Root:soil adhesion in the maize rhizosphere: the rheological approach

This study was designed to investigate the strength of attachment of plant seedling roots to the soil in which they were grown. The study also assessed the effects of differing soil textures and differing soil matric potentials upon the strength of the root:soil attachment. A device for growing roots upon a soil surface was designed, and was used to produce roots which were attached to the soil. In order to quantify root:soil adhesion, roots of maize seedlings, grown on the soil surface, were subsequently peeled off using a universal test machine, in conjunction with simultaneous time-lapse video observation. To clarify the partitioning of energy in the root:soil peeling test, separate mechanical tests on roots, and on two adherent remoulded topsoil balls were also carried out. The seedling root was characterised by a low bending stiffness. The energy stored in bending was negligible, compared to the root:soil adhesion energy. The mechanical properties of two adherent remoulded topsoil balls were a decrease of the soil:soil adhesion energy as the soil:soil plastic energy increased. These two parameters were therefore interdependent. Using a video-camera system, it was possible to separate the different processes occurring during the root:soil peeling test, in particular, the seed:soil adhesion and the root:soil soil adhesion. An interpretation of the complex and variable force:displacement curves was thus possible, enabling calculation of the root:soil interfacial rupture energy. At a given suction (10 kPa), the results of the peeling test showed a clear soil texture effect on the value of the root:soil interfacial rupture energy. In contrast, for the same silty topsoil, the effect of the soil water suction on the value of the interfacial rupture energy was very moderate. The root:soil interfacial rupture energy was controlled mainly by a product of microscopic soil specific surface area and the macroscopic contact surface area between the root and the soil. Biological and physical interactions contributing to root:soil adhesion such as root:soil interlocking mechanics were also analysed and discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of soil aggregate size on early shoot and root growth of maize (Zea mays L.)

Differences in plant growth arising from differences in aggregate size in the seedbed are normally atributed to limitations in nutrient or water supply during the early growth period. This study was initiated to determine if these were the only mechanisms by which aggregate size influences plant response. Four different aggregate size fractions (less than 1.6 mm, 1.6 to 3.2 mm, 3.2 to 6.4 mm and 6.4 to 12.8 mm diameter) were sieved from a silt loam soil. Nutrients were added to the soil and maize was grown in the aggregates for eighteen days after seedling emergence. Soil matric potential was maintained between — 3 and −20 kPa. Shoot dry weight declined by 18% as aggregate size increased from less than 1.6 mm to 1.6–3.2 mm. There was little further decline as aggregate size increased to 6.4–12.8 mm. Final leaf area showed a similar decline. The availability of nutrients or water were not limiting. Total root length in the coarsest aggregate system was less than 60% of that in the finest system. Main axes of seminal and nodal roots were longer in the coarser aggregate systems, the length of primary laterals was not affected, and length of secondary laterals was lower in the coarser systems. A greater proportion of the roots penetrated the larger aggregates than the smaller aggregates; however, the larger aggregates offered greater resistance to penetration by a rigid micropenetrometer (150 μ diameter probe). Diameter of the main axes roots were greatest in the largest two aggregate fractions. it is speculated that a combination of increased endogenous ethylene in roots in the finest aggregate system due to entrapment by water and increased mechanical resistance in the coarsest aggregate system accounts for the observed effects on root norphology.     

Root tensile strength and root distribution of typical Mediterranean plant species and their contribution to soil shear strength

