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Effects of soil compaction on the development of rice and maize root systems
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EFFECTS OF SOIL COMPACTION ON THE DEVELOPMENT OF RICE AND MAIZE ROOT SYSTEMS M. IIJIMA, Y. KONO, A. YAMAUCHI and J. R. PARDALES,JR School of Agriculture, Nagoya University, Chikusa, Nagoya 454,Japan (Received 25 November 1990; accepted in reVlsedform 12 March 1991)
IrJIMA M., KONO Y., YAMAUCHI A. and PARDALESJ. R .. JR Effects ofsod compaction on the develupment of rice and maize root ~Yl'tems. ENVIRONMENTAL AND EXPERIMENTAL BOTANY 31, 333-342. 1991. The aim of this study was to determine the effects of soil compaction on the development of root system components of rice (Of)'za sativa L.) and maize (Zea mays L.). Plants were grown for 4 weeks in root boxes (24 em long x 2 em wide x 40 em deep) with soil bulk densities of 1.33 g/cm 3 (control) and 1.50 g/cm 3 (compact). In the compact treatment the main root axes of rice never penetrated beyond the 10-15 em soil layer even by the fourth week, while seminal and seminal adventitious roots of maize had penetrated 30-35 em deep by the third week. Generally, growth of higher order (second and third) lateral roots compensated for the restricted growth of the main root axes in both species. The ratio of first order L-type laterals producing higher order laterals on their axes was greater in the compact treatment for rice, while that of maize was not significantly increased. The root growth responses of rice and maize to soil compaction were different in the downward penetration of the main axis and the growth of the higher order laterals.
INTRODUCTION
and sorghum, and a concentrated type for rice and Job's tears. An analysis of the responses of these two different types of root systems to a compact soil condition would provide an understanding of how these plants manage their growth. This study's goal was to determine interspecific differ ences between rice and maize in terms of the development of their different root system com ponents in mechanically impeded soil.
changes, including the restric tion ofroot extension, increase in radial expansion and proliferation of lateral roots, occur in plants grown in compact soils or mechanically impeded growth media.~.Il-I3' Comparative studies of 2 week-old cereals in compacted soils revealed that (I) the length and number ofhigher order laterals were increased, (2) the lateral roots that have higher elongation growth under the control con ditions exhibited more restricted growth and (3) MATERIALS AND METHODS shoot and root growth ofsorghum and maize were more restricted than that ofrice and Job's tears. r3) Rice (Oryza sativa L., cv. upland rice, Norin In contrast, 4-week-old plants did not clearly 11) and maize (Zea mays L., cv. Robust 30-71), exhibit interspecific differences in growth restric- which have typical concentrated and scattered type tion. This suggests that some growth recovery root systems, respectively,' 151 were selected as test mechanism may exist for sorghum and maize plants. They were grown in root boxes (24 cm between 2 and 4 weeks of growth. long x 2 cm wide x 40 cm deep). Air dried loamy The four species examined have different types sand (87% sand, 9.6% silt, 3.4% clay) was mixed of root systems,' Ib) that is, a scattered type for maize with powdered compound synthetic fertilizer 333 MORPHOLOGICAL
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M. IIJIMA et al.
(12% N; 8% P20 S ; 10% K 2 0) at the rate of 0.2 g/kg of soil. The soil was contained in the root boxes at bulk densities ofl.33 g/cm 1 (control) and 1.50 g/cm 3 (compact). Seeds of both species were pregerminated at 30°C for 2 days and then sown on 14 May 1989 (day 0). The root systems were sampled using the root box pinboard method(7) at weekly intervals until day 28 (fourth week). The root boxes were placed inside a concrete trench under a vinyl shed. The plants were grown under natural light conditions. The weekly aver age temperatures during the experimental period from the first to the fourth week were 18.6, 19.5, 21.1 and 21.4°C, respectively. Watering of the soil was accomplished by submerging the entire root box in water for I hr at I-week intervals. Weekly submersion enabled a regular fluctuation of soil: water content. This method of watering allowed an even distribution of moisture in the soil profile inside the root box. A detailed examina tion of the physical environment of the soil inside the root box has been reported elsewhere. (3.4) For the root boxes sampled on the first week, four seedlings of each species were grown in one box. For the rest of the boxes sampled from the second to the fourth week, three pregerminated seeds of each species were sown near the center of each root box. The seedlings were thinned to one plant per box at 4 days after planting. Each species and treatment had eight replications sampled during the first week, and four replica tions for the rest of the samplings. A total of 56 root boxes were used. Shoots were oven dried at 80°C for 72 hr to obtain dry weight values. Shoot nitrogen content was determined by the Kjeldahl method, and shoot phosphorus content by colorimetric deter mination following NAKAMURA'S procedure. (10) For determination of shoot nitrogen and phos phorus content of the first and second week samples, all replicate plants were mixed due to the small amounts of sample tissues. Photographs of the sampled root system were taken. After this all the root samples were preserved in FAA (for malin, acetic acid, 70% ethanol; 1: I: 18 parts by volume). For each of the species, soil bulk density treat ment and sampling time, only one representative root system (i.e. showing average shoot dry weight, leaf number and seminal, nodal and lat
