Journal of Experimental Botany, Vol. 52, No. 354, pp. 167-171,
January 2001
© 2001 Oxford University Press
Short Communications |
Partial mechanical impedance can increase the turgor of seedling pea roots
1 Biochemistry and Physiology Department, IACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK
2 Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK
3 Crop and Weed Science Department, IACR-Rothamsted, Harpenden, Hertfordshire AL5 2JQ UK
Received 4 April 2000; Accepted 21 August 2000
Abstract
Roots of 3-d-old pea seedlings (Pisum sativum L.) were mechanically impeded using a sand core apparatus, which allowed mechanical impedance to be varied independently of aeration and water status. Turgor of root cortical cells was then measured using a pressure probe. In seedlings grown in sand cores for 1 d, impedance had little effect on turgor, but in seedlings grown in the sand cores for 2 d, impedance increased turgor by 0.18 MPa in the apical 6 mm.
Key words: Mechanical impedance, pea, Pisum sativum L., pressure probe, turgor.
Introduction
Mechanically strong soils decrease root elongation in many crop species (see Bengough and Mullins, 1990
; Atwell, 1993, for reviews). This can be a serious agricultural problem, as the ability of the root system to access water and nutrients from the deeper soil layers is restricted. While some soils are naturally strong, traffic associated with tillage operations can compact the soil beneath the tilled layer and increase its strength.
Roots penetrate soil by exerting a growth pressure (
) which deforms the soil ahead of, and around, the root ( is equal in magnitude to the soil pressure which opposes root growth). Cell turgor pressure in the root apex allows the root to generate . Roots with greater turgor would therefore be expected to generate greater , but there have been few studies where turgor has been measured directly in mechanically impeded roots using a pressure probe. It has been found that soil compaction did not increase the turgor of lupin (Lupinus angustifolius L.) roots (Atwell and Newsome, 1990). However, interpretation of data from such experiments is difficult, as turgor may change after the roots are removed from the soil (Atwell and Newsome, 1990). This problem was avoided (Clark et al., 1996) by making in situ measurements of turgor in impeded seedling pea (Pisum sativum L.) roots. The roots were completely mechanically impeded (so that elongation stopped) using a shear beam apparatus. This allowed of the impeded roots to be measured, and permitted access to the impeded root with the microcapillary of the pressure probe so that turgor could be measured. Complete mechanical impedance increased turgor in the apex of pea roots from 0.55 MPa to 0.78 MPa, and the turgor did not decrease for at least 90 min after seedlings were removed from the apparatus and incubated in 0.5 mol m-3 CaSO4. This suggested that turgor in partially impeded pea roots (in soil, for example) could be studied by removing them from the impeding environment and subsequently measuring turgor.
Here, this approach is used to obtain turgor measurements in partially impeded pea roots. Compacted soil was not used to impose the impedance due to the complex interactions of impedance with water and aeration. Compaction increases soil strength but also decreases aeration, while changes in soil water content change soil strength. Instead, a sand core system (Materechera et al., 1991) was used, which allows mechanical impedance to be varied independently of aeration and water status.
Materials and methods
Plant material
Seeds of Pisum sativum L. (cv. Meteor) were soaked in distilled water for 6–8 h at room temperature, then set to germinate for 3 d on two sheets of wet filter paper in a Petri dish at 20±1 °C. The Petri dishes were covered with aluminium foil to exclude light. After 3 d, the root (radicle) was 15–20 mm long.
Sand core apparatus
The apparatus used loaded or unloaded sand cores to change the mechanical impedance of the sand with negligible effects on aeration and water supply. Loading of the sand cores causes negligible compaction of the sand, but increases the resistance of the sand particles to displacement. Sand cores were made in plastic cylinders 100 mm long. The inside surface of the plastic cylinder was lined with PTFE, giving an inside diameter of 100 mm. The PTFE served to minimize wall friction and to ensure a uniform pressure distribution in the sand. The sand was RH 65 grade silica sand (Hepworth Minerals and Chemicals Ltd, Sandbach, UK). The particle size distribution of the sand (% weight retained) was 0.5 mm, 0.8%; 0.355 mm, 7.3%; 0.250 mm, 28.3%; 0.180 mm, 41.5%; 0.125 mm, 19.2%; 0.090 mm, 2.4%;<0.090 mm, 0.5%. The cores rested on a sand table with a mean grain size of 0.06 mm, which was kept saturated with 0.5 mol m-3 CaSO4.