In Mediterranean environments, gully erosion is responsible for large soil losses. It has since long been recognized that slopes under vegetation are much more resistant to soil erosion processes compared to bare soils and improve slope stability. Planting or preserving vegetation in areas vulnerable to erosion is therefore considered to be a very effective soil erosion control measure. Re-vegetation strategies for erosion control rely in most cases on the effects of the above-ground biomass in reducing water erosion rates, whereas the role of the below-ground biomass is often neglected or underestimated. While the above-ground biomass can temporally disappear in semi-arid environments, roots may still be present underground and play an important role in protecting the topsoil from being eroded. In order to evaluate the potential of plant species growing in Mediterranean environments to prevent shallow mass movements on gully or terrace walls, the root reinforcement effect of 25 typical Mediterranean matorral species (i.e. shrubs, grasses herbs, small trees) was assessed, using the simple perpendicular model of Wu et al. (Can Geotech J 16:19–33, 1979). As little information is available on Mediterranean plant root characteristics, root distribution data were collected in SE-Spain and root tensile strength tests were conducted in the laboratory. The power root tensile strength–root diameter relationships depend on plant species. The results show that the shrubs Salsola genistoides Juss. Ex Poir. and Atriplex halimus L. have the strongest roots, followed by the grass Brachypodium retusum (Pers.) Beauv. The shrubs Nerium oleander L. and the grass Avenula bromoides (Gouan) H. Scholz have the weakest roots in tension. Root area ratio for the 0–0.1 m topsoil ranges from 0.08% for the grass Piptatherum miliaceum (L.) Coss to 0.8% for the tree Tamarix canariensis Willd. The rush Juncus acutus L. provides the maximum soil reinforcement to the topsoil by its roots (i.e. 304 kPa). Grasses also increase soil shear strength significantly (up to 244 kPa in the 0–0.1 m topsoil for Brachypodium retusum (Pers.) Beauv.). The shrubs Retama sphaerocarpa (L.) Boiss. and Anthyllis cytisoides L. are increasing soil shear strength to a large extent as well (up to 134 and 160 kPa respectively in the 0–0.10 m topsoil). Whereas grasses and the rush Juncus acutus L. increase soil shear strength in the topsoil (0–0.10 m) to a large extent, the shrubs Anthyllis cytisoides (L.), Retama sphaerocarpa (L.) Boiss., Salsola genistoides Juss. Ex Poir. and Atriplex halimus L. strongly reinforce the soil to a greater depth (0–0.5 m). As other studies reported that Wu’s model overestimates root cohesion values, reported root cohesion values in this study are maximum values. Nevertheless, the calculated cohesion values are used to rank species according to their potential to reinforce the soil.     

Root growth as influenced by aggregate size

Summary This study examined the effects of aggregate size on root impedance and developed an equation to describe the root pressure necessary to avoid deflection around an aggregate. This critical root pressure was predicted to increase with increasing aggregate size, decreasing root diameter, and decreasing deflection angle. In growth chamber experiments, maize (Zea mays L.) seedlings were grown in A horizon material of Groseclose silt loam (Clayey, mixed, mesic, Typic Hapludult). The soil had been moist sieved into different aggregate sizes (0–1, 1–2, 2–3, and 3–6 mm diameter). The larger aggregates did constitute a slight root impedance as roots were deflected around them. Diameters of roots grown in 3–6 mm aggregates increased significantly, whereas root lengths were not always signficantly decreased. The smaller aggregates did not impede root growth and were readily displaced by roots. Large aggregates were more of an impedance to lateral roots than to main axes.     

Influence of root density on the critical soil water potential

Estimation of root water uptake in crops is important for making many other agricultural predictions. This estimation often involves two assumptions: (1) that a critical soil water potential exists which is constant for a given combination of soil and crop and which does not depend on root length density, and (2) that the local root water uptake at given soil water potential is proportional to root length density. Recent results of both mathematical modeling and computer tomography show that these assumptions may not be valid when the soil water potential is averaged over a volume of soil containing roots. We tested these assumptions for plants with distinctly different root systems. Root water uptake rates and the critical soil water potential values were determined in several adjacent soil layers for horse bean (Vicia faba) and oat (Avena sativa) grown in lysimeters, and for field-grown cotton (Gossypium L.), maize (Zea mays) and alfalfa (Medicago sativa L.) crops. Root water uptake was calculated from the water balance of each layer in lysimeters. Water uptake rate was proportional to root length density at high soil water potentials, for both horse bean and oat plants, but root water uptake did not depend on root density for horse bean at potentials lower than −25 kPa. We observed a linear dependency of a critical soil water potential on the logarithm of root length density for all plants studied. Soil texture modified the critical water potential values, but not the linearity of the relationship. B E Clothier Section editor     

