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Crop Science 41:363-371 (2001)
© 2001 Crop Science Society of America

CROP PHYSIOLOGY & METABOLISM

The Response of Root/Shoot Partitioning and Root Morphology to Light Reduction in Maize Genotypes

Y. Héberta, E. Guingob and O. Loudetc

a INRA Station d'amélioration des plantes fourragères, F-86600 Lusignan
b Pioneer Génétique (SARL), F-35830 Betton
c INRA Laboratoire de biologie cellulaire, Route de St-Cyr, F-78026 Versailles cedex

Corresponding author (hebert{at}lusignan.inra.fr)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In maize (Zea mays L.), increasing sowing density in early maturing genotypes modifies the root/shoot ratio, which generates more frequent lodging. The effects of available light on root emergence and biomass were thus studied to investigate the genetic variation and its interaction with light in traits related to root lodging. Isolated plants of eight hybrids of contrasting maturities and lodging resistance were grown in pots in outdoor conditions, in full sunlight and under neutral shading which captured 61% of the incident light. Plants were harvested at four sampling dates for root and shoot dry matter determination and measurement of some major traits involved in the architecture of the adventitious root system. The partitioning of biomass between root and shoot varied among genotypes. Biomass allocation was significantly affected by light treatment, and the effects varied among genotypes. Some genotypes seemed better able to maintain biomass allocation to the shoot, without major effects on root morphology, than were others. At silking, the final number of emerged roots from the first six internodes was similar across treatments. On the other internodes, the number of roots was lower in the shaded treatment, but the reduction observed (between 8 and 26%) depended on genotype and earliness. In addition, some genotypes with similar root biomass clearly differed in root morphology (58% variation in the average weight of root primary axes under shading), indicating a variation in the structure of the root system. The relationships between root biomass and root number indicated significant interactions between genotypes and shading, with possible consequences on root lodging resistance in the field.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
AS A POSSIBLE CONSEQUENCE of the great genetic progress made in maize yield over the past years, more attention must be paid to the problem of root lodging, a phenomenon that implicates the mechanical characteristics of root anchorage. As shown for wheat (Triticum aestivum L.) by Siddique et al. (1990), it seems that modern cultivars allocate a lesser proportion of the total dry matter to their root system than do older cultivars. A similar effect of selection is likely to have occurred in maize, as a consequence of the general increase observed in grain yield and harvest index (Derieux et al., 1987).

The genetic progress achieved over the last four decades proves that the classical selection procedure against root lodging works quite well; lodging rankings made over a large number of locations are very efficient to screen lines and hybrids for the best resistance to lodging. This procedure is very expensive, however, since a number of locations are required for the assessment to be accurate. Further advances in screening efficiency may depend on a more precise understanding of the lodging phenomenon itself. This is of major importance in the Northern areas of Europe, where crop densities are usually high, which increases the susceptibility to root lodging. However, this effect is largely dependent on the genotype (Derieux et al., 1987; Boyat et al., 1990).

When water and nutrients are not limiting, reduction in light is the most important environmental constraint arising when plants are cultivated in dense plantings. In particular, the assimilation and redistribution of carbon are greatly affected by plant density. It has been clearly established that, at high densities, competition for light changes the relationships between the aerial part and the root system, together with the morphology (size, architecture) of the plant (Pellerin and Demotes-Mainard, 1992). Under light-limited conditions, the growth of roots is reduced more than the growth of the aerial parts, which leads to a decrease in the root/shoot ratio.

The effects of light quantity on root development have seldom been investigated (Nakata and Kono, 1990; Pellerin, 1991; Demotes-Mainard and Pellerin, 1992; Hoppenstedt and Geisler, 1992). On plants of a single maize genotype cultivated under reduced light conditions, Pellerin (1991) and Demotes-Mainard and Pellerin (1992) observed a reduction in the number of internodes bearing elongated roots, together with a lower number of roots on the upper root internodes. In soybean [Glycine max (L.) Merr.], Buttery and Stone (1988) and Del Castillo et al. (1989) noticed that the number of lateral roots was reduced under carbon-limited conditions. The effects of light reduction on root mass and morphology may partly explain why lodging susceptibility is increased at high plant density, since several authors have shown that root lodging behavior is associated with several characteristics of the underground parts and the base of the plant stem: particularly, the number of roots on the upper internodes (Nass and Zuber, 1971; Hébert et al., 1990; Hébert et al., 1992; Duparque and Pellerin, 1994; Guingo and Hébert, 1997; Sanguineti et al., 1998), the diameter of roots (Hébert et al., 1992; Stamp and Kiel, 1992; Guingo and Hébert, 1997), and the number of internodes with emerged roots (Duparque and Pellerin, 1990; Pellerin et al., 1990).

