maximum relative growth rate RGRmax (kg kgâ1dâ1) and having a nitrogen content NC (g kgâ1). The product (P, kg) is the plant part related to processes that ...
Annals of Botany 86: 1073±1080, 2000 doi:10.1006/anbo.2000.1268, available online at http://www.idealibrary.com on
Regulation of Growth at Steady-state Nitrogen Nutrition in Lettuce (Lactuca sativa L.): Interactive Eects of Nitrogen and Irradiance A . R . D E P I N H E I RO H E N R I Q U E S and L . F. M . M A R C E L I S * Plant Research International, Bornsesteeg 65, P.O. Box 16, 6700 AA Wageningen, The Netherlands Received: 31 January 2000 Returned for revision: 30 May 2000 Accepted: 28 July 2000 The morphological and physiological adaptation of Lactuca sativa L. (`Vegas') to dierent irradiance levels and rates of nitrogen supply was analysed in such a way that eects of irradiance were clearly distinguished from the eects of nitrogen. Lettuce was grown in a glasshouse in aerated nutrient solutions containing all necessary nutrients except nitrogen. Nitrogen was supplied in excess and at limiting rates in relation to plant growth to provide steady state nutrition. Shading the plants created the low irradiance level. The eects of nitrogen supply and irradiance on growth showed a marked interaction. Dry matter production decreased strongly with decreasing nitrogen supply at high irradiance, but decreased only slightly at low irradiance. Nitrogen had no eect on radiation use eciency except for the lowest nitrogen treatment at high irradiance. The eect of nitrogen on growth was mainly mediated by its eect on leaf area development and hence on light interception. Decreases in leaf area were due to decreases in speci®c leaf area and dry matter partitioning towards the leaves, while the decrease in speci®c leaf area was the result of an increase in leaf dry matter percentage at low nitrogen supply. Dry matter and nitrogen partitioning, and nitrate concentration were closely related to plant nitrogen concentration. Irradiance did not aect these relationships. Irradiance in¯uenced partitioning only indirectly by aecting plant nitrogen concentration. The demand for organic nitrogen per unit leaf area was lower at lower irradiance. Organic nitrogen per unit leaf area appeared to be adjusted to the irradiance level, independently of the nitrogen supply, suggesting priority of nitrogen usage in photosynthesis. # 2000 Annals of Botany Company Key words: Lactuca sativa L., lettuce, growth, irradiance, leaf area, nitrogen, radiation use eciency, partitioning.
I N T RO D U C T I O N Plants have a remarkable capability to adapt and grow eectively in a wide range of environments. They do so by changing their morphological and physiological characteristics in response to the environmental conditions of growth (Lambers et al., 1990). Light and nutrition are two such features of the environment that aect growth. Growth is determined by the available incident radiation, the proportion of radiation absorbed by the canopy and the eciency by which absorbed radiation is converted into biomass (Barneix, 1990). Eects of nitrogen on growth might result from eects on photosynthetic rate and/or on leaf area (Novoa and Loomis, 1981; Grindlay, 1997). Hirose and Werger (1987) showed the existence of an asymptotic relationship between photosynthetic rate and leaf nitrogen content. This means that there is a range over which relatively large increases in leaf nitrogen content will give only marginal increases in the photosynthetic rate and carbon gain. However, carbon gain can also be increased by nitrogen investments in canopy expansion resulting in more light interception. In several studies (Lambers et al., 1981; Waring et al., 1985; McDonald et al., 1986a), the main eect of nitrogen on growth was in fact through eects on leaf area. Photosynthesis was of secondary importance in the attainment of crop biomass. The ®nal eect will depend * For correspondence. Fax 31 317 423 110, e-mail l.f.m.marcelis@ plant.wag-ur.nl
0305-7364/00/121073+08 $35.00/00