eral development) was selected from among the replicates and the number and length of all the component roots were determined. It took about 800 hours to measure manually all the com ponent roots from the representative samples. Replicated measurement was not, therefore, con ducted. The root distribution along the soil depth was determined on the photographs of the rep resentative root systems by an image processing system (lEAS II, Zeiss-Kontron, Germany). It was expressed as area covered by the roots at every 5 cm layer starting from the soil surface. For the rest of the replicated samples, the num ber and length ofmain root axes (seminal, seminal adventitious, hypocotyl, and nodal roots), and the number of the two types(3,6,16 1 of first order lateral roots on the seminal axis were measured. The two types of lateral roots are the L-type, which is relatively long, thick and has an ability to produce higher order laterals on its axis, and the S-type, which is relatively short, thin and does not have the same ability as the L-types to produce higher order laterals. In rice, these two types of laterals were reported to differ anatomically. (6) The mean root elongation rate of the main axes was calculated by the following formula: (LIZ~LIZ~ I )/7, where LIZ is the average of the total main axes (or seminal axis) length at the n-th week. The data taken from the replicated samples were subjected to an analysis ofvariance. RESULTS
Shoot growth tended to be restricted in the compact treatment as shown in Table 1; the dry weight ofrice for compacted soil was about 80°'0 that obtained with control soil, and did not show time course changes during the experimental period. In contrast, maize shoot dry weight for compacted soil was significantly reduced especi ally by the second week, but thereafter showed recovery. Shoot nitrogen content tended to follow that of shoot dry weight in both species. In con trast, the ratio of rice shoot phosphorus content decreased with time, while that of maize did not. In the compact treatment, downward rooting was restricted, but there appeared to be no restric tion in horizontal root spread relative to control (Fig. I). Roots of rice in the compact treatment
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS
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EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS Table I. Shoot growth parameters
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16.9±1.2 (84.7)* 556 (94.1) 72.9 (84.4)
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73.8±6.4 (78.5)* 2822±200 (89.9)NS 119.8±6.5 (64.3)**
155.4± 12.3 (82.9)NS 5567 ±544 (81.3)NS 150.6±4.0 (40.3)*
59.2±3.1 (66.9)** 1783 (63.2) 161.9 (81.2)
181.0± 12.7 (84.8)NS 4812±465 (83.I)NS 212.8± 16.2 (94.6)NS
303.4±28.7 (83.4)NS 7050±732 (81.5)NS 267.2±21.0 (72.7)NS
SDW, Shoot dry weight; SNC, shoot nitrogen content; SPC, shoot phosphorus content. Values are means of 4-8 replicates ( ± S.E.). NS, not significant at 5° 0; *, ** significant at P = 0.05 and 0.01, respectively.
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never penetrated the soil layer deeper than lO 15 em, even by the fourth week; maize roots pene trated the same layer during the second week, and reached depths of30-35 em by the third week (Fig. 2). In maize the seminal and/or seminal adventitious roots penetrated deepest. The nodal roots that emerged from the first node followed these latter roots, and reached depths of 15-20 em. By the fourth week, 21 % of the area covered by the roots was below the IS-em soil layer. Elongation of the main root axes of rice and maize was similarly restricted in the compact soil; i.e. 44% and 50%, respectively, by the fourth week (Fig. 3). In contrast, seminal root axis elon gation was restricted differently for the two species; in rice, it was severely restricted through out the experiment, but in maize, the restriction was much less. By the fourth week maize seminal root length in the compact treatment was no
longer significantly different from that of the control. Likewise, although the ratio in the elonga tion rate of the total main axes decreased with time for both species, that of the seminal axes also differed between the two species (Fig. 4). In rice it decreased with time and did not change during the third and fourth weeks. In maize a large amount of restriction was shown by the second week, but thereafter growth was appar ently promoted by soil compaction by the third and fourth weeks. The total number of components in a root sys tem and their total length were restricted in the compact treatment for both species (Fig. 5). Total length ofmain root axes and the first order laterals were severely restricted; the total number and length of the second and third order lateral roots were much less restricted, possibly even promoted. The promotion was particularly
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS
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remarkable in the third order laterals of rice. Based on Fig. 5, percentages of number and length of the second and third order laterals to those of the whole root system were calculated (Table 2). For rice the percentages in the compact treatment were higher than those in the control. The same trend was evident for maize except on the fourth week when the values became lower in the compact treatment. Second and third order laterals are produced on the first order L-type laterals. The production ratio of the first order L-type laterals (number of first order L-type laterals/total number of first order laterals) was therefore determined to com pare the ability of the two species to produce higher order laterals. The ratio was determined for the seminal axis, which was the oldest among
the main axes and whose age was about the same in both species (Fig. 6). The ratio in the compact treatment for rice increased rapidly and reached its maximum by the third week, and remained significantly higher throughout the experiment. For maize the ratio was not significantly higher (P = 0.05) in the compact than in the control treatment.
