Several controlled environmental and field experiments were carried out to define the critical nitrogen dilution curve for winter oilseed rape, cultivar Goeland.
Annals of Botany 81 : 311–317, 1998
Determination of a Critical Nitrogen Dilution Curve for Winter Oilseed Rape C. C O L N E N N E*, J. M. M E Y N A R D†, R. R E A U‡, E. J U S T ES§ and A. M E R R I EN‡ * Institut Superieur d ’Agriculture de Lille—F59064 Lille cedex, † INRA, Station d ’Agronomie—F78850 ThieralGrignon, ‡ CETIOM—F75116 Paris, § INRA, Unite d ’Agronomie ChaW lons-Reims—F51686 Reims cedex 2 Received : 14 July 1997
Accepted : 20 October 1997
Several controlled environmental and field experiments were carried out to define the critical nitrogen dilution curve for winter oilseed rape, cultivar Goeland. This curve is described by the following power equation : N ¯ 4±48 W −!±#&, where N is the total nitrogen concentration in the shoot biomass and W the shoot biomass. This curve has been validated over the range of shoot dry matter of 0±88 to 6±3 t ha−". For lower shoot biomasses this equation overestimated the critical nitrogen concentration ; we propose a constant value of 4±63 (N is expressed in reduced N, which is a more stable N fraction in the shoot at these stages of development). These results have been validated in several pedoclimatic conditions in France on a single variety in 1994 and 1995. The higher position of this curve relative to the C species reference curve (Greenwood et al., Annals of Botany 67 : 181–190, 1990) can be explained $ by the experimental conditions obtained by Greenwood et al. (1990) ; therefore, all their rape data are rather close to the critical curve that we propose. The differences found between wheat and winter oilseed rape critical N dilution curves correspond to their respective leaf : stem dry matter ratio and the specific leaf loss phenomenon occuring in rape. Winter oilseed rape has a higher capacity of N accumulation in its shoot than wheat for the same aerial dry matter. The proportion of nitrate in shoots rises with the nitrogen nutrition index (N.N.I.) and is more important for rapeseed than for wheat for the same N.N.I. This difference is especially high at the beginning of flowering when the shade provided by the canopy of rapeseed flowers decreases nitrate reductase activity. # 1998 Annals of Botany Company Key words : Winter oilseed rape, Brassica napus L., plant N concentration, nitrate, reduced N, shoot biomass, critical nitrogen concentration, dilution curve, N productivity.
INTRODUCTION Lemaire and Salette (1984 a) developed the concept of the critical N concentration in aerial biomass which corresponds, at any moment of vegetative growth, to the minimum concentration of N necessary to achieve maximum above ground biomass. This concentration is represented by a power equation : (1) N ¯ aW −b where W is the total shoot biomass expressed in t ha−", N is the total N concentration in shoots expressed as a percentage of the shoot dry matter, and a and b are positive constants. The curve defined by eqn (1) discriminates three different types of N status. Below the curve growth is limited by N, above it growth is not limited by N, and on the curve the N concentration is optimum. During the early stages of growth, the value of parameter b in eqn (1) decreases and remains very low (0±12–0±15 ; Lemaire and Gastal, 1997) because of the absence of competition for light between plants. As a consequence, Lemaire and Gastal (1997) considered that the critical N concentration takes a constant value according to this very low N dilution phenomenon occurring at the beginning of growth. This value differs among plants and is 4±8 % N and 4±4 % N for grassland and wheat, respectively (Lemaire and Salette, 1984 a ; Justes et al., 1994). Justes et al. (1994) have shown that this critical N concentration level can be used up to a value of 1±55 t ha−" for wheat, when self shading of leaves has already started (Meynard, 1985). 0305-7364}98}02031107 $25.00}0
After flowering, the allometric relationship between N concentration and shoot dry weight changes as the plant ages (loss of leaves, increase in lignified tissues) and also with changes in the biochemical nature of storage materials such as starch accumulation in wheat (Lemaire and Gastal, 1997) or lipid in oilseed rape. Greenwood et al. (1990) proposed two reference curves for C and C plants which are : $ % (2) C plants : N ¯ 5±7 W −!±& $ ± − ! & C plants : N ¯ 4±1 W (3) % Nevertheless, it seems necessary to define the specific values of the critical N dilution curve coefficients for each species according to the different histological and morphological plant characteristics compartments (Lemaire and Gastal, 1997). This has been done for grassland (Lemaire and Salette, 1984 a), potatoes (Greenwood et al., 1990), wheat (Justes et al., 1994) and maize (Plenet, 1995). These critical N dilution curves could be used to determine the plants’ N requirements and to calculate the nitrogen nutrition index (N.N.I.) which quantifies the nitrogen status of the plants (Lemaire, Gastal and Salette, 1989 ; Lemaire and Meynard, 1997), and can be used in dynamic models to take account of the effect of nitrogen on growth and yield (Justes, Jeuffroy and Mary, 1997). The objective of this study is to determine the critical N dilution curve for winter oilseed rape during vegetative growth, from emergence to the beginning of flowering, and to validate it in different pedoclimatic conditions.