To obtain the impeded treatment, a uniform vertical stress was applied to the sand by placing a 17 kg mass on top of the core. The mass was not placed directly on the sand, but on a 3 mm thick, 89 mm diameter plastic disc, in order to ensure an equal pressure distribution across the surface of the sand. The disc had five 2.5 mm holes, one in the centre for penetrometer measurements, and the other four equi-distant at a radius of 25 mm for the pea seedlings. The mass had corresponding holes, 20 mm in diameter. Controls used the disc without the 17 kg mass. Cores were set up by pouring dry RH 65 sand into the cylinders and covering them with a disc. The water table of the sand table was raised until it was just flooded and the sand in the cylinders was allowed to wet up by capillary rise. When this sand was wet, the water table was lowered to 30 cm below the top of the cylinder, giving a matric potential of -3 kPa. The air-filled porosity was approximately 0.1.
Strength of the sand cores
The strength of the sand cores was measured using a stainless steel 5° semi-angle penetrometer. The penetrometer had a relieved shaft and the diameter at the base of the penetrometer cone was 2 mm. The penetrometer resistance (qP) was calculated by dividing the force required to push the penetrometer into the sand at 1 mm s-1 by the cross-sectional area of the base of the cone. The resistance to penetration of a sharp penetrometer is considered to be directly proportional to (Greacen and Oh, 1972; Bengough and Mullins, 1990), which is calculated from qP as the pressure required to expand a cylindrical cavity using the equation
 | (1) |
is the penetrometer cone semi-angle. The coefficient of soil–metal friction was calculated using the ring method, where the torque required to rotate a loaded metal ring is measured (Koolen and Kuipers, 1983). It was verified using a 30° semi-angle penetrometer that (when a hole was made first to simulate the planting procedure) there was negligible variation of qP with depth between 20 and 40 mm from the top of the core (results not shown).
Growth experiments
The length of the root was measured and the seedlings planted in the sand cores, after using forceps to make a hole in the sand core just deep enough to accommodate the root. The seed rested on top of the hole in the disc. Dry RH 65 sand was brushed into any gaps between the root and the hole in the sand core. The 17 kg mass was placed on the impeded treatment, and seedlings in both treatments covered with dry RH 65 sand, which was immediately watered. The planting procedure anchored the seedling and prevented subsequent root growth from pushing the seed out of the sand. The sand table was loosely covered with a wooden lid and left for either 1 d or 2 d at 20±1 °C. Seedlings were then removed from the sand cores, noting the length of time that the seedling had been in the core, and the root length measured. Roots were washed free of sand in 0.5 mol m-3 CaSO4 before making turgor measurements with the pressure probe.
Turgor measurements
Turgor in individual cortical cells was measured using a pressure probe (Hüsken et al., 1978). Roots were removed from the sand cores as described above and bathed in 0.5 mol m-3 CaSO4. Measurements of turgor were made from 3 mm to 14 mm from the root apex, up to 90 min after removal of the roots from the sand cores. The zone of elongation in unimpeded roots of cv. Meteor is 0.5–5 mm from the apex (Clark et al., 1996). There was no evidence for radial gradients of turgor within the cortex (data not shown).
Statistical analyses
The significance of the effect of impedance on root elongation rate and cell turgor were determined with analyses of variance, using completely randomized designs. For elongation rate, each root was treated as a single replicate. For turgor, where several measurements were made on different cells of the same root, each cell was treated as a single replicate. There was a greater variation in turgor between cells on the same root than between mean values of turgor for different roots.
Results
Strength of sand cores
The qp of the control cores was 0.19±0.004 MPa (±s.e., n=20), which corresponded to a of 0.054 MPa (from equation 1). The addition of the 17 kg mass greatly increased the mechanical strength of the sand core, giving a qp of 1.40±0.065 MPa (±s.e., n=14) and a calculated of 0.40 MPa.
Effect of impedance on root elongation rate and turgor
The impeded treatment decreased the root elongation rate of 4-d-old seedlings by about 50% (Table 1). Impedance caused only a small (but statistically significant) increase in turgor along the root profile (Fig. 1). In the whole root profile, the mean turgor of cells from impeded roots was 0.04 MPa greater than cells from control roots (Table 1).
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) roots. Data were obtained from 11 control roots and 11 impeded roots.In 5-d-old seedlings, the impeded treatment also decreased the root elongation rate by 50% (Table 2
). Impedance increased turgor to a much greater extent than in 4-d-old seedlings, especially nearer the apex (Fig. 2). In cells less than 6 mm from the apex, the mean turgor of cells from impeded roots was 0.18 MPa greater than cells from control roots (Table 2). In cells more than 6 mm from the apex, the mean turgor of cells from impeded roots was 0.07 MPa greater than cells from control roots.