DISCUSSION
The development of the root system was gen erally restricted by compacted soil in maize and rice. However, the effects of soil compaction differed in degree, depending on the root system component and its species. Compacted soil par
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ticularly affected the downward penetration of the main root axes and the growth ofhigher order laterals. An interspecific difference was most obvi ous in the growth of the first order L-type laterals . During the 4-week experiment for rice in compact soil, the higher order laterals became the more significant components of the root system. This type of change in the root system of maize was observed during the second and third weeks but was only temporary; by the fourth week the maize root system in the compact treatment was not substantially different from that of the control. A previous study that compared lateral root growth offour cereal species in compact soil indi cated that the higher the root elongation rate in the non-compacted soil, the greater the growth restriction in compacted soilY: The same seemed to occur for a given root at different growth stages, e.g. in maize, seminal axis elongation rate in the compact treatment recovered by the third and fourth weeks. At the same time (third and fourth weeks) control seminal axis elongation was much lower compared to that of the first and second weeks. For the seminal axis of rice, this phenom enon was not apparent because the restriction was so severe that root elongation almost ceased by the fourth week. Control seminal axis elongation rate declined by the third week for maize and by the fourth week for rice. However, it should be noted that during these times seminal axes reached the bottom wall of the root box ; root elongation could have been affected by the physi cal stress imposed by the wall.
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Time (weeks after sowing) FIG. 5. Number and total length of each root system component of rice and maize. Main, 1st, 2nd and 3rd indicate main root axes, first order laterals, second order laterals and third order laterals, respectively. Values are taken from the representative root system samples presented in Fig. I.
Table 2. Percentages of number and length of higher order laterals (second and third order laterals) to the whole r(Jot system
Time (weeks after sowing) ._---
Nurn ber of roots
Rice
C T Maize C T
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C, Control; T, compact. Values are taken Ii'om the representativc root systcm samples shown in Fig. 1.
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS
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Time (weeks after sowing) FIG. 6. Production ratio of L-type lateral roots (per cent to the total first order laterals) for seminal root axes of rice and maize. Values arc the means of eight replications for first week sampling and four replications per week for the remain der. Vertical bars are S.E. **. *** indicate statistical significance at P = 0.01 and O.OOL respectively.
GOSS,'2) who used a controlled environment to apply constant pressure to plant roots, reported that the percentage reduction in seminal axes elongation for 22-day-old barley was similar to that of 7-day-old plants. In contrast, SOMAPALA and WILLAT' 14, reported that plant age seemed to influence maize root penetration into soil "aggre gates". Root elongation rate, however, is influ enced by many factors such as the initiation of laterals on the same axis.'9,171 Because the initiation of laterals was modified by compacted soil, main axis elongation rates in the compact treatment may also have been changed with time. Further experiments to examine the phenomenon are required. In this study, the soil moisture condition fluc tuated regularly at weekly intervals. As the liquid component decreased with time, the penetration resistance increased, as measured by a needle type penetrometer developed especially for the narrow root box method.,si As a result, plant root systems were subjected to a soil penetration resist ance gradient from 0 to 0.9 MPa in the control and from 0 to 2.9 MPa in the compact treatment between weekly irrigation intervals. Even though the liquid and gaseous components in the com pact treatment were lower than those of the control, mechanical impedance was considered to be a primary factor that modified the morphology of the root systems.'4' Higher mechanical impedance of the soil in the compact treatment reduced the elongation
growth ofroots, resulting in a smaller rooted zone. The restriction of shoot growth in the compact treatment was most likely caused by restricted water and/or nutrient collection from the reduced soil volume explored by the root system. Recovery of shoot dry weight of maize to control values, starting in the third week, may be explained by the elongation of the main axes (seminal, seminal adventitious) to the deeper zone of the soil during this period. The ratio of shoot dry weight in the compact treatment of rice relative to controls was nearly constant during the experimental period (Table I). This may probably be attributed to an enhanced growth of higher order laterals that compensated for the restricted growth ofthe main root axes, resulting in a densely proliferating root system within a limited soil volume (Figs I and 5). Depletion of phosphorus within the root hair cylinder is essentially complete a few days after a root enters the soil.,1,81 :Moreover, uptake of phosphorus per unit length of root would be related to the increased mass of soil.' I, Thus, as the restriction of downward rooting increased with time in rice, absorption of phosphorus decreased. In this study, the effects of soil compaction on modification of the root system components such as the main root axes (seminal, seminal adven titious, nodal), and different orders of laterals with distinguished L- and S-types were examined. Developmental processes ofeach component were
342
M. IIJIMA et al.
assessed as the growth of the plant progressed. The response of the root system development to soil compaction was clearly different between rice and maize. The functions ofeach root component, such as water and nutrients absorption, need to be studied further to determine their contribution to the growth ofthe whole plant under compacted soil conditions.
