AOBPreview originally published online on April 8, 2004
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Annals of Botany 93: 699-710, 2004
© 2004 Annals of Botany Company
Effect of 3D Nitrogen, Dry Mass per Area and Local Irradiance on Canopy Photosynthesis Within Leaves of Contrasted Heterogeneous Maize Crops
1 Unité Mixte de Recherche INRA-INAPG Environnement et Grandes Cultures, BP 01, 78850 Thiverval-Grignon, France and 2 INRA Unité Agropédoclimatique de la zone caraïbe, Domaine Duclos, 97170 Petit-Bourg, Guadeloupe, France
* For correspondence. E-mail Jean-Louis.Drouet{at}grignon.inra.fr
Received: 24 October 2003; Returned for revision: 17 November 2003; Accepted: 13 February 2004; Published electronically: 8 April 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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• Background and Aims Nitrogen partitioning within stands has been described fairly comprehensively, especially for C3 plants in dense stands where the horizontal heterogeneity of foliage distribution is relatively small. Nitrogen has been shown to be distributed vertically and in parallel to light, maximizing carbon assimilation and stand productivity. Conversely, row crops such as maize (C4 plants) are characterized by strong horizontal heterogeneity of foliage distribution, and a three-dimensional (3D) approach is required to investigate the combined effect of spatial distribution of nitrogen and light on canopy photosynthesis.
• Model The 3D geometry of maize canopies was modelled with varying densities and at different developmental stages using plant digitizing under field conditions. For lamina parts, photosynthesis was measured and nitrogen content per unit area (Na) was described from analysis of nitrogen content per unit mass (Nm) and dry mass per unit area (Ma). Hyperbolic relationships between photosynthesis at irradiance saturation (Pmax) and Na were established as well as a linear relationship between dark respiration (Rd) and Na, whereas quantum efficiency (
) was found to be independent of Na.
• Key Results and Conclusions Nm, Ma and Na were shown to change over time vertically (i.e. between laminae), which has been largely reported previously, and horizontally (i.e. within laminae), which has scarcely been described previously. Even if Ma played a major role in Na, a strong relationship between Na and Ma could not be demonstrated, whereas several previous studies have found that Na was essentially related to Ma rather than Nm. From simulations of radiative exchange using a 3D volume-based approach and lamina photosynthesis using a hyperbola, it was shown that real patterns of Na partitioning could increase daily crop photosynthesis by up to 8 % compared with uniform patterns of Na, especially for the earliest stages of stand development.
Key words: 3D plant architecture, heterogeneous crop, dry mass per unit area, irradiance, lamina, nitrogen content per unit area, nitrogen content per unit mass, maize, photosynthesis, virtual plant, Zea mays.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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Nitrogen supply affects plant growth and productivity by altering both leaf area and photosynthetic capacity (Novoa and Loomis, 1981). About 75 % of leaf nitrogen is involved in the photosynthetic processes (Evans, 1989) and its partitioning inside canopies has been largely described (see Grindlay, 1997, for review). Several authors have observed vertical gradients of leaf nitrogen in crops (Shiraiwa and Sinclair, 1993; Connor et al., 1995; Dreccer et al., 2000), herbaceous plants (Field, 1983; Hirose and Werger, 1987, Lemaire et al., 1991) and trees (DeJong and Doyle, 1985; Hollinger, 1996). Leaf nitrogen is distributed in parallel to light distribution (Anten et al., 1995; Kull and Jarvis, 1995), which corresponds to the optimal distribution of leaf nitrogen that maximizes carbon assimilation and crop productivity (Mooney and Gulmon, 1979; Field, 1983; Gutschick and Wiegel, 1988; Dreccer et al., 2000). Chen et al. (1993) proposed an alternative theory called coordination where canopy nitrogen is distributed so that a balance is maintained between two processes depending on nitrogen content: the Rubisco-limited rate of carboxylation and the electron