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Keywords:

  • border cell;
  • cell production;
  • cell release;
  • root cap;
  • root meristem;
  • soil compaction;
  • soil mechanical impedance;
  • maize (Zea mays)

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
  • • 
    To assess the influence of mechanical impedance on cell fluxes in the root cap, maize (Zea mays) seedlings were grown in either loose or compacted sand with penetration resistances of 0.2 MPa and 3.8 MPa, respectively. Numbers of cap cells were estimated using image analysis, and cell doubling times using the colchicine technique.
  • • 
    There were 5930 cells in the caps in the compact and 6900 cells in the loose control after 24 h growth in sand. Cell production rates were 2010 cells d−1 in compact and 1570 cells d−1 in loose sand.
  • • 
    These numbers represent accumulations of 4960 and 3540 detached cells d−1 around the cap periphery following the two types of treatment. The total number of detached cells was estimated as sufficient to completely cover the whole root cap in the compact sand, but only 11% of the root cap in the loose sand.
  • • 
    In conclusion, mechanical impedance slightly enhanced meristematic activities in the lateral region of the root cap. The release of extra border cells would aid root penetration into the compact sand.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Cells are continuously released from the periphery of the root cap. Together with the root-cap mucilage, with which these cells are associated, a boundary layer is created between the plant root and the soil. Previously, the number of cells released into this boundary layer from caps of maize (Zea mays) roots was found to increase 1.7-fold when the roots were grown for 1 d in compacted sand compared with the number of cells obtained from caps of roots grown in loose sand (Iijima et al., 2000). The additional numbers of cells released are considered to diminish the friction between the compacted sandy growth medium and the surface of the root.

The maize root cap has a discrete meristem, which produces new cap cells. Each new generation of cells is progressively displaced away from the meristem by continued growth and divisions in this zone. A question arises, therefore, as to how the higher rate of cell detachment from the cap in compacted soil conditions is regulated, and whether this is compensated for by an enhanced rate of cell production in the cap meristem. Another possibility is that cell production in the meristem and cell release at the cap periphery are events that are independent of one another. In this case, conditions that affect the production of new cells need not necessarily affect the detachment of peripheral cells, and vice versa.

Cell production rates in the root cap meristem have been studied extensively in roots of species, such as Allium sativum (Thompson & Clowes, 1968; Taylor & Clowes, 1978), Convolvulus arvensis (Phillips & Torrey, 1972), Lycopersicon esculentum (Barlow, 1992) and Z. mays (Clowes, 1961, 1971, 1981; Barlow & Macdonald, 1973), growing under laboratory conditions. Rates of cell detachment from the cap have been considered in only a few studies. However, Barlow (1977) showed that about three layers of cells were lost per day from the exterior of the cap of maize seminal roots grown in solution culture. Both Clowes & Woolston (1978) and Zhao et al. (2000) found differences in the number of cells detaching from root caps when the roots were grown in various environmental conditions in the laboratory. In no case, however, have the rates of cap cell production or cap cell detachment been estimated in relation to the media, such as abrasive soil or sand, in which roots actually grow.

It is well known that when plant roots are grown in compact soil they become thickened (Iijima & Kono, 1992; Iijima et al., 1991) and the root surface is often distorted (Baligar et al., 1975). Anatomical changes associated with mechanical impedance have been documented by Barley (1965, 1976), Peterson & Barber (1981), Veen (1982) and Wilson et al. (1977). However, the dimensions of the root cap have not been analysed fully, even though the cap is the portion of the root apex that experiences the peak mechanical stress (Kirby & Bengough, 2002) during its passage through soil. One possible consequence of impedance is that soil particles more readily abrade cells from the surface of the cap. The size of root cap may thus be decreased, even if the rate of new cap cell production remains constant. We tested the hypothesis that the rate of new cap cell production is maintained, or even increased, in compacted sand. In the present paper, estimates are given of the rate of new cell production by the maize root cap meristem, the number of cells contained in various zones of the root cap, and the rate of cell production (or loss) from the external surface of the cap. These findings further our understanding of the behaviour of the root cap when maize seedlings are grown in mechanically impeding sand.

