JXB Advance Access published online on May 4, 2007
Journal of Experimental Botany, doi:10.1093/jxb/erm065
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RESEARCH PAPER |
Experimental evaluation of an efflux–influx model of C exudation by individual apical root segments
1UMR INPL(ENSAIA)-INRA Agronomie et Environnement Nancy-Colmar, 2 avenue de la forêt de Haye, F-54500 Vandoeuvre lès Nancy, France
2Unité de recherche Plantes et Systèmes de Cultures Horticoles, INRA Domaine Saint-Paul - Site Agroparc, F-84914 Avignon, France
* Present address and to whom correspondence should be sent: UMR 1220 INRA-ENITAB Transfert sol-plante et cycle des éléments minéraux dans les écosystèmes cultivés, 71, avenue Edouard Bourlaux, BP 81, F-33883 Villenave d'Ornon, France. E-mail: Christophe.Nguyen{at}bordeaux.inra.fr
Received 21 December 2006; Revised 28 February 2007 Accepted 6 March 2007
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Abstract |
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The aim of this study was to evaluate if a model describing the efflux and the influx of C through the root surface could be fitted to experimental short-term kinetics of carbon (C) exudation by individual apical root segments in maize (Zea mays L.). The efflux of C was set constant or modelled by a power function of the distance from the apex to simulate the greater release of C around the root tip commonly reported in the literature. The influx was proportional to the C concentration in the external solution to simulate the active re-uptake of exudates by the root. Plants were exposed to full light or to shade to manipulate C allocation to roots. The model with a constant efflux gave satisfactory fits to the kinetics of exudation (average R2=0.66). The average gross efflux was then 2.1 µg C cm–2 root surface h–1. The model was improved if exudation was set more intense towards the root apex (average R2=0.74). The estimated gross efflux decreased then from 5.2 µg C cm–2 h–1 at the apex to 1.8 µg C cm–2 h–1 for the region located 5–25 cm from the root tip. The decrease in net exudation of individual roots due to the shading of plants was weak, which may indicate that the import of C by the primary roots studied was not reduced significantly. By describing the exudation of an apical root segment of variable length and diameter, the model is a first step in linking exudation to root system architecture models and to whole plant functioning.
Key words: Carbon, exudation, model, root apex, root length, root diameter
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Introduction |
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Proliferation of micro-organisms in the rhizosphere has major impacts on soil processes such as nutrient cycling, bioavailability of pollutants, and soil health (Curl and Truelove, 1986). Rhizosphere micro-organisms proliferate from the continuous release of organic compounds from roots, in particular, diffusible root exudates which are an important source of easily available substrates (Newman, 1985; Nguyen, 2003). Therefore, the release of root exudates is a key issue in the modelling of the growth of micro-organisms in the rhizosphere and of the processes mediated by them (Newman and Watson, 1977; Darrah, 1991; Toal et al., 2000). Unfortunately, the study of root exudates in soil is particularly difficult because of their interactions with soil particles and because they are rapidly consumed by micro-organisms (Todorovic et al., 2001; Kuzyakov, 2002). Consequently, root exudation has mainly been studied with plants cultivated in sterile nutrient solution. This enabled the amount of carbon (C) released from roots to be estimated (Barber and Gunn, 1974; Prykril and Vancura, 1980; Kraffczyk et al., 1984; Jones and Darrah, 1993a; Gransee and Wittenmayer, 2000). However, compared with soil conditions, hydroponics probably underestimates exudation because of the lack of micro-organisms and mechanical impedance, both of which have been shown to increase exudation (Barber and Gunn, 1974; Prykril and Vancura, 1980). In addition, in the last decade, it has been shown that estimates of exudation from hydroponics were largely underestimated because exuded sugars and amino acids are actively re-taken up by transporter proteins of the root plasmalemma if the nutrient solution is not renewed (Jones and Darrah, 1993a; Farrar et al., 2003) and this was not considered experimentally. These authors calculated that re-uptake could be as high as 80–98% of the gross efflux of C. Hydroponics cultures also enabled it to be demonstrated that root exudation depends on whole plant functioning. Exudation is affected by C allocation to roots (Dilkes et al., 2004) and it is closely linked to root system architecture because it is more important at the root tips (McDougall and Rovira, 1970; McCully and Canny, 1985; Trofymow et al., 1987; Darwent et al., 2003). Consequently, modelling of root exudation at the plant scale requires that both the growth and architecture of the root system and the exudation of individual roots are described.