transport rate of carboxylation. They found that carbon assimilation gain was similar, using either the theory of coordination or optimization (Hirose and Werger, 1987). These vertical gradients of nitrogen were essentially observed within dense stands and the combined effect of light and nitrogen on canopy photosynthesis was assessed by using a vertical description of these two variables (i.e. by canopy layer, a 1D approach). However, studies within heterogeneous canopies have been scarce, especially in row crops such as maize, sorghum, wheat and sunflower (see Grindlay, 1997), and have seldom dealt with the relationships between nitrogen and light gradients by using a 3D approach (e.g. in maize, Drouet and Bonhomme, 1999). The effect of canopy heterogeneity on light interception has been assessed previously (Drouet et al., 1999), but the combined effect of spatial distribution of nitrogen and light on canopy photosynthesis within row crops has not been investigated. Since light interception is an area-based phenomenon (Biscoe and Gallagher, 1978), leaf photosynthesis has more often than not been simulated from leaf nitrogen expressed per unit leaf area, Na (Field and Mooney, 1986; Sinclair and Horie, 1989). Na is the product of leaf nitrogen per unit mass, Nm, and dry mass per unit area, Ma, and numerous studies have dealt with relationships between Na and Ma, and between Na and Nm. Many authors have shown that changes in Na were related to changes in Ma (i.e. anatomical changes, Gulmon and Chu, 1981; DeJong et al., 1989; Hirose et al., 1989; Ellsworth and Reich, 1993; Rosati et al., 2000; Le Roux et al., 2001; Meir et al., 2002), whereas various results have been found with changes in Na related to Nm (i.e. chemical changes, Kull and Niinemets, 1993; Grassi et al., 2002). Moreover, results concerning nitrogen partitioning related to irradiance and concerning relationships between Na, Nm and Ma have been described predominantly on C3 plants and rarely on C4 plants such as maize. Those two groups of plants differ with regard to leaf nitrogen content and photosynthesis: leaf nitrogen content is lower for C4 plants than for C3 (Lemaire and Gastal, 1997) and nitrogen use efficiency is higher for C4 plants than for C3 (Gosse et al., 1986; Sage and Pearcy, 1987; Sinclair and Muchow, 1999).
In an earlier paper (Drouet and Bonhomme, 1999), it was shown that local leaf irradiance plays a major role in leaf nitrogen partitioning within maize crops. In the present study, two questions are addressed: (1) do nitrogen gradients within and between laminae in relation to local irradiance increase canopy photosynthesis compared with uniform nitrogen partitioning, and (2) what is the source of the variations in nitrogen per unit area, Na: changes in nitrogen concentration, Nm, or in dry mass per unit area, Ma, or both?
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MATERIALS AND METHODS |
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Experimental design
Two field experiments were carried out in Grignon (France, 48°N, 2°E) using the maize (Zea mays L.) hybrid ‘Déa’. For the first one, maize was sown in the early summer of 1996 at two initial densities: 20 plants m–2 (D-density) and 10 plants m–2 (d-density). Each trial area comprised 40 rows, 50 m long. Two ‘open-thinned’ plots at low density were obtained by removing plants during stem elongation from the area of initial density 10 plants m–2, as follows. (1) Three plants out of four were removed from within each row, which resulted in a final density of 2·5 plants m–2 (d10
2·5-density), and (2) one row out of two was removed as well as nine plants out of ten within the remaining row, which resulted in a final density of 0·5 plants m–2 (d100·5-density). For the second experiment, maize was sown in the early summer of 1999 at two initial densities: 30 plants m–2 and 10 plants m–2. One plot at low density was obtained by removing plants during stem elongation from one plot of initial density 10 plants m–2. Three plants out of four were removed within each row, which resulted in a final density of 2·5 plants m–2. In both experiments, one amount of mineral nitrogen (5·5 g m–2) was applied at sowing. The plots were weeded and plants were kept free of water stress by liberal drip irrigation.