on the relative advantages for whole plant carbon gain of using nitrogen to increase the leaf photosynthetic capacity or to produce more leaf area for light interception (Grindlay, 1997). Two parameters important in the attainment of leaf area are the fraction of plant dry mass partitioned to the leaves (LMR, kg kg ÿ1) and the amount of leaf area per leaf dry mass (SLA, m2 kg ÿ1) (McDonald, 1990). These parameters which depend on the growth environment can describe plant morphological adaptation to this environment; therefore, they can be of importance in explaining growth. Novoa and Loomis (1981) proposed that the demand for nitrogen is determined by growth rate and the nitrogen composition of new tissue. In view of the relation between nitrogen and photosynthesis, it has been suggested that the nitrogen content per leaf area is adjusted to the irradiance experienced during growth in order to make full use of the intercepted radiation (Grindlay, 1997). This provides an important link between nitrogen nutrition and radiation, indicating that dierences in nitrogen content per leaf area and hence in nitrogen demand might also be expected at dierent irradiance levels. When studying the eects of nitrogen and irradiance on growth it is important to ensure one can clearly distinguish between the eects of each factor. According to Ingestad (1982), Ingestad and McDonald (1989) and McDonald (1990), this is impossible unless the required stability of plant nutrition (constant nutrient concentration in the # 2000 Annals of Botany Company
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De Pinheiro Henriques and MarcelisÐNitrogen Nutrition in Lettuce
plant) has been established. Only then is it ensured that any changes observed are associated with decreased nitrogen supply (and not just plant size or age) allowing that the eect of a further variable such as irradiance becomes quanti®able. Moreover, only when the condition of steady state nutrition is satis®ed can quantitative comparisons with results in literature be made in a conclusive way (Ingestad and McDonald, 1989). The aim of this work was to quantify parameters that describe morphological and physiological adaptation of lettuce to nitrogen nutrition and irradiance levels, in such a way that eects of irradiance were clearly distinguished from the eects of nitrogen nutrition. We investigated whether the eects of nitrogen nutrition and light intensity on growth were mediated through eects on radiation use eciency (RUE) or leaf area development and also whether eects on leaf area development were the result of eects on dry matter partitioning or speci®c leaf area, given that eects on speci®c leaf area might be the result of dry matter percentage or leaf thickness. Furthermore, adaptation of the plant nitrogen concentration was studied and we investigated whether morphological and physiological parameters showed unique relationships to the plant nitrogen concentration. M AT E R I A L S A N D M E T H O D S Plant growth conditions Lettuce seeds (Lactuca sativa L. `Vegas') were sown in moist vermiculite at 158C on 16 February. After germination, seedlings were transferred to a glasshouse where they were grown on an aerated nutrient solution. The composition of the nutrient solution was: 3.02 mM H2 PO4ÿ , 8.51 mM K , 5.47 mM Ca2 , 2.43 mM Mg2 and 10.575 mM SO4ÿ , traceelements and Fe-EDTA (Steiner, 1984). Until the start of the treatments, nitrogen
NO3ÿ concentration was 10 mM. Afterwards nitrogen was supplied by adding a mixture of NO3ÿ , Ca2 , Mg2 and K to the nitrogen-free nutrient solution, maintaining a constant ratio between the cations, the rate of supply being dependent on the treatment. The pH was kept between 5.7 and 6.5; electrical conductivity (EC) of the nutrient solution was 2 dSm ÿ1 for the low nitrogen treatments and 3 dSm ÿ1 for the excess nitrogen treatments. Daily average global radiation outside the glasshouse was 10.3 MJ m ÿ2 d ÿ1; average temperature was 16.58C; average relative humidity was 79 %; and plant density was 27 plants m ÿ2. Nitrogen and irradiance treatments Treatments were initiated 28 d after sowing. Two levels of irradiance in combination with four rates of nitrogen supply were applied. Nitrogen was supplied in excess (12 mM NO3ÿ ) or daily at rates of 100, 85 and 70 % of the daily nitrogen demand to create steady state nutrition. Except for the treatment with excess N, we checked regularly that plants completely took up the nitrate from the nutrient solution within 1 d using analytical nitrate test strips (Merck). To create the low irradiance level, plants
