The architecture of the root system is related to its water and mineral uptake. In this paper, the number, growth, and branching of first-order lateral roots are ...
Plant and Soil 164: 169-176, 1994. (~) 1994 Kluwer Academic Publishers, Printed in the Netherlands,
Evaluation of parameters describing the root system architecture of field grown maize plants (Zea mays L.) II. Density, length, and branching o f first-order lateral roots
Loic P a g b s and S y l v a i n P e l l e r i n INRA, Centre d'Avignon Unit6 de Recherches en Ecophysiologie et Horticulture, Domaine St-Paul, B.P. 91 84143 Mon(favet Cedex, France and INRA, Station de recherches Grandes Cultures Laboratoire d'Agronomie 28, rue de Herrlisheim, B.P. 507 68021 Colmar, France Received 14 July 1994. Accepted in revised form 3 August 1994
Key words: branching, growth, lateral roots, maize, root morphology, Zea mays L.
Abstract The architecture of the root system is related to its water and mineral uptake. In this paper, the number, growth, and branching of first-order lateral roots are studied on field grown maize (early maturing cultivar 'Dea'), mainly in relation to the depth and to the rank of the bearing phytomer. The soil was a deep clay loam, without any barrier until 1.80 m. The branching density was studied along axile roots until 1.40 m from the base, on a sample of individually excavated axile roots. A strong gradient of density was shown: the mean branching density decreased from 12 roots.cm -~ near the base to 4 roots.cm -l at a 60 cm depth. Seminal roots were less densely branched than nodal roots. The mean difference was about 4 roots.cm- ~. The length and branching density of lateral roots were studied on mature parts of the root systems where the growth and branching of the laterals were completed, using samples extracted from large soil monoliths. The length distribution of lateral roots was highly asymmetrical, for every source phytomer (mean: 25 mm; median: 16 mm). Many lateral roots were very short, and only 2 % reached a length higher than 10 cm. Only 29 % of all the laterals bore second-order lateral roots. Vigorous laterals branched more systematically and more profusely: the branching density varied from 2 to 5 roots.cm-J according to the length of the mother lateral root. Both the number and length of lateral roots appeared to be affected by the soil bulk density which varied with the depth.
Introduction Many recent works have emphasised the need to describe the root system architecture to provide a basis for improved understanding of the uptake of water and minerals (Clarkson, 1991; Fitter, 1991; Habib et al., 1991). A better knowledge of the structure of the root system is necessary because of the diversity in root functioning (Waisel and Eshel, 1991), and because of the diversity in their morphogenetic capacities (Fitter, 199 l; Waisel and Eshel, 1991). Important progress have been made during these last years in the relationship between the structure and the uptake of individual roots (Clarkson, 1991). On maize partic-
ularly, the water and mineral transfer resistances are better identified (McCully and Canny, 1988; Peterson et al., 1993) and the importance of the root connections have been demonstrated recently (McCully and Mallet, 1993). The variations in the morphogenetic capacities between maize roots have been studied according to various criteria: live duration (Fusseder, 1987), apical diameter and growth rate (Cahn et al., 1989; Feldmann, 1979), length (Iijima et al., 1991), and anatomy (Jordan et al., 1993; Varney et al., 1991). All these works pointed out the interest of an architectural approach. In the first paper of this series (Pellerin and Pages, 1994), we have confirmed the indeterminate and continuous growth of the axile roots (i.e. both seminal
170 and nodal roots). For seminal roots (from phytomers 0 and 1), and for nodal roots originating from phytomer 2 to phytomer 5 (P2 to P5, see terminology in the first paper), the growth rates decreased gradually until silking. Initial growth rate values were about 3.8 mm per growing degree day (base temperature: 6°C). This indefinite and continuous growth resulted in the exploration of a large soil volume: the extension radius was about 60 cm and the maximum depth was about 130 cm at silking. Unlike the axile roots, lateral roots (or branch roots) of maize have a determinate growth pattern (Cahn et al., 1989; Fusseder, 1987; Varney et al., 1991; Varney and McCully, 1991), with a short growth duration (a value of two days is given by Fusseder (1987) for laterals on the primary root, and of 2.4 days by Cahn et al. (1989) as a mean value for laterals on all axile roots), and a lower growth rate (Cahn et al., 1989). Their final lengths are generally small (2.2 cm according to Cahn et al. (1989); from 1 to 10 cm on the primary root according to Fusseder (1987); 3 cm according to Varney et al. (1991). Their individual lengths are highly variable (Iijima et al., 1991; Jordan et al., 1992; Varney et al., 1991). These roots are very numerous, since the branching densities along the axile roots vary from 7 to 12 roots per centimetre of axile roots (Jordan et al., 1992; Morita et al., 1992; Varney et al., 1991). Thus, the cumulated length of these lateral roots is very high, about 15 to 35 times the length of axile roots. According to Cahn et al. (1989) the total length of second and third order laterals could be neglected compared to the length of first-order laterals. Because of their high total length, and their morphological characteristics, the first-order lateral roots have a predominant role in the uptake process. Therefore, it appears necessary to quantify the development of these lateral roots in field conditions in order to assess the parameters of architectural models such as suggested by Pages et al. (1989). First, their density along the axile roots has to be specified. Existing data obtained under field conditions are available only for the proximal 35 to 45 centimetres of samples of axile roots (Jordan et al., 1992; Morita et al., 1992; Varney et al., 1991). Moreover, these data suggest possible gradients of the branching density along the axile roots. It is also necessary to get quantitative data about their length distribution. Very few data exist on this subject: only Varney et al. (1991) presented a global distribution histogram of the lateral root length for the proximal zone of the mother axile roots. Given the large variations of these lengths, and the asymmetry of
the distributions, the mean values (Cahn et al., 1989) are not sufficient to describe their real character at the root system level. The branching density of the lateral roots, which has not received attention yet, will be also considered in this study. For all these aspects, special attention will be given to the gradients from the basal to the distal end of the root, and to the influence of the originating phytomer.