Root effects on soil water and hydraulic properties

Plants can affect soil moisture and the soil hydraulic properties both directly by root water uptake and indirectly by modifying the soil structure. Furthermore, water in plant roots is mostly neglected when studying soil hydraulic properties. In this contribution, we analyze effects of the moisture content inside roots as compared to bulk soil moisture contents and speculate on implications of non-capillary-bound root water for determination of soil moisture and calibration of soil hydraulic properties. In a field crop of maize (Zea mays) of 75 cm row spacing, we sampled the total soil volumes of 0.7 m × 0.4 m and 0.3 m deep plots at the time of tasseling. For each of the 84 soil cubes of 10 cm edge length, root mass and length as well as moisture content and soil bulk density were determined. Roots were separated in 3 size classes for which a mean root porosity of 0.82 was obtained from the relation between root dry mass density and root bulk density using pycnometers. The spatially distributed fractions of root water contents were compared with those of the water in capillary pores of the soil matrix. Water inside roots was mostly below 2–5% of total soil water content; however, locally near the plant rows it was up to 20%. The results suggest that soil moisture in roots should be separately considered. Upon drying, the relation between the soil and root water may change towards water remaining in roots. Relations depend especially on soil water retention properties, growth stages, and root distributions. Gravimetric soil water content measurement could be misleading and TDR probes providing an integrated signal are difficult to interpret. Root effects should be more intensively studied for improved field soil water balance calculations. Presented at the International Conference on Bioclimatology and Natural Hazards, Pol’ana nad Detvou, Slovakia, 17–20 September 2007.     

Use of a microtensiometer technique to study hydraulic lift in a sandy soil planted with pearl millet (Pennisetum americanum [L.] Leeke)

A pot experiment was carried out with pearl millet (Pennisetum americanum [L.] Leeke) growing in a sandy soil in which the upper (topsoil) and lower (subsoil) parts of the pots were separated by a perlite layer to prevent capillary water movement. Using microtensiometers a study was made to establish whether it was possible to measure hydraulic lift by which the upper part of the soil was rewetted when water was supplied exclusively to the lower part of the soil.Hydraulic lift occurred during the first seven days of the period of measurement, with a maximum water release to the soil of 2.7 Vol. % during one night (equivalent to 10.8 mL water in the top 10 cm of the soil profile). This magnitude was obtained at very high root length densities, so that water release from the roots would be expected to be much smaller under field conditions.Hydraulic lift ceased when the soil matric potential in the topsoil dropped below-10 kPa at the end of the light period and could not be re-established, neither by extending the dark period, nor after rewatering the topsoil. The disappearance of hydraulic lift could be explained in part through osmotic adaptation of plant roots and, thus prevention of water release from the roots in the topsoil. It is concluded that hydraulic lift may affect nutrient uptake from drying topsoil by extending the time period favourable for uptake from the topsoil.     

Effect of cyclic wetting and drying of a soil on root hair growth of maize roots

The objective of this study was to investigate the effect of cyclic soil wetting and drying on maize (Zea mays L.) root hair growth. Three soils, Chalmers silty clay loam (Typic Haplaquolls), Raub silt loam (Aquic Argiudolls) and Aubbeenaubbee sandy loam (Aric Ochraqualfs) and two soil moisture contents, −175 (M₀) and −7.5 kPa (M₁), were used to study root hair growth in a controlled-climate chamber. Increasing soil moisture after 7d from M₀ and M₁ resulted in a cessation of root hair growth behind the root cap while drying the soil after 7d from M₁ and M₀ promoted root hair growth on new but not old or existing roots. By maintaining liquid continuity under cyclic wetting and drying of a soil, root hairs may be of far greater significance to the nutrition of the plant than originally thought. Journal Paper No. 11023, Purdue Univ. Agric. Exp. Stn., W. Lafayette, IN 47907. Contribution from the Dep. of Agron.     