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Colnenne et al.—Nitrogen Dilution Cure in Winter Oilseed Rape MATERIALS AND METHODS
Field experiments
Several N fertilization experiments were carried out with rates of N supposed to be insufficient and with others assumed to be non-limiting for growth. Three types of experiment were required to cover these conditions from emergence until the beginning of flowering. The first experiments, in the early development stages, were conducted in controlled environment conditions to give the value of the critical N concentration used for low biomasses (until the beginning of competition for light). Two other types of experiments, combining rosette growth and stem extension development, were carried out in field conditions during autumn and spring, and these provided information about the critical N dilution curve. Controlled enironment experiments At the beginning of the growth period, from stage two to stage six green leaves open (B2-B6 ; Jung, 1992), the experiments had to be performed in artificial soil conditions to obtain early N deficiency. Seeds were sown in a mixture of # sand and " vermiculite and were irrigated by rain water. $ $ Three nitrogen treatments were defined according to the minimum requirements of the different mineral elements for growth (Merrien, Palleau and Maisonneuve, 1988) and in respect of the ionic balance (Table 1). Nutrients were given regularly, the thermal and radiation intensity glasshouse conditions were specified for optimal development, and the experiment was carried out twice. Shoot dry matter and reduced nitrogen concentration data have been collected on a sample of 0±44 m# from each plot at three and six green leaves open (B3–B6). These data allowed us to determine the critical N concentration, defined as a constant, for low biomasses. T 1. Characteristics of mineral nutrients solutions
K Ca Mg P S N
Low N
Medium N
High N
469±2 228±2 73±9 185±8 144±3 100±9
692±2 558±7 153±8 185±8 144±3 504±4
1055±7 961±6 146±6 185±8 144±3 1008±9
Quantities (mg) by pot required until B6 (Jung, 1992). An extra mix composed of oligo-elements : Fe, Cu, Mn, Zn, Co (Hortrilon, Ceiba Geiby) was added to all solutions. For the second experiment, a fourth spread equivalent to " of the total supply was $ added.
Three autumn and six spring field experiments were carried out during two growing seasons (1994 and 1995) under different pedoclimatic conditions in France. To obtain nitrogen deficiencies in autumn, the sowings were early, following an underfertilized crop and on a shallow soil. Each experiment included six nitrogen fertilizer levels applied at emergence (Table 2). In the spring experiments, all treatments received 0 or 40 kg N ha−" at sowing (depending on the experimental conditions) and, after winter, different nitrogen fertilizer amounts were applied to combine limiting and non-limiting growth conditions (Table 3). Shoot dry matter and total nitrogen concentration were determined on three samples of 0±8 m# from each plot between three–four green leaves open (B3–B4) until the beginning of flowering (F1). The data collected in the autumn and spring permitted estimation of the slope of the critical N dilution curve at growth stages between rosette and stem extension. In all three types of experiment, the experimental design took the form of randomized complete blocks with four replicates ; the plant density (cultivar Goeland) was 45 m−# with 0±4 m between rows. Analytical methods The critical N dilution curve was determined by Lemaire and Salette (1984 a) using the Kjeldahl method to measure the total N concentration of the plants. Because there is uncertainty about the total N concentration measured by this method (Guiraud and Fardeau, 1977), the critical N dilution curve for rape is expressed with the total N concentration measured by the Dumas method. At early stages of development, the shoot nitrate concentration is higher and more variable than during stem elongation (Justes et al., 1994). This phenomenon induces an important variability in the total N concentration measurements. According to these results, the critical constant value, for low shoot biomasses at early stages, is expressed with the reduced N concentration. The Dumas method involved combustion of plant powder at about 1800 °C, reduction of N oxides by reduced Cu at 600 °C and analysis of N by catharometry (Carlo Erba NA # 1500 analyser). Nitrate was extracted from dried plant tissues with a 1 KCl solution. The nitrite formed after nitrate reduction on a cadmium column was measured by spectrophotometry at 540 nm using the Griess method (see Justes et al., 1994, for details). The reduced N concentration