Acknowledgement -- We arc grateful to the Ministry of Education, Science and Culture ofJapan for its support of this work under Grant-in-Aid for Co-operative Research (A) 02304017.
7.
8.
9.
10.
REFERENCES
II.
1. CORNISH P. S., So H. B. and MCWILLIAM J. R. (1984) Effects of soil bulk density and water regimen on root growth and uptake of phosphorus by ryegrass. Aust. j. Agric. Res. 35, 631-644. 2. Goss M.j. (1977) Effects of mechanical impedance on root growth in barley (Hordeum vulgare L.). I. Effects on the elongation and branching ofseminal root axes. j. expo Bot. 28, 96-111. 3. hJIMA M. and KONO Y. (1991) Interspecific differences of the root system structures of four cereal species as affected by soil compaction. Jap. j. Crop Sci. 60, 123~131. 4. hJIMA M., KONO Y. and TATSUMIj. (1990) Mutual relationships between patterns of soil penetration resistance and root radial expansion growth of upland rice. Envir. Control Bwl. 28, 53-60. 5. hJIMA M., TATSUMIJ. and KONo Y. (1990) Devel opment of a device for estimating penetration resistance of the soil in the root box. Envzr. Control Biol. 28, 41 ~51. 6. KONO Y., IGETA M. and YAMADA N. (1972) Studies on the developmental physiology of the
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lateral roots in rice seminal roots. Jap. J. Crop Scz. 41, 192~204 (in Japanese with English summary). KONO Y., YAMAUCHI A., NONOYAMA T., TATSUMIJ. and KAWAMURA N. (1987) A revised experimental system of root~soil interaction for laboratory work. Envir. Control Biol. 25, 141~151. LEWIS D. G. and QUIRK j. P. (1967) Phosphate diffusion in soil and uptake by plants. IV. Com puted uptake by model roots as a result ofdiffusive flow. Pl. Soil 26, 454--468. MAY L. H., RANDLES F. H., ASPINALL D. and PALEG L. G. (1967) Quantitative studies of root development. II. Growth in the early stages of development. Aust. j. biol. Sci. 20, 273~283. NAKAMURA M. (1950) Colorimetric determination of phosphorus. Nzppon Nogezkagaku Kazshz 24, 1-5 (in Japanese with English summary). RUSSELL R. S. (1977) Plant root 5.-ystems: theirfunction and interaction wIth the soil. McGraw-Hill, London. RUSSELL R. S. and Goss M. j. (1974) Physical aspects of soil fertility-the response of roots to mechanical impedance. Neth. j. Agnc. Sci. 22, 305~ 318. SCHUURMAN J. J. (1965) Influence of soil density on root development and growth of oats. Pl. SOIL 22,352-373. SOMAPALA H. and VVILLAT S. T. (1979) Effect of plant age on root penetration. Pages 349~361 in R. LAL and D. j. GREENLAND, eds Soil pkvsical propertIes and crop productIOn In the trojnn.John "Viley, Chichester, U.K. YAMAUCHI A., KONO Y. and TATSUMI j. (1987) Quantitative analysis on root system structures of upland rice and maize. Jap. j. Crop Sci. 56, 608 617. YAMAUCHI A., KONO Y. and TATSUMI j. (1987) Comparison of root system structures of 13 species of cereals. Jap. j. Crop SCI. 56,618-631. YORKEJ. S. and SAGAR G. R. (1970) Distribution of secondary root growth potential in the root sys tem of Pisum sativum. Can. j. Bot. 48, 699~704.
\01 H .\Iv J, pp ')'11 '1+2,
I~Yl
Utlg8-8.nl~11 ~3
Prmlt'o III Grf',tt BnL.llll
'J
(JO+(J (J(
I qq I Pergetrnun Prt'... ~ ph
EFFECTS OF SOIL COMPACTION ON THE DEVELOPMENT OF RICE AND MAIZE ROOT SYSTEMS M. IIJIMA, Y. KONO, A. YAMAUCHI and J. R. PARDALES,JR School of Agriculture, Nagoya University, Chikusa, Nagoya 454,Japan (Received 25 November 1990; accepted in reVlsedform 12 March 1991)
IrJIMA M., KONO Y., YAMAUCHI A. and PARDALESJ. R .. JR Effects ofsod compaction on the develupment of rice and maize root ~Yl'tems. ENVIRONMENTAL AND EXPERIMENTAL BOTANY 31, 333-342. 1991. The aim of this study was to determine the effects of soil compaction on the development of root system components of rice (Of)'za sativa L.) and maize (Zea mays L.). Plants were grown for 4 weeks in root boxes (24 em long x 2 em wide x 40 em deep) with soil bulk densities of 1.33 g/cm 3 (control) and 1.50 g/cm 3 (compact). In the compact treatment the main root axes of rice never penetrated beyond the 10-15 em soil layer even by the fourth week, while seminal and seminal adventitious roots of maize had penetrated 30-35 em deep by the third week. Generally, growth of higher order (second and third) lateral roots compensated for the restricted growth of the main root axes in both species. The ratio of first order L-type laterals producing higher order laterals on their axes was greater in the compact treatment for rice, while that of maize was not significantly increased. The root growth responses of rice and maize to soil compaction were different in the downward penetration of the main axis and the growth of the higher order laterals.
