species-specific (COUTTS, 1983; MATERECHERA, 1991). Much less investigated have been the mechanical effects exerted by a slope on root growth, probably ...
Downloaded By: [Univ Studi Insubria] At: 09:27 18 July 2007
PLANTBIOSYSTEMS,137 (1) 47-56, 2003
Modification of root morphological parameters and root architecture in seedlings of Fraxinus ornus L. and Spartium junceum L. growing on slopes D. CHIATANTE, M. SARNATARO,S. FUSCO, A. DI IORIO and G. S. SCIPPA
received 9 February 2002; revised version accepted 1 July 2002
ABSTRACT - A computerised image analysis system was used to investigate several root morphological parameters in young seedlings of broom (Spartium junceum L.) and manna ash (Fraxinus ornus L.) growing in three different environmental conditions: slope, terrace and plane. Data show that slope, to a greater extent than terrace, affects the root system of both species by increasing significantly the length and number of root apices of first-, second- and thirdorder lateral roots. Besides these variations in morphological parameters, the overall architecture of the root system results modified on a slope. In fact, an asymmetrical distribution of lateral roots was observed around the tap root and along two main directions: up-slope and down-slope. The morphological parameters, analysed separately on the two portions of the root system, present significant differences. The following hypothesis is put forward: in response to a slope, the two species reinforce their anchorage strain by changing the organisation of the root system, particularly in the up-slope direction. In these two species, the occurrence of slight differences in response to a slope suggests that plant anchorage might require species-specific adaptations. KEY WORDS - root architecture, slope, Spartium junceum L., Fraxinus ornus L. ABBREVIATIONS - TL = total root length; NA= number of root apices; RV=root volume; R_A= root area; subclass 1= third- and second-order lateral roots; subclass 2= firs>order lateral roots; subclass 3= taproot
Root growth is genetically controlled but it can be modified by the rooting environment (CALDWELL, 1994; COURTS, 1989; FITTER, 1994; LYNCH, 1995; SULTAN, 1995; VIA et al., 1995). This bio-adaptive response has been called "edaphoecotropism" (BELL & SULTAN, 1999; GEDROC et al., 1996; PIGLIUCCI et al., 1996; VANICEK, 1973) and depends upon several factors, some of which are strictly related to soil characteristics, such as temperature, aeration, moisture and mechanical
impedance (COUTTS, 1989; FORTIN & POFF, 1990). The architecture of a root system can be defined as that particular three-dimensional network of roots resulting from the processes of growth (axial and radial) and the branching of numerous individual root axes (THALER& PAGES, 1998). Therefore, it is possible that alterations in root growth may, consequently, also affect root architecture. In the literature, it has been shown that mechanical
Downloaded By: [Univ Studi Insubria] At: 09:27 18 July 2007
48 D, CHIATANTEet al.
stresses imposed by wind and slope affect root system organisation. The response of a root system to wind has often been studied in nurseries by growing trees in a wind-tunnel (STOKES et al., 1995) or by simulating mechanical stress by flexing the trunk (STOKES et al., 1997). In all these studies it has been observed that root systems of artificially stressed trees present a particular adaptive growth with an asymmetric allocation of biomass in two preferential directions, i.e. towards and away from the origin of the mechanical stress (BLACKWELLet al., 1990). Wind also induces alterations in secondary root growth, i.e. woody roots change in shape from oval to forms approximating an I- or T-beam (GOODMAN & ENNOS, 1996, 1997, 1999; NICOLL et al., 1995; NICOLL & RaY, 1996; STOKES et al., 1997). The response of root systems to wind stress seems to be species-specific (COUTTS, 1983; MATERECHERA, 1991). Much less investigated have been the mechanical effects exerted by a slope on root growth, probably because of the difficulties inherent in such field studies. For instance, it can be difficult to transport the hea W equipment required to excavate an intact root system up a slope. Nevertheless, from the few studies conducted with tree species, it emerges that roots growing up-hill are stronger than those extending down-hill (SCHIECHTL, 1980). With the aim of gathering information on the mechanical effects of slope upon root systems, we compared the root system of two plants species, Fraxinus ornus L. and Spartium junceum L., growing in three different rooting environments: slope, terrace and plane. In our study, we used young plants because the soil environment is known to influence root growth at an early stage of development (RUSSELL, 1977) when the direction of growth, with respect to the vector of gravity, is determined (CHAMPAGNATet al., 1974). From an experimental point of view, we decided to adopt a new protocol which enabled the rapid measurement of several morphological parameters by means of a computerised image analysis system.
