Báo cáo khoa học: "Analysis and simulation of the architecture of a growing root system: application to a comparative study of several tree seedlings"

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Analysis and simulation of the architecture of a growing root system: application to a comparative study of several tree seedlings M. L. E. 1 Dreyer H.Joannes 1 Colin-Belgrand 2 Pages 1 INRA, Centre de Recherches Foreshores, BP 35, 54280 Seichamps, and 2 INRA, Station d’Agronomie, Domaine-de-St-Paul, 84140 Montfavet, France. Introduction root architecture Modeling It has been that the The basis of root architecture modeling is frequently suggested extension of root an adequate definition of branching termi- and the shape spatial systems markedly influence the rate and nology. In this respect, two main patterns of nutrient uptake from the soil. approaches may be outlined. The first one is based on a topological or morphometric Many nutrient and water uptake models description of ramifications. Fitter (1987) have been proposed, based on root distri- applied this approach to describe and bution patterns; for instance, spatial simulate root systems of various herba- (mostly vertical) distribution of roots may ceous species. Basic structural units are be related to physical and chemical prop- the links, straight segments between suc- erties of successive soil layers as in the cessive nodes (branching points). The empirical model of Gerwitz and Pages order of these links is counted from the (1973). Parameters describing extension, periphery of the branching structure such as total root length, explored soil towards the primary axis (hypocotyl). Main volume and rooting density, are frequently parameters are either topological (like used. magnitude) or geometric (like link lengths, On the other hand, a root system may branch spacing, branching angles). The also be described as a network of resis- main limitation of this approach is that it is tances to nutrient and water transfers. It purely descriptive and cannot be used to appears therefore important not only to describe growth. quantify root distribution, but also to ana- The second approach is based on de- lyze the spatial ramified architecture, in velopmental analysis beginning from the other words, the connecting links between root origin and evolving with growth and the different parts of the root system.
increasing complexity. First-order roots ori- growing root system with all its dynamic a ginate from the hypocotyl and bear aspects (Belgrand et al., 1987). It is also a second-order laterals and so on (Hackett a root is defined developmental approach: and Rose, 1972). In this way, each root the non-branched structure formed as member has a distinctive identity and through the activity of a single apical each order of roots has specific dimen- meristem. The growth and architecture of sions, properties and branching patterns growing root systems of young tree seed- (Rose, 1983). In a developmental model, lings are studied by direct and non-de- the simulation of root growth and ramifica- structive observations in ’minirhizotrons’, tion is based for each root-order on time of where root growth occurs at the interface emergence of the successive axis, elon- between the lower wall of rhizotrons and gation rate and rate of lateral branching the soil. (Lungley, 1973; Rose, 1983). The data acquisition system, presented More recently, new developmental greater detail in this volume, is roof in models were proposed in which the move- segment based. In our method, synthetic ment of root tips through the soil is de- parameters of root growth and architec- scribed (Pages and Aries, 1988; Diggle, specified in terms of growing time ture are 1988). These models differ from the pre- for each order (number of axis, time of vious ones because they all have root tips emergence, elongation rate, branching growing during each time step rather than characteristics, such as interbranch dis- having each tip growing individually for the tance and length of the apical non-branch- entire duration. ing zone, defined by the region from the most visible apical n + 1 order laterals to We have recently developed a new the axis tip). Statistical studies of these method which allows a detailed analysis of
data allow the determination of elongation Taproot branching patterns may be de- scribed through the interbranch distance laws and branching patterns. They may then be integrated into a deterministic distribution and the length of the apical non-branching zone (LAnbr). The inter- three-dimensional model (Pages and branch distance is rather similar for the 2 Aries, 1988). oak species (0.4-0.5 cm) and for the 2 This method has been applied to the acacias (0.6-0.9 cm). No systematic analysis of root growth in several different changes in branch spacing were deter- tree species seedlings in order to explore mined with time; the differentiation of later- the different architectural models. Two al roots occurs in a strictly acropetal order groups of species were used, oaks