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Discussion
The high impedance treatment
The use of the sand core system, rather than compacted soil, means that any changes due to the high impedance treatment can be ascribed to mechanical impedance, rather than to changes in aeration or root–soil contact. The mechanical impedance was also uniform at depths where the roots were growing. The penetrometer resistance of the high impedance sand was relatively low compared with some studies of root growth in compacted soil, where a qp of 3 or 4 MPa has been recorded (Materechera et al., 1992). However, the high impedance treatment in the present study was sufficient to halve the rate of root elongation. The calculated (0.40 MPa) is also more than half the maximum value of reached by completely impeded pea roots, which is about 0.60 MPa for cv. Meteor (Clark et al., 1999).
Response of turgor to impedance
Interpretation of turgor measurements reported in the present study is based on the assumption that the measurements accurately reflect the turgor status of the roots before they were removed from the sand cores. This assumption is valid in that the turgor of completely mechanically impeded 4-d-old pea roots did not change for at least 90 min after they were removed from the impeding environment (Clark et al., 1996). There is also a ‘lag’ between the relief of mechanical impedance and an increase in root elongation rate in pea (Bengough and Young, 1993; Croser et al., 2000). A similar ‘lag’ was observed after the relief of chilling in maize roots (Pritchard et al., 1990). These authors observed that cells whose expansion was arrested by chilling did not resume expansion after re-warming (despite their elevated turgor), but that root elongation recovered by the expansion of cells newly produced by the meristem. Such observations highlight the importance of cell wall biochemical properties in growth.
Direct measurements of the response of root turgor to mechanical impedance due to soil compaction have been reported before. Lupins were grown in soils with two levels of compaction, then the roots removed and turgor measured with a pressure probe (Atwell and Newsome, 1990). The compacted treatment decreased the elongation rate by 35%, but the rate of root volume expansion was unaffected, i.e. the roots were shorter and thicker. Although compaction made the osmotic potential more negative, it did not increase turgor in the zone of elongation, which was 0.36 MPa. There was also a large (0.6–0.9 MPa) discrepancy between turgor and osmotic potential, which they suggested was partly due to a decrease in turgor when the roots were removed from soil (the plants were well-watered). In wheat, however, Atwell and Newsome (unpublished data, cited in Atwell, 1990) found that soil compaction increased turgor from 0.52 to 0.73 MPa.
Recently, it was reported that a 3-d impedance treatment did not affect turgor in pea roots, although osmotic potential was made more negative by about 0.2 MPa (Croser et al., 2000). The approach used in these experiments was similar to that reported here in that impedance was imposed in sand cores and turgor was measured shortly after removing roots from the sand. Their method differs in that the sand was wetted up before being placed in the plastic tubes and packed to different bulk densities for the control and impeded treatments. It is therefore possible that the roots were responding to changes in aeration and root–soil contact as well as to the change in mechanical impedance.
It was found that a 1-d complete impedance treatment increased turgor in the apical 5 mm of 4-d-old pea roots by a mean of 0.23 MPa (Clark et al., 1996). The results presented here show that partial mechanical impedance can also increase the turgor of pea roots. The effect was very small in 4-d-old roots that had been impeded for 1 d, but in 5-d-old roots that had been impeded for 2 d, there was a mean increase of 0.18 MPa in the apical 6 mm. The extent to which mechanical impedance affects turgor in seedling pea roots appears to depend on the magnitude of the impedance and on the length of time for which it is imposed. This is consistent with increases in turgor being due to solute accumulation resulting from the decreased rate of volume expansion of the root (Atwell, 1988). It is therefore suggested that the increase in turgor in partially impeded pea roots was caused by solute accumulation resulting from decreased volume expansion, with a feedback mechanism limiting the increase in turgor. There is direct evidence that turgor can control solute fluxes in other tissues, such as sugar beet (Bell and Leigh, 1996).
Although partial mechanical impedance has been shown here to increase root turgor, other studies show that mechanical impedance caused by compaction does not necessarily lead to an increase in turgor. While these different responses may be due to the other changes in soil physical conditions caused by compaction, they may also indicate that changes in turgor are not central to the response of roots to impedance.
Acknowledgments
IACR-Rothamsted and Silsoe Research Institute are grant-aided by the Biotechnology and Biological Sciences Research Council. We thank Richard Cope, Silsoe Research Institute for technical assistance with the sand tables.
Notes
4 To whom correspondence should be addressed at Silsoe. Fax: +44 1525 860156. E-mail: lawrence.clark{at}bbsrc.ac.uk 
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