INTRODUCTION
and sorghum, and a concentrated type for rice and Job's tears. An analysis of the responses of these two different types of root systems to a compact soil condition would provide an understanding of how these plants manage their growth. This study's goal was to determine interspecific differ ences between rice and maize in terms of the development of their different root system com ponents in mechanically impeded soil.
changes, including the restric tion ofroot extension, increase in radial expansion and proliferation of lateral roots, occur in plants grown in compact soils or mechanically impeded growth media.~.Il-I3' Comparative studies of 2 week-old cereals in compacted soils revealed that (I) the length and number ofhigher order laterals were increased, (2) the lateral roots that have higher elongation growth under the control con ditions exhibited more restricted growth and (3) MATERIALS AND METHODS shoot and root growth ofsorghum and maize were more restricted than that ofrice and Job's tears. r3) Rice (Oryza sativa L., cv. upland rice, Norin In contrast, 4-week-old plants did not clearly 11) and maize (Zea mays L., cv. Robust 30-71), exhibit interspecific differences in growth restric- which have typical concentrated and scattered type tion. This suggests that some growth recovery root systems, respectively,' 151 were selected as test mechanism may exist for sorghum and maize plants. They were grown in root boxes (24 cm between 2 and 4 weeks of growth. long x 2 cm wide x 40 cm deep). Air dried loamy The four species examined have different types sand (87% sand, 9.6% silt, 3.4% clay) was mixed of root systems,' Ib) that is, a scattered type for maize with powdered compound synthetic fertilizer 333 MORPHOLOGICAL
334
M. IIJIMA et al.
(12% N; 8% P20 S ; 10% K 2 0) at the rate of 0.2 g/kg of soil. The soil was contained in the root boxes at bulk densities ofl.33 g/cm 1 (control) and 1.50 g/cm 3 (compact). Seeds of both species were pregerminated at 30°C for 2 days and then sown on 14 May 1989 (day 0). The root systems were sampled using the root box pinboard method(7) at weekly intervals until day 28 (fourth week). The root boxes were placed inside a concrete trench under a vinyl shed. The plants were grown under natural light conditions. The weekly aver age temperatures during the experimental period from the first to the fourth week were 18.6, 19.5, 21.1 and 21.4°C, respectively. Watering of the soil was accomplished by submerging the entire root box in water for I hr at I-week intervals. Weekly submersion enabled a regular fluctuation of soil: water content. This method of watering allowed an even distribution of moisture in the soil profile inside the root box. A detailed examina tion of the physical environment of the soil inside the root box has been reported elsewhere. (3.4) For the root boxes sampled on the first week, four seedlings of each species were grown in one box. For the rest of the boxes sampled from the second to the fourth week, three pregerminated seeds of each species were sown near the center of each root box. The seedlings were thinned to one plant per box at 4 days after planting. Each species and treatment had eight replications sampled during the first week, and four replica tions for the rest of the samplings. A total of 56 root boxes were used. Shoots were oven dried at 80°C for 72 hr to obtain dry weight values. Shoot nitrogen content was determined by the Kjeldahl method, and shoot phosphorus content by colorimetric deter mination following NAKAMURA'S procedure. (10) For determination of shoot nitrogen and phos phorus content of the first and second week samples, all replicate plants were mixed due to the small amounts of sample tissues. Photographs of the sampled root system were taken. After this all the root samples were preserved in FAA (for malin, acetic acid, 70% ethanol; 1: I: 18 parts by volume). For each of the species, soil bulk density treat ment and sampling time, only one representative root system (i.e. showing average shoot dry weight, leaf number and seminal, nodal and lat
eral development) was selected from among the replicates and the number and length of all the component roots were determined. It took about 800 hours to measure manually all the com ponent roots from the representative samples. Replicated measurement was not, therefore, con ducted. The root distribution along the soil depth was determined on the photographs of the rep resentative root systems by an image processing system (lEAS II, Zeiss-Kontron, Germany). It was expressed as area covered by the roots at every 5 cm layer starting from the soil surface. For the rest of the replicated samples, the num ber and length ofmain root axes (seminal, seminal adventitious, hypocotyl, and nodal roots), and the number of the two types(3,6,16 1 of first order lateral roots on the seminal axis were measured. The two types of lateral roots are the L-type, which is relatively long, thick and has an ability to produce higher order laterals on its axis, and the S-type, which is relatively short, thin and does not have the same ability as the L-types to produce higher order laterals. In rice, these two types of laterals were reported to differ anatomically. (6) The mean root elongation rate of the main axes was calculated by the following formula: (LIZ~LIZ~ I )/7, where LIZ is the average of the total main axes (or seminal axis) length at the n-th week. The data taken from the replicated samples were subjected to an analysis ofvariance. RESULTS
Shoot growth tended to be restricted in the compact treatment as shown in Table 1; the dry weight ofrice for compacted soil was about 80°'0 that obtained with control soil, and did not show time course changes during the experimental period. In contrast, maize shoot dry weight for compacted soil was significantly reduced especi ally by the second week, but thereafter showed recovery. Shoot nitrogen content tended to follow that of shoot dry weight in both species. In con trast, the ratio of rice shoot phosphorus content decreased with time, while that of maize did not. In the compact treatment, downward rooting was restricted, but there appeared to be no restric tion in horizontal root spread relative to control (Fig. I). Roots of rice in the compact treatment
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS
335
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2 weeks old
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FIG. 1. Root system profiles of rice and maize grown in root box at two different bulk densities of
1.33 g(cm 3 (control) and 1.50 g(cm 3 (compact).