were shaded by cheesecloth with a light transmittance of 41 %. Steady state nutrition Nitrogen demand of the plants was determined daily from non-destructive measurements of fresh mass increase and calculations of the demanded nitrogen concentration of the plant (as described below). Daily increase in fresh mass of the plants was determined non-destructively every other day by weighing four plants per treatment, while immersing the roots in water. This measurement yielded the fresh mass of the shoot (Mshoot). Assuming a constant shoot mass ratio (SMR, ratio of shoot to total plant mass) of 0.18 (value determined in previous lettuce experiments) during the experiment, fresh mass of the shoot was used as a measure of the fresh mass of the plant (M) according to: M Mshoot =SMR To estimate the demanded nitrogen concentration of the plant (comparable to Caloin and Yu, 1984), the plant mass was assumed to be the sum of two plant parts each having dierent physiological functions and biochemical compositions. The production capacity (C, kg) is the metabolically active plant part, producing plant biomass with the maximum relative growth rate RGRmax (kg kg ÿ1d ÿ1) and having a nitrogen content NC (g kg ÿ1). The product (P, kg) is the plant part related to processes that do not lead to increase in biomass, having a nitrogen content NP (g kg ÿ1). For optimal growth, NC and NP in the fresh matter were assumed to be 2.75 and 1.75 g kg ÿ1, respectively, as determined in previous lettuce experiments. The nitrogen content (N, g) of plants grown with excess nitrate was described by: N NP P NC C and the daily nitrogen demand (dN/dt, g d ÿ1) of those plants by: dN=dt NP dP=dt NC dC=dt The mass of C was calculated from non-destructive measurements of fresh growth rate of a plant (dM/dt) and assuming a RGRmax of 0.215 kg kg ÿ1d ÿ1, as determined in previous experiments on lettuce: C
dM=dt=RGRmax and P as: PMÿC
Experimenal design and statistical analysis The treatments were arranged in a split-plot design with two blocks. Irradiance level was randomized over the whole-plots within each block, nitrogen level over the plots within each whole-plot and harvest date over sub-plots within each plot.
De Pinheiro Henriques and MarcelisÐNitrogen Nutrition in Lettuce Data were analysed by analysis of variance using generalized linear models with binomial distribution for the leaf mass ratio, root mass ratio, leaf nitrogen ratio and root nitrogen ratio and normal distribution for other variables. Dierences between treatments were tested by Students' t-test (P 4 0.05).
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R E S ULT S Dry matter production Dry matter production decreased strongly with decreasing rate of nitrogen supply at high irradiance, but only slightly at low irradiance. Reducing irradiance decreased dry matter production (Fig. 1).
Growth analysis
Leaf development
At weekly intervals six plants per treatment, three from each block, were harvested. Plants were separated into leaves, stems and roots. Fresh mass and leaf area were determined. Dry mass was measured after samples were oven dried at 1058C for 48 h. Samples were stored for subsequent chemical analysis.
Reducing nitrogen supply markedly reduced leaf area index (LAI) at high irradiance, while it had only a small eect at low irradiance. Reducing irradiance decreased LAI (Fig. 2). The speci®c leaf area (SLA) decreased linearly with decreasing plant nitrogen concentration (g N kg ÿ1). Supplying nitrogen at a rate of 70 % of plant demand decreased SLA by 18 % at high irradiance and by 13 % at low irradiance. Reducing irradiance increased SLA by about 52 % (Fig. 3A).