In most studies, the growth of the root system and its architecture were considered with no attention paid to the effect of the genotype (Del Castillo et al., 1989; Pellerin, 1991; Demotes-Mainard and Pellerin, 1992). Nevertheless, several points should be raised concerning the connection between the genetic variability of shoots and roots under light-limited conditions. First, genetic variation in root morphology (numbers of root primary axes, internode length, root diameter, root growth direction, etc.), which is now considered an important factor contributing to better lodging resistance, must be checked for any interactions with environmental effects. Second, there might exist several different biomass allocation patterns among genotypes in response to density. If confirmed, this could help to account better for the genotype x density interaction. Last, to analyze the genetic variation which could be used to improve root lodging resistance in high density conditions, there is a need to investigate the relationships between root biomass and root geometry when carbon allocation changes because of environmental effects. The differences among genotypes in response to reduced light availability might explain the genotypic differences in root lodging susceptibility.

This paper restricts its focus to the effects of the reduction of light quantity on root distribution and development, and to the ratio between the root system and the shoot. This information may help elucidate the genotype x density interaction, and may be useful to breeders selecting genetic material capable of high root lodging resistance as well as high yield and early maturity.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Eight F1 commercial or experimental maize hybrids were studied in 1995 and 1996 at Lusignan, France (46°26'N, 0°07'E, 145 m elevation). They were chosen for their divergence in root lodging susceptibility (Table 1). These hybrids were grown outdoors as isolated plants to avoid the effects of differences in self-shading induced by contrasting earliness among genotypes. The average total radiation received by the plants varied between 3 and 26 MJ m-2 d-1 over the period of the experiment. Artificial shading was chosen to provide light stress, and enable growth comparisons between shaded plants and non-shaded plants. The shade treatment (a neutral shade cloth which transmitted 31% of the 400- to 700-nm radiation, 29% of the 350- to 500-nm radiation, and shifted the red/far-red ratio from 1.18–1.11) (Ets Puteaux, Le Chenay, France) was installed at the 10-leaf stage, given that competition for radiation between plants can be considered negligible before this stage (Duparque et al., 1990). Several authors have addressed the difference between shading and increased density (Huld and Andersson, 1997), emphasizing effects on grain production (Andrade et al., 1993). The average apparent photosynthesis in leaves was equally reduced, with similar consequences on dry matter accumulation (Hashemi-Dezfouli and Herbert, 1992), even though an increase in density leads to heterogeneity of irradiance intensity between leaves, which is not observed in shading experiments. Comparing several genotypes, Gerakis and Papkosta-Tasopoulou (1980) found similar changes in the relative performance of hybrids both by artificial shading and by self-shading. Undoubtedly, the pattern of dry matter accumulation of a given plant is differently affected by shade compared with increasing crop density, but the major advantage of shade treatment lies in the elimination of earliness-induced side effects in comparing the changes in dry matter accumulation and partitioning between roots and the shoot among genotypes.


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  		Table 1. The hybrids used in these experiments and their agronomic traits

