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Agronomy Journal 92:690-697 (2000)
© 2000 American Society of Agronomy

CORN

Phosphorus Deficiency Affects the Rate of Emergence and Number of Maize Adventitious Nodal Roots

Sylvain Pellerin, Alain Mollier and Daniel Plénet

INRA, Unité d'Agronomie, 71, avenue Edouard-Bourleaux, B.P. 81, 33883 Villenave d'Ornon Cedex, France

pellerin{at}bordeaux.inra.fr


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 
Root growth is critical for P uptake, especially when soil P availability is low. We studied the effects of P deficiency on the rate of appearance and number of adventitious nodal roots of field-grown maize plants (Zea mays L.). Experiments were conducted in 1995, 1996, and 1997 on a long-term P fertilization trial with three P fertilization regimes, located on a sandy soil in southwest France. Phosphorus deficiency had a negative effect on leaf area index (LAI). The amount of photosynthetically active radiation (PAR) absorbed by the canopy and plant growth were reduced, especially during the first phases of the crop cycle (between the 7- and 14-visible-leaf stage). The emergence of adventitious roots was delayed for P-deficient plants, but the synchrony between root and leaf emergence was not disturbed. The final number of roots was significantly lower for P-deficient plants for phytomers 4 to 7. These phytomers were those for which root differentiation occurred when the PAR absorbed by the canopy was most severely reduced relative to the fertilized P treatments. A unique relationship was found for all years and P treatments between the cumulative amount of PAR absorbed by the plant and the number of emerged adventitious roots. We concluded that the reduced number of adventitious roots for phytomers 4 to 7 on P-deficient plants may be caused by the negative effect of P deficiency on LAI and its subsequent effect on PAR absorption and C nutrition of plants during the period of root emergence from specific phytomers.

Abbreviations: LAI, leaf area index • PAR, photosynthetically active radiation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 REFERENCES
 
THE GROWTH AND DEVELOPMENT of root systems involve production of new roots, root elongation, and branching. In most field experiment studies, however, root system growth is analyzed in terms of total root mass or length, with little attention paid to developmental processes. This lack of relevant experimental data hampers progress in modeling the root system architecture as proposed by Diggle (1988), Pagès et al. (1989), and Jourdan and Rey (1997). These models consider the root system as a set of connected axes, and simulate the production of new roots and their elongation and branching in a three-dimensional coordinate system. In the future, these models will be very useful tools for modeling water and nutrient uptake by plants since they provide very relevant data for water and nutrient uptake calculation, such as the total root length per branching order or age and its distribution within a soil volume (Berntson, 1994; Clausnitzer and Hopmans, 1994). Their use in field conditions, however, requires a better knowledge of the influence of environmental factors on root system morphogenesis.

The root system of maize is made up of the radicle, the seminal adventitious roots appearing on the scutellar node, and the nodal adventitious roots appearing on the stem, at the bottom part of the successive phytomers (Fig. 1) . All these roots carry first-order laterals, which themselves carry second-order laterals. On each internode, primordia of nodal adventitious roots develop from dedifferentiated cells of the stem parenchyma, just behind the stem cortex and below the intercalary meristem of the overlying internode (Hoppe et al., 1986). A time interval elapses between the end of the root primordium differentiation and its emergence out of the stem. Picard et al. (1985) and Pellerin (1993) have shown that a linear relationship exists between the rank of the phytomer from which root emergence occurs and the plant leaf stage. This relationship was very robust for several environmental conditions (years, locations, sowing dates, planting densities, and N fertilization). In contrast, the number of nodal roots per phytomer was found to vary with environmental conditions, especially for the upper phytomers (Hoppe et al., 1986; Jordan et al., 1988; Pellerin, 1994; Thomas and Kaspar, 1995). For these phytomers, Pellerin (1991) and Demotes-Mainard and Pellerin (1992) have shown that the number of roots produced was lower when C nutrition of plants was reduced either by shading or by increasing competition for light between plants.