T 2. Characteristics of autumn field experiments
Location
Year
N fertilizer applications (N in kg ha−", applied at emergence)
Surgeres Surgeres Saint Florent}Cher
1994 1995 1995
0-20-40-60-80-100 0-20-40-60-80-100 0-20-40-60-80-100
Number of samplings 4 4 3
Sampling stages (Jung, 1992) B7-B8}B9}B9}B9 B5}B7}B7}B7 B6}B6-B8}B6-B7
Colnenne et al.—Nitrogen Dilution Cure in Winter Oilseed Rape
313
T 3. Characteristics of spring field experiments
Location Surgeres Surgeres Saint Pathus Surgeres Saint Florent}Cher Saint Pathus CETIOM (Reau, Colnenne and Wagner, 1996) Cha# lons (Gabrielle et al., 1997)
Year (sowing date) 1994 (26 Aug. 1993) 1994 (15 Sep. 1993) 1994 1995 1995 1995 1994 1995 1994
N fertilizer applications (N in kg ha−", applied at recovery growth)
Number of samplings
Sampling stages (Jung, 1992)
0-30-60-90-120-150-180
3
C2}F2}G2
0-30-60-90-120-150-180
3
D1}F1}G2
0-35-70-105-140-175 0-30-60-90-120-150-180 0-30-60-90-120-150-180 0-40-80-120-160-200-240 Variable for different experimentations 0-135-272
4 4 4 4 13
C1-D1}D1-D2}D2-E}F1 C1}D1}F1}G2 Between C1 and F1 C1}D1-D2}D2-E}F1 Between B8 and C1
4
Between rosette stage and beginning of flowering
results from the difference between the total N and nitrate concentrations. Statistical analyses were conducted with the STATGRAPHICS software (Manugistics, USA ; 1993).
a power regression equation was fitted to these theoretical critical points to determine the equation of the critical N dilution curve. RESULTS
Data processing The data selected come from experiments for which leaf loss during winter was very low, meaning a steadily decreasing N concentration. At each measurement date and for each experiment, shoot biomasses and total N concentrations of the different N treatments were analysed by analysis of variance and the test of Newman and Keuls. The results allowed us to distinguish the N-limiting from the non-N-limiting treatments. The N-limiting growth treatment is defined as a treatment for which a supplement of N application leads to a significant increase in shoot biomass (P ! 0±05). The non-N-limiting growth treatment is defined as a treatment for which a supplement of N application does not lead to an increase in shoot biomass and, at the same time, induces a significant increase in N concentration (P ! 0±05). These data were used either to define the critical N dilution curve (Justes et al., 1994), or to validate it. If, at the same measurement date, statistical analyses distinguished at least one N-limiting treatment and one non-N-limiting treatment among the different N treatments, this series of data can be used to define the critical N dilution curve. The series which present only N-limiting or non-N-limiting treatments are used to validate the critical curve. The procedure to define the critical N dilution curve has been proposed by Justes et al. (1994). For one date of sampling, a theoretical critical point was defined as follows : (a) each N treatment was characterized by a shoot biomass and a total N concentration ; (b) the data of limiting N growth conditions were fitted by a simple linear regression ; (c) the data of non-limiting N situations were used to calculate the maximum shoot biomass as the average of observed data ; (d) the theoretical critical point was characterized by the calculated maximum shoot biomass, and its total N concentration as the ordinate of the maximum shoot biomass in the simple linear regression ; (e)
