Journal of Experimental Botany, Vol. 52, No. 364, pp. 2127-2133,
November 1, 2001
© 2001 Oxford University Press
Original Papers |
The position of localized soil compaction determines root and subsequent shoot growth responses
1 Centre for Horticulture and Plant Sciences, University of Western Sydney, Hawkesbury, Richmond, NSW 2753, Australia
2 School of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
Received 23 October 2000; Accepted 2 July 2001
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Abstract |
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Plants growing in soils typically experience a mixture of loose and compact soil. The hypothesis that the proportion of a root system exposed to compact soil and/or the timing at which this exposure occurs determines shoot growth responses was tested. Broccoli (Brassica oleracea var. italica cv. Greenbelt) seedlings were grown in pot experiments with compact, loose and localized soil compaction created by either horizontal (compact subsoils 75 or 150 mm below loose topsoil) or vertical (adjacent compact and loose columns of soil) configurations of loose (1.2 Mg m-3) and compact (1.8 Mg m-3) soil. Entirely compact soil reduced leaf area by up to 54%, relative to loose soil. When compaction was localized, only the vertical columns of compact and loose soil reduced leaf area (by 30%). Neither the proportion of roots in compact soil nor the timing of exposure could explain the differing shoot growth responses to localized soil compaction. Instead, the strong relationship between total root length and leaf area (r2=0.92) indicated that localized soil compaction reduced shoot growth only when it suppressed total root length. This occurred when isolated root axes of the same plant were exposed to vertical columns of compact and loose soil. When a single root axis grew through loose soil into either a shallow or deep compact subsoil, compensatory root growth in the loose soil maintained total root length and thus shoot growth was unaffected. These contrasting root systems responses to localized soil compaction may explain the variable shoot growth responses observed under heterogeneous conditions.
Key words: Compaction, mechanical impedance, root length, split-root, soil strength.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Soil compaction reduces root elongation (Barley, 1965
; Taylor and Ratliff, 1969) and can also cause reductions in shoot growth (Schuurman, 1965). Field experiments have suggested that soil compaction reduces shoot growth by restricting the volume of soil explored by the root system and hence the availability of water and nutrients to the plant (Bennie and Botha, 1986; Taylor and Brar, 1991). However, laboratory studies of plants growing in compact soil have not revealed a clear decrease in water or nutrient acquisition prior to a growth suppression (Masle and Passioura, 1987). Instead, phytohormonal messages, produced in the mechanically impeded roots and transferred to the shoot, may act to reduce shoot growth. The identity and behaviour of these phytohormonal messages are still the subject of investigation (Tardieu et al., 1992; Hartung et al., 1994; Mulholland et al., 1999). The shoot growth responses attributed to messages produced by compact soil include a reduction in mature cell sizes in leaves (Beemster and Masle, 1996) and a reduction in leaf number (Mulholland et al., 1999). The clearest evidence of compact soil reducing shoot growth is obtained when plants are grown in entirely compact soil. But soil physical conditions are typically heterogeneous (Hamblin, 1985).
Under field conditions plant root systems encounter considerable spatial variation in mechanical impedance. Even in compact soils, areas of lower mechanical impedance will occur due to shrinkage cracks and channels formed by earthworms or previous root growth (Tardieu, 1988). Furthermore, in cultivated soils, dense compact subsoils frequently underlie the loosened topsoil. Under these conditions root systems encountering hard compact zones of soil have the opportunity to proliferate in zones of looser soil. Such plasticity in root system development, in response to heterogeneous soil conditions, has been reported in both pot (Garcia et al., 1988) and field experiments (Bennie and Botha, 1986; Montagu et al., 1998) resulting in the total root length per plant being maintained. The influence of such root system plasticity, under localized soil compaction, on shoot growth has not been determined.
