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CROP ECOLOGY, PRODUCTION & MANAGEMENT

Root Development of Maize (Zea mays L.) as Observed with Minirhizotrons in Lysimeters

^a ETH Zürich, Institute of Plant Sciences, FEL, Eschikon 33, CH-8315 Lindau, Switzerland
^b ETH Zürich, Institute of Plant Sciences, ETH-Zentrum, Universitästr. 2, CH-8092 Zürich, Switzerland

markus.liedgens{at}ipw.agrl.ethz.ch

	ABSTRACT

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Knowledge of root growth patterns is relevant for agronomic management practices. The growth of maize roots (Zea mays L.) was studied by means of minirhizotrons in lysimeters. Ten minirhizotrons (60-mm outer diameter) were placed horizontally at soil depths between 5 and 100 cm, perpendicular to the maize row. The root density (roots cm^-2) on images (2.43 cm²) was observed at weekly intervals. During the growing season, the root density increased to a maximum and then decreased, a pattern observed in 3 yr and at all depths and positions relative to the plant row. Growing degree days, compared with chronological time, did not improve the description of temporal root growth patterns. Maximum root density did not occur at the same time as pollen shed, in contrast to maximum leaf area. At shallow depths, the maximum root density was observed at the same time as pollen shed, while at a depth of 100 cm, the maximum root density occurred 2 wk after pollen shed. The maximum root density was observed about 10 d after pollen shed at most positions relative to the plant row. In conclusion, the maximum root density and hence resource uptake capacity of the root system was reached with some delay in relation to the vegetative development of the crop.

Abbreviations: GDD, growing degree days • LAI, leaf area index • LOESS, local regression • T_b, base temperature

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results REFERENCES

 
THE DEVELOPMENT of the root system of agricultural crops is not as well known as that of the shoot. However, roots also are important in the overall growth processes of the plant as well as for the uptake of nutrients and water. Knowledge of timing and location of root development is necessary for planning and executing agricultural operations such as timing and placement of fertilizers and irrigation, mechanical weed control, and for evaluating below-ground interactions of crops and weeds in conventional agriculture or among intercropped species in more complex agricultural systems or alley cropping. The capacity of the maize root system to change its shape, as reflected by changes in morphological characteristics, such as root dry weight (Derieux et al., 1994), length (Shalhevet et al., 1995), diameter (Teyker and Hobbs, 1992), branching (Liang et al., 1996), and orientation (Nakamoto, 1989), as well as its relation to the shoot (Demotes-Mainard and Pellerin, 1992) is fundamental for coping with a very variable soil environment.

The difficulties associated with observing roots in an inaccessible environment promoted the development of many investigative techniques (Böhm, 1979). Our limited knowledge of root systems is further restricted by the specific biases inherent to these experimental techniques. Destructive techniques are of limited value for investigating the developmental processes that occur at a spatial resolution several times smaller than the size of the sampling unit, while all non-destructive techniques include a certain degree of artificiality and violate the basic scientific assumption that there is no interaction between the experimental method and the studied object.

On the other hand, root development and growth are seldom studied. Non-destructive techniques such as observations of roots through transparent soil interfaces make it easier to separate the spatial and temporal variability of root growth as compared with destructive studies (McMichael and Taylor, 1987). Restricting root investigations to descriptions of depth and temporal patterns may be enough to characterize the root systems of homogeneous canopies (pastures, grasslands, and crops with small row intervals). For row crops, however, water and nutrient uptake is also largely influenced by the establishment of the root system in the interrow (Tardieu, 1988).

The most commonly studied aspect of maize rooting is the vertical distribution of roots in the soil profile (Reeves et al., 1992; Sharp and Davies, 1985; Wiesler and Horst, 1994). The objective of this work, therefore, was to characterize the time course of maize root development at several vertical and horizontal locations in the soil. This information is needed for prescribing the timing of agricultural management operations such as nitrogen fertilization with respect to optimizing resource utilization, and minimizing root system damage by mechanical weed control. The temporal aspects of roots spreading from near the soil surface in the plant row downwards and into the interrow were investigated for 3 yr by means of minirhizotrons that were placed horizontally at different depths in drainage lysimeters with repacked soil.