Models for root growth and architecture are available (Pagès, 1999; Bidel et al., 2000; Pagès et al., 2000; Dunbabin et al., 2002). By contrast, very little modelling of root exudation has been undertaken. A simulation model has been proposed by Jones and Darrah (1993a) to demonstrate the importance of exudate re-uptake by roots. This model considered fixed rates of gross exudation for root tip and non-root tip regions and the recapture of exudates by the whole root as described by Michaelis–Menten kinetics. A more mechanistic model was suggested by Farrar et al. (2003) for glucose. In this model, both gross exudation and re-uptake also operate, but exudation was modelled as the passive diffusion of glucose from the root cell cytoplasm to the soil solution due to the steep gradient between these two compartments. Unfortunately, none of these models were tested and parameterized experimentally. Therefore, the aim of this study was to evaluate if a model describing the efflux of total C and its re-uptake by roots could fit the experimental kinetics of exudation measured at the individual root level. The second objective was to set up a protocol to determine the exudation from individual roots. Root boxes were designed for isolating roots from maize plants, while the rest of the root system continued growing in soil. The kinetics of exudation by apical root segments having different lengths was measured in order to investigate the variation in exudation with distance from the root apex. In addition, plants were cultivated in full light or in shade to manipulate C allocation to roots. The kinetics of exudation were then used to parameterize and evaluate the efflux–influx model.
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Materials and methods |
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The efflux–influx model of root exudation
The model studied in this work was adapted from the efflux–influx model described by Farrar et al. (2003) for uncharged compounds. The model described root exudation as the difference between the gross exudation of C (E, µg C h–1) driven by passive diffusion from the root cytoplasm and the active uptake of C-compounds (I, µg C h–1) described by Michaelis–Menten kinetics:
 | (1) |
 | (2) |
 | (3) |
If roots are allowed to exude in a solution that is not renewed, the concentration of C in the root bathing solution at equilibrium is low, typically 4–10 µg C cm–3 or 0.3–0.8 mM C (recalculated from Prikryl and Vancura, 1980; Jones and Darrah, 1993a; E Personeni, unpublished results) because the efficiency of exudates re-uptake is high (Jones and Darrah, 1993a). By comparison, sugar concentrations in roots as high as 100–1000 mM sucrose in phloem have been reported in the literature (Farrar, 1985; Bidel et al., 2000) and 40 mM glucose in other root cells (Farrar et al., 2003). Consequently, it can be assumed that Ci>>Ce. Moreover, Ci was assumed to be constant with time because exudation measurements were performed, on average, 7 h after the beginning of the photoperiod and, therefore, the allocation of C to roots was in steady-state (Throughton and Currie, 1977). From these simplifications, the gross exudation E (equation 1) can be assumed to be independent of Ce and constant with time. By contrast, E does not increase linearly with root length because the local efflux of C is higher close to the apex. To simulate this, the local efflux of C 
E(x) was modelled as a power function of the distance x to the apex:
 | (4) |
is the C efflux rate at the apex (µg C cm–2 h–1) and 
is a parameter. This mathematical model was chosen because it allows a wide range of efflux profiles to be simulated, including the case where the efflux was the same along the root axis (=0).
Then, the exudation of an apical root segment of length L (cm) and of radius r (cm) is given by
 | (5) |
rL.
The kinetics of root uptake of sugars and amino acids are multiphasic, suggesting that multiple carriers with different affinities operate for a given substrate (Reinhold and Kaplan, 1984). This allows the transport of the substrate for a broad range of concentrations. For example, Xia and Saglio (1988) reported two different Michaelis constants for hexose uptake by maize root tips, and examination of the kinetics curves indicates that for concentrations below 5 mM, the velocity of hexose uptake could be approximated by a linear function of the hexose concentration. Hence, it is reasonable to assume that for C concentrations in the root bathing solution below 0.8 mM (see above), the re-uptake of exudates is proportional to the C in solution and to the root surface and then equation (2) can be rewritten as:
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After integration, equation (3) gives:
 | (7) |
Plant growth
Maize plants were grown in root boxes made of PVC (Fig. 1A). They consisted of a soil compartment (29x17.5x1 cm) separated from a free space of the same size by a plastic grid (4 mm square mesh). The soil compartment was filled with moist soil (520 g dry weight, humidity: 25 g water 100 g–1 dry soil) sieved at 5 mm. An agricultural topsoil was used (0–30 cm) collected at the INRA experimental station at Mirecourt (Vosges, France) with the following characteristics: clay, 191 g kg–1; silt, 466 g kg–1; sand, 343 g kg–1; C, 13 g kg–1; N, 1.3 g kg–1; pH (H2O), 6.5. Two germinated seeds of Zea mays L. (cv. DEA) were placed at a 1 cm depth in the soil compartment of each root box (Fig. 1A). The free space was covered with a transparent altuglass part maintained with clips. The root boxes were transferred into a growth chamber with the following conditions: 22/18 °C day/night temperature, 16 h photoperiod, 200 µmol m–2 s–1 light intensity, and 70% relative humidity. In the shade treatment, light was reduced one week before exudation measurements by using a greenhouse shade cloth, which gave a light intensity of 40 µmol m–2 s–1 at the plant level. The root boxes were inclined at 30° above horizontal to facilitate the downward extension of roots through the plastic grid towards the free space. In this compartment, roots were in permanent contact with the water that condensed on the transparent cover. Root boxes were supplied with 0.25 strength Hoagland's nutrient solution (Hoagland and Arnon, 1950) to maintain the soil at the above-mentioned humidity.