Description of plant structure, lamina nitrogen and lamina photosynthesis
In the first experiment, the 3D geometric structure of the plants was measured with a magnetic digitizing device (Polhemus, 1993; see Sinoquet and Rivet, 1997, and Drouet, 2003, for more details). For each plant, the coordinates along the axis of the stem and the midrib of each lamina were recorded. The number of points per axis varied from ten to 30 according to the length and the curvature of the organ. To examine densities of 10 and 20 plants m–2, data were obtained from 20 plants (four rows with five plants per row) at three stages of development: beginning of stem elongation (60 days after sowing, DAS), end of stem elongation (74 DAS) and post-silking (90 DAS). In both low density plots (2·5 and 0·5 plants m–2), data were measured at post-silking on 12 plants (four rows with three plants per row). For each plot, the corresponding leaf area index (LAI) is shown in Table 1. Measurements were taken in the morning to minimize possible effects of wind, water stress and solar position.
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In the first experiment, total nitrogen analyses were carried out on the six central plants of each plot (two rows with three plants per row). To study nitrogen partitioning between laminae, the even-numbered laminae from 6 to 14 were analysed. Nitrogen partitioning within laminae was assessed from analysis of nitrogen content on laminae 6 (located at the bottom of the canopy), 10 (in the middle of the canopy) and 14 (at the top of the canopy). The velum of each lamina was separated from the midrib and then subdivided into two or three parts of equal length, according to the length of the lamina. Dry mass was determined after oven drying at 70 °C for at least 2 d. After crushing, total nitrogen (mineral and organic) content per unit mass (Nm) was determined with a carbon–nitrogen analyser (ANA 1500 CN; Thermoquest, Les Ulis, France) based on the Dumas method (dry process method, Dumas, 1831).
In both experiments, carbon dioxide assimilation was measured at the lamina level by using a LI-6400 portable photosynthesis system (LI-COR, Inc., NB, USA). Photo synthesis–irradiance response curves were obtained by measuring photosynthesis successively at 500, 200, 150, 100, 50, 500, 1000, 1500 and 2000 µmol PAR m–2 s–1. Lamina photosynthesis at irradiance saturation was measured at 2000 µmol PAR m–2 s–1 (P2000). Samples were taken from the bottom (i.e. lamina 6) to the top (i.e. lamina 14) of the canopy, at several dates around silking within the plots at densities of 20 and 10 plants m–2 in 1996, and within the plots at densities of 30, 10 and 2·5 plants m–2 in 1999 (Table 2). All measurements of lamina photosynthesis were carried out between 0800 and 1100 h (see Bunce, 1990). Total nitrogen analyses were carried out on all samples where photosynthesis was measured.
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Reconstruction of 3D shoot structure and restitution of lamina nitrogen
The reconstruction of the 3D shoot structure (Fig. 1A, B) and the restitution of lamina nitrogen partitioning (Fig. 1C, D) has been described in Drouet and Bonhomme (1999) and Drouet (2003). Dry mass per unit lamina area (Ma) was evaluated from dry mass and lamina area estimated from allometric relationships between lamina length and maximal width (Bonhomme and Varlet-Grancher, 1978). Nitrogen content per unit area (Na) was determined from Nm and Ma. Each plant was geometrically represented by a set of about 1000 triangles and individual values of Nm, Ma and Na were assigned to each triangle.
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Simulation of light distribution and canopy photosynthesis
A 3D volume-based version of the light transfer model RIRI (Radiation Interception in Row Intercropping, Sinoquet and Bonhomme, 1992) was used to calculate irradiance distribution inside the canopies. It had previously been validated using radiation measurements for several crops, especially maize canopies (see Sinoquet and Bonhomme, 1989, 1992). This model is based on the turbid-medium analogy. In this method, the canopy structure is abstracted by an array of 3D cells (0·1 m wide), which may contain foliage or be empty. For each canopy cell, the lamina area density, the lamina angle distribution (not shown) and the lamina nitrogen content (Fig. 1C, D) are calculated from the area, the orientation and the nitrogen content of the triangles (Fig. 1A, B) (B. Andrieu, pers. comm.). The model deals with direct and diffuse incident radiation and scattered radiation, which makes it possible to obtain, within each cell of the canopy, the instantaneous lamina irradiance for sunlit and shaded lamina area (for more details, see Sinoquet and Bonhomme, 1992; Drouet, 1998). For each plot, simulations were performed in the photosynthetic active radiation waveband (PAR, 400–700 nm) from six values of daily global radiation (Table 3). Instantaneous values of direct radiation and diffuse radiation were simulated for each time step (0·1 h) according to Spitters et al. (1986) and J.-M. Allirand ( pers. comm.). For each cell of the canopy, the daily average lamina irradiance (Id,a) was computed from the instantaneous values (Fig. 1E, F). The model relies upon the optical properties of the foliage. We found that reflectance and transmittance were independent of Na, at least during the period studied (data not shown). Consequently, only one average value of reflectance/transmittance was used (0·07). Soil reflectance was set equal to 0·10 (for more details, see Drouet, 1998).