Materials and Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Plant growth

Surface-sterilized maize (Z. mays L. cv. Mephisto) caryopses were germinated at 19°C in darkness. Seedlings were then grown under conditions described by Iijima et al. (2000).

FS1 sand (Fife Silica Sand Ltd, Burrowin Quarry, Fife, Scotland), with particle sizes of 0.125–710 mm, was wetted to a water content of 6 g water per 100 g sand (matric potential of −4.4 kPa). The sand was packed into a seedling holder made from a disposable Pasteur pipette at two different compaction levels, here termed ‘loose’ and ‘compact’, with respective estimated penetrometer resistances of 0.21 MPa and 3.8 MPa. Penetrometer resistance was estimated following the method described by Iijima et al. (2000). In this study, the growth of roots in loose sand was regarded as the control for evaluating soil compaction effects. Filter paper grown roots were not used as the control because of the many physical, chemical and biological factors that could cause differences in growth compared with sand. One seedling with a single seminal root 18–25 mm long was transplanted into each seedling holder (Fig. 1 in Iijima et al., 2000) and allowed to grow in darkness at 19°C for 24 h.

image

Figure 1. Median longitudinal sections (×100 magnification) of root caps typical of this study of the effect of sand compaction. (a) Root tip after 64 h of growth on moistened filter paper (day 0 sample). (b Root tips grown for 24 h in loose sand. (c) Root tips grown for 24 h in compact sand. Bar, 500 µm.

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Colchicine treatment and root section preparation

Following a 24-h growth period, the lower cylinder of the seedling holder was detached to expose the seminal root, which was then placed in aerated 1 mm colchicine solution in a darkened container at 20–22°C in order to accumulate metaphases for the determination of potential cell doubling times within discrete regions of the root meristem (Evans et al., 1957). Six apices, 3–5 mm long, from roots grown under either loose or compact sand conditions were harvested and fixed in FAA (formalin–acetic acid−70% ethanol, 1 : 1 : 18 parts by volume) after being exposed to colchicine for either 0, 1.5, 3 or 4.5 h. Six apices were similarly fixed on day 0, before any exposure to sand culture commenced. The aim was to evaluate the short-term effects of soil mechanical impedance on cap cell production rates.

The fixed apices were dehydrated, embedded in paraffin wax, and sectioned longitudinally at a thickness of 10 µm. The sections were placed on microscope slides and, after dewaxing, were stained by Feulgen reaction and counterstained with 0.25% Fast Green before being finally mounted under a cover slip in Canada Balsam.

Root cap dimensions and cell numbers

Median sections from root tips sampled on day 0 and day 1 (Fig. 1) were located and the lengths and breadths of the root caps were measured, as shown in Fig. 2a. In the case of the day 1 samples, cap dimensions were estimated using the sections of the 1.5-h colchicine-treated roots because these tips were free of adhering sand particles and gave better quality sections than those at the start (0 h) of the treatment. The total number of cells in each root cap was estimated according to a morphometric method described in Bengough et al. (2001). The numbers of cells in each of the first four tiers of the cap meristem – both its columella and its lateral portions (Fig. 2b1,2) – were estimated similarly. Dimensions of cap cells in meristem, columella, lateral, and peripheral region were measured using an eyepiece graticule.

image

Figure 2. (a) Root cap dimensions D1, D2, L1 and L2. (b) Cellular skeleton of the root cap (Redrawn with permission from Fig. 3B in Annals of Botany87: 697). 1, Columella portion of cap meristem; 2, lateral portion of cap meristem; 1,3,5 cap columella; 2,4,6 lateral cap; 5, outer layer of cap columella; 6, outer layer of lateral cap; 7, root–cap junction (7). Cap production rates were estimated from the cell cycles of cap meristem. Cell production rates were calculated from the numbers and cell production rates of both root and cap meristem. Cell release rate was the number of border cells released per day.