	Materials and methods

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The Experiment
The experiment was conducted for 3 yr (1994–1996) at the Experimental Station of the Institute of Plant Sciences, Swiss Federal Institute of Technology, at Eschikon (ZH). Soil processes and plant growth were studied in a lysimeter facility. In the year before the experiment started (1993), aboveground drainage lysimeters (square surface area of 1 m² and soil depth of 110 cm) were uniformly repacked with the topsoil of a slightly alkaline (pH 7.2–7.5) sandy loam soil with a low soil organic matter content. Ten minirhizotrons (between 5- and 100-cm soil depths), crossing the lysimeter horizontally, were installed during soil filling in vertical rows at 12.5, 37.5, 62.5, and 87.5 cm from the lysimeter side, providing a horizontal distance of at least 50 cm between minirhizotrons at successive soil depths. Maize (cv. Atlet; FAO 250) was planted each year in four replications. To allow for root decomposition, lysimeters were kept without plant cover in the year before experimentation. Soil preparation was limited to surface scarification to avoid damage to the uppermost minirhizotrons. The lysimeters were kept weed-free in the year before the experiments to ensure root-free soil at the beginning of the investigations. Sowing dates were 10 May 1994, 16 May 1995, and 21 May 1996. Maize was planted in a row in the middle of the lysimeter at a density of seven plants per square meter. Lateral shading screens were mounted at the edges of the lysimeters and parallel to the maize row to simulate the presence of neighboring rows. During the growing season, the plots were irrigated with 20 mm of water whenever tensiometer measurements in the topsoil fell below -0.08 MPa. Nitrogen fertilizer (NH₄NO₃ placed in the maize row) was applied twice: 50 kg N ha^-1 at planting and 60 kg N ha^-1 when plants reached the stage of three fully expanded leaves. Other nutrients were broadcast at planting (42 kg ha^-1 P, 240 kg ha^-1 K, 17 kg ha^-1 Mg, 61 kg ha^-1 Ca, and 26 kg ha^-1 S). Crop protection was carried out as necessary. Maize harvests were on 16 September 1994, 2 October 1995, and 30 September 1996.

Data Sampling
Plant shoots were characterized by leaf area growth and by phenological development (Ledent et al., 1990). The leaf area of fully expanded leaves of all plants was calculated from measurements of leaf length and maximum width as follows:

where LA is leaf area (cm²); a and b are coefficients (0.75 and 0, respectively); L is leaf length (cm); and W is maximum leaf width (cm). Leaf senescence was visually estimated according to the percentage of lost or yellow leaf area. Green leaf area was calculated for each plant according to the difference in the total formed and total lost leaf area and summed over plots. The measurements of leaf length and width as well as the recording of the developmental stages were made two to three times per week. Leaf area was transformed to leaf area indices (LAI) on a plot basis.

Roots were observed at the soil interface of minirhizotrons (outer diameter 60 mm). The tubes were installed horizontally at depths of 5, 10, 15, 20, 25, 30, 45, 60, 80, and 100 cm, perpendicular to the plant row. Root images were recorded weekly (with the exception of the late 1994 season, when technical problems made recording impossible) using a video system from Bartz Technology Company (Santa Barbara, CA). Individual images (13.5 mm high and 18 mm wide, giving an area of 2.43 cm²) were recorded along a strip (40 cm long) starting at the plant row and showing the upper side of the minirhizotrons. The images were evaluated in the laboratory, where the recorded tapes were displayed on a monitor.

The root members in each image were counted according to Upchurch and Ritchie (1983). The numbers of root members in each minirhizotron were averaged over six segments (positions relative to the plant row) with five consecutive images each. Root density was defined as number of root members per square centimeter of the surface area of the minirhizotron.

The temporal progression of root density is represented chronologically. These time trends are complemented by non-parametric local regression (LOESS) curves (Cleveland, 1993) that enable improved visualization of data (reduction of the large variation in mean root density on successive sampling dates) but not a quantitative time trend analysis. An alternative time scale, based on growing degree days (GDD), was considered. GDD were calculated from air temperature, with a base temperature (T_b) of 8°C (Derieux and Bonhomme, 1990).