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Collection of root exudates
Six to eight weeks after sowing, roots that had grown in the free space of the root box were used for exudation studies. Exudation was determined 6–7 h after the start of the photoperiod. In order to study the possible longitudinal variation of exudation, an apical segment of primary root of variable length and diameter (Table 1) and free of soil particles and of laterals was carefully inserted into a 4 mm internal diameter silicon tube attached to the plastic grid. Below the root apex, the tube was connected to a 1.5 cm long silicon tube stopped with a 4 mm glass bead (Fig. 1B). This tube was used to inject and to withdraw the CaCl2 (5 mM) root bathing solution by using a syringe+needle. The thickness of the tube wall was 2 mm to prevent leaks of the solution from the needle cut. Preliminary tests confirmed that silicon tubes did not release any detectable amount of C. The CaCl2 was used to stabilize the root membranes (Neumann and Römheld, 2000).
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A protocol was set up to rinse the root segments to eliminate the C possibly in the apoplast. Roots (n=9) were repeatedly rinsed as follows: the whole tube was filled with the CaCl2 solution, which was left for 2 min and then withdrawn. This rinsing procedure was repeated to perform a total of five rinsings. The rinsing solutions were immediately frozen (–20 °C) and individually analysed for their organic C content after acidification and centrifugation (see below). The t test for paired samples was used to test for significant differences in the organic C in the solution between two consecutive rinsings. In a separate experiment, roots were rinsed by a continuous flow of CaCl2 (5 mM). In this case, roots were inserted into a ‘T’ connector, assembled on one side to an open-end silicone tube. The horizontal branch of the connector was used to percolate the CaCl2 solution through the tube+root for 15, 30, or 45 min (3.5 ml min–1). Afterwards, the silicone tube was stopped as described above and the root was bathed in CaCl2 for 2 min. The organic C concentration in this solution was used to evaluate the efficiency of the rinsing by percolation using a one-way ANOVA with the duration of percolation as a factor (n=4).
The kinetics of exudation were determined for apical root segments repeatedly rinsed five times and incubated in the CaCl2 solution for 5, 15, 30, 60, 120, and 180 min. The root bathing solutions were then collected and frozen (–20 °C) before organic C determination. After the last sampling, root segments within the tube were excised and their length and average diameter were determined after scanning and image analysis (WinRHIZO, Regent Instruments Inc., Québec, Canada). Fifteen roots were examined for the full light treatment and 14 roots for the shaded plants, corresponding to 7–8 root boxes per light treatment.
A separate experiment was dedicated to estimate the microbial degradation of root exudates. Eight roots were allowed to exude for 6 h and the individual root bathing solutions were incubated in the dark at 22 °C with 50 µl 14C-[U]-glucose (1900 Bq, 17 ng C) in a 240 ml airtight incubator that contained 25 ml NaOH 1 M to trap the 14CO2. The radioactivity in the NaOH was determined by liquid scintillation counting after 1, 3, 4, and 6 h incubation.
Determination of organic C in the root bathing solution
When possible, a 1 ml aliquot of the root bathing solution was subsampled and mixed with 100 µl HCl 2 M to evacuate the dissolved inorganic C. Otherwise the volume of HCl was adapted to represent 1:10 of the sample volume. The samples were centrifugated at 16 000 g for 15 min to sediment possible root cells. One ml of the surpernatant was collected and stored at –20 °C until C analysis. Using a microscope, it was checked that the supernatants were free of root debris.
Organic C determination was performed by a TOC analyser (model TOC-V CSH/CSN, Shimadzu, Japan) equipped with a manual injection kit. All samples were corrected for the background C of the CaCl2 solution+HCl, which was typically around 1 µg C cm–3. The volume of injection was 70 µl and injections were replicated until the coefficient of variation was less than 5%. It was checked that four washings of the injection syringe with ultra pure water were sufficient to avoid contamination between samples.
Experimental kinetic of exudation were fittted to equation (7) using the Proc Model of SAS v9.1 (The SAS Institute, Cary, NC, USA) and parameters were estimated by the ordinary least squares method. The estimates for parameters and and their associated standard deviations were used to simulate the gross efflux of C as a function of the distance from the root apex (E(x), equation 4). For each position on the root segment (step=0.1 cm), 1000 simulations were performed by drawing random values of and in a normal distribution.