MATERIALS AND METHODS Samplings and excavation Five young saplings of both manna ash (E ornus) and broom (S. junceum) were collected from three different sites located in Molise (Italy). The three sites chosen for our investigation were: plane, terrace and slope (40°). The terrace was a sub-horizontal surface on a slope of 40 ° , and it represented an intermediate condition between slope and plane. The soil characteristics (according to the Soil Survey Staff, 1998) were as fol-
lows. Plane: eutrudept loamy; deep soil on colluvial deposit, medium textured, surface litter 2-cm thick. Terrace: eutrudept loamy, deep soil on colluvial deposit, medium textured, surface litter 3-5-cm thick. Slope: eutrudept loamy, skeletal soil on colluvial deposit, medium textured, skeletal, surface litter absent. With regard to organic matter and water retention characteristics, these three sites resulted similar. The organic matter was: abundant in the first 10 cm, medium from 10 cm down to 50 cm, and scarce below 50 cm. The water retention curves showed little differences in waterdrainage capacity, and no substantial differences in water availability. Based on the number of growth rings 3-4-year old seedlings were collected for both species.
In situ excavations To reduce mechanical damage, the root systems were excavated by hand and using different size trowels. The roots were cleaned from soil by using a paint brush. During excavation, the root systems were stained with bright watercolours to enhance image quality in photographs. Three-dimensional drawings were also made of the root systems in situ during excavations. After excavation, in-scale illustrations were made of the spatial distribution of the root system in the horizontal and vertical planes.
Computemed image analysis of morphological parameters The root morphological parameters were measured with a computerised image analysis system. This consisted of a scanner and a WhinRhizo 3.10 version analytical software package purchased from Regent Ltd., Canada. This system allowed the measurement of morphological parameters, such as total root length (TL), number of root apices (NA), root volume (RV), and root area (RA), which describe root system organisation. An interesting advantage of this computerised image analysis system was that it enabled us to measure the root morphological parameters in each category of branching order. For this purpose, it was necessary to preliminarily select a series of root diameter intervals which included the lateral roots belonging to the same branching order. After several attempts, we selected the following diameter intervals: 0.0-1.0 m m (named subclass 1), 1.0-2.7 m m (named subclass 2), and >2.7 mm (named subclass 3) for S. junceum; and 0.0-0.6 mm (named subclass 1), 0.6-2.7 mm (named subclass 2), and >2.7 mm (named subdass 3) for E ornus. In both species, subclass 1 included: third- and second-order lateral roots; subclass 2 the first-order lateral roots, and subclass 3 the taproot. It was impossible to find a specific diameter interval which
Downloaded By: [Univ Studi Insubria] At: 09:27 18 July 2007
Root system of seedlings growing on slope 49
could enable us to distinguish between the third- and second-order lateral roots, probably because these two branching orders included roots whose diameters are too similar. For this reason, subclass 1 included secondand third-order lateral roots. Where indicated, the analysis of morphological parameters by means of the computerised image analysis system was repeated separately for the lateral roots which had grown in the up-slope or down-slope direction.
a abb
g 250 v
-J
F-
0
Statistical analysis Analysis of variance (ANOVA) and significant multiple comparison test (Bonferroni), with significance accepted at the p