 
To reduce plant-to-plant variability, seeds were weighed and graded prior to sowing, and only seeds of comparable sizes were pregerminated at 20°C on wet blotting paper. To make it possible to measure root/shoot biomass ratio, it was necessary to be able to measure root biomass. Thus, the seedlings were transplanted to cylindrical pots filled with 25% natural soil (sandy-loamy Alfisol) and 75% silica sand, 0.6- to 1.3-mm mesh size (one plant per pot). Twelve-liter containers (0.27-m diam, 0.22-m height) were used, as a compromise between the room needed by plants to grow and easy pot manipulation. The containers in which plants are grown have known effects on the spatial geometry of roots, but their effects on root biomass are small (Kjellstrom and Kirchmann, 1992; Schuch and Pittenger, 1996) and they could be used to measure accurately root/shoot biomass allocation. The pots were watered every 2 h by an automated system providing the necessary macro- and micro-nutrients for optimal growth (N: 5 mmol/L, P: 0.3 mmol/L, K: 2.2 mmol/L, Ca: 0.55 mmol/L, Cl: 0.25 mmol/L, Na: 0.1 mmol/L, Mg: 0.25 mmol/L, S: 0.12 mmol/L; micro-nutrients).

The experimental design was a split-plot with two independent replicates nested within the light treatment plots. The containers were arranged at 0.7-m intervals in both directions. In such conditions, the quantity of light energy as well as the light quality could be considered the same among plants.

The daily average temperature of the substrate in the two treatments was measured at 100-mm depth in five randomly distributed pots. The base temperature used for calculations of accumulated thermal units was 6°C (Bloc and Gouet, 1973). To measure the changes in growth of shoots and roots, six plants per replicate and per genotype were observed at four developmental stages: 10-leaf stage, 12-leaf stage, 15-leaf stage, and silking (Table 2). The duration of the sowing to silking interval was known from previous thermal time recordings for the genotypes used here. On that basis, the period from the 10-leaf stage to silking was split into three parts, and the boundaries of the intervals made up the dates of sampling for each genotype. This procedure was likely to account better for the growth of the genotypes regardless of their earliness.


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  		Table 2. Observation stages in degree-days (upper lines) and leaf numbers (lower lines), averaged over the 1995 and 1996 experiments

 
We focused on the assessment of the amount of biomass accumulated and on some major traits related to the morphology of the primary root system. The traits related to biomass accumulation were as follows: (i) the dry weights of shoots and roots, (ii) the total dry weight of the plant, (iii) the root/shoot biomass ratio, and (iv) the average weight of primary root axes, which is equivalent to the average amount of root dry matter accumulated per primary adventitious root axis.

The experimental conditions necessary to estimate root biomass (i.e., the containers) did not allow for a full description of the actual root architecture. Consequently, root information was reduced to some traits easily accessible through pot culture. These traits mainly described the quantity of roots produced, together with how the root system was structured. The following traits were measured: (i) the number of adventitious roots on Internodes 2 to 11, (ii) the total number of adventitious roots, (iii) the number of internodes carrying elongated primary root axes, and (iv) the average diameter of the basal part of the primary roots on Internode 7 at silking (determined on the basis of previous measurements and the strong correlations observed between internodes for root thickness; the roots on Internode 7 are considered average roots of a given plant with regard to thickness).

The root internodes were identified by considering that maize stem consists of successive phytomeres made of internodes which bear roots at their basal ends, and nodes producing the leaves at their upper ends (Kiesselbach, 1949; Girardin et al., 1986).

The data from the 2 yr were pooled, and submitted to analysis of variance. The computations were done according to a three-level split-plot design,with using the years as replications (confirmed by the non significant genotype x year interaction—data not shown). The light conditions were used as main plots, the blocks as subplots, and the genotypes as sub-subplots. The magnitude of genotype x light interaction was evaluated by computing the ratio of the genotype mean square over the genotype x light mean square (called the interaction ratio in the following). The relative contribution of each genotype to the genotype x light interaction was estimated by use of Wricke's ecovalence method (Wricke, 1962). The analyses of variances were performed with the S-Plus package (Venables and Ripley, 1994).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Biomass Partitioning
The total biomass, the biomass of each part of the plant, and the root/shoot ratio exhibited significant differences among genotypes and light conditions at all sampling dates (Table 3). The genotype x light condition interaction was significant at the 1% level for the dry weight of aerial part at the third and last sampling dates, and for the dry weight of root biomass from the second sampling date onwards (Table 3). At silking, shoot and root dry weight both exhibit very different interaction ratios (respectively 253.1 and 2.6), as shown by the mean squares reported in Table 3; the interaction for shoot biomass was distinctly lower compared to root biomass. For root biomass, the genotypes that were the most responsible for the interaction varied during growth period: at the third sampling date, two hybrids (W117 x F288 and Eva) accounted for 88% of the interaction sum of squares, whereas at silking Mammouth accounted for more than half of the interaction sum of squares (Table 4).