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS Table I. Shoot growth parameters
(~o
337
of conllol) of rice and mal:::e m the compact soil treatment Time (weeks after sowing)
Rice
SDW (mgjplant) (:~ of control SNC (,ugjplant) 00 of control SPC (,ugjplant) o II of control
5.4±0.1 (83.3)** 169 (84.5) 22.5 (87.0)
Maize SDW (mgjplant) ~ ~ of control SNC (,ugjplant) 00 of control SPC (,ugjplant) 0,<) of control
16.9±1.2 (84.7)* 556 (94.1) 72.9 (84.4)
2
3
4
22.7±2.4 (78.6)NS 861 (78.6) 57.9 (73.4)
73.8±6.4 (78.5)* 2822±200 (89.9)NS 119.8±6.5 (64.3)**
155.4± 12.3 (82.9)NS 5567 ±544 (81.3)NS 150.6±4.0 (40.3)*
59.2±3.1 (66.9)** 1783 (63.2) 161.9 (81.2)
181.0± 12.7 (84.8)NS 4812±465 (83.I)NS 212.8± 16.2 (94.6)NS
303.4±28.7 (83.4)NS 7050±732 (81.5)NS 267.2±21.0 (72.7)NS
SDW, Shoot dry weight; SNC, shoot nitrogen content; SPC, shoot phosphorus content. Values are means of 4-8 replicates ( ± S.E.). NS, not significant at 5° 0; *, ** significant at P = 0.05 and 0.01, respectively.
5 !! 10 15 20 25 30 35 40
(Rice) 1st week
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5 10 15 20 25 30 35 40
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(Maize) 1st week
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o Area (mm 2 ) FIG. 2. Distribution of area covered by roots of rice and maize within the root box (24x40 em). Values are taken from the representative root system samples presented in Fig. I.
338
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Maize
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Time (weeks after sowing) FIG. 3. Length of total main root axes (upper) and seminal root axes (lower) of rice and maize. Values are the means of eight replications for first week sampling and four replications per week for the remainder. Vertical bars are S.E. *, **, *** indicate statistical significance at P = 0.05, 0.01 and 0.001, respectively.
never penetrated the soil layer deeper than lO 15 em, even by the fourth week; maize roots pene trated the same layer during the second week, and reached depths of30-35 em by the third week (Fig. 2). In maize the seminal and/or seminal adventitious roots penetrated deepest. The nodal roots that emerged from the first node followed these latter roots, and reached depths of 15-20 em. By the fourth week, 21 % of the area covered by the roots was below the IS-em soil layer. Elongation of the main root axes of rice and maize was similarly restricted in the compact soil; i.e. 44% and 50%, respectively, by the fourth week (Fig. 3). In contrast, seminal root axis elon gation was restricted differently for the two species; in rice, it was severely restricted through out the experiment, but in maize, the restriction was much less. By the fourth week maize seminal root length in the compact treatment was no
longer significantly different from that of the control. Likewise, although the ratio in the elonga tion rate of the total main axes decreased with time for both species, that of the seminal axes also differed between the two species (Fig. 4). In rice it decreased with time and did not change during the third and fourth weeks. In maize a large amount of restriction was shown by the second week, but thereafter growth was appar ently promoted by soil compaction by the third and fourth weeks. The total number of components in a root sys tem and their total length were restricted in the compact treatment for both species (Fig. 5). Total length ofmain root axes and the first order laterals were severely restricted; the total number and length of the second and third order lateral roots were much less restricted, possibly even promoted. The promotion was particularly
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS
339
Total main axes
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Time (weeks after sowing) FIG. 4. Root elongation rate of total main root axes (upper) and seminal root axes (lower) of rice and maize. Values are the means of eight replications for first week sampling and four replications per week for the remainder. Values in parentheses are percentages of the compact to control.