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Results |
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Rinsing of roots
Before exudation measurements, individual roots were rinsed to remove C possibly contained in the apoplast (Farrar, 1985; Xia and Saglio, 1988). If roots were rinsed repeatedly, the concentration of C in the bathing solution (Ce) decreased significantly for the first three rinsings (Fig. 2). At the first rinsing, the quantity of C in the solution C1 (µg C) was related to the root volume Vr (cm3) by the relationship C1 = 20V
(R2=0.83), where Vr=r2L is the root volume. This relationship indicates that small apical root segment released relatively more C at the first rinsing than larger roots. After three rinsings, Ce stabilized and was, on average, 2.4 µg C cm–3 (standard deviation: 0.89) (Fig. 2). If roots were rinsed by continuous percolation of the CaCl2 solution for 15, 30, or 45 min, the concentration of C in the bathing solution after a 2 min exudation immediately following the rinsing was 2.1 µg C cm–3 (standard deviation: 1.51) and was not significantly changed by the duration of the percolation (data not presented).
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Microbial degradation of exuded C
The microbial degradation of the C exuded by roots was evaluated by incubating the root bathing solution with 14C-[U]-glucose. The production of 14CO2 was low: 0.4±0.32, 1.0±1.04, 1.5±1.64, and 2.9±3.18% of the added 14C, after 1, 2, 3, 4, and 6 h, respectively. This indicates that the microbial activity in the root bathing solutions was very weak.
Kinetics of root exudation
The characteristics of the apical root segments used for determining the kinetics of exudation are given in Table 1. The length did not exceed 25 cm because of the limitation by the size of the root box. The root diameters were, on average, smaller in the shade treatment.
Soluble organic C in the root bathing solution (Ce) increased with time according to a hyperbolic kinetics (Fig. 3A). Before 30 min of incubation, Ce increased rapidly by 4 µg C cm–3, which corresponded to a net exudation of 1.9 µg C cm–2 h–1 for a mean apical root segment of 11 cm length, 0.16 cm diameter, bathing in 1.1 cm3 solution. After 5 min and 15 min of incubation, Ce was significantly higher for plants exposed to full light. Afterwards, Ce reached a plateau at 7–8 µg C cm–3 at the end of the experiment.
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Model fits
If no longitudinal variation in the efflux of C was considered (
set to 0, equation 4), the model explained, on average, 66% of the variability in the net root exudation (Table 2). Then, the average gross efflux was 2.1 µg C cm–2 h–1 (parameter ). The residuals (observed–predicted values) were negatively correlated with the length of the root segment (r= –0.29, P=0.02; r= –0.4, P <0.001 for the full light and shade treatments, respectively). This indicated that the model moderately underestimated exudation of small root segments and overestimated that of long segments. Consistent with this, if the efflux was allowed to decrease with the distance from the apex, (
0, equation 4), the goodness of fit increased, on average, by 8%, the increase being more important in the full light treatment (Table 2). Then, the absolute value of the residuals was on average below 1 µg C cm–3 (Fig. 3B) and was not dependent on time. The parameter estimates were similar between the two treatments. The estimated efflux at the root apex was around 5.2 µg C cm–2 h–1 (parameter ).
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The simulation of the efflux of C at different distance from the root apex (equation 4) indicated an important decrease between 0 cm and 5 cm (Fig. 4). Between 5–25 cm from the apex, the efflux decreased little and was typically around 1.8 µg C cm–2 h–1.
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Discussion |
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Protocol for determining exudation from a single root
In order to test the model, the roots were rinsed to remove the organic solutes that possibly accumulated in the apoplast during root growth (Farrar, 1985; Xia and Saglio, 1988). The carbon concentration in the solution was significantly higher for the first two rinsings, indicating that a pool of carbon of a limited size was rapidly released from roots (Fig. 2). The size of this pool was on average 6.8 µg C and was too important to originate solely from the diffusion of C from the cytoplasm through the plasmalemma according to the calculation done for glucose (Farrar et al., 2003). This rapid release of C was interpreted as mainly originating in the diffusion of some soluble C accumulated in the apoplast. This is supported by the work of Farrar (1985), who reported that the rate constant for the loss of sugars from the apoplast of barley roots was seven times higher than that for the loss from cytoplasm. The non-linear relationship found between the release of C at the first rinsing and the volume of the root segments suggests that this accumulation of C was more important close to the apex.