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Cell production rates

Numbers of mitotic and interphase cells were counted in each of the four most proximal tiers in both the columella and the lateral portion of the cap meristem using a ×100 oil-immersion lens. No cell divisions could be observed beyond the four most proximal tiers. Counts of mitotic and interphase cells were also made in the meristem of the root proper at a distance 400–500 µm behind the root cap junction. A linear fit to the colchicine–metaphase accumulation data was statistically better than an exponential fit. Hence, using the data gathered at each location, cell doubling times were estimated as the inverse of the rate of accumulation of metaphase plus restitution nuclei (Fig. 3). Restitution nuclei were identified as those whose chromosomes were reverting from metaphase to interphase at the time of fixation (Barlow & Woodiwiss, 1992; Barlow, 1992).

image

Figure 3. Changes in the mitotic index (squares) and the mean percentage of cells in prophase (diamonds), metaphase (circles), ana-telophase (triangles) and restitution (crosses) in roots sampled during treatment with 1 mm solution of colchicine. (a,b) Root meristem after loose sand and compacted sand treatments, respectively. (c,d) Root cap meristem after loose sand and compacted sand treatments, respectively. Bars are standard errors of the mean derived from six replicate plants.

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Cell loss to the rhizosphere

The number of cells released from the cap periphery to the rhizosphere soil was estimated based on the cap cell production rates and the net loss of cells from the total cell population of the cap. The number of cell layers of root cap released per day was evaluated together with the cap renewal time by using the cell population lost to the rhizosphere. The intact and broken cell populations released to the rhizosphere soil were also identified using the published relation between penetration resistance and border cells recovered (Iijima et al., 2000). The fraction of the surface area of the root cap and elongation zone that was covered with border cells was calculated from the area of the root cap, the rate of border cell production, border cell dimensions and the root elongation rate, using the method described by Bengough & McKenzie (1997). The dimensions of 50 intact border cells were measured, to estimate the area of the root covered by each border cell. Surface area of the root caps was estimated by assuming that the caps were shaped as half spheroids. The fraction of the root elongation zone (at a distance of 4 mm behind the root apex) covered with border cells was similarly estimated by assuming that the elongation zone was cylindrical.

Statistical analysis

Six roots were harvested at each sample time and mean values of a given variable, together with an SE of the mean, were estimated. Differences between two treatments were subjected to an analysis of variance, and differences among three treatments were subjected to Duncans multiple range test. For estimation of cell production rates and cell loss, combination of the SE value was calculated to acquire the associated errors from the original metaphase accumulation data in each sampling time. The SE value became large because of the error accumulation in each data conversion. Therefore, significance at P < 0.10 was used for the statistical differences. Most of the published studies dealing with the cap cell production rate did not show statistics because data conversion is necessary to acquire the cell doubling time. Moreover, because of the greater fluctuation of cap cell numbers, error factors may become too large for the statistical evaluation of cell production. For this analysis, a Student's t-test was used because application of anova was inappropriate.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Root cap dimensions

After 1 d in sand, roots from the compacted treatment were 80% shorter than those from the loose sand treatment. The compacted roots were also approx. 50% thicker, at around 3–5 mm from the root tip (Table 1).

Table 1.  Root growth and root cap dimensions of Zea mays after 24 h root elongation in compacted or loose sand
TreatmentEstimated penetrometer resistanceRoot elongation rateRoot diameter at 4 ± 1 mm from the root tipRoot cap dimension (mm)
LengthDiameter
L1L2D1D2
  1. Values are means ± SE from six replicates. **, Significant difference between loose and compact treatments at P < 0.01, tested by anova. ns, Not significant difference at P < 0.05, tested by anova. For an explanation of L1, L2, Di and D2 dimensions, see Fig. 2a.

Loose0.2235.2 (± 1.8) **1.18 (± 0.01) **0.532 (± 0.017) **0.441 (± 0.0436) **0.506 (± 0.012) ns0.624 (± 0.008) **
Compact3.837.2 (± 0.6)1.79 (± 0.07)0.462 (± 0.005)0.275 (± 0.019)0.477 (± 0.009)0.563 (± 0.020)

The dimensions of the root caps were compared following exposure to the two growth conditions (Fig. 1). Dimensions L2 and D2 are the lengths and diameters of the apex of the root that lies under the cap, whereas L1 and D1 are the dimensions of the cap itself (Fig. 2a). The first pair of values (L2 and D2) were decreased more than the latter pair (P < 0.01), possibly because they are affected by changes in the amount of lateral root cap, brought about by impedance. Although the root diameter at 3–5 mm from the apex increased significantly, the diameter of the root cap was narrower in the compacted sand treatment. Indeed, in the compacted sand, the dimension L2, which relates to the portion of root apex covered by lateral cap cells, was only 62% of the value estimated from root apices in the loose sand treatment.