Total nitrogen and nitrate contents of the leaves were determined on every harvest, while those of the stems and roots were determined on the second, fourth, sixth and eigth harvest. Total nitrogen content was determined with an element analyser after incinerating the samples (9508C), according to the Dumas method (ROBOPREP-CN, Biological Sample Converter, Europe Scienti®c, Crewe, UK). After extraction in water, nitrate was determined with a continuos-¯ow analyser system [TRAACS 800, Bran Luebbe Analysing Technologies, Elmsford (NY), USA]. Per treatment, two samples were analysed, each sample consisted of three replicate plants per block.
20 Dry matter production per plant (g)
Chemical analysis
Calculations
Iabs Io
1 ÿ r
1 ÿ e ÿkL Io was estimated from the measured solar radiation outside the glasshouse assuming a constant light transmission of the glasshouse of 51 % and an average of 45 % photosynthetically active radiation in the global radiation. For the low irradiance treatments, the light transmission of cheesecloth (41 %) was used in the calculations. To estimate radiation use eciency (RUE, g MJ ÿ1) the slope of the linear relationship between cumulative dry matter production and cumulative absorbed radiation by the canopy was calculated (Gosse et al., 1986).
12 8 4 0 16 Mar.
05 Apr.
25 Apr.
15 May
F I G . 1. Time response of dry matter production per plant for dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. e, r, excess nitrogen supply; s, d, 100 % nitrogen supply; n, m, 85 % nitrogen supply; h, j, 70 % nitrogen supply; e, s, n, h, high irradiance; r, d, m, j, low irradiance. Each data point is the mean of six replicates. Standard errors of means were smaller than symbol size.
20 16 LAI (m2 m2)
Absorbed photosynthetically active radiation (Iabs , MJ m ) by the canopy was calculated from the estimated photosynthetically active radiation above the canopy (Io , MJ m ÿ2 d ÿ1) assuming an exponential light extinction with increasing leaf area index (LAI, m2 m ÿ2) (Monsi and Saeki, 1953). The canopy re¯ection coecient (r) was 0.02 and the extinction coecient (k) 0.65 (values measured for lettuce in previous experiments; these parameter values were not aected by increasing plant density from 17 to 27 plants m ÿ2 or shading by 30 %). ÿ2
16
12 8 4 0 16 Mar.
05 Apr.
25 Apr.
15 May
F I G . 2. Time response of leaf area index (LAI) for dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. Symbols and details as in Fig. 1.
De Pinheiro Henriques and MarcelisÐNitrogen Nutrition in Lettuce 1.0
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60 kg1
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F I G . 4. Relationship between the fraction of plant mass partitioned to leaves (LMR) and roots (RMR) and the plant nitrogen concentration at dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. Symbols and details as in Fig. 3.
0.4 0.3 0.2
1.0
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Plant N concentration (g N
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F I G . 3. Relationship between speci®c leaf area (SLA) and plant nitrogen concentration (A) and between leaf thickness and plant nitrogen concentration (B) at dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. e, r, Excess nitrogen supply; s, d, 100 % nitrogen supply; n, m, 85 % nitrogen supply; h, j, 70 % nitrogen supply; e, s, n, h, high irradiance; r, d, m, j, low irradiance. Data points are means over the whole experimental period. Standard errors of means were smaller than symbol size.
Nitrogen partitioning (kg kg1)
SLA (m2 kg1 DM)
80
Dry matter partitioning (kg kg1)
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0.8 LNR
0.6 0.4
RNR 0.2 0.0 0
Leaf thickness (ratio of leaf fresh mass to leaf area, kg m ÿ2) decreased linearly with decreasing plant nitrogen concentration at high irradiance, but only slightly at low irradiance (Fig. 3B). Leaf thickness decreased with decreasing irradiance by 17 % for the 70 % nitrogen treatment and 25 % for the excess nitrogen treatment. The irradiance eect on SLA as well as on leaf thickness was larger than that of nitrogen. Dry matter partitioning The leaf mass ratio (LMR, fraction of dry matter partitioned to the leaves) decreased with decreasing rate of nitrogen supply (Fig. 4). Supplying nitrogen at a rate of 70 % of plant demand decreased LMR by 19 % at high irradiance and by 11 % at low irradiance. Reducing irradiance increased LMR for rates of nitrogen supply of 70, 85 and 100 % N, while it had no eect at excess nitrogen supply. LMR decreased with decreasing plant nitrogen concentration following a curvilinear relationship. Irradiance did not aect this relationship between dry matter partitioning and plant nitrogen concentration (Fig. 4).