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  		Fig. 1 Structure of the maize root system

 
Phosphorus availability has been shown to affect root growth, notably on maize. Authors generally agree that P deficiency leads to a higher root-to-shoot ratio (Anghinoni and Barber, 1980; Khamis et al., 1990; Rosolem et al., 1994). Absolute effects on root length and biomass, however, were complex and sometimes controversial. Narayanan and Reddy (1982) observed on 15-d-old maize plants higher primary and secondary but lower tertiary root length when plants were grown on a nutrient solution without P. Baligar (1987) also observed in 22-d-old maize plants a higher root weight when plants were grown at low P concentration (100 µmol L-1) compared with a higher P concentration (1000 µmol L-1). Using a P starvation experiment, Anghinoni and Barber (1980) observed an increased root length and dry weight on 12-d-old maize plants when the duration of P starvation increased between 1 and 6 d. However, Khamis et al. (1990) observed on 3-wk-old maize plants no effect of P deprivation on root biomass, even after 22 d. In a pot experiment, Schenk and Barber (1979) observed almost no effect of the P fertilization level on root length and root weight of 23-d-old maize plants. Finally, there are reports of P fertilization increasing root length and biomass (Hajabbasi and Schumacher, 1994; Rosolem et al., 1994). These contrasting results show that the root system response to P availability is complex and likely depends on the range of P availability explored, the time scale of the experiment and the root parameter under consideration. Most studies were conducted on young plants in pot experiments, so results may be influenced by the particular conditions of such experiments (low light, physical constraints for root growth, etc.). In contrast, information on the morphological response of the root system to P deficiency in field situations is scarce.

Our objective was to investigate the effect of P deficiency on the rate of appearance and number of adventitious nodal roots of field-grown maize. Since nodal adventitious roots form the framework of the whole root system, this work is the logical first step in studying the maize root system morphological response to P deficiency.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 REFERENCES
 
Experimental Field
The study was conducted in 1995, 1996, and 1997 on a long-term P fertilization trial located at Carcarès Sainte Croix (43°52' N, 0°44' W, 55 m above sea level) in southwest France on a sandy soil (6% clay, 13.5% silt, 80.6% sand, 1.78% organic matter, pH in water 5.9 in the 0–0.25 m top layer). The trial was initiated in 1972 and continuously cultivated in maize harvested for grains. The experimental design was a randomized complete block with each experimental treatment replicated four times. Individual plots were 6 m wide and 30 m long. The experimental fertilization treatments applied since 1972 were (i) no P (treatment T0), (ii) 1.5 times the amount of P exported annually from the field by the grains (treatment T1.5), and (iii) three times the amount of P exported annually from field (treatment T3). In 1995, the amount of extractable Olsen-P in the 0- to 0.25-m top layer was 23 mg P kg-1 in T0, 49 mg P kg-1 in T1.5, and 66 mg P kg-1 in T3 (Olsen and Sommers, 1982). In 1995, 1996, and 1997, the amount of P supplied was 0 kg P ha-1 yr-1 in T0, 52.5 kg P ha-1 yr-1 in T1.5, and 111 kg P ha-1 yr-1 in T3, respectively.

Maize, cultivar Volga, was sown on 13 Apr. 1995, 10 Apr. 1996, and 15 Apr. 1997, respectively. Plant density was 8.3, 8.0, and 8.8 plants m-2 for the three experimental years, respectively. The distance between rows was 0.80 m. Techniques for tillage were those usually applied in this area: crushing of crop residues just after harvest and incorporation by disking in February, plowing in March, broadcast application of P [Ca(H2PO4)2] and K (KCl) at the beginning of April, seed bed preparation just before sowing, and cultipacking just after. Nitrogen was applied at time of sowing (50 kg N ha-1 as NH4NO3) and at the end of May (200 kg N ha-1 as NH4NO3). The total amounts of N and K supplied (250 kg N ha-1, 96 kg K ha-1) were determined so as to be nonlimiting. Weeds were controlled before emergence by application of 6 L ha-1 of alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)acetamide] (336 g L-1) and atrazine (2-chloro-4-ethylamino-6-isopropylamine-s-triazine) (144 g L-1) and at the 4- to 6-visible-leaves stages by application of 2.5 L ha-1 of bentazone [3-isopropyl-1H-2,1,3-benzo-thiadiazin-4(3H)-one 2,2-dioxide] (480 g L-1). Irrigation was conducted in order to compensate for moisture deficits. Grain harvest was performed on 11 Oct. 1995, 8 Oct. 1996, and 25 Sept. 1997.