and (Fig. 2a) and is also regular along the several acacias, which show marked dif- taproot length. The LAnbr is also rather ferences in shoot growth and ramification. constant; it seems there was no trend of evolution of the LAnbr with either time or taproot length (Fig. 2b). Yet, there are specific differences, especially for A. Materials and Methods albida (Table I). Long lateral roots appear 3 mo after ger- Acorns of oaks (Quercus petraea Liebl., Q. mination when the taproot reaches the rubra du Roi) and seeds of acacias (Acacia bottom of the minirhizotron. Specific dif- albida Del., A. holosericea) were germinated on ferences can be observed between oaks the same substrate (a homogeneous mixture of sandy clay and peat) in minirhizotrons with 4 and acacias (Table I). replicate plants per species. The seedlings were grown under controlled climate in a growth cabinet (150 pmol of PAR 22/16°C , 1 s 2 m- ’ day/night temperature regime, 16 h daily photo- Discussion and Conclusion period). Root growth was monitored every second day for 2 mo (Belgrand et al., 1987). in Mean values of root characteristics given are At the seedling stage, we did not observe Table I. differences between growth models strong of the observed root systems. It should be noted that the values of the different archi- tectural parameters, like branch spacing, Results are quite constant for seedlings, although the taproot elongation rate is very dif- The forms of the root systems, as they ferent. All shown species may be describ- appeared 2 mo after germination are ed as having a fast growing and regularly drawn in Fig. 1. Root configuration is very ramifying taproot, bearing more or less similar for all presented species: a fast plagiogeotropic laterals with very restricted growing and orthogeotropic taproot bear- growth. ing short second-order roots with plagio- At this stage, we cannot differentiate geotropic and restricted growth; their final distinct architectural models, but the num- lengths never exceeded 10 cm. ber of long lateral roots could contribute to the expression of architectural models on Taproot elongation is always linear and older plants. There are 2 phases in the non-rhythmic, with a daily rate of about architecture setting: the first one, with 1.4-1.9 cm/d for oaks, 1.2 cm/d for A. taproot setting and an acropetal initiation holosericea and cm/d A. 1.5-2.2 for and a limited development of lateral roots; albida (Table 1).
the second one with a strong plagiotropic References root differentiation in non-acropetal order (Kahn, 1977). Our results concerning the Belgrand M., Dreyer E., Joannes H., Velter C. & development of long lateral roots could Scuiller 1. (1987) A semi-automated data pro- lean in the same way. cessing system for root growth analysis: appli- cation to a growing oak seedling. Tree Physiol. On the other hand, the influence of soil 3, 393-404 properties may be overriding on the Diggle A.J. (1988) ROOTMAP - a model in changes of root architecture. The influ- three-dimensional coordinates of the growth ence of physical soil properties is well and structure of fibrous root systems. Plant known: for instance, number of lateral Soil 1 05, 169-178 roots and rate of extension are greatly Fitter A.H. (1987) An architectural approach to the comparative ecology of plant root systems. increased by mutilation of the taproot tip New Phytol. 106 (suppl.), 61-77 (Hackett, 1971). In the same way, effects Gerwitz A. & Page R. (1973) An empirical of water stress on lateral root initiation and mathematical model to describe plant root sys- elongation have been reported (Jupp and tems. J. Appl. Ec:ol. 11, 773-781 Newman, 1987). An analogous effect of Hackett C. (1971) Relations between the waterlogging can be observed (Riedacker dimensions of the barley root system: effects of and Belgrand, 1983). However, in these mutilating the rcot axes. Aust. J. Biol. Sci. 24, examples, there are no details in terms of 1057-1064 root architecture. Our new method could Hackett C. & Rose D.A. (1972) A model of the extension and branching of a seminal root of be used for this kind of analysis.
and its in Lungley D.R. (1973) The growth of root sys- relations between barley, studying use root dimensions. II. Results and inferences from tems. A numerical computer simulation model. manipulation of the model. Aust J. Biol. Sci. 25, Plant Soil 38, 145-159 669-679 L. & Aries F. (1988) SARAH: mod6le de Pages simulation de la croissance, du d6veloppement A.P. & Newman E.I. (1987) Morphological Jupp et de I’architecture des syst6mes racinaires. and anatomical effects of severe drought on the Agronomie 8, 888-897 roots of Lolium perenne L. New Phytol. 105, 393-402 Riedacker A. & Belgrand M. (1983) Morphog6- nese des syst6mes racinaires des semis et bou- Kahn F. (1977) Analyse structurale des sys- tures de chêne pédonculé. Plant Soil 71 , 131-146 t6mes racinaires des plantes ligneuses de la Rose D.A. (1983) The description of the growth for6t tropicale dense humide. Candollea 32, of root systems. Plant Soil 75, 405-415 5 321-358