remarkable in the third order laterals of rice. Based on Fig. 5, percentages of number and length of the second and third order laterals to those of the whole root system were calculated (Table 2). For rice the percentages in the compact treatment were higher than those in the control. The same trend was evident for maize except on the fourth week when the values became lower in the compact treatment. Second and third order laterals are produced on the first order L-type laterals. The production ratio of the first order L-type laterals (number of first order L-type laterals/total number of first order laterals) was therefore determined to com pare the ability of the two species to produce higher order laterals. The ratio was determined for the seminal axis, which was the oldest among
the main axes and whose age was about the same in both species (Fig. 6). The ratio in the compact treatment for rice increased rapidly and reached its maximum by the third week, and remained significantly higher throughout the experiment. For maize the ratio was not significantly higher (P = 0.05) in the compact than in the control treatment.
DISCUSSION
The development of the root system was gen erally restricted by compacted soil in maize and rice. However, the effects of soil compaction differed in degree, depending on the root system component and its species. Compacted soil par
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ticularly affected the downward penetration of the main root axes and the growth ofhigher order laterals. An interspecific difference was most obvi ous in the growth of the first order L-type laterals . During the 4-week experiment for rice in compact soil, the higher order laterals became the more significant components of the root system. This type of change in the root system of maize was observed during the second and third weeks but was only temporary; by the fourth week the maize root system in the compact treatment was not substantially different from that of the control. A previous study that compared lateral root growth offour cereal species in compact soil indi cated that the higher the root elongation rate in the non-compacted soil, the greater the growth restriction in compacted soilY: The same seemed to occur for a given root at different growth stages, e.g. in maize, seminal axis elongation rate in the compact treatment recovered by the third and fourth weeks. At the same time (third and fourth weeks) control seminal axis elongation was much lower compared to that of the first and second weeks. For the seminal axis of rice, this phenom enon was not apparent because the restriction was so severe that root elongation almost ceased by the fourth week. Control seminal axis elongation rate declined by the third week for maize and by the fourth week for rice. However, it should be noted that during these times seminal axes reached the bottom wall of the root box ; root elongation could have been affected by the physi cal stress imposed by the wall.
Rice (Compact)
0 6000 5000
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Time (weeks after sowing) FIG. 5. Number and total length of each root system component of rice and maize. Main, 1st, 2nd and 3rd indicate main root axes, first order laterals, second order laterals and third order laterals, respectively. Values are taken from the representative root system samples presented in Fig. I.
Table 2. Percentages of number and length of higher order laterals (second and third order laterals) to the whole r(Jot system
Time (weeks after sowing) ._---
Nurn ber of roots
Rice
C T Maize C T
---
Total root length
2
:3
4
2
3
4
20.4 52.5 6.8 19.6
64.5 65.7 29.1 39.1
51.0 69.9 65.1 50.2
5.7 17.4 1.6 4.5
28.6 37.5 8.2 18.9
27.3 43.1 31.5 30.6
C, Control; T, compact. Values are taken Ii'om the representativc root systcm samples shown in Fig. 1.
EFFECTS OF SOIL COMPACTION ON ROOT SYSTEMS
-
-
~
~
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0
CD
a.
40
40
Rice
30
30
20
20
10
10
0
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341
Maize
control
-0
2
compact
4
Time (weeks after sowing) FIG. 6. Production ratio of L-type lateral roots (per cent to the total first order laterals) for seminal root axes of rice and maize. Values arc the means of eight replications for first week sampling and four replications per week for the remain der. Vertical bars are S.E. **. *** indicate statistical significance at P = 0.01 and O.OOL respectively.
GOSS,'2) who used a controlled environment to apply constant pressure to plant roots, reported that the percentage reduction in seminal axes elongation for 22-day-old barley was similar to that of 7-day-old plants. In contrast, SOMAPALA and WILLAT' 14, reported that plant age seemed to influence maize root penetration into soil "aggre gates". Root elongation rate, however, is influ enced by many factors such as the initiation of laterals on the same axis.'9,171 Because the initiation of laterals was modified by compacted soil, main axis elongation rates in the compact treatment may also have been changed with time. Further experiments to examine the phenomenon are required. In this study, the soil moisture condition fluc tuated regularly at weekly intervals. As the liquid component decreased with time, the penetration resistance increased, as measured by a needle type penetrometer developed especially for the narrow root box method.,si As a result, plant root systems were subjected to a soil penetration resist ance gradient from 0 to 0.9 MPa in the control and from 0 to 2.9 MPa in the compact treatment between weekly irrigation intervals. Even though the liquid and gaseous components in the com pact treatment were lower than those of the control, mechanical impedance was considered to be a primary factor that modified the morphology of the root systems.'4' Higher mechanical impedance of the soil in the compact treatment reduced the elongation
growth ofroots, resulting in a smaller rooted zone. The restriction of shoot growth in the compact treatment was most likely caused by restricted water and/or nutrient collection from the reduced soil volume explored by the root system. Recovery of shoot dry weight of maize to control values, starting in the third week, may be explained by the elongation of the main axes (seminal, seminal adventitious) to the deeper zone of the soil during this period. The ratio of shoot dry weight in the compact treatment of rice relative to controls was nearly constant during the experimental period (Table I). This may probably be attributed to an enhanced growth of higher order laterals that compensated for the restricted growth ofthe main root axes, resulting in a densely proliferating root system within a limited soil volume (Figs I and 5). Depletion of phosphorus within the root hair cylinder is essentially complete a few days after a root enters the soil.,1,81 :Moreover, uptake of phosphorus per unit length of root would be related to the increased mass of soil.' I, Thus, as the restriction of downward rooting increased with time in rice, absorption of phosphorus decreased. In this study, the effects of soil compaction on modification of the root system components such as the main root axes (seminal, seminal adven titious, nodal), and different orders of laterals with distinguished L- and S-types were examined. Developmental processes ofeach component were