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  		Table 3. Mean squares of traits evaluated at silking stage and their significance

 

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  		Table 4. Wricke's ecovalence for root biomass at third and last sampling dates, and the total number of roots at silking

 
The growth analysis performed with consideration to the allometric relationship between root dry weight and shoot dry weight indicated a highly significant (P < 0.001) effect of shade on the relationship between root and shoot. The values of the linear regression coefficient, 1.0127 and 0.7900 in sun light and shade, respectively, were both significantly different from zero (Table 5). A significant (P < 0.01) impact of genotype on the allometric relationship between root and shoot dry weight was also detected. A plot of the logarithms of root vs. shoot dry weight is given in Fig. 1 .


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  		Table 5. The allometric relationship between root and shoot dry weight. Linear equations of the form y = a + bx, where y is the natural logarithm of root dry weight and x is the natural logarithm of shoot dry weight

 


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  		Fig. 1. Allometric relationships between shoot and root dry weight, together with the original data and the regression coefficients per treatment across genotypes (with standard errors)

 
The root/shoot ratio of non-shaded plants was relatively stable across sampling dates, at an average of 0.5 (Table 6 and Fig. 2) . In the shade treatment, growth was slowed and, from the second sampling date onwards, the dry weight of both aerial and root parts and the total dry matter were significantly reduced by shading. At the last two stages, the root/shoot ratio was lower in the shaded treatment, which showed that root growth was reduced more than shoot growth (Table 6).


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  		Table 6. Root and shoot dry weight at each sampling date. Mean over the genotypes in the sunlight and shaded treatments

 


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  		Fig. 2. Root/shoot biomass ratio (log scale) as a function of the total biomass of the plant (vertical bars are the confidence intervals of the data from harvest at silking, at the 0.05 probability level)

 
The genotype x light interaction of the root/shoot ratio, was significant from the second sampling date onwards, and was found the most important at silking (Table 3). Figure 2 indicates that this interaction was associated with an important reduction in the genotypic variation of the trait. Some genotypes strongly contributed to this effect: LG2304 only exhibited a small reduction in the root/shoot ratio under shading, compared with LH74 x F271 and F113 x Co255, which showed a severe decrease in aerial biomass and had the lowest root/shoot ratio among the studied material. These hybrids showed an example of association between root lodging behavior and the root/shoot ratio. In contrast, some hybrids did not significantly contribute to the interaction: Mammouth, Eva, and Népal maintained a high root/shoot ratio whatever the conditions, and F7001 x F1772, with a low root/shoot ratio, behaved a similar way. In these cases, no relations with the lodging habits of the varieties were observed.

Morphology of the Root System
The average total number of adventitious roots was strongly reduced by shading from the second sampling date, but the number of internodes carrying roots did not appear very different between the light conditions, even though significance was detected (Table 7). No effect of shading on root counts of the different internodes appeared at the 10-leaf stage. From the 12-leaf stage onwards, fewer roots emerged and elongated under shaded conditions. A significant reduction in root numbers was successively revealed on internode 7, then on internode 8, and finally on internodes 9 and 10 only at silking (Table 3). The analyses of variance indicated no effects of maturity (cumulated degree-days to silking) on root counts across treatments and sampling dates. The diameter of the roots on Internode 7 was smaller under shaded conditions (Table 7).


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  		Table 7. Characteristics of the adventitious root system in the sunlight and shaded treatments at silking. Means over the genotypes

 
The genotype x light interaction was significant for the total number of adventitious roots (Table 3). The varieties LG2304 and Mammouth accounted for most of the interaction for this trait (Table 4). At silking, the root numbers exhibited significant interactions on some internodes, mainly Internodes 9 and 10 (Table 3), with interaction ratios of 4.6 and 2.1, respectively. The genotype x light interaction was not significant for root diameter and the number of internodes carrying emerged roots (Table 3).