When plants are grown in soil with localized compaction, shoot growth is often similar to rates observed for plants grown in entirely loose soil (Schuurman, 1965; Kirkegaard et al., 1992; Hartung et al., 1994; Montagu et al., 1998), although there are some reports of reduced shoot growth under the above conditions (Andrade et al., 1993; Blaikie and Mason, 1993; Hussain et al., 1999). The strength of shoot growth responses to localized soil compaction could be influenced by either the proportion of roots experiencing compact soil or the timing of root entry into compact soil in relation to ontogeny (Montagu et al., 1998). The latter is supported by the observation that the response to soil compaction was determined by the developmental stage of wheat leaves when the root impedance was imposed (Beemster and Masle, 1996).
The aim of this study was to investigate how root systems, and subsequent shoot growth, respond to localized soil compaction by growing broccoli seedlings in horizontal and vertical combinations of loose and compact soil. Particular attention was paid to the timing of root growth into the compact soil zones and the differing proportion of roots in each soil zone and whether this could account for the observed variations in shoot growth.
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Materials and methods |
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Soil and growth conditions
Broccoli plants (Brassica oleracea var. italica cv. Greenbelt) were grown in two experiments with differing spatial arrangements of compact (1.8 Mg m-3) and loose (1.2 Mg m-3) soil. A third experiment was conducted to determine root elongation rates in loose soil and thus estimate when roots contacted the compact subsoil in horizontally layered treatments. Soil bulk densities reflected those observed in a field experiment from whence the soil was obtained (Montagu et al., 1998
). The soil was a sandy loam (46% coarse sand, 37% sand, 9% silt, 8% clay, and 0.61% organic carbon) which was air-dried and passed through a 6 mm sieve and phosphorus (100 mg P kg-1) applied as CaHPO4. During the experiments a total of 90 mg N pot-1 was applied as a nutrient solution (7.5 g Ca(NO3)2 l-1, 2.1 g KNO3 l-1, 2.7 g Mg(NO3)2 l-1). Micronutrients were added by applying a total of 50 ml per pot of the following micronutrient solution (415 mg CuSO4.5H2O l-1, 365 mg ZnSO4.7H2O l-1, 15 mg Na2MoO4.2H2O l-1, 125 mg H3BO3 l-1, 500 mg EDTA–Iron (III) sodium salt l-1). In both experiments, plants were grown in a controlled environment chamber (Thermoline, Australia) with a 13 h photoperiod, 23.5/16.2 °C day/night, and a photon flux density of 900 µmol photons m-2 s-1 at pot level. The relative humidity ranged between 58–70% during the light phase and 78–90% during the dark phase. Plants were watered daily to bring the pot to 90% of field capacity as determined by weight.
Treatments
Experiment one (horizontal soil layers):
In the first experiment soil was packed into cylindrical PVC pots (240 mm high, 84 mm diameter), to generate the following treatments: loose; shallow compact subsoil (compact subsoil 75 mm below loose topsoil); and deep compact subsoil (compact subsoil 150 mm below loose topsoil). Soil bulk densities were achieved by manually packing a known dry weight of soil into a set volume using a metal piston of a diameter equal to the internal diameter of the pot. Each treatment was replicated four times and arranged in a randomized block design. Pregerminated broccoli seeds were transplanted into each pot (1 d after germinating) and thinned to one plant per pot 5 d after planting (5 DAP).
Experiment two (vertical soil columns):
In the second experiment, rectangular pots (each half 100x50 mm in cross-section and 350 mm high) were glued together, allowing soil to be packed into the adjacent columns at different bulk densities. Three treatments were generated as follows: loose (both columns loose); compact (both columns compact) and, compact/loose, where one column of the soil was compact and the other column loose. The treatments were replicated four times and arranged in a randomized block design. To split the dicotyledonous root system between the two columns, seedling radicals were manipulated to obtain four lateral roots of similar length. This was achieved by germinating and growing the seeds on filter paper saturated with water in an incubation cabinet (12 h light and 25 °C constantly) until radicals were 10–15 mm long. About 2–3 mm of root apex was then excized to promote lateral root formation. Eight days later, seedlings which had developed four lateral roots of approximately equal length (15–20 mm) were planted into split-pots with two roots in each side. Because the root system was split between vertical columns of loose and compact soil, water extraction from each half of the split-pots was measured using 200 mm buriable waveguides attached to a time domain reflectometry instrument (Trase System, Model 6050XI, Soilmoisture Equipment Corp, USA). This allowed each half of the pot to be maintained at 90% of its water-holding capacity, despite different water extraction rates from each half.