20
40
60
Plant N concentration (g N kg1 DM) F I G . 5. Relationship between the fraction of nitrogen partitioned to leaves (LNR) and roots (RNR) and the plant nitrogen concentration at dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. Symbols and details as in Fig. 3.
Nitrogen partitioning Like the leaf mass ratio, the leaf nitrogen ratio (LNR, fraction of nitrogen partitioned to the leaves) decreased curvilinearly with decreasing plant nitrogen concentration (Fig. 5). Irradiance did not aect this relationship between LNR and plant nitrogen concentration (Fig. 5).
Dry matter percentage The dry matter percentage of the plant increased with decreasing plant nitrogen concentration and increasing irradiance (Fig. 6). Dry matter percentage of the leaves was similarly aected (data not shown).
De Pinheiro Henriques and MarcelisÐNitrogen Nutrition in Lettuce
6
4
2
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30
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0
0
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Plant N concentration (g N
DM)
F I G . 6. Relationship between plant dry matter percentage ( %) and plant nitrogen concentration at dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. Symbols and details as in Fig. 3.
Leaf nitrogen The nitrate concentration in the leaf dry mass (g N-NO3 kg ÿ1 DM) decreased linearly with decreasing plant nitrogen concentration regardless of the irradiance level (Fig. 7A). However, at a given rate of nitrogen supply, nitrate concentration was highest at the low irradiance level (Fig. 7A). The organic nitrogen concentration in the leaf dry mass (g N kg ÿ1 DM) decreased with decreasing plant nitrogen concentration regardless of the irradiance level (Fig. 7A). The nitrogen content per unit leaf area (g N m ÿ2) decreased with decreasing plant nitrogen concentration and irradiance level (Fig. 7B). The organic nitrogen content per unit leaf area (g N m ÿ2) was not aected by the rate of nitrogen supply but decreased with decreasing irradiance (Fig. 7C). Eciency of conversion of absorbed radiation into dry matter Nitrogen supply had no eect on radiation use eciency (RUE) except for the 70 % nitrogen treatment at high irradiance, for which RUE was reduced by 10 %. The RUE of plants grown at low irradiance (4.9 g MJ ÿ1) was 30 % higher than that of plants grown at high irradiance (3.5± 3.9 g MJ ÿ1) (Fig. 8). DISCUSSION Eciency of conversion of absorbed radiation into dry matter Radiation use eciency was not aected by nitrogen supply except for the 70 % nitrogen treatment at high irradiance, when the nitrogen content per unit leaf area dropped as low as 0.7 g m ÿ2. Rates of nitrogen supply higher than 85 % of the plant demand, leading to nitrogen contents per unit leaf area larger than 0.9 g m ÿ2 (0.9±1.2 g m ÿ2), were of no advantage to RUE, suggesting that RUE had reached its maximum. This is consistent with the results of Sinclair and Horie (1989) that showed an asymptotic relationship between RUE and nitrogen content in the leaf. However,
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Nitrogen per unit leaf area (g N m2)
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Leaf organic N concentration (g N kg1 DM)
Leaf nitrate concentration (gN-NO3 kg1 DM)
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Organic nitrogen per unit leaf area (g N m2)
Dry matter percentage (%)
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0
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C
0.8 0.6 0.4 0.2 0.0
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Plant N concentration (g N
60 kg1
DM)
F I G . 7. Relationship between leaf nitrate concentration (± ± ±), leaf organic nitrogen concentration (ÐÐ) (A), nitrogen
organic NO3 ÿ per unit leaf area (B), organic nitrogen per unit leaf area (C) and plant nitrogen concentration at dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. Symbols and details as in Fig. 3.