Plant Sampling and Measurements Performed on Shoots and Roots
Plant stages were followed between emergence and silking by recording weekly the number of visible and expanded leaves on 10 plants per individual plot. A leaf was considered fully expanded when the ligule was visible. On five of these plants, the length (Lm, in m) and the maximum width (Wm, in m) of expanded leaves were recorded. Only the length (Lv) was recorded for visible, not fully expanded leaves. Leaf areas were calculated under the assumption that a maize leaf consists of a rectangle (Lm/2 long and Wm wide) combined with an isosceles triangle (Lm/2 high and of Wm base length). Details of the calculation are given in Fig. 2 . Total leaf area per plant was calculated by adding area of individual leaves. Leaves for which more than half of their area was dried out were not considered in the calculation of the total plant leaf area. The green leaf area index (LAI, in m2 m-2) was calculated by multiplying the leaf area per plant by the number of plants per unit land area.



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  		Fig. 2 Diagram of a maize leaf and measurements used for calculating its surface area. Lm (maximum leaf length) and Wm (maximum leaf width) were measured when the leaf was fully emerged. During leaf emergence (ligule not visible), only the visible leaf length (Lv) was measured (e.g., Lv1 at time 1 and Lv2 at time 2). When Lv is the visible leaf length, the visible surface area (A) was calculated as follows (after Plénet et al., 2000a):

(surface area of visible part of the isosceles triangle)

(surface area of the isosceles triangle)

(surface area of the isosceles triangle and of the visible part of the rectangle)

(surface area of the isosceles triangle and of the rectangle) (ligulate leaf)

 
About every week between emergence and silking, 14 plants per individual plot were sampled for measurement of above-ground dry biomass and root observations. In each individual plot, only the two middle rows were used for plant sampling. Plants were sampled including their root clump corresponding to a 100-mm-diam. half-sphere containing the stem basis, the basal part of adventitious roots and the associated soil. Root clumps were then separated from aerial parts and washed. The dry biomass of aerial parts was measured after crushing and drying at 80°C for 72 h. On root clumps, the rank of the last phytomer carrying emerged adventitious root was recorded (in 1996 and 1997 only). A root was considered an emerged root as soon as it had broken through the covering leaf sheath. Phytomers were numbered as proposed by Girardin et al. (1986) (Fig. 1). The mesocotyl was considered the first phytomer (P1). Therefore, the internode between the first and the second leaf was designated as the second phytomer (P2). The following internode was designated as the third phytomer (P3), and so on. Roots emitted from the internode of the phytomer Pi were defined as Pi roots. In 1997, additional observations were performed on adventitious root primordia differentiation. The rank of the last phytomer carrying root primordia and the number of root primordia per phytomer were recorded by very carefully removing leaf sheaths of phytomers which did not carry emerged roots at the observation date. Root primordia then appeared as small dots just below the intercalary meristem of the overlying internode. Pellerin (1993) has shown that no more root primordia differentiate on a given phytomer once root primordia differentiation has begun on the following phytomer. Therefore, the final number of root primordia on a phytomer was calculated by considering all plants on which at least one root primordia was visible on the following phytomer.

At harvest, the final number of roots per phytomer was counted on 10 plants sampled per individual plot.

Meteorological Data and Calculations
Daily minimum and maximum air temperature (in °C) and daily global solar radiation (in MJ m-2) used for calculations were recorded at the meteorological station of Mont-de-Marsan, which was 15 km from the experimental field. In 1997, however, a meteorological station was set up in the experimental field and air temperature (HMP35A Vaisala Oy temperature probe, Helsinki, Finland) and incident photosynthetically active radiation (PARi) (JYP 1000 sensors, SDEC, France) were recorded. Data were measured every 10 min, and average values per hour were stored in a data-logger (CR10X, Campbell Scientific). Climatic conditions in Mont-de-Marsan were compared with those of the experimental site in 1997. The deviation was only 1% (21 degree-days) for thermal time and 3.3% (100 MJ m-2) for cumulated global solar radiation during the sowing-maturity period. Thermal time after sowing (TT, in degree-days, °Cd) was calculated on a daily basis as follows (Bonhomme et al., 1994):

(1)
with n being the number of days after sowing, TX the maximum daily air temperature (in °C), TN the minimum daily air temperature (in °C), and Tb the base temperature (6°C). Any maximum temperatures exceeding 30°C were set at 30°C. The daily amount of photosynthetically active radiation absorbed by canopy (PARa, in MJ m-2 d-1) was calculated as proposed by Bonhomme et al. (1982):