Experiment three (root elongation rates in loose soil):
Ten germinated seeds were transplanted into each of four pots containing loose soil and grown under the conditions outlined above. The plants were subsequently harvested every 3 d to determine the rate of early root penetration into loose soil.
Shoot measurements
In both experiments the length of the 1st, 2nd and 3rd leaves was measured periodically. Leaf area (Delta-T Devices Ltd, Decagon Devices Inc. England) and number and shoot dry masses were determined 28 and 30 d after planting in Experiments 1 and 2, respectively.
Immediately prior to harvest, the first leaf was removed and leaf water potential measured (Scholander Pressure Chamber, Arimad-2, Israel) using methods outlined earlier (Turner, 1981). On the same leaf, concentrations of P, K, Ca, Mg, Mn, Fe, Zn, and Cu were measured by ICP emission spectroscopy (ARL model 3520B) and concentrations of N were measured by thermal conductivity (Leco FP-428 Nitrogen Analyser).
Root measurements
In Experiments 1 and 2, the soil was washed from root systems and root length and dry mass were determined at harvest. Where treatments combined loose and compact soil, roots from each zone were measured separately. Root length was determined on a subsample of at least 10% of the root fresh mass, by the line intercept method (Böhm, 1979). The root subsample was dried at 70 °C and specific root length (m root g-1 dry mass) calculated for each soil zone. Total root length for each soil zone was calculated combining the specific root length from each subsample with total root dry mass for that zone.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Shoot growth
In Experiments 1 and 2, entirely compact soil profiles reduced shoot dry mass and leaf area of broccoli by 25–54% (P<0.01), compared to loose soil profiles (Tables 1
, 2). The reduced leaf area arose from both fewer and smaller leaves (P<0.05).
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When plants were grown in a combination of compact and loose soil, the spatial arrangement of the zones determined the shoot response. Plants grown with compact subsoil (shallow and deep: Experiment 1) produced a similar leaf area and shoot dry mass as plants grown in an entirely loose profile (Table 1
). By contrast, the compact/loose treatment (Experiment 2) reduced both leaf area and shoot dry mass (P<0.05; Table 2) to a level intermediate between the loose and compact treatments; leaf area was reduced by 30% in the compact/loose treatment and 54% in the entirely compact treatment, as compared to the loose treatment.
The reduction in leaf area caused by the compact and compact/loose treatments arose from both a delay in leaf production leading to fewer leaves than in the loose treatment and the production of smaller leaves (Tables 1, 2; Figs 1, 2). In both experiments, the difference between compact and loose treatments were apparent even in leaf 1; neither compact subsoil treatments reduced leaf length in Experiment 1 hence mean lengths for these two compact subsoil treatments are plotted in Fig. 1. By contrast, in Experiment 2, the leaves of plants growing in compact/loose soil elongated more slowly than when the soil was loose, but not as slow as in the compact treatment (Fig. 2).
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Differences in leaf length between treatments were not as marked as differences in the final leaf area for two reasons (Tables 1
, 2; Figs 1, 2). Firstly, reductions in leaf number contributed to the reduced plant leaf area and secondly, the length of an individual leaf was not as sensitive to the soil treatment as was leaf area. For example, the area of leaf 1 of the compact treatment was reduced by 27%, compared to loose soil, while the length was reduced by only 11%. Width growth must also have been reduced by the compact and compact/loose treatments, but because of the lobed nature of the leaves, repeatable measurements of this dimension were not practicable. While leaf length measurements underestimated the effects of the various soil treatments they do provide useful information on leaf expansion dynamics. In both experiments leaf water potential was -0.017±0.006 MPa with no detectable differences between treatments. Foliar nutrient concentrations at the end of the experiments were also unaffected by soil impedance treatments with all nutrients in the range considered adequate for growth (data not shown).