Influence of maize root mucilage on soil aggregate stability

This study was undertaken to determine the effects of root exudates on soil aggregate stability. Root mucilage was collected from two-month old maize plants (Zea mays L.) Mucilage and glucose solutions were added at a rate of 2.45 g C kg⁻¹ dry soil to silty clay and silt loam soils. Amended soils, placed in serum flasks, were incubated for 42 d with a drying-wetting cycle after 21 d. Evolved CO₂ was measured periodically as well as the water-stable aggregates and soluble sugar and polysaccharide content of the soil. In mucilage-amended soils CO₂ evolution started with a lag phase of 2–3 days, which was not observed in glucose-amended soils. There was then a sharp increase in evolved CO₂ up to day 7. During the second incubation period there were only small differences in evolved C between treatments. Incorporation of mucilage in both soils resulted in a spectacular and immediate increase in soil aggregate stability. Thereafter, the percent of water-stable aggregates quickly decreased parallel to microbial degradation. On completion of the incubation, aggregate stability in the silty clay soil was still significantly higher in the presence of mucilage than in the control. This work supports the assumption that freshly released mucilage is able to stick very rapidly to soil particles and may protect the newly formed aggregates against water destruction. On the silty clay, microbial activity contributes to a stabilization of these established organo-mineral bounds.     

Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem

The radial movement of cis-abscisic acid (ABA) has been investigated in young excised roots of Zea mays L. and Helianthus annuus L. which were grown hydroponically. In addition to the symplastic path, ABA was largely translocated across the root apoplast by solvent drag with the water in the transpiration stream. On the apoplastic path ABA may even cross the endodermis. Depending on the ABA concentration of the medium (range: 5–500 nM) and in the root apoplast, the solvent-drag component of the flow of ABA counteracted the dilution of ABA in the xylem caused by transpirational water flow. Acidification of the rhizosphere and of the root apoplast increased the apoplastic transport component. In sunflower, the apoplastic flow of ABA was significantly weaker than in maize roots. This was also indicated by the larger apparent reflection coefficient (σ_ABA) of sunflower roots for ABA (sunflower: σ_ABA = 0.97 ± 0.02, n = 6 roots; maize: σ_ABA = 0.68 ± 0.06, n = 6 roots; ±SD). For both species, σ_ABA was smaller than unity. Root reflection coefficients were affected by factors such as pH, ABA concentration of the medium, and by the suction force applied to excised root systems. Due to the complex composite structure of the permeation barrier in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected the absolute value of the reflection coefficient, σ_ABA has been termed `apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) modify the intensity of the root-to-shoot signal which is influenced by an apoplastic bypass flow of ABA. The latter may be substantially affected by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper. Received: 25 February 1998 / Accepted: 16 July 1998     

Effects of soil structural discontinuity on root and shoot growth and water use of maize

The role of roots penetrating various undisturbed soil horizons beneath loose layer in water use and shoot growth of maize was evaluated in greenhouse experiment. 18 undisturbed soil columns 20 cm in diameter and 20 cm in height were taken from the depths 30–50 cm and 50–70 cm from a Brown Lowland soil, a Pseudogley and a Brown Andosol (3 columns from each depth and soil). Initial resistance to penetration in undisturbed soil horizons varied from 2.5 to 8.9 MPa while that in the loose layer was 0.01 MPa. The undisturbed horizons had a major effect on vertical arrangement of roots. Root length density in loose layer varied from 96 to 126 km m^-3 while in adjacent stronger top layers of undisturbed horizons from 1.6 to 20.0 km m^-3 with higher values in upper horizons of each soil. For specific root length, the corresponding ranges were 79.4–107.7 m g^-1 (on dry basis) and 38.2–63.7 m g^-1, respectively. Ratios of root dry weight per unit volume of soil between loose and adjacent undisturbed layers were much lower than those of root length density indicating that roots in undisturbed horizons were produced with considerably higher partition of assimilates. Root size in undisturbed horizons relative to total roots was from 1.1 to 38.1% while water use from the horizons was from 54.1 to 74.0%. Total water use and shoot growth were positively correlated with root length in undisturbed soil horizons. There was no correlation between shoot growth and water use from the loose layers.     