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Lamina photosynthesis in response to irradiance was simulated by using a rectangular hyperbola (Chartier, 1966, 1969; Thornley, 1976):
Pn,i,a(I) = (Pmax I)/(Pmax + I) – Rd(1)
where Pn,i,a is net instantaneous photosynthesis per unit lamina area, I is PAR, Pmax is photosynthesis at irradiance saturation, is quantum efficiency and Rd is dark respiration.
Pachepsky et al. (1996) indicated that such a model is quite adequate to predict quantitatively the biomass production of crops and has the advantage of requiring few parameters. These authors pointed out that it is not necessary to introduce into crop models more complicated and more sophisticated descriptions of photosynthesis. Pmax, and Rd were fitted by non-linear regression between Pn,i,a and I.
Pmax as a function of Na was modelled by using a hyperbola (Sinclair and Horie, 1989). As photosynthesis was measured at 2000 µmol PAR m–2 s–1, this model was used to simulate P2000 as a function of Na:
P2000(Na) = P2000,max [2/{1 + exp[–ß (Na – N0)]} – 1](2)
where P2000,max, ß and N0 are three parameters fitted by non-linear regression to the median P2000 value for each 0·2 g m–2 range in Na (Muchow and Sinclair, 1994).
Pmax derives from P2000, and Dwyer and Stewart (1986) pointed out that the variability in P2000 is lower than that in Pmax.
and Rd were assumed to vary linearly with Na (Hirose and Werger, 1987, Muchow and Sinclair, 1994):
(Na) = a + b Na(3)
Rd(Na) = aRd + bRd Na(4)
where a, b, aRd and bRd are parameters.
For each cell of the canopy, the net daily photosynthesis per unit lamina area (Pn,d,a, Fig. 1G, H) was computed by summing the instantaneous values (Pn,i,a). The net daily photosynthesis of the whole canopy (Pn,d,c) resulted from the sum of Pn,d,a (de Wit, 1965).
To quantify the photosynthetic benefit of real Na partitioning, we compared Pn,d,c values between real and associated hypothetical canopies. For each studied canopy, two hypothetical canopies were generated by redistributing Na from the real partitioning. In the first one, Na was redistributed uniformly along the velum by using, for each lamina, its mean Na value. In the second one, Na was redistributed uniformly within the whole canopy corresponding to the mean value of Na within the canopy.
Data were analysed (regression tests, mean comparisons, graphs) using the S-PLUS computer package (S-PLUS, 1996).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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Photosynthetic capacity of laminae
The relationship between P2000 and Na was described by using the hyperbolic model proposed by Sinclair and Horie (1989) (Fig. 2A, B). Before silking, fitted values of the parameters ß and N0 (Table 4) were close to those found by Muchow and Sinclair (1994): 2·45 vs. 3·68 for ß and 0·27 vs. 0·20 for N0. Conversely, values of P2000,max strongly varied between our fitted values and those found by these authors (36·8 vs. 52·0). This difference can be explained by different climatic conditions, and consequently different photosynthetic capacity of laminae, between the two experimental sites, especially the air temperature: 21–22 °C in Grignon (48°N) vs. 28–30 °C in Australia (14°S). After silking, the values of the parameters ß and N0, and consequently the shape of the P2000(Na) curve, changed: 1·41 after silking vs. 2·45 before silking for ß and 0·43 vs. 0·27 for N0, while P2000,max changed slightly (40·2 vs. 36·8).