(2)
with k being the extinction coefficient of the radiation in the canopy [ for maize crop according to Varlet-Grancher et al. (1982)]. In 1995 and 1996, the incident PAR (PARi in MJ m-2) was calculated from global solar radiation (GR, in MJ m-2 d-1) according to Varlet-Grancher et al. (1982):

(3)

Statistical Tests
Statistical tests were performed using the GLM procedure of the SAS computer program (SAS Inst., 1994). Means corresponding to the three P treatments were separated using the Newman-Keuls multiple comparison test. A Student's t-test was used when only two means were compared.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 REFERENCES
 
Plant Stage, Leaf Area Index, Photosynthetically Active Radiation Absorption, and Aerial Biomass Accumulation
Detailed results of the effects of P treatments on growth of aerial parts were given in (Plénet et al., 2000a, 2000b). Only results for interpreting root data are presented here.

Figure 3 shows the number of visible leaves as a function of thermal time after sowing for the three P treatments and the three experimental years during the sowing-flowering period. The rate of appearance of leaves was delayed in the T0 treatment in all three years. No difference was observed between the T1.5 and T3 treatments. The difference between the T1.5 and T0 treatments reached a maximum of two leaves between the 8- and 14-leaf stages (i.e., between approximately 400 and 600°Cd). The final number of leaves was not affected. In treatment T1.5, the silking stage (50% visible silks) was observed on 14 July (992°Cd) in 1995, 12 July (1007°Cd) in 1996 and 13 July (988°Cd) in 1997. In the T0 treatment, flowering was delayed by 2 to 4 d (33 to 65°Cd) depending on the year.



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  		Fig. 3 Number of visible leaves as a function of thermal time after sowing (basis 6°C) for the three P treatments (T0, T1.5, and T3) and the three experimental years (1995, 1996, and 1997) during the sowing-flowering period

 
Figure 4a shows the green leaf area index as a function of thermal time after sowing for the three P treatments and the three years. Figure 4b shows the LAI of T0 or T3 relative to the LAI of T1.5. For each P treatment, the LAI values were very close between years except at the end of the growing season (after 1700°Cd). This was due to a quicker leaf senescence in 1996 and 1997 (Plénet et al., 2000a). The T3 and T1.5 treatments were very close, although the T3 treatment was slightly higher than the T1.5 treatment between 350 and 650°Cd in 1995 and 1996 (Fig. 4b). Unexpectedly, the LAI was very slightly lower in the T3 treatment in 1997. The main difference between treatments was observed between the T0 and the T1.5 and T3 treatments. The difference between the T0 and T1.5 treatments was significant at the threshold value during practically the entire maize cycle (from 150 to 1500°Cd). The relative reduction in LAI in T0 reached a maximum level between 300 and 650°Cd (between 7 and 14 visible leaves) and became smaller thereafter. Because of this reduced leaf area, the daily and cumulative amount of photosynthetically active radiation absorbed by the canopy were reduced in the T0 treatment (Fig. 5a and 5b) . The maximum reduction in daily PARa was observed about 400°Cd after sowing. It was even more pronounced in 1996 and 1997. The effect of P treatments on the daily absorbed PAR disappeared after 800°Cd because the LAI reached in all treatments allowed for nearly maximum radiation interception.



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  		Fig. 4 Green leaf area index (LAI, m2 green leaf [m2 soil]-1) for the three P treatments and the three years as a function of thermal time after sowing (basis 6°C). (a) Absolute values for T0, T1.5 and T3; (b) relative values calculated as the ratio between the LAI of treatment T0 or T3 and the LAI of treatment T1.5

 


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  		Fig. 5 Comparison of absorbed photosynthetically active radiation (PARa) between P treatments in 1995, 1996, and 1997. Relative values were calculated as the ratio between the PARa in the T0 or T3 treatment and the PARa in the T1.5 treatment. (a) Daily values (dPARa). (b) Cumulative values since maize emergence (cPARa)