Critical N dilution cure Following the computation method of Justes et al. (1994), 15 groups of experimental data between 1±43 t ha−" and 6±47 t ha−" of shoot dry mass allowed us to calculate the theoretical critical N points (Table 4, Fig. 1). The critical N dilution curve results form a bivariate regression (Dagnelie, 1975) to take into account both the variability of the shoot dry matter and the total N concentration. It is represented by the following relationship : N ¯ 4±48 W −!±#&
(5)
where N is the total N concentration in the shoot biomass and W the shoot biomass expressed in t ha−". The 95 % confidence interval of the mean (Dagnelie, 1992) lies between 3±95–4±25 % N for a shoot dry mass of 1±43 t ha−" and between 2±70–2±92 % N for a shoot dry mass of 6±47 t ha−" (Fig. 1). Critical N constant alue Five groups of sampling data from controlled environment and field conditions were used to determine the critical N concentration value for low biomass defined as a constant. These data concern earlier stages of growth, when the plants can be considered as isolated, and correspond to 0±03 to 1±18 t ha−" shoot biomass. For these aerial biomasses, the critical N concentration value has not been determined by the same statistical method because the very high slope of the linear regression resulted in a highly variable estimate (Justes et al., 1994). In consequence, it has been defined as the mean value of the minimum N concentration of the nonN-limiting situation (4±71 % N) and the maximum N concentration of the N-limiting situation (4±55 % N). The critical N-value proposed is 4±63 % N, and can be considered validated by the other results collected. The intersection of
314
Colnenne et al.—Nitrogen Dilution Cure in Winter Oilseed Rape T 4. Experimental treatments selected from statistical analysis and theoretical critical N points Field experiment (sowing date)
Location Surgeres Surgeres Surgeres Surgeres Surgeres Surgeres St Pathus Surgeres Surgeres Surgeres Surgeres Surgeres St Pathus St Florent}Cher St Florent}Cher
Autumn Autumn Autumn Spring (26 August) Spring (15 September) Spring (15 September) Spring Autumn Autumn Spring Spring Spring Spring Spring Spring
Year
Date of measurements
1994 1994 1994 1994 1994 1994 1994 1995 1995 1995 1995 1995 1995 1995 1995
14 2 17 21 22 21 28 6 26 23 16 31 12 27 16
Oct. 1993 Nov. 1993 Nov. 1993 Mar. 1994 Feb. 1994 Mar. 1994 Mar. 1994 Oct. 1994 Oct. 1994 Feb. 1995 Mar. 1995 Mar. 1995 Apr. 1995 Feb. 1995 Mar. 1995
Total N concentration (% W)
Calculated total N concentration (%)
Sampling stages (Jung, 1992)
1±43 2±71 3±80 6±18 2±06 4±90 1±88 1±35 3±14 2±13 3±68 6±38 6±47 1±83 2±74
4±40 3±60 3±11 2±83 3±76 3±09 3±88 3±98 3±55 3±61 3±10 2±79 3±13 3±70 3±44
B7-B8 B9 B9 F2 D1 F1 F1 B7 B7 C1 D1 F1 F1 C1-D1 D1-F1
the critical N constant value (4±63 % N) and the critical N dilution curve [eqn (5)] defined the limit of these two fields. The abscissa of shoot dry matter is 0±88 t ha−".
6 5 4
Experimental alidation of the critical N dilution cure
3 2 1 0
1
2 3 4 5 Shoot dry matter (t ha–1)
6
7
F. 1. Calculated critical N points (_) used to define the critical N dilution curve : —— N ¯ 4±48 W −!±#& ; – – – – – confidence band (P ¯ 0±95) ; — – — – — schematic representation of the distribution of measured values in different nitrogen fertilization treatments at a given date which allow calculation of the critical N point.
Total N concentration (% W)
Calculated shoot dry matter (t ha−")
Nitrate proportions in the plants according nitrogen status
7
To characterize the N status of plants, the nitrogen nutrition index defined by Lemaire et al. (1989), was used as follows : (6) N.N.I. ¯ Nt}Nc
6 5 4 3 2 1 0
This critical N dilution curve was tested in several pedoclimatic conditions. One hundred and forty two situations, characterized as either N-limiting or non-Nlimiting growth conditions, were selected in the range of 0±1 t ha−" to 6±9 t ha−" corresponding to stage two green leaves open (B2) to the beginning of flowering (F1). These two populations of points were well discriminated by the critical N dilution curve (Fig. 2). These results confirm the validity and stability of this critical curve in different pedoclimatic conditions also found by Lemaire and Salette (1984 a), Justes et al. (1994) and Plenet (1995) for grassland, wheat, and maize, respectively.