The huge amount of root images stored on video tapes did not allow the construction of time series of images, as proposed by Dubach and Russelle (1995). This circumstance limits our characterization of root growth to time trends of net root density. "Time to maximum root density," representing the time interval between planting and the observation of maximum root density, was considered to be the most relevant root developmental parameter. To compare root and shoot growth the "relative time of maximum root density" was introduced and represents the time lag between the observation of the maximum number of roots and the observation of pollen shed, indicating the degree of lack of synchronization of shoot and root development. Root developmental parameters were calculated on a plot basis (for the evaluation of the effect of year), on a position basis (for the evaluation of the effects of position and the position x year interaction), on a soil depth basis (for the evaluation of the effects of soil depth and the soil depth x year interaction), and for each soil depth x position combination (for the evaluation of the effect of the soil depth x position interaction).

Statistical Analysis
Data were analyzed according to the mixed effects model theory by SAS PROC MIXED (Littell et al., 1996). Years were considered fixed and replications random effects for all analyses except for the interaction of soil depths and positions, where both years and replications were considered random effects. For shoot developmental parameters, plants within plots, and for root developmental parameters, positions and soil depths were introduced into the analysis as repeated measurements.

	Results

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Root Development
Figure 1 shows the time course of maize root development at depths of 15, 45, and 80 cm, located 0 to 67, 135 to 202, and 337 to 405 mm away from the plant row, respectively. The same general time trend can be observed for LOESS fits at all depths, positions relative to the plant row, and years: an initial phase with a small root density, a phase of rapid increase towards a maximum, followed by a final decrease. The missing sampling dates at the end of the 1994 growing season limit the analysis of growth characteristics in this year. Unexpectedly minimum root densities (evident from raw and fitted data) were observed sometimes after planting at various soil locations. These observations are believed to be artifacts (possibly caused by decaying roots of the previous crop or weeds).

View larger version (29K): [in this window] [in a new window]   Fig. 1 Mean root density (roots cm-2) as observed with minirhizotrons. Data from various depths (15, 45, and 80 cm), positions relative to the maize row (0–67, 135–202, and 337–402 mm), and years (1994, 1995, and 1996) are shown. Symbols and lines represent observed means and non-parametric LOESS fits, respectively

Shoot Development
The development of maize shoots is described according to phenological stages and the development of leaf area (Fig. 2) . Early phenological development was faster in 1996, a consequence of the warmer weather after sowing (Fig. 3) , but differences disappeared by the time of pollen shed (Table 1) . Leaf area development was similar in the 3 yr: it follows a sigmoidal pattern, with slow early growth, intense growth towards the middle of the season (start of reproductive growth), and a slower decay towards harvest. The comparisons of the LAI and the time until pollen shed are shown in Table 1. Leaf area differences among the years were large, but only the values in 1996 were significantly lower. The lack of statistical significance in the comparison of leaf areas between 1994 and 1995 despite a difference of 17% may have been a consequence of high variability among plots: the establishment of the stand was affected by non-uniform germination and damping-off of some plants, while transplanting seedlings did not help to make the plant stand more uniform. The timing of pollen shed varied less among years than the leaf area indices recorded at the same time.

View larger version (22K): [in this window] [in a new window]   Fig. 2 Characterization of shoot development during the three experimental years. (A) Plant developmental stages (according to Ledent et al., 1990). The numbers represent leaf stages (number of fully developed leaves). PS: pollen shed; S: silking; (B) Leaf area index

View larger version (31K): [in this window] [in a new window]   Fig. 3 Characterization of meteorological conditions of the three experimental seasons. (A) Cumulative GDD. (B) Cumulative precipitation

Overall, year and soil depth showed strong effects on root developmental parameters, while the effect of positions (relative to the plant row) was only marginal.

Effect of Factor Interactions on Root Development Parameters
Time to maximum root density and relative time of maximum root density showed similar characteristics. Thus, the evaluation of factor interactions on root developmental parameters will be restricted to root growth duration.