Root distributions in a Grevillea robusta-maize agroforestry system in semi-arid Kenya

Limited knowledge of root distributions in agroforestry systems has resulted in assumptions that various tree species are more suited to agroforestry than others, because they are presumed to have few superficial lateral roots. This assumption was tested for Grevillea robusta when grown with maize (Zea mays) in an agroforestry system in a semi-arid region of Kenya. At a site with a shallow soil, root lengths of both species between the soil surface and bedrock were quantified by soil coring, at intervals over four cropping seasons, in plots containing sole stands and mixtures of the trees and crop; the trees were 4–6 years old and they were severely pruned before the third season. Profiles of soil water content were measured using a neutron probe. Prior to pruning of the trees, recharge of soil water below the deepest maize roots did not occur, resulting in significant (P<0.05) suppression of maize root lengths and downward root growth. Maximum root length densities for both species occurred at the top of the soil profile, reaching 1.1–1.7 cm cm^-3 for G. robusta, but only 0.5 cm cm^-3 for maize grown with trees. Root populations in mixed plots were dominated by G. robusta at all times, all depths and all distances from trees and maize and, thus, there was no spatial separation of the rooting zones of the trees and crop. Competition between G. robusta and maize for soil water stored near the surface was unavoidable, although pruning reduced its impact; complementary use of water by the trees and crop would only have been possible if alternative sources of water were available. This revised version was published online in June 2006 with corrections to the Cover Date.     

Abscisic acid and hydraulic conductivity of maize roots: a study using cell- and root-pressure probes

Using root- and cell-pressure probes, the effects of the stress hormone abscisic acid (ABA) on the water-transport properties of maize roots (Zea mays L.) were examined in order to work out dose and time responses for root hydraulic conductivity. Abscisic acid applied at concentrations of 100–1,000 nM increased the hydraulic conductivity of excised maize roots both at the organ (root Lp_r: factor of 3–4) and the root cell level (cell Lp: factor of 7–27). Effects on the root cortical cells were more pronounced than at the organ level. From the results it was concluded that ABA acts at the plasmalemma, presumably by an interaction with water channels. Abscisic acid therefore facilitated the cell-to-cell component of transport of water across the root cylinder. Effects on cell Lp were transient and highly specific for the undissociated (+)-cis-trans-ABA. The stress hormone ABA facilitates water uptake into roots as soils start drying, especially under non-transpiring conditions, when the apoplastic path of water transport is largely excluded. Received: 26 February 2000 / Accepted: 17 August 2000     

Drought adaptation of field grown wheat in relation to soil physical conditions

In order to investigate the effects of soil texture on possible non-hydraulic signals under field conditions, spring wheat plants (Triticum aestivum L. cv. Cadensa) grown in sand and loam soils and with a well developed root system were exposed to slow soil drying in the late vegetative stage of growth. Soil water potential and content were measured daily at different depths and plant responses were measured in flag leaves. When the average soil water potential in the top soil layers (0–25 cm depth in sand and 0–45 cm depth in loam) dropped to –60 or –70 kPa and the lower soil layers were still at field capacity, morning xylem [ABA] (0.03–0.04 vs. 0.06–0.08 mmol m-3) and midday leaf ABA concentration increased (250–300 vs. 400–450 ng/g DW) and leaf conductance decreased relatively to well-watered (control) plants (0.75–0.88 vs. 0.64–0.70 mol m-2 s-1). These responses took place before any decrease in leaf water potential occurred as compared with control plants, indicating that they were triggered by root-borne signals due to reduced root water status in the top soil layers. At this stage the soil water content was as low as 6% by volume, the fraction of roots in ‘wet’ soil was 0.12 and relative available soil water was 45% in sand and still high 20%, 0.48 and 70%, respectively, in loam of the whole soil profile indicating that roots were responding to soil water availability and not soil water content at a certain evaporative demand. In addition, similar responses occurred at high and low evaporative demands (3.4–5.2 vs. 0.6–4.0 mm/day of potential evapotranspiration). This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of constant and fluctuating soil temperature on growth,development and nutrient uptake of maize seedlings