Relationships between Total Number of Roots and Root Biomass
On average, the average weight of root axes was affected by shade (despite the low power of the F-test for split-plot), which reduced it to a value lower than 0.5 (Table 7). Significant differences among genotypes, together with the interaction with light treatment, were observed (Table 3).

Root biomass was relatively more affected by the shade treatment than the total number of adventitious roots (Fig. 3) . Root biomass was reduced by 50 to 70%, regardless of the absolute value of the genotype under normal conditions. Comparing the variations in root count and the differences in biomass, we noticed Mammouth's contrasting behavior: its total number of adventitious roots was almost the same in the two treatments, whereas its root biomass was greatly reduced by shading. However, the genotype x light interaction was significant even when the computations were done without the genotype Mammouth.



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  		Fig. 3. Number of roots per plant (upper part of the figure) and root biomass per plant (lower part) at growth stage 4 as a function of the genotype and the treatment. Left-hand bars stand for normal conditions, right-hand bars for each genotype are the values under shade. The percentages refer to the relative decrease in trait under shade versus sunlight

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Root/Shoot Partitioning
Our work confirms previous findings which report that shading significantly reduces the growth of both roots and shoots, and also decreases the root/shoot ratio (Lambers and Posthumus, 1980; Brouwer, 1983; Demotes-Mainard and Pellerin, 1992). Given that there was no limitation for water or mineral nutrients in this experiment, such observations could be explained by an effect of competition for light. In the genotypes studied, the growth of roots was generally more affected than the shoot by a reduction in the radiate energy available to a given plant. This result confirms a general finding for various species (Troughton, 1956; Brouwer, 1966; Hunt and Burnett, 1973). When the plants were not subjected to any limitations, a unique allocation pattern applied, and the root/shoot ratio remained at the value of a constant allometric coefficient regardless of the genotypes and the sampling dates.

Under light competition, the root/shoot biomass ratio decreased with growth, similarly to what several other authors observed (Lambers and Posthumus, 1980; Del Castillo et al., 1989; Demotes-Mainard and Pellerin, 1992). This result shows that shaded isolated plants behave as plants under increased density conditions. Regardless of the hybrid, shoot biomass accumulation is less inhibited than root biomass accumulation by light competition, compared with full illumination.

An important result of the present work is that the partitioning of biomass between shoots and roots cannot be supposed to be constant among genotypes. We have shown that some hybrids allocate a higher proportion of biomass to roots than others. This result could be dependent upon the genetic material used, since other published results do not strictly corroborate ours (Wiesler and Horst, 1994). Our findings suggest that different genetic types could exist, which would strongly contribute to the amount of genotype x light interaction found: Mammouth and Eva were hybrids with high root/shoot biomass but were very much affected by light reduction; LG2304 and F7001 x F1772 exhibited the smallest effects of light competition on the root/shoot ratio. This lodging-resistant hybrid appeared less affected by shading; similarly, two lodging-susceptible hybrids exhibited the lowest root/shoot ratios among the material studied. It could be hypothesized that the root/shoot ratio is likely to account partly for the lodging behavior of some hybrids. Hence, part of the ability of some genetic material to resist lodging under high crop density could reside in the capability of these genotypes to maintain root growth while shaded. Their rooting ability would enable the root system to better counterbalance the mechanical effects of wind on the shoot. It is important to notice that different root/shoot ratios can be obtained with roughly similar aerial biomass.

Root System Architecture
The study of the patterns of root distribution along the stem clearly revealed that the main impact of carbon limitation through light reduction was more a delay in root emergence and elongation than a strong limitation in root emergence. The internodes exhibiting a reduced number of roots under shading conditions were higher on the stem as the plants grew. On the lowest internodes, no significant differences in root counts were noticed; some may have arisen under more severe light limitation, but the present result is relevant to normal crop densities, where competition for light seldom occurs before the 8- or 10-leaf stage. We demonstrated that the numbers of root axes on the different internodes were not qualitatively modified.