Material and methods Only the main points will be briefly reported here, as the cultural and the environmental conditions have been presented in detail in the first paper (Pellerin and Pag6s, 1994).
Culture (The study was carried out on maize (Zea mays L., cultivar 'Dea') grown at Colmar (Haut Rhin), East of France (4803 , N, 7°2 , E, 200 m altitude). Techniques for soil preparation were those usually applied in this area: ploughing during the autumn, fertilisation at 14 g m -2 of N, 6 g m -2 of P, and 12 g m -2 of K, seedbed preparation just before sowing (22 April 1992), and culti-packing just after (24 April 1992). The plant density was 8.9 plants.m -2, with 75 cm between rows and 15 cm between plants in the row. The soil was a deep clay loam (1.80 m without any barrier) whose detailed characteristics are given in Table 1 and Figure 2 of the first paper. The timedependant variations of the air temperature and the soil water potential are given on Figures 3 and 4 of the first paper. Measurements Axile roots were gently excavated in order not to damage the branch bases. This was done from 1.20 m deep trenches with vertical walls. The bearing phytomer, and the maximum horizontal distance from the plant (horizontal reach) were recorded for each axile root. The distribution of the sample roots according to the bearing phytomer is given in Table 1. After a gentle wash in tap water, the "completely branched" part of the root (i.e. taking off the basal part without any branches, and the young part where branching was not completed) was cut into 2-cm segments. All lateral roots were counted on each of those segments. Two soil monoliths were excavated for measuring the length and counting the branches on the laterals:
171 Table
1.
Number of axile roots sampled per phytomer
Phytomer number
0
1
2
Number of axile roots for lateral
6
5
7
Counting Number of axile roots for lateral
1
--
2
3
4
5
6
9
9
10
11
4
4
4
6
length and branching measurements
the first one excavated on the 18 th of June was 40 cm deep and 30 cm wide; and the second one excavated on the 23 rd of July was 60 cm deep and 30 cm wide. These monoliths were excavated by making two parallel trenches 15 cm apart from one row. A tunnel was dug between the two adjacent trench walls in order to set a horizontal board as a base for carrying the monolith. The last two faces of the monolith were cut with a long knife. The monoliths were surrounded by a wooden frame to support them during transport to the laboratory, and were left overnight in a big container with salted water. The next day, the softened soil was gently washed out. The measurements were done on axile roots with a sufficient length (greater than 15 cm) inside the monolith. The axile roots were again divided into 2-cm segments, on which the length of laterals and number of branches (second-order laterals) were recorded. These measurements were made with a ruler in a water filled tray. Laterals that had been damaged, because they were close to the sides of the monolith, were discarded. The measurements have been made only on segments with a maturity sufficient for assuming that the final length of the laterals was reached. In these parts of the axile roots, the apices of the laterals were rounded and the hairs were very close to or covered the tip (as already observed by Varney et al., 1991). The distribution of the sampled axile roots according to the bearing phytomer is given in Table 1.
Results
Branching density on axile Patterns of branch root density along axile roots are presented in Figure 1. The general trend of this scatterplot is illustrated by the smoothed curve and consists of decreasing density toward the distal end of the root. The average density varied from 12 roots.cm - l , close to the base, to 6 roots.cm -1 at 40 cm. This gradient and the variability declined steadily toward the apex. A similar pattern appears in the relationship between branching density and depth (Fig. 2). Note that there appears to be a slight depression in the smoothed curve close to 40 cm-depth, which matches with the position of the plough pan. The relationship between the branch density and depth on one hand, and the relationship between soil bulk density and depth on the other (Fig. 2 in the first paper), showed a similar (but opposite) variation pattern. These branch density variations were not fortuitous since they were encoun-