1
5 2 3 4 Shoot dry matter (t ha–1)
6
7
F. 2. Experimental validation of the critical N dilution curve : D, Non-N-limiting growth conditions ; E, N-limiting growth conditions ; —— critical N dilution curve ; – – – – – ‘ envelope ’ curves : Nmax (Nmax ¯ 6±18 W −!±#") and Nmin (Nmin ¯ 2±07 W −!±"().
where Nt is the total N concentration measured in the aerial parts, and Nc the critical nitrogen concentration for the same shoot biomass. For a N.N.I. equal to 1, the N nutrition is considered as optimum, with higher values indicating excess N, and lower values indicating deficiency. The NO − proportion was measured for an important range $ of crop N status (0±45 ! N.N.I. ! 1±42) and for various stages. Figure 3 shows a positive relationship between the N.N.I. and the NO − proportion. The NO − proportion $ $ varies from 0±01 to 0±06 when N.N.I. is lower than 0±8 (significant N deficiency), from 0±01 to 0±14 when N.N.I. is
Colnenne et al.—Nitrogen Dilution Cure in Winter Oilseed Rape Total N concentration (% W)
Nitrate proportion (NO3–/Nt)
0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02
7 6 5 4 3 2 1 0
0 0.4
0.6 0.8 1 1.2 1.4 Nitrogen nutrition index (N.N.I.)
1.6
F. 3. Nitrate proportion in the aerial biomass of winter oilseed rape for different N.N.I. : E, before flowering ; D, at flowering. Comparison with the wheat data : max envelope curves for wheat : —— before flowering ; – – – – – at flowering (data from Justes et al., 1994).
close to 1 (near optimal N nutrition) and from 0±01 to 0±18 when N.N.I. is significantly higher than 1 (N excess).
315
1
5 2 3 4 Shoot dry matter (t ha–1)
6
7
F. 5. Comparison of different critical N dilution curves for winter oilseed rape (——) (N ¯ 4±48 W −!±#&) and wheat (– – –) (N ¯ 5±35 W −!±%%) (Justes et al., 1994) and the Nmax and Nmin envelopes for winter oilseed rape (——) and wheat (– – –) (Justes et al., 1994).
values were close to the result of eqn (5) : the mean deviation of the curve was 0±15 % N. Comparison with other critical N dilution cures
DISCUSSION Validity of the critical N cure
Total N concentration (% W)
The critical N dilution curve is a tool for N status diagnosis in plants (Lemaire and Meynard, 1997) ; but particularly for the winter oilseed rape, some factors have to be taken into account before its use. After cold conditions, plants can lose most of their oldest leaves which are the lowest N concentration organs of the plant. Then, the N concentration of the aerial biomass increases as a result of maintaining the richest N concentration tissues, i.e. the youngest leaves. In addition, the rape redistributes N from the dying parts to the living leaves. These two phenomena are particularly prominent in Cruciferae. In such cold conditions, the N status of plants defined by the critical curve may be not very accurate. However, in two experimental series where the plants lost their leaves after cold conditions, the measured
7 6 5 4 3 2 1 0
1
5 2 3 4 Shoot dry matter (t ha–1)
6
7
F. 4. Comparison of the critical N dilution curves defined for C $ plants : – — – — – N ¯ 5±67 W −!±& (Greenwood et al., 1990) and for winter oilseed rape : —— N ¯ 4±48 W −!±#& ; D, specific rape data (Greenwood et al., 1990).