Starting from the third washing, there was a significant and constant amount of C in the rinsing solution: around 2.4 µg cm–3. Because a similar quantity of C was also recovered after rinsing the roots by percolation for 15, 30, or 45 min, this residual C was not related to the efficiency of the rinsing. This C was unlikely to be root cap cells+mucilage because samples were centrifugated and checked under the microscope. The most probable explanation for this residual C is the rapid partial replenishment of the stock of C in the apoplast between two rinsings. Indeed, after a rinsing, the gradient of C between the apoplast and the cytoplasm (Van Bel, 2003) is significantly increased, particularly at the phloem level: 100–1000 mM sucrose in phloem sieve tubes (Farrar, 1985; Bidel et al., 2000) and 40 mM glucose in other root cells (Farrar et al., 2003). Moreover, the apoplast represents a small volume of the root (5%; Farrar, 1985), but the exchange surface with the cytoplasm is very large. Consequently, when the root was no longer in contact with the solution between two rinsings, the C acccumulated rapidly in the apoplast and it was then recovered in the bathing solution during the subsequent rinsing or at t=0 for the kinetics of exudation. Hence, it is considered that three sequential rinsings would be enough to wash the roots. For roots growing in soil, the C stocked in the apoplast is expected to have little influence on root exudation as long as the roots are permanently in contact with the soil solution. Then, the root exudation is likely to derive mainly from the diffusion of cytoplasmic C, the apoplast being a route for solutes towards the outer soil solution.
The significance of exudates degradation by micro-organisms was evaluated by incubating the root bathing solution with a small amount of 14C-[U]-glucose and by monitoring its mineralization. Glucose is a very common sugar found in root exudates of maize grown in hydroponics (Kraffczyk et al., 1984; Jones and Darrah, 1996) and its mineralization was assumed to be representative of the microbial consumption of the C released from the roots in our experimental conditions. The mineralization of 14C-[U]-glucose was very low, less than 2% of the added 14C within 3 h of incubation, which was the longest time of exudation. After 6 h, the mineralization was less than 5%. Consequently, it is concluded that the microbial degradation of root exudates was negligible in these conditions. This was possible because the roots were carefully chosen to be free of any visible soil particles.
Kinetics of root exudation
If passive diffusion alone was responsible for the increase in C concentration in the root bathing solution (Ce), the Ce at equilibrium should have been around the root concentration in neutral compounds: 40 mM of glucose for example (Farrar et al., 2003), which gives 240 mM C. It was determined that after 1 h of exudation, Ce stabilized around 7–8 µg C cm–3 or 0.6–0.7 mM C, which is similar to the total organic C concentration determined by Jones and Darrah (1993a) for maize cultivated in hydroponics without renewal of the nutrient solution. Therefore, in our work, the plateau value of Ce was too low to have resulted from passive diffusion alone and it indicates that an active re-uptake of root organic solutes must have operated and limited the net efflux of C. The re-uptake of exudates by roots has clearly been demonstrated for sugars and amino acids (Jones and Darrah, 1993b, 1996). Our results confirmed that if the root bathing solution is not renewed, the net amount of C released from roots is low (Jones and Darrah, 1993a). In soil conditions, net exudation is expected to be greater because re-uptake is probably limited by the rapid utilization of root exudates by soil micro-organisms (Nguyen and Guckert, 2001).
Shading moderately decreased the C concentration in the root bathing solution before 0.5 h but it had no effect at equilibrium (>1 h) indicating that both the efflux and the re-uptake must have been altered. Indeed, the restriction of C allocation to roots by shading is likely to impact both the diffusible C and the energy available for the transporters that operate the uptake. Based on previous works that showed the dependency of root exudation on C allocation to roots (Hodge et al., 1997; Dilkes et al., 2004), the effect of shading observed here was unexpectedly low, despite light intensity being reduced by 80%. The lack of strong effects of light treatment on root exudation has two probable origins. Firstly, the difference in C availability in plant tissues between the two treatments may not have been important because of the moderate light intensity applied in the full light (200 µmol m–2 s–1). In maize, the gross assimilation response to light intensity is generally maximum below 500 µmol m–2 s–1 (Lizaso et al., 2005) and, therefore, the effect of light treatment on root exudation would probably have been stronger if the treatments had covered a larger range of light intensities. Secondly, plants may have acclimatized to shade, resulting in little change in C availability for the roots that were used for exudation determination. Indeed, the reduction of root diameter observed in the shaded plants may have adjusted the root sink strength to C availability (Thaler and Pagès, 1998), possibly minimizing the impact of shading on the C concentration in roots and consequently on exudation. It is also reported in the literature that, in barley, darkening of plants for up to 8 d had little effect on root growth or on their carbohydrate content, because they continued to import C despite the fact that photosynthesis and metabolic activity of shoots had declined rapidly (Farrar and Jones, 2000). Finally, limitation of C allocation to roots first reduces the growth of laterals because the low diameter of their protophloem tubes generates a higher resistance to assimilate transport (Bidel et al., 2000). Consequently, primary roots are the last roots that are affected by C limitation and in the present study, they may have not been significantly C-limited by shade. Because only primary roots were studied, this may be an additional reason why shade had little effect on root exudation.