 
Rate of Differentiation and Appearance of Adventitious Roots on the Successive Phytomers
Table 1 gives the rank of the last phytomer carrying root primordia (1997 only) or emerged roots (1996 and 1997) for each P treatment and sampling date. Root differentiation and root emergence were slightly delayed in the T0 treatment; the rank of the last phytomer carrying root primordia was significantly lower in T0 at the fourth and fifth sampling dates (391.6 and 493.5°Cd after sowing, respectively). The rank of the last phytomer carrying emerged roots was significantly lower in T0 at the fourth and fifth sampling dates in 1996 (416.8 and 478.4°Cd after sowing) and at the third sampling date in 1997 (312.1°Cd after sowing). Almost no difference was observed between the T1.5 and T3 treatments. The period during which root differentiation and emergence were delayed in T0 (between approximately 300 and 500°Cd after sowing) roughly corresponds to the period during which leaf appearance was also delayed. This delay in root appearance subsequently disappeared.


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  		Table 1 Average rank of the last phytomer carrying root primordia (1997 only) or emerged roots (1996 and 1997) for each P treatment and sampling date

 
Figure 6 shows the rank of the last phytomer carrying root primordia (squares) or emerged roots (triangles and circles) as a function of the number of visible leaves for all P treatments and sampling dates. In accordance with results found by others, both relationships were almost linear except at the end of root emergence on upper phytomers (Picard et al., 1985; Pellerin, 1993, 1994). Regression lines between the rank of the last phytomer carrying emerged roots and the number of visible leaves were slightly different between years but not between P treatments . This shows that P deficiency delayed leaf and root appearance but did not strongly alter the synchrony between both processes.



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  		Fig. 6 Rank of the last phytomer carrying root primordia (RP) (squares, 1997) or emerged roots (ER) (triangle, 1996; circle, 1997) as a function of the number of visible leaves (NFV). Regression lines were RP = 0.709NVF + 0.656, n = 15, R2 = 0.98; ER = 0.544NFV + 0.048, n = 19, R2 = 0.97 in 1996; ER = 0.438NFV + 0.618, n = 15, R2 = 0.97 in 1997. Regression lines were calculated for NFV < 16

 
Number of Roots per Phytomer
Table 2 lists the final number of adventitious roots observed at harvest on each phytomer for the three experimental years and P treatments. In 1995, the number of roots was significantly lower in T0 on phytomers 5 to 7. In 1996 and 1997 it was significantly lower in T0 on phytomers 4 to 7. The difference between P treatments disappeared on phytomer P8 for all years. Conversely, the number of roots was higher in T0 on phytomer 9 in 1995 and 1996. For all years, no statistically significant difference was observed between T1.5 and T3 treatments.


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  		Table 2 Final number of roots per phytomer for each year and P treatment

 
Table 3 compares in 1997 the final number of emerged adventitious roots per phytomer to the number of root primordia that were differentiated. In order to be sure that the final number of root primordia was reached, only plants having at least one root primordia on the phytomer i + 1 were considered for calculating the number of root primordia on the phytomer i. In most cases the final number of emerged roots was close to the number of root primordia which were differentiated. Unexpectedly, the number of emerged roots was slightly but significantly higher than the number of root primordia in a few cases: T1.5, P6; T1.5, P7; T3, P6; and T3, P7. This may be due to sampling problems. The number of emerged roots was significantly lower than the number of root primordia in only four cases: T0, phytomer 4; T1.5, phytomer 8; T3, phytomer 4; and T3, phytomer 8. Except on P4, no statistically significant difference was found in T0 between the final number of emerged roots and the number of root primordia. This shows that P deficiency affects the number of roots at an early stage of their development.


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  		Table 3 Final number of root primordia and emerged roots per phytomer and P treatment in 1997

 
Relationships between the Amount of Photosynthetically Active Radiation Absorbed by Plants and the Number of Adventitious Roots
Phytomers whose number of roots was reduced by P deficiency were those on which root differentiation and emergence occurred when LAI and PAR absorption by plants were also reduced. In Fig. 7 , we have plotted the relative number of roots on each phytomer (i.e., the number of roots in T0 divided by the number of roots in T1.5) versus thermal time when root primordia differentiation was finished on the corresponding phytomer in T0. Thermal time at the end of the root primordia differentiation period was used at the x-axis because it was shown that the number of roots was affected by P deficiency at this stage (Table 3). The relative daily PARa (i.e., the daily PARa by plants in T0 divided by the daily PARa by plants in T1.5) was plotted in the same figure. Relatively good agreement was found between both processes, with the most severe reduction of root emission observed on phytomers on which the root differentiated when the daily PARa was most reduced. The recovery observed on phytomer 8 consistently coincides with the amount of PARa recovered per plant. In Fig. 8 , we have plotted for all sampling dates until silking the total number of emerged adventitious roots on all phytomers as a function of the cumulative amount of PARa per plant. A common relationship was observed for the three experimental years and P treatments.