Cell sizes in the cap were estimated by a morphometric method. The most notable difference between the cell length distributions at day 0 and those measured in the loose and compacted treatments is a decrease in the numbers of cells with lengths between 20 µm and 50 µm (Fig. 4). Cells of this size are mostly nonmeristematic cells, but are not the cells in the peripheral region of the cap. The decrease in number of these cells indicates that, after transfer to sand culture, the majority of the nonmeristematic cells diminishes, except for the peripheral cells. The number of meristematic cells did not differ among the treatments (Table 2). By contrast, there were fewer large cells (> 70 µm long) in the caps from the 1-d compacted sand treatment than there were from the loose sand treatment (Fig. 4). Many of these larger cells would be included in the peripheral layers of the cap; their lower numbers would be consistent with the additional reduction in size of the cap (and hence a diminished cap surface area) due to the compaction treatment. The total number of cells comprising the cap in the 1-d compacted treatment was significantly reduced by soil compaction (Table 3). The transfer to sand stimulated a release of cells from the cap periphery, the release being 49% greater in the compacted sand treatment: 2950 cells were released in this treatment compared with 1970 cells released in the loose sand treatment (Table 3).

image

Figure 4. Frequency distribution of cell lengths derived by image analysis of a two-dimensional section (see Fig. 2b) for entire root caps on day 0, and after 1 d of loose and compacted sand treatments (data from total of six roots per histogram).

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Table 2.  Cell doubling time and meristematic cell numbers of Zea mays, as affected by sand compaction
 LateralColumellaTotal root capRoot proper
  1. Values are means ± SE from six replicates. *, Significant difference at P < 0.10 by t-test. Values with same letters are not significantly different at P < 0.05 by the Duncan's multiple range test. ns, Not significant.

Mean cell doubling time (h)
Loose 28.1 ± 4.441.1 ± 10.3 31.4 ± 3.321.4 ± 2.2
Compact 19.9 ± 2.4*53.2 ± 23.4 ns   26.4 ± 2.4 ns25.7 ± 5.4 ns
Cell numbers in meristem
Day 01558 ± 88 a 206 ± 17 a1764 ± 101 a 
Loose1663 ± 40 a 255 ± 23 a1918 ± 56 a 
Compact1567 ± 90 a 267 ± 34 a1834 ± 109 a 
Table 3.  Estimated cap cells broken during the sloughing off and cap renewal time in Zea mays
 (a) Total cap cell number(b) Difference in cell number between day 0 and day1(c) Cell production rates (d1)(d) Cells lost in rhizosphere (b + c)Sloughed cap cellsCap renewal time (d) a/c
Intact cells1Broken cells (d–e)
  1. Values are means ± SE derived from six replicates. *, Significant difference at P < 0.10, by t-test, between loose and compact treatment. 1Estimated from the data of Iijima et al. (2000).

Day 08873 ± 388      
Loose6901 ± 2911972 ± 4851570 ± 2303542 ± 53719101632 (46%)4.4 ± 0.7
Compact5926 ± 3832947 ± 5452012 ± 2564959 ± 60223202639 (53%)2.9 ± 0.4
Probability (t-test)*     *
Ratio (compact : loose)0.86 1.28 1.211.620.66

Cell production rates

Treatment of roots with colchicine solution resulted in a linear accumulation of metaphases, some of which restituted to interphase during the 4.5-h collection period (Fig. 3). The rates of accumulation permitted estimation of potential cell doubling times. In the lateral portion of the cap meristem, the potential cell doubling time in compacted sand was significantly faster for compact (20 h) than that in loose sand (28 h) at P < 0.10 (Table 2). There was no significant difference between the two treatments for the cell doubling times in the columella region of the cap meristem. For the cells sampled in the main body of the root meristem, the potential doubling times, although 20% longer in the compacted treatment, were not significantly different.