Sinclair and Horie proposed that RUE would reach a maximum at a nitrogen content per unit leaf area of 1.2±2 g m ÿ2 while in the present work RUE reached its maximum at a much lower value of 0.9 g m ÿ2. For the low irradiance treatments, the nitrogen content per unit leaf area varied between 0.6 and 0.9 g m ÿ2, thus never reaching the 1.2 g m ÿ2 proposed by Sinclair and Horie (1989). These results indicate that the leaf nitrogen content per unit leaf area that maximizes RUE shifts to lower values with decreasing irradiance. The small eect of nitrogen on RUE suggests that the leaf photosynthetic rate remained constant, which is in
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Cumulative dry matter production (g m2)
500 400 300 200 100 0
0
50
100
150
Cumulative PARabs (MJ m2) F I G . 8. Relationship between cumulative dry matter production and cumulative absorbed PAR (PARabs) for plants grown at dierent rates of nitrogen supply ( % of plant demand) and irradiance levels. The ®tted linear relationships through the origin are y 3:9x; R2 0.993 for excess, 100 % and 85 % nitrogen supply at high irradiance; y 3:5x; R2 0.988 for 70 % nitrogen supply at high irradiance; y 4:9x; R2 0.998 for all nitrogen treatments at low irradiance. The constant terms represent radiation use eciency (RUE). Each data point is the mean of six replicates. Symbols as in Fig. 1.
agreement with studies of Lambers et al. (1981), Waring et al. (1985) and McDonald et al. (1986a) showing little dependence of leaf photosynthesis on nitrogen supply. Increasing the rate of nitrogen supply increased the SLA and the organic nitrogen concentration in the leaf tissue. As a result, nitrogen supply had no eect on the organic nitrogen per leaf area. As this is the nitrogen fraction related to the photosynthetic machinery (Evans, 1996), the hypothesis that photosynthetic rate remained constant can be supported, which explains the small eect of nitrogen on RUE. For the 70 % N treatment at high irradiance, the organic nitrogen per unit leaf area decreased by 10 %. This decrease might have resulted in a decreased photosynthetic capacity, which explains the lower RUE observed for this treatment. The curvilinear relationship between net photosynthesis and irradiance (Evans, 1996) might explain the higher RUE observed at low irradiance. Moreover, these plants showed a lower organic nitrogen content per unit leaf area than plants grown at high irradiance, which is associated with lower rates of respiration (Van der Werf, 1996). From the present data, it can be concluded that dierences in growth observed among nitrogen treatments were mainly due to the eects of nitrogen on the leaf area and hence on light interception. Leaf development Low LAI is normally associated with decreased nitrogen availability (Freijsen and Veen, 1990; McDonald, 1990; Grindlay, 1997), as was also observed in our study. The two variables that are important in the attainment of leaf area are the leaf mass ratio and the speci®c leaf area (McDonald, 1990). SLA can be further divided into leaf thickness (kg FM m ÿ2), and into the dry matter percentage (ratio of dry
mass to fresh mass) of the leaf tissue (DM %Leaves , kg DM kg ÿ1 FM) (1/SLA Leaf thickness DM %Leaves) (Dijkstra, 1990; Ryser et al., 1997). The present study showed that leaf area decreased with decreasing rate of nitrogen supply due to decreases in both LMR and SLA. The decrease in SLA with decreasing nitrogen supply was due, in turn, to the increase in leaf dry matter percentage. The eect of nitrogen on LMR was smaller at low irradiance than at high irradiance explaining why nitrogen had only a small eect on the leaf area at the low irradiance level. These results are consistent with results of Corre (1983). The high SLA at low irradiance was due to the low dry matter percentage and the small leaf thickness. The smaller leaf thickness at low irradiance was probably due to fewer mesophyll cell layers (Waring et al., 1985). The increase in SLA at decreasing irradiance giving plants an increased ability to intercept light, the limiting resource, was not followed by any increase in the LMR at the excess nitrogen supply and only by 12 % increase in the