The determination of critical N dilution curves has already been realized for different species. Greenwood et al. (1990) have specifically worked on the definition of a general C species reference curve [eqn (2)]. The comparison $ of eqn (2) and eqn (5) shows a difference between Greenwood et al.’s C curve and the critical curve proposed for winter $ oilseed rape. The C curve is higher for early stages of $ development and lower for high aerial biomasses (Fig. 4). The difference between the two models reaches 1±2 % N for a shoot biomass of 1 t ha−" and 0±6 % N for 6±5 t ha−". These differences could be explained by the fact that : (1) Greenwood et al. (1990) obtained their results from experiments where there was uncertainty about the N status of the crops (see Greenwood et al., 1990). In the absence of statistical analysis to distinguish limiting or non-limiting N treatments, these authors have used data from multi-harvest experiments which are not expected to have been limited by N. In these conditions, the C curve parameters could have $ been overestimated ; or (2) this relationship has been defined with different morphological plants such as summer cabbage, spring rape and french beans. The shoot dimorphisms of these different species could cause considerable uncertainty about the C parameters of the curve [eqn (2)]. To $ get more precision, we extracted the specific rape measurements from the data base of Greenwood et al. (1990). The position of these results (Fig. 4) are closer to the rape critical N dilution curve than the C curve ; this illustrates the low $ precision of the generic C curve for a specific C species. $ $ A more specific analysis has been done on wheat (Justes et al., 1994). The comparison with the winter oilseed rape dilution curve is shown in Fig. 5. The difference between the two species is significant ; it could be explained by different phenomena. For low aerial biomasses (up to 2±5 t ha−"), the wheat critical N dilution curve is higher than that of oilseed rape. This higher position corresponds to plants which have a high leaf : stem dry mass ratio, i.e. a bigger proportion of
316
Colnenne et al.—Nitrogen Dilution Cure in Winter Oilseed Rape growth or indirectly associated with it via N metabolism (see Justes et al., 1994). For the highest shoot biomasses, Nmax could have been underestimated because the mineral N availability in the soil is rarely very high at the end of the regrowth period. In Fig. 5, the winter oilseed rape and wheat maximum N concentrations (Justes et al., 1994) are compared. For aerial biomasses greater than 3±5 t ha−", the oilseed Nmax curve is higher than that for wheat. This gap can be explained by the specific value for each species of the proportionality coefficient k linking the maximal N absorption rate and the growth rate as follows :
Leaf : stem dry matter ratio (R)
2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
1
5 2 3 4 6 –1 Shoot dry matter : W (t ha )
7
8
F. 6. Comparison of the leaf-stem dry matter ratio for winter oilseed rape (E) (R ¯ 11±63 W −!±&#) and wheat (D) (R ¯ 29±36 W −!±&)) (wheat data from M. H. Jeuffroy, unpubl. res.) during growth.
tissues rich in N (Fig. 6). This has already been reported by Lemaire and Salette (1984 a). For aerial dry mass higher than 2±5 t ha−", the rapeseed critical N dilution curve is higher than the wheat curve, although its leaf : stem dry mass ratio remains lower than for wheat. The higher position for rapeseed could be explained by : (1) the loss of the oldest leaves, i.e. the less rich N concentration organs ; and (2) the reallocation of N from dying to living parts. The different positions of the wheat and winter oilseed rape curves allow us to characterize the level of their N productivity. According to A/ gren (1985), the nitrogen productivity, i.e. the quantity of assimilated carbon per unit of absorbed N, increases with crop growth above a threshold at a ground biomass of 1 t ha−". The lower position of the wheat critical N dilution curve for shoot dry matter higher than 2 t ha−" indicates a higher N productivity than for winter oilseed rape. The difference between the two crops indicates that winter oilseed rape requires 16 % more N than wheat when both have a shoot dry matter of 6±5 t ha−". Variability of the N concentration in the aerial biomass The experimental results show a large variability for the N concentration in the shoot biomass. We have defined two limit curves which characterize a first estimation of the maximum (Nmax) and the minimum (Nmin) N concentration in the aerial biomass. The Nmax and Nmin curve equations are : (7) Nmax ¯ 6±18 W −!±#" Nmin ¯ 2±07
W −!±"(
(8)
These curves are proposed for shoot dry matter from 0±3 to 6±9 t ha−". For these biomasses, the total N concentration can vary by a factor of 2±15 and 1±74 for 0±3 and 6±9 t ha−", respectively. The variability found above and below the critical N dilution curve is very similar : 0±43 % N above and 0±50 % N below the critical curve (Fig. 2). The Nmax curve corresponds to an estimate of the maximum N accumulation capacity in the shoot. In these cases, the N absorption rate would be regulated by mechanisms directly associated with
(dNa}dt)max ¯ k(dW}dt)max
(9)
