Relationships between soil structure, root distribution and
water uptake of chickpea (
Cicer arietinum
L.). Plant growth
and water distribution
Alessandra Pardo
a, Mariana Amato
a,*, Fabrizio Quaglietta Chiaranda`
baDipartimento di Produzione Vegetale,Uni
6ersita‘della Basilicata,6ia Nazario Sauro 85,85100 Potenza, Italy
bDipartimento di Ingegneria del Territorio ed Agronomia Ambientale,Uni
6ersita‘di Napoli Federico II,Napoli, Italy Received 6 July 1999; received in revised form 5 January 2000; accepted 14 March 2000
Abstract
Root clustering as a consequence of soil compaction has been hypothesised as a cause of reduction in both water uptake and growth of plants. On the basis of such a hypothesis models of water uptake that take root spatial arrangement into account have been proposed. Data to validate such models are scarce, particularly for leguminosae. This research was conducted on chickpea with the aim of studying the spatial distribution of roots and water in growth media of different degrees of compaction. Chickpea var. Sultano was grown in pots containing silty-clay soil with aggregates of B0.5 cm in diameter for treatment TF (fine soil), and fine soil + a large clod (representing 30 – 33% of the total soil dry weight) for treatment TF+Z. Each pot was brought to field capacity and covered with mulch to avoid evaporation losses. Plants were grown on stored water. The treatment with homogeneous soil (TF) showed a higher above-and below-ground growth and water uptake compared with those of TF+Z. A hyperbolic relation was found between root density and soil resistance penetration. Roots were distributed quite homogeneously in the fine soil of both treatments, but they colonised only the outer parts of the clods. At the end of the experiment treatment TF+Z showed unextracted water in the clods. The spatial distribution of roots and the plant ability to take up water were strongly affected by soil structural conditions. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Chickpea; Spatial distribution of roots; Soil compaction
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1. Introduction
The effects of soil tillage on crops are manifold and of different relevance according to pedocli-matic conditions. In particular, research on the
role of root systems on the behaviour of plants in untilled soils has focused on different mechan-isms of reduced root functionality and water up-take.
The classical view is that root systems in com-pacted soils are more concentrated in the surface layers and cannot access the water stored in deep soil (Hamblin, 1985). Other researchers have worked on direct effects of soil compaction on the
* Corresponding author. Tel.:+39-971-474268; fax:+ 39-971-474269.
E-mail address:amato@unibas.it (M. Amato)
reduction of shoot growth, probably mediated by chemical signals from roots (Masle and Passioura, 1987), thereby suggesting that a low uptake of water may be a consequence rather than a cause of reduced plant size. Another subject of specula-tion and experimental work is that of spatial distribution of roots within a layer of soil. It has been shown that roots tend to cluster in areas of lower penetration resistance of soils, such as non-compacted inter-rows, or cracks in swelling clays (Tardieu and Manichon, 1986a,b, 1987a,b; Am-ato, 1991). As a consequence of a clustered root distribution there are areas of soil where the root density is very low and water is not taken up at a rate useful for plants. This may amount to a relevant reduction in uptake compared to a soil in which the same root system is more uniformly distributed (Tardieu, 1987; Amato, 1991). This concept provides the basis for models of root water uptake that specifically take root spatial distribution into account (Passioura, 1985; Lafolie et al., 1991; Petrie and Hall, 1992).
Data to validate such models are still scarce, though, and especially those documenting the dis-tribution of water around roots and root clusters at a small scale (i.e. cm). Amato (1991) has pro-vided maps of roots and soil water content for maize grown on different media and used them in a model to test the hypothesis that the spatial distribution of roots could account for incomplete extraction of water in case of clustered roots. There is no such data on legumes, though. Previ-ous research on the comparison of wheat and
chickpea under different tillage methods has shown that the legume crop is more sensitive than wheat to soil compaction in terms of root distri-bution (Pardo, 1998). In order to further investi-gate the mechanisms of yield reduction and water uptake of chickpea in response to soil com-paction, this research studies the effect of soil structural status and decreasing water availability on the spatial distribution of roots and water in the soil, and on total water uptake and plant growth of chickpea.
2. Materials and methods
The experiment was conducted on chickpea (Cicer arietinum L. var. Sultano), grown in cylin-drical pots of 7000 cm3volume placed in a green-house at Potenza (Italy) on a silty-clay soil (Table 1), taken from the field at Corleto Perticara (Potenza). Two treatments were compared with six replicates; TF (fine soil), with sieved soil com-posed of aggregates smaller than 0.5 cm of diame-ter, and TF+Z (fine soil+clod), with the same soil in the centre of which a large clod was placed. The clod weighed 30 – 33% of the total soil dry weight.