LMR at 70 % nitrogen supply. Changes in leaf area were due to eects on both SLA and LMR. A low rate of nitrogen supply decreased SLA due to the increase in leaf dry matter percentage. Shading increased SLA due to the decrease in both leaf thickness and leaf dry matter percentage. Dry matter and nitrogen partitioning The partitioning of dry matter to the leaves and roots was strongly related to the plant nitrogen concentration, which is consistent with data of Ingestad and McDonald (1989), Boot et al. (1992) and Van der Werf et al. (1993). Irradiance in¯uenced dry matter partitioning but only indirectly because nitrogen concentration in the dry matter was aected: plants grown at low irradiance showed higher plant nitrogen concentration than those grown at high irradiance. Our data, in combination with data from the literature (Ingestad and McDonald, 1989), suggest the existence of a unique relationship between the plant nitrogen concentration and the partitioning of dry matter to the leaves and roots. The generally-held idea that shading increases LMR (Brouwer, 1962) can be explained by the fact that irradiance aects nitrogen concentration of the plant. However, irradiance did not aect LMR at excess nitrogen supply, which does not support Brouwer's `functional equilibrium'. Poorter and Van der Werf (1998) showed similar results concerning the lack of an eect of irradiance on LMR for a large number of species, although there were some exceptions. This could be related to the pattern of the relationship between dry matter partitioning and plant nitrogen concentration. In the present work, dry matter partitioning showed a curvilinear relationship with plant nitrogen concentration as McDonald (1990) and Van der Werf et al. (1993) proposed, whereas, for example, Freijsen and Veen (1990) and Boot et al. (1992) found a linear relationship. The relationship between LMR and plant nitrogen concentration may vary, depending on the plant species, between a linear and curvilinear relationship, explaining why in certain cases irradiance aected dry
De Pinheiro Henriques and MarcelisÐNitrogen Nutrition in Lettuce matter partitioning at excess nitrogen supply while in other cases it did not. Nitrogen and dry matter partitioning were tightly coupled to each other. The relationship between nitrogen partitioning and plant nitrogen concentration was curvilinear, as was the relationship between dry matter partitioning and plant nitrogen concentration. Our data suggest the existence of a unique relationship between plant nitrogen concentration and dry matter and nitrogen partitioning. Irradiance in¯uenced partitioning, but only indirectly by aecting plant nitrogen concentration. Ratio of fresh mass to dry mass As discussed previously, dry matter percentage is an important variable explaining, in part, the eects of nitrogen on growth. Water and nitrate uptake by plants seem to be closely related to each other (CaÂrdenas-Navarro et al., 1999). The low nitrate concentration at low nitrogen supply and at high irradiance could partly explain the high dry matter percentage under these conditions. Fewer nitrates being stored in the vacuole would represent a reduction in osmotically active compounds, resulting in less water uptake by the cells and increased dry matter percentage. Assuming that NO3ÿ (and similarly the concentration of other monovalent cations) is the only osmotic compound aected by the treatments, and that the osmotic potential of lettuce leaves is a constant 285 mosmol g ÿ1 fresh mass (Blom-Zandstra, 1985), one can calculate whether the eects on nitrate concentration explain the eects on dry matter percentage. At high irradiance, it can be calculated that decreasing the nitrate concentration from 20.6 g NO3ÿ kg ÿ1 DM (excess nitrogen supply) to 2.4 g NO3ÿ g ÿ1 DM (70 % nitrogen supply) could increase the dry matter percentage from 4.3 to 4.7 %, thereby explaining 20 % of the actual increase in dry matter percentage. At excess nitrogen supply, increasing the nitrate concentration from 20.6 mg NO3ÿ g ÿ1 DM (high irradiance) to 25.9 mg NO3ÿ g ÿ1 DM (low irradiance) could decrease the dry matter percentage from 4.3 to 4.2 %, explaining about 50 % of the actual