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  		Fig. 7 Relative number of roots (number of roots in T0/number of roots in T1.5) on phytomers 2 to 8, and relative daily absorbed photosynthetically active radiation (dPARa) by plants (daily PARa T0/daily PARa T1.5) as a function of thermal time after sowing (in °Cd). Relative numbers of roots were plotted against thermal time when root primordia differentiation was finished on a given phytomer in T0 (i.e., thermal time when at least one root primordium was observed on the following phytomer). Dates calculated in 1997 were used for all three years. The arrows show the end of root primordia differentiation on phytomers 2 and 8, respectively

 


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  		Fig. 8 Total number of adventitious nodal roots (TNR) as a function of the cumulative absorbed photosynthetically active radiation (cPARa) by plant for the three experimental years and the three P treatments. Fitted curve: TNR = 5.19 + 43.59[1 - exp (-0.098cPARa)], n = 66, R2 = 0.98

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
REFERENCES
 
Results obtained in this experiment showed that P deficiency severely reduced the leaf area index of maize. As shown by Plénet et al. (2000a), this reduced LAI was the consequence both of the delayed appearance of leaves on P-deficient plants and of a reduction of their final surface area. This major effect of P deficiency on leaf appearance and growth is consistent with observations of other authors on several species (Atkinson, 1973; Sicher and Kremer, 1988; Fredeen et al., 1989; Rao and Terry, 1989; Lynch et al., 1991; Cromer et al., 1993; Hajabbasi and Schumacher, 1994; Rodriguez et al., 1998a, 1998b, and 1998c). Plénet et al. (2000b) showed that the lower biomass accumulation in the T0 treatment was mainly explained by this effect of P deficiency on leaf growth and its subsequent effect on PAR absorption. The radiation use efficiency (aerial dry biomass produced per unit of PARa) was not very different between P treatments (Plénet et al., 2000b).

Phosphorus deficiency delayed nodal root appearance and reduced the final number of emerged roots on lower phytomers (phytomers 4 to 7). A similar observation was reported by Hajabbasi and Schumacher (1994), who also observed a delayed appearance of nodal roots on maize grown under P deficiency. The synchrony between leaf and root emergence was not disturbed, however, which shows the robustness of the relationship between both processes. The negative effect of P deficiency on the number of emerged adventitious roots was in agreement with the reduction in leaf area index and PARa by plants (Fig. 7). This suggests that the lower number of emerged adventitious roots on phytomers 4 to 7 on P-deficient plants may be the consequence of the reduced LAI and PAR interception by plants at the time of phytomer differentiation. This hypothesis is consistent with results from Demotes-Mainard and Pellerin (1992), who have shown that the number of adventitious roots on maize depends on the C nutrition of the plant. Moreover, a common relationship was found between the total number of emerged adventitious roots and the cumulative amount of PARa by plants for all three years and P treatments (Fig. 8). This strengthens our hypothesis that the effect of P deficiency on nodal root emergence may be accounted for by the effect of P deficiency on leaf growth and solar radiation interception.

The negative effect of P deficiency on nodal root appearance occurred quite early during the plant cycle and may have hampered the subsequent P uptake capacity of plants. Indeed, when soil P availability is low, root growth is critical for P uptake (Silberbush and Barber, 1983). Of course, a more complete analysis of the root system morphological response to P deficiency would be necessary to assess this indirect effect of P availability on root growth and subsequent P uptake capacity. According to Mollier and Pellerin (1999), P deficiency does not reduce the elongation rate of adventitious nodal roots, but does severely lower the elongation rate of laterals. These results strengthen the idea that modeling P uptake by plants on a time scale corresponding to the plant cycle should take into account this effect of P availability on root growth.SAS Institute 1994


   ACKNOWLEDGMENTS
 
Thanks are due to Jean-Marie Esvan, Eric Martin, and Christian Barbot for technical assistance. This study benefited from the financial support of the Institut National de la Recherche Agronomique, France.

Received for publication August 5, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 REFERENCES
 




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