Earlier maximum root densities were observed in 1994 at most soil depths (Fig. 4A) , with significant differences compared with the other years at shallow soil depths (5 and 10 cm) and at all positions relative to the plant row (Fig. 5A) . A steady increase in time to maximum root density with depth (Fig. 4A) was only observed in 1994, but not for any position relative to maize row (Fig. 4B). In the other years and at some positions time to maximum root density increased with depth in the upper soil layers (5–20 cm), with little or inconsistent variation towards deeper layers. Significant differences in time to maximum root density among depths were observed especially in 1994 (at the shallow soil depths of 5 and 10 cm maximum root density occurred often earlier, while at 60, 80, and 100 cm it was reached often later than at the other soil depths), but only seldomly in 1995 and not at all in 1996. The statistical analysis of the effect of depths on time to maximum root density at varying positions relative to the plant row showed the following pattern: near the row (0–135 mm) there was no difference among depths; at intermediate distances (135–270 mm) maximum root density was observed significantly later at deep soil layers (80 and 100 cm); at greatest distances (270–405 mm) maximum root density occurred significantly earlier at shallow soil depths (5 and 10 cm). The variation in time to maximum root density at varying distances from the maize row was small at all depths (Fig. 5B), although some extreme values increased the variability, and it was very small in all experimental years (Fig. 5A). Only in 1995 did the differences in time to maximum root density among positions show a consistent and significant pattern: at positions near the plant row (0–202 mm) maximum root density was reached earlier than at positions further away (202–405 mm). At no depth was the effect of horizontal position on time to maximum root density significant. However, the small sample size (3 yr times four replications) available to compare two soil locations (characterized by their soil depths and positions relative to the plant row) and high variability between adjacent soil locations may be reasons for the low frequency of significant differences in time to maximum root density among soil locations.

View larger version (21K): [in this window] [in a new window]   Fig. 4 Effect of soil depth on mean root growth duration of maize as observed with minirhizotrons. (A) Effect within years. (B) Effect at selected positions relative to the plant row

View larger version (21K): [in this window] [in a new window]   Fig. 5 Effect of position (relative to the plant row) on mean root growth duration of maize as observed with minirhizotrons. (A) Effect within years. (B) Effect at selected soil depths

There was a large variation in the root density between successive sampling dates. The evaluation of mini-rhizotron images in spatial series, instead of temporal series, may have been a source of sampling variability. Another reason for a high sampling variability may have been the difficulty distinguishing between dead and living roots (Smit and Zuin, 1996) and identifying new roots, which may appear transparent (Harris et al., 1995). The high variation in the root density between successive sampling dates may also have resulted from high turnover rates. Roots with short life spans (visible for less than 14 d) have been observed for tomato (Lycopersicon esculentum Mill.) at transparent interfaces (Reid et al., 1996). There are some indications that maize roots, too, have a short life span: short maize root branches were reported to grow for only 2.5 d (Cahn et al., 1989) and start senescence within a few days after appearance (Fusseder, 1987). It was postulated that short life cycles of roots reflect local root dynamics which are independent of overall root growth (Reid et al., 1993). Localized responses of roots to irregular availability of resources have been reported for maize (Anghinoni and Barber, 1988; Zhang and Barber, 1992). Overall root growth may be regulated at a coarser resolution, which requires larger observation areas than those available in minirhizotrons. The discussed sources of variation may have influenced intersampling root variability and may have prevented the observation of clearer and more consistent patterns of net root growth.

The description of temporal patterns of net root growth was not improved by relating it to growing degree days instead of to the chronological time scale, in contrast to the description of shoot data made by, e.g., Russelle et al. (1984). Some researchers reported the use of thermal time to describe the growth of maize roots (Pellerin and Pagès, 1994), but Zur et al. (1992) preferred to use the chronological time scale to represent their root data. The fact that the description of net root growth on the basis of GDD was not improved in the present study may reflect a limited relationship between air temperature and net root growth. Roots develop in an environment characterized by distinct vertical temperature gradients (Richner et al., 1996), making it difficult to select a unique temperature source to describe net root growth. The importance of choosing an adequate temperature reference for calculating GDD is evident for shoot growth, which is more closely related to soil temperature than to air temperature during germination and early development (Swan et al., 1987), because the maize shoot apex is located below the soil surface during this growth period.