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A similar behaviour was found between the pre- and post-silking photosynthetic measurements for quantum efficiency,
(Fig. 2C), on the one hand and for dark respiration Rd (Fig. 2D) on the other. No effect of Na was found on , while Rd varied linearly with Na (Table 4). These results are in accordance with the literature (e.g. Schieving et al., 1992; Connor et al., 1993; Anten et al., 1995), except for findings by Hirose and Werger (1987) and Muchow and Sinclair (1994), who proposed a linear relationship between and Na, and for findings by Pons et al. (1989) who proposed a curvilinear relationship between and Na.
The model of lamina photosynthesis was globally assessed by comparing the simulated values of Pn,i,a (Pn,i,a,sim) from P2000(Na), and Rd(Na) with the observed values (Pn,i,a,obs). We obtained Pn,i,a,sim = 1·001 ± 0·01 Pn,i,a,obs – 0·251 ± 0·211 (P value = 0·0001, R2 = 0·97). Thus, the model with its parameterization is a suitable description of lamina photosynthesis.
Nm, Ma and Na partitioning within and between laminae
Results for Na have been described in a previous paper (Drouet and Bonhomme, 1999). The main trends for Na are recalled here in order to point out the links between Na, Nm and Ma.
Partitioning along the velum (Table 5).
At the beginning of stem elongation (60 DAS) under both density conditions (D- and d-density), Nm and Ma increased from the base to the tip of velums 6 and 10, especially between the base and the middle of the velum for Nm (significant at P < 0.05) and between the middle and the tip of the velum for Ma (P < 0.05). This produced a significant increase in Na from the base to the tip of velums 6 and 10. At the end of stem elongation (74 DAS), the same behaviour was observed for laminae 10 and 14 in both densities: Nm and Ma increased from the base to the tip of the velum (only between the middle and the tip of the velum for Ma), producing an increase in Na. At post-silking (90 DAS), slight variations in Nm and Ma in the opposite direction generated slight variations in Na along velum 6. This behaviour was also observed for velum 10, but only in the D-density. For velum 10 in d-, d102·5- and d100·5-densities, Nm and Ma also varied slightly in the opposite direction and no significant change was observed for Na. Nm remained constant along velum 14 in all densities, while Ma decreased significantly. A significant decrease in Na was also observed from the base to the tip of velum 14, except in the D-density.
Partitioning between laminae (Fig. 3).
Regarding changes in Nm, Ma and Na between laminae over time (D-density, Fig. 3, first line), significant differences were observed (P < 0.05) in Nm between laminae and between dates of measurements. Nm varied slightly around the mean values. Whatever the date, no strong gradient in Nm was observed between laminae. On the contrary, Ma in lower laminae, which had finished their growth (i.e. laminae 6 and 8), varied very slightly with no significant difference between mean values. Growing laminae had lower values of Ma than mature ones (e.g. for lamina 12, mean values of Ma increased from 33·2 g m–2 at 60 DAS to 56·6 g m–2 at 90 DAS). Strong gradients in Ma were observed between laminae (e.g. at 90 DAS, mean values of Ma varied from 40·7 g m–2 for lamina 6 to 56·6 g m–2 for lamina 12). Consequently, Na varied slightly over time in the laminae that had finished their growth (i.e. laminae 6 and 8) and increased in growing laminae (i.e. laminae 10, 12 and 14 between 60–90 DAS). At post-silking (90 DAS), a gradient in Na was observed from the bottom to the top of the canopy. This may be the result of nitrogen remobilization from the lower laminae to the upper ones, nitrogen absorption and growth in Ma (and lamina thickness) of developing laminae. Regarding changes in Nm, Ma and Na between laminae as a function of plant density at post-silking (Fig. 3, second line), Nm increased significantly (P < 0.05) from the bottom to the top of the dense canopies (D- and d-density). Variations in Nm were not apparent in the open-thinned canopies (d102·5- and d100·5-density). Consequently, an increase in Nm from dense to open canopies was observed for lower laminae (i.e. laminae 6 and 8), while Nm remained relatively constant between dense and open canopies for upper laminae (i.e. laminae 10–14). Similar vertical profiles of Ma were observed between densities, with higher values of Ma in the open canopies (d102·5- and d100·5-density) than in the dense ones (D- and d-density). Consequently, the profiles of Na partitioning were characterized by a stronger vertical gradient in the dense canopies (D-density) than in the open ones.