where Na is the uptake amount of N in the shoot and W the shoot dry matter expressed in t ha−". The winter oil seed rape k-value, calculated according eqn (7) and eqn (9), is : k ¯ 48±7 W −!±#". The lower wheat k-result, k ¯ 46±5 W −!±%%, validated from 1±5 to 14 t ha−" (Justes et al., 1994), indicates that oilseed rape has a higher capacity of N accumulation in its shoots than wheat, for the same aerial dry matter. For situations below the Nmax curve, N uptake would depend on the mineral N availability in the soil. The lowest points correspond to the minimum N taken up by plants in our experimental fields. A slight overestimation of the initial Nmin concentration values could have been induced by the difficulties of obtaining low mineral N soil availabilities in field conditions at the beginning of the growth stages. According to Penning de Vries (1982), Nmin is the inferior limit at which the metabolism would cease to function. In Fig. 5, the comparison between wheat and winter oilseed rape Nmin curves shows a relatively higher position for wheat results (Nmin ¯ 2±2 W −!±%% ; Justes et al., 1994). For a shoot biomass of 6±8 t ha−" the oilseed rape and what Nmin are, respectively, 1±5 % and 0±95 %. Two main hypotheses can be proposed : (1) our experimental conditions did not permit shoot biomass N concentration measurements to fall as low as those obtained for wheat by Justes et al. (1994). This could be due to the more important winter oilseed rape shoot dry matter and a higher root growth rate than for wheat in autumn ; this could induce a higher N absorption capacity for oilseed rape ; or (2) the important loss of the senescent leaves and the N reallocation from these dying parts to the youngest leaves increase the N concentration in oilseed rape shoots. Magnitude of reduced N and nitrate proportion : comparison with wheat results In the case of optimal N fertilization, near the critical N dilution curve, the concentration of nitrate in shoots is generally relatively low : 0±1 % (Gastal, Belanger and Lemaire, 1992 ; Justes et al., 1994), and corresponds to the optimum N assimilation. When the plants grow with high fertilization rates, a part of the N uptake is not immediately assimilated. This extra N is in the NO − form for early stages $ and in the reduced N form after the beginning of elongation in wheat (Justes et al., 1994). Figure 3 shows that regardless of N.N.I. level or development stage (before and at the beginning of flowering), the proportion of nitrate was higher for oilseed rape than for wheat (Justes et al., 1994).
Colnenne et al.—Nitrogen Dilution Cure in Winter Oilseed Rape This could be interpreted either as : (1) a greater assimilation effectiveness for wheat than for rapeseed ; or (2) a higher capacity for oilseed rape to sequester N which does not directly participate in the N nutrition of the plant. At the beginning of flowering, the smaller reduction could result from the shade provided by the canopy of flowers. Merlo et al. (1995) have shown a great variability in nitrate reductase activity (N.R.A.) in Brassica campestris according to different light conditions. Triboı$ -Blondel (1988) obtained a decrease in N.R.A. in oilseed rape from the beginning of flowering in relation to the flower canopy development. Thus, the assimilation of nitrate is less efficient at the beginning of flowering, and the proportion of nitrate increases and becomes higher than that for wheat. CONCLUSIONS We defined a critical N dilution curve for winter oilseed rape. This curve has been validated in several pedoclimatic conditions in France. However, the parameters have only been determined for one specific cultivar, i.e. Goeland. Thus, it will be necessary to investigate whether it can be used for other winter oilseed rape varieties. However, the critical N dilution curve parameters have been validated for other species such as wheat, maize and grassland (Lemaire and Salette, 1984 b ; Justes et al., 1994 ; Plenet, 1995) regardless of cultivar. It is therefore proposed that eqn (5) could be used for other winter rapeseed cultivars. This curve can be used as a tool for N diagnosis of winter oilseed rape from emergence to the beginning of flowering. However, after loss of leaves during cold periods—an important phenomenon in Cruciferae—the shoot N concentration becomes higher because of the loss of old tissues and N reallocation from older parts. In these conditions, the N diagnosis could be quite imprecise. Wheat is characterized by a higher N productivity than winter oilseed rape. This point is illustrated by the higher position of the critical N dilution curve for rape and by the higher nitrate proportion values found for oilseed rape than for wheat. In all cases, N uptake appears less reduced and therefore less productive for winter oilseed rape than for wheat. A C K N O W L E D G E M E N TS We thank M. H. Jeuffroy (I.N.R.A.) who authorized the use of her data ; and A. M. Triboı$ -Blondel (I.N.R.A.), F. Caceres, L. Champolivier, P. Fauvin, G. Sauzet (C.E.T.I.O.M.),N. Moray(C.N.R.S.)andB. Dubart(I.S.A.) for their participation in data collecting and for advice. LITERATURE CITED AI gren GI. 1985. Theory for growth of plants derived from the nitrogen productivity concept. Physiologia Plantarum 64 : 17–28.
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