root border cells play a significant mechanical role in decreasing frictional resistance to root penetration, and a conceptual model for this function is discussed. Root and border cell exudates influence specific interactions between plant hosts and soil organisms, including pathogenic fungi. The area of exudates and border cell function in soil is an exciting and developing one that awaits the production of appropriate mutant and transgenic lines for further study in the soil environment.     

Do roots mind the gap?

Aims

Roots need to be in good contact with the soil to take up water and nutrients. However, when the soil dries and roots shrink, air-filled gaps form at the root-soil interface. Do gaps actually limit the root water uptake, or do they form after water flow in soil is already limiting?

Methods

Four white lupins were grown in cylinders of 20 cm height and 8 cm diameter. The dynamics of root and soil structure were recorded using X-ray CT at regular intervals during one drying/wetting cycle. Tensiometers were inserted at 5 and 18 cm depth to measure soil matric potential. Transpiration rate was monitored by continuously weighing the columns and gas exchange measurements.

Results

Transpiration started to decrease at soil matric potential ψ between ?5 kPa and ?10 kPa. Air-filled gaps appeared along tap roots between ψ?=??10 kPa and ψ?=??20 kPa. As ψ decreased below ?40 kPa, roots further shrank and gaps expanded to 0.1 to 0.35 mm. Gaps around lateral roots were smaller, but a higher resolution is required to estimate their size.

Conclusions

Gaps formed after the transpiration rate decreased. We conclude that gaps are not the cause but a consequence of reduced water availability for lupins.