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DISCUSSION |
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Contribution of a 3D approach to simulate canopy photosynthesis within heterogeneous row crops
Changes in Na along the velum (i.e. ‘horizontal’ changes) were essentially observed until stem elongation (Table 5), whereas changes in Na between laminae (i.e. ‘vertical’ changes) were observed after stem elongation (Fig. 3C). The effect of Na partitioning within canopies was quantified by comparing the daily net photosynthesis (Pn,d,c) between real and associated hypothetical canopies. Before stem elongation within the dense canopy (D-density), real Na partitioning (Table 5 and Fig. 3) made it possible to increase Pn,d,c from 1 to 7 % compared with uniform Na along each velum, and from 2 to 8 % compared with uniform Na within the whole canopy (Fig. 4A). After stem elongation, which corresponded to canopy closure, similar behaviour was observed but the increase in Pn,d,c was lower: from 2 to 4 % in the first case and from 4 to 6 % in the second. After silking, gradients along the velum were not significantly different in most canopies (Table 5) that involved no difference in Pn,d,c between the real D-density canopy and its associated hypothetical canopy with uniform Na along the velum. On the contrary, strong gradients in Na between laminae (Fig. 3C) involved increased Pn,d,c from 5 to 7 % compared with uniform Na within the whole canopy (Fig. 4A). This behaviour was also observed within the d-density canopy, but the difference between real and hypothetical canopies was lower: Pn,d,c increased by 2 % after silking (Fig. 4B). For the open-thinned canopies, Na gradients within and between laminae had disappeared, and the real Na partitioning gave similar Pn,d,c to uniform Na within laminae and within the whole canopy (Fig. 4B). The first hypothesis for Na partitioning (vertical Na gradients only) is thus valuable for the dense canopies after stem elongation. The second one, which does not take into account Na partitioning to simulate photosynthesis and biomass production, is valuable for the open-thinned canopies where no horizontal and vertical gradients were observed.
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In heterogeneous canopies, vertical descriptions of Na partitioning can be used in dense ones to simulate canopy photosynthesis (e.g. Hirose and Werger, 1987; Anten at al., 1995; Dreccer et al., 2000), even if a vertical exponential Na partitioning underestimated Pn,d,c (Dreccer et al., 2000). But when lamina area density partitioning varies greatly along an axis perpendicular to the row direction (d-density, Fig. 1A, B), simulations indicated differences of canopy photosynthesis up to 8 % between real and associated hypothetical canopies. That involved taking into account spatial variations in Na (i.e. Na within and between laminae) by using 3D representations of the canopy variables. However, those differences are within the range of error of photosynthesis measurements in canopies.
In the open-thinned canopies (d102·5- and d100·5-density), total shoot nitrogen content increased within the plant compared with the d-density canopy: from 1·41 g per plant to 2·13 g per plant after the moderate thinning and from 1·41 g per plant to 2·30 g per plant after the strong thinning (Table 1). Thinning plants involved uniform Na partitioning, with Na tending towards a maximal value around 2 g m–2 (Fig. 3F, see Drouet and Bonhomme, 1999), which might correspond to the maximal photosynthetic capacity of laminae. Simulations indicated that Na partitioning after removing plants involved an increase in Pn,d,c from 6 to 7 % within the open-thinned canopies compared with the Na partitioning within the d-density canopy. Because of foliage heterogeneity within the open-thinned canopies, 3D representations of the canopy variables were needed. But it was shown that, in our experimental open-thinned canopies, this approach was not finally necessary because of uniformity of Na and Id,a partitioning (see Drouet and Bonhomme, 1999).
Spatial changes in Na partitioning: changes in Nm or regulation by Ma?
Na is the product of Nm and Ma. Increased Ma is the result of increased lamina thickness (Maurice et al., 1997) and increased lamina density (Witkowski and Lamont, 1991). Relationships between Na and Nm and between Na and Ma for all canopies indicated that changes in Na jointly result from changes in Nm and Ma (Fig. 5). However, the relationship between Na and Ma was better (Table 6; RSE = 0·21 and R2 = 0·78) than that between Na and Nm (RSE = 0·28 and R2 = 0·61).