Root growth
In Experiment 3, the main taproot grew at 28 mm d-1 in loose soil for 6 DAP before slowing to 8 mm d-1, between 6 and 9 DAP. The slowing of the taproot elongation rate coincided with the proliferation of lateral roots so that by 12 DAP many lateral roots were the same length as the taproot.
In Experiments 1 and 2, plants grown in the compact treatment had total root lengths 70% lower (P<0.01) than those grown in the loose treatment (Tables 1, 2). The compact soil reduced root length more than dry mass, which decreased by only 12–45%, resulting in roots grown in compact soil having lower (P<0.01) specific root lengths (Tables 3, 4). When root systems were split between loose and compact soil, specific root lengths in each zone were typical of the roots that grew entirely in loose or compact soil (Tables 3, 4).
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The position of the compact soil affected the response of the whole root system to localized compaction. Plants grown in either compact subsoil treatments in Experiment 1 produced a similar or even greater total root length than those plants grown in the loose treatment (Table 1
), even though both compact subsoil treatments had 14–26% of their root system in the compact soil. By contrast, in Experiment 2, the total root length of the compact/loose treatment was intermediate (P<0.05) between the compact and loose treatments (Table 2). This arose because no compensatory growth of roots in loose soil occurred when the root system was split vertically between loose and compact soil.
Relationship between the roots and shoots
In Experiments 1 and 2, only small increases in the proportion of biomass allocated to roots were observed when plants grew in compact soil (Tables 1, 2). The compact/loose and compact subsoil treatments had little effect on the proportion of biomass allocated to roots. However, changes in specific root length induced by the compact soil, resulted in root lengths decreasing considerably more than dry mass. Consequently, a tight relationship between total root length and leaf area was maintained over all treatments in both Experiments 1 and 2 (Fig. 3).
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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The reduction in the shoot growth of broccoli seedlings grown in entirely compact soil (Tables 1
, 2) is consistent with the responses of other species (Masle and Passioura, 1987). But plants typically grow in soils with a mixture of loose and compact soil. These experiments show that when soil compaction is localized shoot growth is reduced in direct proportion to the reduction in total root length per plant (Fig. 3). For example, of the heterogeneous soil treatments created, only the compact/loose treatment in a vertical arrangement decreased total root length and, as a consequence, lowered shoot growth. By contrast, a compact subsoil reduced neither the total root length nor shoot growth (Tables 1–4). Similar relationships between root length and shoot growth have previously been observed when entire root systems have been confined or subjected to compact soils (Richards and Rowe, 1977; Blaikie and Mason, 1993) but the authors are not aware of any studies in which this relationship has been examined under heterogeneous root zone conditions. How the root systems differed in their response to the localized soil compaction is examined below.
The growth of roots in compact soil was always slowed (Tables 3, 4). When soil compaction was localized, the position of the compact soil determined how the root system as a whole responded. In short, compensatory root growth in loose zones of the soil occurred only when the same root axes grew through both loose and compact soil, for example, into compact subsoil. Increased branching of the root axes in the loose soil, as indicated by a high specific root length, compensated for the decrease in root length in the compact subsoil (Table 3). As a result, total root lengths were the same as for root systems from loose treatment. Such compensatory growth is consistent with the observation that impedance of the distal part of the root axes stimulates branching in the proximal root zone (Atwell, 1990). By contrast, when the root system was split vertically, individual root axes experienced only compact or loose soil. The two parts of these root systems did not appear to interact, hence reductions in root length in the compact soil caused an overall reduction in total root length per plant (Tables 2, 4). Compensatory root growth by tomato seedlings in the loose column of a similar vertical split-root system has been observed (Mulholland et al., 1999). However, this related to root dry matter which is considerably less sensitive to soil compaction than root length due to the decreases in specific root length which also occur in response to soil compaction, for example, see Table 4. In these experiments the inability of the root system to compensate for root length reductions in areas of compact soil was strongly correlated with reductions in shoot growth (Fig. 3).