342
M. IIJIMA et al.
assessed as the growth of the plant progressed. The response of the root system development to soil compaction was clearly different between rice and maize. The functions ofeach root component, such as water and nutrients absorption, need to be studied further to determine their contribution to the growth ofthe whole plant under compacted soil conditions.
Acknowledgement -- We arc grateful to the Ministry of Education, Science and Culture ofJapan for its support of this work under Grant-in-Aid for Co-operative Research (A) 02304017.
7.
8.
9.
10.
REFERENCES
II.
1. CORNISH P. S., So H. B. and MCWILLIAM J. R. (1984) Effects of soil bulk density and water regimen on root growth and uptake of phosphorus by ryegrass. Aust. j. Agric. Res. 35, 631-644. 2. Goss M.j. (1977) Effects of mechanical impedance on root growth in barley (Hordeum vulgare L.). I. Effects on the elongation and branching ofseminal root axes. j. expo Bot. 28, 96-111. 3. hJIMA M. and KONO Y. (1991) Interspecific differences of the root system structures of four cereal species as affected by soil compaction. Jap. j. Crop Sci. 60, 123~131. 4. hJIMA M., KONO Y. and TATSUMIj. (1990) Mutual relationships between patterns of soil penetration resistance and root radial expansion growth of upland rice. Envir. Control Bwl. 28, 53-60. 5. hJIMA M., TATSUMIJ. and KONo Y. (1990) Devel opment of a device for estimating penetration resistance of the soil in the root box. Envzr. Control Biol. 28, 41 ~51. 6. KONO Y., IGETA M. and YAMADA N. (1972) Studies on the developmental physiology of the
12.
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lateral roots in rice seminal roots. Jap. J. Crop Scz. 41, 192~204 (in Japanese with English summary). KONO Y., YAMAUCHI A., NONOYAMA T., TATSUMIJ. and KAWAMURA N. (1987) A revised experimental system of root~soil interaction for laboratory work. Envir. Control Biol. 25, 141~151. LEWIS D. G. and QUIRK j. P. (1967) Phosphate diffusion in soil and uptake by plants. IV. Com puted uptake by model roots as a result ofdiffusive flow. Pl. Soil 26, 454--468. MAY L. H., RANDLES F. H., ASPINALL D. and PALEG L. G. (1967) Quantitative studies of root development. II. Growth in the early stages of development. Aust. j. biol. Sci. 20, 273~283. NAKAMURA M. (1950) Colorimetric determination of phosphorus. Nzppon Nogezkagaku Kazshz 24, 1-5 (in Japanese with English summary). RUSSELL R. S. (1977) Plant root 5.-ystems: theirfunction and interaction wIth the soil. McGraw-Hill, London. RUSSELL R. S. and Goss M. j. (1974) Physical aspects of soil fertility-the response of roots to mechanical impedance. Neth. j. Agnc. Sci. 22, 305~ 318. SCHUURMAN J. J. (1965) Influence of soil density on root development and growth of oats. Pl. SOIL 22,352-373. SOMAPALA H. and VVILLAT S. T. (1979) Effect of plant age on root penetration. Pages 349~361 in R. LAL and D. j. GREENLAND, eds Soil pkvsical propertIes and crop productIOn In the trojnn.John "Viley, Chichester, U.K. YAMAUCHI A., KONO Y. and TATSUMI j. (1987) Quantitative analysis on root system structures of upland rice and maize. Jap. j. Crop Sci. 56, 608 617. YAMAUCHI A., KONO Y. and TATSUMI j. (1987) Comparison of root system structures of 13 species of cereals. Jap. j. Crop SCI. 56,618-631. YORKEJ. S. and SAGAR G. R. (1970) Distribution of secondary root growth potential in the root sys tem of Pisum sativum. Can. j. Bot. 48, 699~704.