Hence, further investigations are needed to elucidate, at the plant scale, the link between C availability and root exudation (Hodge et al., 1997; Dilkes et al., 2004). Availability of assimilates can change the exudation of individual roots by changing their content in diffusible C. Availability of assimilates can also change the exudation of the root system by altering the root diameter and the number of root tips, which determine the root surface area through which exudates diffuse and the number of sites with high exudation rates.
The model of single root exudation
In the present study, exudation was modelled regardless of the biochemical nature of the compounds released from roots. The model assumed an efflux of C constant with time originating in the important gradient of C concentration between the root and the external solution. The model also describes an influx of C proportional to the concentration of C in the solution. Exudates are of various nature (Grayston et al., 1996) and they are not all released by passive diffusion since some compounds such as enzymes are actively secreted. Moreover, apart from sugars, amino acids (Jones and Darrah, 1993a, b) and phytosiderophores (Neumann and Römheld, 2000), the re-uptake of root-released C has yet to be proven. Nevertherless, the model evaluated here was successfully fitted to the kinetics of C release from individual roots with R2 ranging from 0.56 to 0.81. This may indicate that here, the root-released C was mainly diffusible sugars. Consistent with this, Kraffczyck et al. (1984) and Jones and Darrah (1993a) determined that sugars contributed to 65% and 77% of the C exuded by maize roots, respectively. In some particular situations, where sugars are not the major part of the C released from roots, the model proposed here may need to be reconsidered. For example, the model might fail to simulate total C exudation correctly in the case of P deficiency, which was reported to trigger the important release of organic acids by facilitated diffusion without the re-uptake of these compounds having been demonstrated (Jones, 1998).
The goodness of fit was increased by 8% if the gross efflux of C E was allowed to decrease with the distance from the apex (equation 4; Table 2), which enabled the model to simulate that short roots exuded relatively more than longer roots. This result is consistent with previous work that determined a greater exudation at the root tip (McDougall and Rovira, 1970; McCully and Canny, 1985; Darwent et al., 2003). Root tips are richer in solutes, particularly sugars (Jones, 1998; Freixes et al., 2002) and this may be a reason why their exudation was more important. In addition, the permeability of root tissues to organic solutes might also be higher near the apex but this has yet to be proven. Therefore, considering the longitudinal variation of the efflux when modelling exudation was relevant. The model suggests that two root systems having the same total root length may exude differently according to the number of roots and, consequently, according to the importance of branching. This is in agreement with the work of Henry et al. (2005), who observed a positive relationship between number of apices and the amount of C exuded by Lolium multiflorum.
The longitudinal variation of the gross efflux (E) simulated from the parameter estimates indicated a steep decrease between 0–5 cm (Fig. 4). It is in agreement with the sugar concentration in root tissues, which was reported to be much stronger in the apical 4–5 cm segment of the root (Jones, 1998). It has to be emphasized that the minimum length used in this study was 1.8 cm (Table 1) and, therefore, the gross efflux calculated for distances from the apex <1.8 cm was extrapolated. The increase in exudation close to the apex may not be as important as the simulated increase because some works reported profiles of sugar concentration in root tissue that showed a plateau close to the root tip (Jones, 1998; Freixes et al., 2002). The gross efflux at the apex estimated here was on average 5.2 µg C cm–2 h–1. By comparison, Jones and Darrah (1993a) proposed a root tip exudation rate derived from the work of Trofymow et al. (1987) of 1.2 µg C tip–1 h–1, but they gave no information concerning the tip length and diameter. Assuming a tip length of 0.5 cm and an average diameter of 0.1 cm, this gives 7.6 µg C cm–2 h–1, which is close to the value found here. Jones and Darrah (1993a) also proposed a rate of gross exudation of 0.24 µg C cm–1 h–1 for non-root tip segment. Assuming a root diameter of 0.1 cm, this gives 0.75 µg C cm–2 h–1, which is, by contrast, lower than the efflux calculated here at 5–25 cm from the root apex: around 1.8 µg C cm–2 h–1 (Fig. 4).
Hence, the model enabled root exudation to be calculated by considering both the efflux and the influx of C, which improves the determination of exudation as compared to the simple quantification of C in the root bathing solution. The gross efflux of C could be assumed to be a minimum for the actual root exudation in soil, provided that the soil behaves as a zero sink due to the rapid consumption of exudates by soil micro-organisms. In soil conditions, the actual exudation may be greater for at least two reasons. The environment of the roots used for exudation determination lacked of mechanical constraints, which was shown to increase root exudation (Barber and Gunn, 1974). Micro-organisms also influence exudation by affecting the membrane permeability, by damaging the root tissues, or by altering the C partitioning to roots, particularly in the case of symbiotic micro-organisms (Grayston et al., 1996).