Shoot growth responses to localized soil compaction in this study are analogous to shoot responses to localized soil water deficits. When root axes experience both wet and dry soils, as in horizontally layered experiments, shoot growth was not reduced (Gallardo et al., 1990; Phillips and Riha, 1994), whereas shoot growth is reduced when separate root axes of the same plant experience wet and dry soil columns, as in split-root experiments (Gowing et al., 1990; Fort et al., 1997). A similar response has also been observed when nutrients are withdrawn from one half of a split-root system (Baker and Milburn, 1965). The nature of shoot growth responses to spatial arrangements of water availability, compaction and nutrition is intriguing and might reflect a universal principle of how developmental signals are transduced through whole plants. The capacity of root axes simultaneously experiencing different soil conditions (e.g. wet/dry; loose/compact) to respond with compensatory growth in favourable areas may be important in not only maintaining the total root length of the plant but also determining subsequent shoot growth responses.
The original hypothesis of this study was that the inhibition of shoot growth by compact soil is a function of either the proportion of the root system exposed to the compact soil and/or the timing at which this occurs. In wheat, the leaf responses to localized soil compaction could be described by the timing of root growth into the compact soil and the spatial distribution of roots between soil zones (Masle, 1998). In our experiments, neither the arrival of roots in compacted zones, nor the proportion of roots residing in those zones explained shoot behaviour. For example, 14–26% of roots were present in the compact soil of the compact subsoil treatments without a reduction in growth (Table 3) whereas shoot growth was reduced when 17% of a root system was present in a column of compact soil (Table 4). Clearly, the proportion of the root system exposed to compact soil did not explain the variations in shoot growth.
A repeated observation of plants growing in entirely compact soil is the reduction in early shoot growth (Masle and Passioura, 1987; Hartung et al., 1994). When only the subsoil is compacted, root exposure to the compact soil is delayed. This has led to suggestions that shoots might become less responsive to inhibitory root messages with age (Montagu et al., 1998). However, no evidence was found that the timing of root growth into the compact soil determined the shoot response in these experiments. When shoot growth was unaffected, the delay in broccoli roots reaching the compact subsoil varied from 4–7 d after planting (compact subsoil 75 or 150 mm below the loose soil, respectively). Thus, in all treatments with compact subsoil, roots had come into contact with compact soil prior to the emergence of the first true leaf 10–15 d after planting. It is unlikely that the 4 or 7 d delay in contacting the compact subsoil was sufficient time for the shoot to become unresponsive to any inhibitory root message. This is supported by finding that the leaf growth of 5–7-d-old wheat seedlings remained sensitive to increases in mechanical impedance of the medium roots were growing in (Young et al., 1997). Consequently, the timing of root growth into compact soil could not explain the differing shoot growth responses observed in these experiments.
In this study, neither the proportion of the root system growing in the compact soil nor the timing at which this occurred could explain the differing shoot responses to localized soil compaction. Instead, the strong correlation between root length and leaf area indicated that shoot growth was reduced only when the total root length was reduced by the soil conditions. That is, aggregate root length was important in determining the shoot growth response to localized soil compaction. The contrasting responses of root systems in these experiments might help to explain the variable shoot growth responses that are observed in response to localized soil compaction.
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Acknowledgments |
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The senior author was supported by a postgraduate scholarship provided by the Rural Industries Research and Development Corporation.
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Notes |
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3 Present address: Forest Research and Development Division, State Forests of New South Wales, PO Box 100, Sydney, New South Wales 2119, Australia. Fax: +61 8 9871 6941. E-mail: Kelvinm{at}sf.nsw.gov.au
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