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For each canopy, the relationship between Na and Nm was slightly stronger than that between Na and Ma at the beginning of stem elongation. At this stage, we can consider that Nm and Ma contributed equally to the variability of Na. Nm and Ma decreased simultaneously from the bottom to the top of the D-density (Fig. 3A, B) and d-density canopies (data not shown), which produced a decrease in Na. At the end of stem elongation, the tendency reversed: the relationship between Na and Nm was slightly weaker than that between Na and Ma. The vertical partitioning of Nm remained constant, whereas Na changed between laminae in the same way as Ma, which corresponded to vertical gradients of Id,a.
At post-silking, the relationship between Na and Ma was better than that between Na and Nm, except in the D-density canopy. Vertical Na distribution (Fig. 3C) corresponded with vertical Id,a distribution (see Drouet and Bonhomme, 1999), except at the top of the canopy where low Na values were observed within the most illuminated laminae (Fig. 1F). That might be due to Na remobilization from not only lower but also upper laminae towards the grains already developed at this period. This hypothesis agrees with results from several authors (e.g. Crafts-Brandner and Poneleit, 1987; Thomas and Smart, 1993; Sadras et al., 1993, 2000), who indicated an effect of reproductive growth on lamina senescence and lamina nitrogen partitioning. The latter in particular showed that the important changes in the profile of lamina nitrogen in maize and sunflower during grain filling were unrelated to the light regime. Vertical gradients were also observed in Ma (Fig. 3B), whereas vertical gradients in Nm were not apparent (Fig. 3A). In the open-thinned canopies, no correlation was found between Na and Nm (R2 = 0·09), whereas clear relationships were observed between Na and Ma (Table 6). These observations suggest that changes in Na might result from changes in Ma rather than changes in Nm, but this result was not as obvious as in previous studies (e.g. Hirose et al., 1989; Ellsworth and Reich, 1993; Rosati et al., 2000; Le Roux et al., 2001). Ma distribution therefore cannot provide a means of distributing Na within plant canopies, contrasting with results from Ellsworth and Reich (1993), for example, who found that the distribution of Ma in tree canopies explained 95 % of the distribution of Na in Acer saccharum. Furthermore, Hirose et al. (1988) showed that Ma in the canopy was controlled not only by light climate but also by nitrogen availability to each lamina. Light distribution therefore cannot provide a means of predicting Ma (and consequently Na) distribution, even if light plays a major role in Ma and Na distribution.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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By using one cultivar of maize grown during a single season, it was shown that Na partitioning was closely related to the heterogeneity of irradiance within maize canopies until silking. Using a 3D approach to describe lamina nitrogen and irradiance, it was shown that the real patterns of Na partitioning could increase daily canopy photosynthesis by up to 8 % compared with uniform patterns of Na. That was especially observed for the earliest stages of development where lamina area density partitioning varies greatly along an axis perpendicular to the row direction. In row heterogeneous crops, the classical approach of vertical gradients for nitrogen and light within dense canopies therefore could not be applied and a 3D description of the canopy variables (irradiance, Nm, Ma, Na) would be required. Most studies have shown that Na is related to dry mass per unit area (Ma) rather than nitrogen concentration (Nm). Our data indicated that, even if Ma plays a major role in Na, we cannot evidence a strong relationship between Na and Ma. Na partitioning resulted mostly from Ma partitioning but also from Nm partitioning.
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ACKNOWLEDGEMENTS |
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The authors are grateful to Dr V. O. Sadras and Dr N. P. R. Anten for valuable suggestions when reviewing the manuscript. We acknowledge Dr H. Sinoquet for valuable consultations concerning his RIRI model and, with P. Rivet, on the digitizing system, Polhemus. We thank Dr B. Andrieu for providing us with his ‘surface-to-volume’ procedure. Thanks to F. Lafouge for the nitrogen analysis, as well as P. Bonchrétien, M. Lauransot, C. Civet, A. Fortineau and M. and J. Chartier for their technical assistance. S. Tanis-Plant gave us editorial advice.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSLITERATURE CITED |
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