The study of three factors influencing temporal patterns of net root growth showed that year and soil depth had a marked effect, while position in relation to the plant row had only a marginal effect.

The time of maize flowering has been considered to be one of the key events in the classification of root development (Foth, 1962). Unfortunately, most studies of root development do not use crop developmental references other than days after planting or emergence dates, thus making it difficult to check Foth's (1962) postulate. The present experiment revealed that maximum root density occurred around flowering (pollen shed) but varied with years and rooting depth. For the maize shoot, the formation of reproductive structures implies the end of vegetative development as leaf formation ceases and tassel formation is initiated (Ledent et al., 1990). For the root system, the formation of nodal roots has been shown to be directly linked to shoot development (Zur et al., 1992), with the consequence that the youngest nodal roots penetrate the soil around flowering (Hoppe et al., 1986). The tighter (anatomical) control of the development of the shoot components (leaves and reproductive organs), as compared with root development, can be seen from the small variation in days until pollen shed (3 d across 3 yr) and the proportionally greater annual variation in the period of time until the maximum number of roots is observed (17 d). Nevertheless, the observation that net root production is completed around flowering shows that not only the nodal roots are affected by the transition from vegetative to reproductive growth, but also that this is an indication that reproductive growth, at least in part, occurs at the expense of the entire root system. The limitation of net root growth during the reproductive phase of the crop probably relies on the functional regulation between shoot and root growth. The start of ear growth (Wagner et al., 1995) has been shown to influence the dry weight of maize roots. In addition, remobilization of nitrogen from dying maize roots has been demonstrated (Fusseder, 1987), especially during reproductive growth when it was enhanced by the demand of the reproductive plant parts (Pan et al., 1995).

The contrasting effects of soil depth and position relative to the plant row on the developmental parameters of roots is interesting. The observation that the time required for root development to reach its maximum increases with depth has been reported previously (Mengel and Barber, 1974). In contrast, no reference on the synchronization of maximum root density at varying distances from the plant row have been reported. The present results suggest that the growth of maize roots first occurs in the topsoil and that roots progressively explore deeper soil layers with time. This is in agreement with the general concept that axillary roots—either seminal or nodal roots, as defined by Hoppe et al. (1986) for maize—of monocotyledonary plants are responsible for the downward exploration of the soil profile (Klepper, 1991). Probably most roots observed on the minirhizotron interface were first or higher order laterals derived from nodal roots. Thus, the observed patterns of root development may be an indirect consequence of nodal root growth. The layered exploration of the soil profile by maize roots is supported by the preferred vertical orientation of its axillary root trajectories (Pellerin and Pagès, 1994), which is further enhanced by increasing soil temperatures and decreasing soil water content (Nakamoto, 1989, 1993; Takahashi and Scott, 1991), which usually decreases with crop development and corresponding increases in leaf area (Foth, 1962). On the other hand, the exploration of the soil between the plant rows results from the more pronounced horizontal component of axillary root trajectories near the plant base (Tardieu and Pellerin, 1990). These findings can explain (i) the increasing likelihood to find roots at greater depths as the growth period increases, while root growth in previously explored shallower layers successively stops and (ii) the longer root growth period near the plant row in the upper soil layers, which are continuously penetrated by newly formed nodal roots, as compared with positions further away from the plant row.

The observation that the maximum root density is found during a 3-wk period around flowering at all depths, positions, and years shows that the maize crop establishes its maximum resource uptake capacity only towards the end of the vegetative growth period. The uptake of soil resources during reproductive growth relies on this framework and on the capacity of the plant to sustain its functioning.

	NOTES

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This research was supported by the Swiss National Science Foundation, Project No. 31-39498.93.

Received for publication September 7, 1999.

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TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results REFERENCES

 

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