Cell production rates from the cap meristem were calculated as the product of the specific cell division rate (cells cell−1 h−1) and the number of cells in the cap meristem (Table 2). The mean production rate of new cells from the entire cap meristem was estimated as 2010 cells d−1 in the compacted sand treatment, whereas it was estimated as 1570 cells per day in the loose sand treatment (Table 3). However, this 21.5% difference in rates was not statistically significant.

In 1 day of loose sand treatment 1570 cells were produced by the cap meristem, but this was accompanied by a net loss of 1970 cells from the total cell population of the cap (Table 3). The number of cells released and accumulating in the cell population surrounding the cap (i.e. cells lost in rhizosphere), therefore increased by 3540 cells in the loose treatment. A corresponding estimate for the compact treatment reveals that the number of cells accumulated around the cap increased by 4960. There were 1300 ± 46 cap cells in the peripheral layer of the root cap at the start of the experiment, so these numbers of accumulated cells correspond to the release of approximately three layers of cells per day from the cap periphery in the loose treatment, and about four layers per day in the compact treatment.

Considering only the rate of cell production rate from the cap meristem (i.e. the rate of cell release from the cap periphery during this 1-d period is disregarded), and assuming that the rate of cell loss is balanced by cell production to maintain a constant cap size in both treatment, the time required to renew the root cap would be 2.9 d in compacted sand, and 4.4 d in the loose sand (Table 3).

It is interesting to compare the numbers of cells released from the cap periphery with the number of cells which could be recovered from the surrounding rhizosphere after the 1-d treatment period, as published in Iijima et al. (2000). Using the published relation between penetration resistance and border cells recovered (Fig. 3 in Iijima et al., 2000), 1910 cells would be expected to be recovered in the loose sand treatment, compared with 2320 cells in the compacted sand treatment (Table 3). Recovery rates during the separation process of cells from the rhizosphere sand was considered in the calculation of border cell numbers. With reference to the estimate of the total number of cells released from the cap periphery, these values correspond to recovery rates of 53% and 46% for the two treatments, respectively. Thus, approximately half of the border cells may have been broken owing to the abrasion with sand particles during the normal growth process. Cell debris was observed amongst the intact border cells, indicating the presence of many damaged cells.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Cap size

Both the total size of the cap and the number of cells of which it is composed were decreased when maize root growth was mechanically impeded. The finding of a smaller cap concurs with results of Wilson & Robards (1979) for barley roots grown among compressed glass beads, and also with the results of Souty (1987) who grew maize roots so that their caps pushed up against a metal plate. In our experiment, the smaller size of caps of impeded roots was associated with fewer cells, especially those large cap cells associated with the cap periphery.

Dynamics of cell production and loss

Mechanical impedance causes a decrease in the root elongation rate, an effect which persists for several days, even after the impedance is removed (Goss & Russell, 1980; Croser et al., 2000). This slowing of root elongation is associated with both a decrease in final cell length and a slower rate at which new cells are produced and added to the cell files that comprise the meristem (Croser et al., 1999). In our experiments, compaction tended to result in a decrease in cell production in the root meristem. By contrast, there was an opposite tendency towards an increased rate of cell division in the cap meristem of compacted roots. These findings support results of Brigham et al. (1998) who suggested that the cap and main root meristems operate largely independently.

The duration of the mitotic cell cycle in the maize root cap meristem has been estimated as between 12 h (by thymidine labelling and by accumulation of colchicine-metaphases; Clowes, 1961) and 14 h for the columella region of the meristem, and 22.5 h for the lateral region (by thymidine labelling; Barlow & Macdonald, 1973). In our experiments, the relatively greater cell production rate from the lateral part of the cap meristem compared with the tip region (28 h and 40 h, respectively) may have resulted from the abrasive nature of the sand compared with the less abrasive sphagnum moss or hydroponics used by Clowes (1961) and Barlow & Macdonald (1973), respectively. However, the cell production rate from the lateral cap meristem region (compared with the central columella zone of the cap) is inherently greater because of the larger number of proliferative cells in this zone.

Steady-state cell kinetics in the root cap predicts that, in a given period of time, the number of cells released from the periphery of the cap should be equal to the number of cells produced by the cap meristem. The total number of cap cells should therefore remain constant throughout that period. However, there was some diminution of the size of the cap and the cell production was not in steady state (Table 3). The accumulated cells correspond to the release of approx. three layers of cells per day from the cap periphery in the loose treatment, and about four layers per day in the compact treatment. These estimates are similar to those of Barlow (1977) for the rate of cap cell loss from maize roots grown in solution culture.