Each pot was fertilised with phosphorus (0.6 g P/pot) and wetted to field capacity. The water content at the onset of the experiment was about 0.29 cm3cm−3in the TF treatment, and 0.31 cm3 cm−3 in the TF+Z treatment. The higher value in this treatment was due to the presence of the clod, more compacted and therefore characterised by a lower percent of large pores. After seedling (03/11/1994) emergence plants were thinned to one per pot. The soil surface was mulched with a 4 cm layer of fritted clay in order to reduce soil evaporation, and the experiment was terminated (04/21/1994) when plants completely stopped their growth. The greenhouse minimum temperature during the growth cycle was 10.2°C and the max-imum temperature ranged between 25 and 30°C. The relative humidity ranged between 40% during the daylight hours and 90% at night.
During the crop cycle plant height and water loss by weighing were measured every 3 – 5 days.
Table 1
Physical and chemical soil characteristics
(0.2B¥B2 mm) 26 g/kg
Organic matter 12.9 g/kg
At the end of the experiment (41 days from sowing) the following characters were determined on plants:
leaf surface area with LI-COR mod 3100 area
meter.
above-ground dry matter after drying at 70°C
until constant weight was reached.
root length density (R.L.D.), on soil samples
collected at the depth of 15 cm, dispersed with Calgon solution and sieved on a 0.2 mm square opening sieve, the length of collected roots was determined with a line intersection method (Newman, 1966). The bulk density of the soil (see below) was then used to calculate length density.
root dry matter, determined after measuring
the mean specific weight of roots per unit length.
Soil measurements were made at 13 – 18 cm depth on soil samples of about 60 g:
gravimetric water content after drying at
105°C.
It was determined in soil clods of about 60 g weight. The values of ms were determined gravi-metrically on separate samples. The bulk volume
VSwas measured by weighing clods in air and in water, after impermeabilization with Saran resin (Blake and Hartge, 1986).
penetration resistance with conic tip
penetrom-eter, with basal diameter of 2.4 mm and angle of 30°.
Each of the above measurements was made on three replications on the bulk soil for three differ-ent regions of soil for treatmdiffer-ent TF+Z: bulk soil, and two layers for the clods, representing two concentric regions of annular width 3 cm. Mea-surements of soil water content of clods were made on three layers, representing three concen-tric regions of annular width 1, 2 and 3 cm starting from the outer layer. Results are reported as referred to the different regions, or as a
weighted average for the whole soil layer in treat-ment TF+Z.
The spatial distribution of roots was deter-mined by in situ cartography (Bo¨hm, 1979). This method consists in locating roots on a horizontal plane. A plane at 15 cm from the soil surface was chosen and prepared according to Bo¨hm (1979). The position of roots was recorded on transparent plastic sheets placed on the observation plane. Root distribution was then analysed using the variance/mean ratio as reported in Grieg Smith (1983), by dividing the observation plane in 2×2 cm cells, by means of a grid. Cells adjacent to the soil-pot interface were not analysed. A t-test was used to compare the values of the variance/mean ratio with each-other and with the theoretical value of 1, corresponding to a random distribu-tion of roots with the hypothesis of a Poisson distribution of the number of root contacts per grid cell (Grieg Smith, 1983).
3. Results
The time-course of plant height is shown in Fig. 1a. It shows at first fast growth, followed by a reduction in growth rate, and significantly higher values for treatment TF starting from day 14. At the end of the experiment plant height was 27.3 cm for treatment TF and 23.2 cm for TF+Z.
The time-course of cumulated water losses (Fig. 1b) shows an initial period (until 22 days from sowing) of similar behaviour of the treatments, and thereafter constantly higher (highly significant differences) losses for treatment TF. This different behaviour was confirmed by the highest amount of unextracted water in the clods of treatment TF+Z, as shown in Fig. 2a, which reports the volumetric water content of the soil at the end of the experiment for both treatments and separately for fine soil and clods in treatment TF+Z.
Fig. 1. Time-course of (a) plant height; (b) soil water uptake in the two treatments. Vertical bars represent LSD forP50.01.
tance were related to root growth, as reported in Fig. 2d, which shows that RLD was not
signifi-Fig. 2. Soil and root characteristics in the different structural regions of the pots for treatment TF and TF+Z: (a) water content; (b) bulk density; (c) penetration resistance; (d) root length density. Vertical bars represented the LSD forP50.01.
growth. This resulted in a lower shoot/root ratio for this treatment. The average root length den-sity at the end of the experiment was significantly higher for treatment TF. The weight per unit length of roots was on average 0.43×10−4
g cm−1, and did not show significant differences between treatments.