Summary The effect of constant and fluctuating soil temperature and two soil moisture regimes on the growth, development, transpiration and nutrient uptake by maize seedlings was studied in a greenhouse investigation. The constant root temperatures were maintained at 30, 34, 35, 36, 37, and 38°C for both 250 and 750 cm of soil moisture suctions. The fluctuating root temperature, for 250 cm of soil moisture suction only, of 30–35, 30–39, 30–40, 30–45 and 30–48°C were maintained to simulate the soil temperature regime under field conditions. The constant root temperature of 35°C and fluctuating temperature between 30–40°C significantly decreased the shoot and root growth and transpiration rate. On the average, there was 1.3 and 0.7 g decrease in fresh shoot weight and 0.36 and 0.30 g in fresh root weight per degree increase in root temperature for 250 and 750 soil moisture suction, respectively. In general, the effect of high soil moisture suction on maize seedlings was more severe when at high root temperature. The shoot and root concentration of N, P, and K decreased while that of B increased with increase in root temperature. The root concentration of Zn also decreased with increase in root temperature.     

Root distribution and morphology of maize seedlings as affected by tillage and fertilizer placement

Suboptimal soil conditions are known to result in poor early growth of maize (Zea mays L.) in no-tillage (NT) systems in contrast with conventional tillage (CT) systems. However, most studies have generally focused on maize roots at later growth stages and/or do not give details on root morphology. In a 2-year field study at two locations (silt loam and loam soils) in the Swiss midlands, we investigated the impacts of tillage intensity, NT vs. CT, and NP-fertilizer sidebanding on the morphology, vertical and horizontal distribution, and nutrient uptake of maize roots at the V6 growth stage. The length density (RLD) and the length per diameter-class distribution (LDD) of the roots were determined from soil cores taken to a depth of 0.5 m and at distances of 0.05 and 0.15 m from both sides of the maize row. The temperature of the topsoil was lower, and the bulk density and penetration resistance were greater in the topsoil of NT compared with CT. The growth and the development of the shoot were slower in NT. RLD was greater and the mean root diameter smaller in CT than in NT, while the vertical and horizontal distribution of roots did not differ between CT and NT. RLD increased in the zone enriched by the sidebanded fertilizer, independent of the tillage system, but LDD did not change. The poorer growth of the roots and shoots of maize seedlings was presumably caused by the lower topsoil temperature in NT rather than by mechanical impedance. The placement of a starter fertilizer at planting under NT is emphasized.     

Soil physical conditions affecting seedling root growth

Summary The role of mechanical impedance, poor aeration and water availability in restricting pea (Pisum sativum L.) seedling root growth in sandy loam soil at three bulk densities and six matric potentials was studied. Mechanical impedance increased both with bulk density and —matric potential. In certain treatments the roots were shorter and thicker as impedance increased but in others shorter, thicker roots were found as impedance decreased and this was attributed to poor aeration. An aeration deficiency index was defined to separate the impedance and aeration effects when the two acted in combination. An aeration effect was found in soils having less than 30, 22, and 11 per cent gas-filled pore space at low, medium and high bulk densities respectively and the distribution and area of the gas/liquid interface probably influenced diffusion resistance. Effects due to restricted water availability were found only at potentials greater than –3.5 bars, the roots being shorter and thinner as moisture stress increased.     