decrease in dry matter percentage. The increase in dry matter percentage as a result of low nitrogen supply can also be related to the nitrate eect on cell wall properties. As suggested by Palmer et al. (1996), nitrate seems able to induce the activity of key enzymes involved in cell wall relaxation leading to a decrease in turgor and water potential and resulting in cell water uptake. Furthermore, nitrate may have aected water uptake through eects on root hydraulic conductance (Hoarau et al., 1996). A large amount of non-structural carbohydrates might also explain the increased dry matter percentage at low nitrogen supply and high irradiance, as carbohydrates are likely to accumulate under such conditions (Lambers et al., 1981; Waring et al., 1985; McDonald et al., 1986a). For example, McDonald et al. (1986a) found that leaf starch concentrations increased by up to 30 % in birch plants grown at low nitrogen nutrition and high irradiance, accounting for a large percentage of leaf dry mass. In the
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present work, an increase in insoluble carbohydrates by 0.33 g g ÿ1 DM would explain the increase in dry matter percentage of plants grown with 70 % nitrogen supply at high irradiance. The increase in the dry matter percentage might also be related to the increase of other carbon-rich compounds such as cellulose and lignin (Waring et al., 1985; McDonald et al., 1986b). The variation in dry matter percentage might be attributable to changes in the concentration of nitrate (acting as an osmoticum, or through eects on cell wall relaxation and root hydraulic conductance), non-structural carbohydrates, cellulose and lignin compounds. Leaf nitrogen usage The contents of organic nitrogen per unit leaf area, 0.50 g m ÿ2 at low irradiance and 0.73 g m ÿ2 at high irradiance, might represent optimal contents that maximized leaf photosynthesis for the irradiance level of growth. Having less than or greater than this optimum might have been costly to the plant as a whole (Grindlay, 1997). Under low irradiance, irradiance generally limits photosynthesis (CorreÂ, 1983). Hence, the observed low organic nitrogen content per unit leaf area at low irradiance may not represent any limitation to the plant, as any extra photosynthetic capacity resulting from a higher organic nitrogen level might not have been used (Grindlay, 1997). At the same time, respiratory losses and the cost of producing and maintaining the photosynthetic apparatus relative to the gain in photosynthate production must be kept as low as possible (Grindlay, 1997). This low organic nitrogen content per unit leaf area might simply be the result of a lower demand per unit leaf area at low irradiance. That the plant nitrogen concentration decreased, but the organic nitrogen content per unit leaf area did not, when nitrogen supply decreased, suggests priority of leaf nitrogen usage in photosynthesis. On the other hand, the high level of nitrate at high nitrogen supply and low irradiance suggests that the nitrate, acting as an osmoticum or involved in processes of cell wall relaxation (Palmer et al., 1996), helped to drive leaf expansion. In this view, it is plausible to consider the existence of a trade-o between the maintenance of a certain content of organic nitrogen per unit leaf area for photosynthesis and expansion of the canopy for light interception, so as to maximize crop dry matter production. Furthermore, the increase in SLA with increasing plant nitrogen concentration and decreasing irradiance can be interpreted as a way of regulating organic nitrogen per unit leaf area via control of leaf expansion. Lettuce plants appeared to adjust the organic nitrogen per unit leaf area to the irradiance level, independently of the nitrogen supply. This suggests that at limiting N supply nitrogen is used with priority for maintaining photosynthesis. AC K N OW L E D G E M E N T S We are grateful to Dr B. Veen for helpful advice and stimulating discussions and Dr A. van der Werf and
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De Pinheiro Henriques and MarcelisÐNitrogen Nutrition in Lettuce
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