The physical characteristics of fine soil were not different between treatments, but they differed significantly between fine soil and the clods of treatment TF+Z (Fig. 2):
the bulk density of treatment TF+Z was 1.32
g cm3
in the fine soil, not different from that of treatment TF, and 27.7% lower than that of the clod (1.69 g cm−3 on average between internal and external areas of the clod, which were not significantly different) (Fig. 2b).
soil penetration resistance was highest in the
clods, especially in the inner part, and lowest in the fine soil (Fig. 2c).
resis-Table 2
Leaf area, shoot and root dry matter, shoot to root ratio and root length density of plant for TF and TF+Z treatment
TF+Z TF
Leaf area (cm2) 122.25 86.991
LSD 0.05P=22
1.434 0.909 Shoot dry matter (g)
LSD 0.05P=0.264
0.540
Root dry matter (g) 0.466
LSD 0.05P=0.069
2.670
Shoot to root ratio 1.950
LSD 0.05P=0.684
2.440 Root length density (cm cm−3) 2.030
LSD 0.05P=0.311
of roots between treatments is reflected in the variance/mean ratio: values were 0.904 for treat-ment TF, and 1.28 for TF+Z. The first value was not significantly different from 1, this indicates a random distribution of roots grown in fine soil; the value of treatment TF+Z was significantly higher than 1 (and different from that of treat-ment TF), because of the aggregate distribution of roots.
As a consequence of the above described distri-bution of penetration resistance and root density, the relationship between root length density and penetration resistance (Fig. 4) was highly signifi-cant. The function was a hyperbolic one (negative power of the independent variable). The density of roots decreases fast until soil resistance to penetration reaches a value of about 3 MPa, and thereafter it tends to stable values of 0.5 cm cm−3.
4. Discussion
Chickpea plants showed a different behaviour when grown on a medium which was uniform in structure compared to one constituted of aggre-gates of different dimensions. On one side plant size was higher in the uniform treatment, and on the other the reduction in growth of the spatially variable treatment was more evident for the top than for the root, as suggested by Aung (1977) for plants experiencing unfavourable soil conditions. This behaviour, which resulted in a lower value of the weight ratio between epigeic and hypogeic
Fig. 3. Root contact points on a horizontal plane at 15 cm in a pot of treatments TF (a) and TF+Z (b).
Fig. 4. Relationship between root density and soil resistance to penetration.
parts for treatment TF+Z, is in general com-mented in terms of response to environmental conditions which impair root functionality, like water or nutrient shortage (Hamblin et al., 1990) and insufficient aeration (Smit et al., 1989). Also, the reduction in plant shoot biomass (37%) and total biomass (30%) was proportionally higher than the reduction in water lost (15%). Since a mulch had been used to reduce evaporation, part of the water used equals transpiration. The exper-iment therefore suggest a reduction in water use efficiency in the TF+Z treatment.
In the presence of soil compaction and water deficiency plants showed a lower water use effi-ciency due to several causes, which partially could have resulted from the increased proportion of carbon lost by the plant because of root exuda-tion, which can reach very high values under stress conditions (Hale and Moore, 1979; Lam-bers, 1987).
The range of values of soil bulk densities of our experiment is similar to that measured in the field by Amato et al. (1994) for the same soil. Penetra-tion resistance was of the same order of magni-tude to those reported by Taylor and Ratliff (1969), although somewhat higher. Both values of bulk density and resistance to penetration indicate that the soil clods were considerably more com-pacted and harder to penetrate than the surround-ing soil, even under soil conditions more favourable to those at the end of the experiment. Roots were not found in clods beyond 2 cm from the surface, showing a tendency to concentrate in fine soil. This kind of behaviour was shown by Amato (1991) for roots ofZea maysin clods: the author reports a distance of about 2 cm as the maximum at which roots can penetrate.
The incomplete colonisation of clods resulted in a lower water uptake in treatment TF+Z. This agrees with reports by Amato (1991) in experi-ments with maize, but gradients of water content measured in the clods were higher in that investigation.
It is important to notice that a gradient in water content within the clod is due to differences in water extraction, while the different values of water content recorded between the fine soil and the clod are also due to the different porosity
indirectly documented by the differences in bulk density.
Tardieu and Manichon (1987b) reported a re-duction in water uptake of Zea mays from com-pacted regions of soil. Such regions are generally associated with both low availability of oxygen and a reduced saturated conductivity of the soil water. Nevertheless there is evidence that a re-duced plant size in compacted soils occurs inde-pendently of the availability of oxygen and water (Russell and Goss, 1974; Masle and Passioura, 1987).
5. Conclusions
The presence of structural obstacles in the growth medium strongly affected the growth of plants and the spatial distribution of roots and water in the soil.
The above-ground growth was reduced more than the below-ground growth, this indicating a lower unit root functionality in the pots contain-ing soil clods.
Gradients of water were measured across soil clods in which roots were found only at the clod surface at the end of the experiment, this indicat-ing a slower extraction of water from soil struc-tural units.
Our experiment cannot discriminate between direct and indirect causes of the reduction in plant size in the presence of clods, but a reduction of the amount of water potentially extractable by the plants through a reduction of the volume of soil colonised by roots was shown.
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