The influence of waterlogging at different temperatures on penetration depth and porosity of roots and on stomatal diffusive resistance of pea and maize seedlings

The 8 days old seedlings of pea (cv. Ilowiecki) and maize (cv. Alma F1) were subjected to differentiated aeration conditions (control — with pore water tension about 15 kPa and flooded treatment) for 12 days at three soil temperatures (7, 15 and 25 °C). The shoots were grown at 25 °C while the soil temperature was differentiated by keeping the cylinders with the soil in thermostated water bath of the appropriate temperature. Lowering the root temperature with respect to the shoot temperature caused under control (oxic) conditions a decrease of the root penetration depth, their mass and porosity as well as a decrease of shoot height, their mass and chlorophyll content; the changes being more pronounced in maize as compared to the pea plants. Flooding the soil diminished the effect of temperature on the investigated parameters; the temperature effect remaining significant only in the case of shoot biomass and root porosity of pea plants. Root porosity of pea plants ranged from 2 to 4 % and that of maize plants — from 4 to 6 % of the root volume. Flooding the soil caused an increase in the root porosity of the pea plants in the entire temperature range and in maize roots at lower temperatures by about 1 % of the root volume. Flooding the soil caused a decrease of root mass and penetration depth as well as a decrease of plant height, biomass and leaf chlorophyll content.     

Wheat root growth and seasonal water use as affected by irrigation under shallow water table conditions

Summary Irrigation experiments with wheat (Triticum aestivum L.) in clay loam, silty clay loam and the silty clay loam. Contrary to this, irrigation at late jointing, and late jointing and milk stages produced deepest root system in the loam. Roots followed the receding water table. was greatest in the loam. Avoiding irrigation at late jointing stage caused shifting of the zone of peak root density downwards and concentration of roots near water table both in the clay loam and the silty clay loam. Contrary to this, irrigation at late jointing, and late jointing and milk stages produced deepest root system in the loam. Roots followed the receding water table. Seasonal evapotranspiration (E) was affected by number of irrigations and water table depths. Water table contribution ranged from 61.6–64.5% of the total E in clay loam, from 39.0–46.8% of the total E in silty clay loam and from 4.0–8.1% of the total E in loam. Irrigations after late jointing contributed largely to the drainage. Yield was significantly higher in the treatments with scheduled irrigations at crown root initiation and late jointing stages in the clay loam and silty clay loam and at crown root initiation, late jointing and milk stages in the loam. This research has been financed in part by a grant made by USDA, ARS, authorized by Public Law-480     

Spatio-temporal Variations in Axial Conductance of Primary and First-order Lateral Roots of a Maize Crop as Predicted by a Model of the Hydraulic Architecture of Root Systems

Rates at which water can be transported along plant roots (axial pathway) vary through time, in part depending on xylem maturation. Because of experimental constraints, the dynamics of root functional heterogeneity under field conditions remains mostly uncharted territory. Recent advances in mechanistic modelling offer opportunities to bypass such experimental limitations. This paper examines the dynamics of local variations in axial conductance of primary and first-order lateral roots of a maize crop using the architecture-based modelling approach developed by Doussan et al. (Annals of Botany: 81, 213–223, 1998). Specifically, we hypothesised that points of major resistance to long distance water transfers could arise from discrepancies between the hydraulic maturity (or water carrying capacity) of main axes and branch roots. To test this assumption, spatial distributions of root axial conductance were tested after 30, 60 and 100 days at soil depths of 10, 50 and 100 cm under a maize (Zea mays L.) crop sown at a density of 8 plants m⁻². As the crop developed, the corresponding root populations encompassed ever increasing amounts of hydraulically mature first-order laterals (branch roots): after a 100-day growth period, the vast majority of laterals had reached their maximum axial conductance at all soil depths down to 100 cm. In contrast, the axial conductance of a large proportion of main axes (primary roots) remained low, even at shallow soil depths and after 100 days of growth. The imbalance between the hydraulic maturity of primary and lateral roots was most conspicuous at soil depths of 100 cm, where ~10% only of the former compared to ~80% of the latter, had reached their maximum axial conductance after a 100-day growth period.