In conclusion, this work showed that a model of C efflux and influx could be fitted to experimental kinetics of net exudation of apical root segments having variable length and diameter. This model enabled root exudation to be estimated by considering the re-uptake of exudates, which is not frequent in the literature. An average gross exudation efflux of 2.1 µg C cm–2 root surface h–1 was determined. The model was improved if exudation was set more intense towards the root apex and the gross C efflux decreased then from 5.2 µg C cm–2 h–1 at the apex to 1.8 µg C cm–2 h–1 in the region located at 5–25 cm from the root tip. The reduction in net exudation due to the shading of plants indicates that further inverstigations are necessary to link exudation with C allocation to roots. By linking exudation to the length and to the diameter of the root and by considering the longitudinal variation of the C efflux, this model provides the potential for upscaling exudation modelling to the whole root system, since models of root architecture provides the number of roots, their length and diameter, along with plant develoment (reviewed in Pages et al., 2000). Merging the present model with root architecture models would enable exudation (amount and localization in soil) to be estimated at the plant scale, in relation to plant development. How different root architectures impact on the amount, localization and dynamics of exuded C could also be explored. Finally, because the root system generally demonstrates a high degree of plasticity (Doussan et al., 2003), the present model may help to explain how environmental conditions of soil-grown plants impact on root exudation through changes in the characteristics of roots. Hence, further developments of the model should help to discover whether changes in exudation of soil-grown plants are due to changes in root architecture or to changes in the exudation of individual roots.
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Barber DA, Gunn KB. The effect of mechanical forces on the exudation of organic substances by the roots of cereal plants grown under sterile conditions. New Phytologist (1974) 73:39–45.[CrossRef][Web of Science]
Bidel LPR, Pagès L, Rivière LM, Pelloux G, Lorendeau JY. MassFlowDyn I: a carbon transport and partitioning model for root system architecture. Annals of Botany (2000) 85:869–886.
Curl EA, Truelove B. The rhizosphere. In: Advanced series in agricultural sciences—Bommer DFR, Sabey BR, Thomas GW, Vaadia Y, Van Vleck LD, eds. (1986) Vol. 15. Berlin, Heidelberg: Springer-Verlag.
Darrah PR. Models of rhizosphere. I. Microbial population dynamics around a root releasing soluble and insoluble carbon. Plant and Soil (1991) 133:187–199.[CrossRef][Web of Science]
Darwent MJ, Paterson E, McDonald AJS, Tomos AD. Biosensor reporting of root exudation from Hordeum vulgare in relation to shoot nitrate concentration. Journal of Experimental Botany (2003) 54:325–334.
Dilkes NB, Jones DL, Farrar J. Temporal dynamics of carbon partitioning and rhizodeposition in wheat. Plant Physiology (2004) 134:706–715.
Doussan C, Pages L, Pierret A. Soil exploration and resource acquisition by plant roots: an architectural and modelling point of view. Agronomie (2003) 23:419–431.[CrossRef][Web of Science]
Dunbabin VM, Diggle AJ, Rengell Z. Simulation of field data by a basic three-dimensional model of interactive root growth. Plant and Soil (2002) 239:39–54.[CrossRef][Web of Science]
Farrar J. Fluxes of carbon in roots of barley plants. New Phytologist (1985) 99:57–69.[CrossRef][Web of Science]
Farrar J, Hawes M, Jones D, Lindow S. How roots control the flux of carbon to the rhizosphere? Ecology (2003) 84:827–837.[CrossRef][Web of Science]
Farrar J, Jones D. The control of carbon acquisition by roots. New Phytologist (2000) 147:43–53.[CrossRef][Web of Science]
Freixes S, Thibaud MC, Tardieu F, Muller B. Root elongation and branching is related to local hexose concentration in Arabidopsis thaliana seedlings. Plant, Cell and Environment (2002) 25:1357–1366.[CrossRef]
Gransee A, Wittenmayer L. Qualitative and quantitative analysis of water-soluble root exudates in relation to plant species and development. Journal of Plant Nutrition and Soil Science (2000) 163:381–385.[CrossRef]
Grayston SJ, Vaughan D, Jones D. Rhizosphere carbon flow in trees, in comparison with annual plants: the importance of root exudation and its impact on microbial activity and nutrient availability. Applied Soil Ecology (1996) 5:29–56.[CrossRef][Web of Science]
Henry F, Nguyen C, Paterson E, Sim A, Robin C. How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth? Plant and Soil (2005) 269:181–191.[CrossRef][Web of Science]
Hoagland DR, Arnon DJ. The water culture: method for growing plants without soil. California Agricultural Experimental Station Circular (1950) 347:1–39.
Hodge A, Paterson E, Thornton B, Millard P. Effects of photon flux density on carbon partitioning and rhizosphere carbon flow of Lolium perenne. Journal of Experimental Botany (1997) 48:1797–1805.