The detached cell population, including broken cells, was estimated as 4960 cells in compacted sand and 3540 cells in loose sand (Table 3). These cells surrounding the cap would be sufficient to cover 103% of the area of the root cap in the compacted sand treatment, but only 11% in the loose sand treatment (see Discussion in Iijima et al., 2000). This suggests that in compacted sand the whole of the root cap is covered in a layer of detached cells. The surface area of the elongation zone of the root will be more sparsely covered with detached cells; 20% of the 1 mm of cylindrical surface area of this zone of the root will be covered in the compacted sand treatment compared with 4% in the loose sand treatment. Thus, the maximum lubricating action of the detached cells will be at the root cap itself, where the stress is greatest (Kirby & Bengough, 2002). This finding would accord with the observations of Bengough & McKenzie (1997) and those of Read et al. (1999) who showed that detached cap cells played some role in modulating the viscosity of maize root cap mucilage, allowing root tips to glide with less effort over a solid supporting medium.

Cap renewal times have been estimated in the seminal roots of maize seedlings grown either in hydroponics or in damp sphagnum moss (Clowes, 1971; Barlow, 1974). In later work (Barlow, 1978), it was shown that cap cells require about 7 d to reach the tip of the columella following their displacement from cap meristem, whereas they required 2–3 d to reach the flank of the cap. In the present experiment, the time taken for all of the nonmeristematic cells in the root cap to be replaced by new cells from the cap meristem was estimated to be 4.4 d in the loose sand treatment and 2.9 d in the compacted sand treatment (Table 3). These estimates rely on the assumption that cap cell production balances cell loss and that caps therefore maintain a constant size. This would imply that cell production and cell loss are coordinated events, as opposed to being regulated independently. Comparison of cell production rates of 2010 and 1570 cells d−1 in compacted and loose sand treatments, with the respective observed losses of 2950 and 1970 cells d−1, suggests that cell release from the cap periphery proceeds faster than cell production from the cap meristem. This accords with earlier results (Barlow, 1977) where meristematic reduplication was slower than the rate of shrinkage of the cap due to cell loss.

The experiments of Brigham et al. (1998) suggest that removal of border cells from the outside of the root cap might stimulate cell division in the cap meristem. They found, for example, that washing cells from the periphery of the cap of pea roots rapidly increased the fraction of cells undergoing mitosis in the cap meristem. In our experiment, the rate of cell production by the cap meristem may have been similarly disturbed by transfer to colchicine solution (a step whereby cells could potentially be washed from the cap), in which case it would not be possible to estimate rates of cap renewal as the values obtained from solution culture might not be comparable with those occurring in a sandy medium. However, the linear nature of the plots in Fig. 3 suggests that entry into mitosis was constant during the course of the colchicine treatment, and that no transient increase of cell production occurred as a result of the exposure to the solution. The slight depression in the percentage of prophase cells during the colchicine treatment suggest that, if there was any effect on cell production, it is likely to have been on the rate of entry into mitosis and, hence, cell production rates obtained by this method could have been slightly underestimated. With regard to the results of Brigham et al. (1998), it is possible that the observed elevation of mitotic index following the washing-off of released peripheral cells represented a transient inhibition of cell division, with cells accumulating in prophase and metaphase before completing anaphase, rather than being an indication of accelerated entry into mitosis. In this respect, the washing process temporarily mimicked the effect of the colchicine in our experiment.

In summary, mechanical impedance does not persistently decrease the production of new cells in the root cap meristem, even slightly enhanced that in the lateral region. The rate of release of border cells may be partly related to the rate of cell production by the cap meristem. An increased rate of release of cells from the periphery of the cap would assist root penetration into compacted soil by decreasing the frictional resistance to elongation growth.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank the Japanese Society for Promotion of Science (B2-12460010), Royal Society and Sasakawa Foundations for funding the visit to SCRI and IACR Long Ashton by M. Iijima. The Scottish Office Agriculture, Environment and Fisheries Department provide grant-in-aid to SCRI.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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