Comparing the root counts on the upper internodes of two consecutive sampling dates (data not shown) suggests, following Pellerin (1991), that shade could simply delay root emergence. However, the intense intra-plant competition for nutrients at silking make it impossible for the plants to recover normal root numbers on the highest internodes, resulting in significantly different root counts in shade compared to standard treatment. These findings corroborate the results of Mawaki et al. (1990) on shaded rice (Oryza sativa L.) plants, and those of Demotes-Mainard and Pellerin (1992) on maize at different plant densities.

Our results also show that a reduction in light availability at the plant scale (for example because of high crop densities) affects the root biomass more than the total number of adventitious roots. This means that the primary root system would not suffer a lot from a reduction in photosynthate (except on the upper internodes, as discussed above). This could result from the particular adaptive value of the root number, since a high number of roots generally ensures more first and second order laterals, and thus contributes to better water and nutrients uptake capacity, as shown in various studies (Jordan and Miller, 1980; Richards and Passioura, 1981; Salih et al., 1999).

In addition to that average effect of competition for light between plants of the same hybrid, the genotype x light interactions of the root/shoot ratio, the root counts on upper internodes (9 and above), and the total number of adventitious roots, strongly indicate that various genetic patterns can be observed in these conditions. Contrasting behavior is presented by Mammouth, which exhibited a significant reduction of its root biomass without any significant change in the number of primary roots on the plants, and by hybrids like LG2304, which is capable of a proportional reduction in both root number and biomass. It must be pointed out that such differences exist without any connection with the variation in earliness between the materials.

The variations of root number and root system weight resulted in an observed reduction in the average specific weight of primary roots, a phenomenon which could possibly be accompanied by other morphological differences such as reduced diameters (as confirmed by our experiments), less branching, and reduced elongation rates of the root main axes. Root length and root branching were not studied in the present work, because the conditions of growth were not appropriate to relevant measurements. But it can reasonably be suspected that these characters were also affected by competition for light. A complete understanding of the variability of root lodging patterns existing among genetic materials would imply taking into account the genetic and environmental variations in those two traits, from which the cohesion between roots and soil strongly depends. The results obtained with the susceptible hybrid Mammouth could possibly be due to a significant influence of available light quantity on these traits.

Consequences for Maize Breeding
Some authors have already hypothesized that, on an average, root systems of cereals could have suffered from intense selection pressure for increased biomass or grain yield (Siddique et al., 1990). Such a phenomenon is likely to occur in maize, as a consequence of the difficulty to measure root development. That could partly account for the need of stronger lodging resistance as the genetic progress makes biomass production increase. The variability among genotypes grown in light limited conditions gives significant information, which meet the breeders' concerns in the field of lodging resistance, and could improve the selection processes.

Genotypic differences in root/shoot ratio have been found independent of the aerial biomass production. The variation in root/shoot ratio among genotypes under light limitation indicates that a more developed root system can be selected from genotypes with a given shoot biomass. Biomass allocation under shade is a non-constant trait; it can potentially be selected, and give way to genetic progress toward better rooted varieties. The existing genetic variability of maize plant growth can give access to material endowed with better rooting capacities and a satisfactory amount of harvestable biomass. In general, it could be hypothesized that a more developed and/or better structured root system would provide the plant easier access to water and nutrients, and thereby better stress resistance. The selection of better root morphology could improve both lodging resistance and water and nutrient stress resistance. Root/shoot ratio would be a problematic selection criterion, but the assessment of some easily accessible traits (e.g., root counts on the upper internodes of the root clump, average root diameter) could help to prevent any decrease in the proportion of the root part of the plant.

Our conclusions on the variability of root morphology are also favorable to significant and low-cost genetic progress in the future. The density stresses allowed the root morphology and architecture to be less affected than the biomass of the total root system. Positive consequences on the selection for lodging resistance can reasonably be expected, since the mechanical quality of plant anchorage mainly resides in the spatial arrangement of primary roots. Breeding for biomass production together with superior root architecture could improve cultivar adaptation to high crop densities.


   ACKNOWLEDGMENTS
 
The authors thank P. Porcheron for assistance in managing the experiments and conducting the studies. They are also grateful to S. Pellerin and C. Varlet-Grancher for useful comments during the preparation of this paper.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This research was supported by a grant from Région Poitou-Charentes (no. 94/RPC-R-138).

Received for publication January 6, 2000.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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