Jones DL. Organic acids in the rhizosphere: a critical review. Plant and Soil (1998) 205:25–44.[CrossRef][Web of Science]
Jones DL, Darrah PR. Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere. II. Experimental and model evidence for simultaneous exudation and re-sorption of compounds. Plant and Soil (1993a) 153:47–49.[CrossRef][Web of Science]
Jones DL, Darrah PR. Influx and efflux of amino acids from Zea mays L. roots and their implications for N nutrition and the rhizosphere. Plant and Soil (1993b) 155–156:87–90.
Jones DL, Darrah PR. Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere. III. characteristics of sugar influx and efflux. Plant and Soil (1996) 178:153–160.[CrossRef][Web of Science]
Kraffczyk I, Trolldeiner G, Beringer H. Soluble root exudates of maize: influence of potassium supply and rhizosphere micro-organisms. Soil Biology and Biochemistry (1984) 16:315–322.[CrossRef]
Kuzyakov Y. Separating microbial respiration of exudates from root respiration in non-sterile soils: a comparison of four methods. Soil Biology and Biochemistry (2002) 34:1621–1631.[CrossRef]
Lizaso JI, Batchelor WD, Boote KJ, Westgate ME. Development of a leaf-level canopy assimilation model for CERES-Maize. Agronomy Journal (2005) 97:722–733.
McCully ME, Canny MJ. Localization of translocated 14C in roots and exudates of field grown maize. Physiologia Plantarum (1985) 65:380–392.[CrossRef]
McDougall BM, Rovira AD. Sites of exudation of 14C-labelled compounds from wheat roots. New Phytologist (1970) 69:999–1003.[CrossRef][Web of Science]
Neumann G, Römheld V. The release of root exudates as affected by the plant's physiological status. In: The rhizosphere, biochemistry and organic substances at the soil–plant interface—Pinton R, Varini Z, Nannipieri P, eds. (2000) Marcel Dekker Inc. 41–93.
Newman EI. The rhizosphere: carbon sources and microbial populations. In: Ecological interactions in soils—Fitter AH, ed. (1985) London: Blackwell Scientific Publications. 107–121.
Newman EI, Watson A. Microbial abundance in the rhizosphere: a computer model. Plant and Soil (1977) 48:17–56.[CrossRef][Web of Science]
Nguyen C. Rhizodeposition of organic C by plant: mechanisms and control. Agronomie (2003) 23:375–396.[CrossRef][Web of Science]
Nguyen C, Guckert A. Short-term utilization of 14C-[U]glucose by soil micro-organisms in relation to carbon availability. Soil Biology and Biochemistry (2001) 33:53–60.[CrossRef]
Pagès L. Root system architecture: from its representation to the study of its elaboration. Agronomie (1999) 19:295–304.[Web of Science]
Pagès L, Asseng S, Pellerin S, Diggle A. Modelling root system growth and architecture. In: Root methods: a handbook—Smit AL, Bengough AG, Engels C, van Noordwijk M, Pellerin S, van de Geijn SC, eds. (2000) Springer. 113–146.
Prikryl Z, Vancura V. Root exudates of plants. VI. Wheat root exudation as dependent on growth, concentration gradient of exudates, and the presence of bacteria. Plant and Soil (1980) 57:69–83.[CrossRef][Web of Science]
Reinhold L, Kaplan A. Membrane transport of sugars and amino acids. Annual Review of Plant Physiology (1984) 35:45–83.[CrossRef][Web of Science]
Thaler P, Pagès L. Modelling the influence of assimilate availability on root growth and architecture. Plant and Soil (1998) 201:307–320.[CrossRef][Web of Science]
Throughton JH, Currie BG. Relations between light level, sucrose concentration, and translocation of carbon 11 in Zea mays leaves. Plant Physiology (1977) 59:808–820.
Toal ME, Yeomans C, Killham K, Meharg AA. A review of rhizosphere carbon flow modelling. Plant and Soil (2000) 222:263–281.[CrossRef][Web of Science]
Todorovic C, Nguyen C, Robin C, Guckert A. Root and microbial involvement in the kinetics of 14C-partitioning to rhizosphere respiration after a pulse labelling of maize assimilates. Plant and Soil (2001) 228:179–189.[CrossRef][Web of Science]
Trofymow JA, Coleman DC, Cambardella C. Rates of rhizodeposition and ammonium depletion in the rhizosphere of axenic oats roots. Plant and Soil (1987) 97:333–344.[CrossRef][Web of Science]
Van Bel AJE. The phloem, a miracle of ingenuity. Plant, Cell and Environment (2003) 26:125–149.[CrossRef]
Xia J, Saglio PH. Characterization of the hexose transport system in maize root tips. Plant Physiology (1988) 88:1015–1020.
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