Correspondence: C.C. de Groot. Present address: Plant Research International, PO Box 16, 6700 AA Wageningen, The Netherlands. E-mail: C.C.deGroot@plant.wag-ur.nl
We studied the effects of phosphorus (P) and light on the physiological and morphological components of growth of young tomato plants (Lycopersicon esculentum Mill. cv. Capita). The importance of dry-mass partitioning and starch accumulation in explaining the effects of P limitation on growth was examined more closely. Plants were grown at a wide range of exponential P supply rates (between 70 and 320 mg g−1 d−1) and one free-access treatment (1 mm). Two light levels (70 and 300 µmol m−2 s−1) were applied. Growth response coefficients (GRCs) were calculated to address the importance of different growth parameters in explaining relative growth rate (RGR). At both light levels, net assimilation rate (NAR) was more important than leaf area ratio (LAR) in explaining the effects of P on growth as indicated by GRCs. At less severe P limitation, LAR became more important and NAR less important. Dry-mass partitioning to both roots and leaves played a minor role in determining the effects of P limitation on growth as indicated by low GRCs. The increase in starch at mild P limitation showed that the assimilate supply was not limiting. At severe P limitation, the rate of photosynthesis was decreased, as suggested by the decrease in starch accumulation.
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Plants can grow in a wide range of environments by adjusting their morphological and physiological characteristics to environmental conditions (Lambers et al. 1990). Phosphorus (P) and light are examples of growth factors that may differ between environments. Phosphorus often limits plant growth (Schachtman, Reid & Ayling 1998), especially in heavily leached soils, for example in tropical parts of South America (Raaimakers et al. 1995) and large parts of Australia (Specht & Specht 1999). Light is a very important resource for plant growth. It drives carbon capture in photosynthesis, the key process for life on earth
When growth is limited by irradiance, the physiological component (NAR) tends to be more important than the morphological component (LAR) in explaining the effects on RGR. For nutrients, the morphological component (LAR) is, on average, more important than the physiological component (NAR) in determining a decrease in RGR due to nutrient limitation (Poorter & Nagel 2000); however, in several experiments, the opposite has been reported (Peace & Grubb 1982; Corré 1983).
This paper addresses the question of whether the effects of P supply on RGR in tomato are determined by the effects on the physiological component (NAR) or by those on the morphological component (LAR), which can be factorized further into SLA and LMR. We tested whether the severity of the P limitation can offer an explanation for the contradictory conclusions reported in the literature. Furthermore, this paper compares P limitation of growth at high light and low light, and discusses interactions between limitation by low light and by low P. We also investigated the effects of P limitation on SLA and tested whether these effects were due to starch accumulation, as expected at low nutrient availability.
Materials and methods
Growth of plants
Seeds of tomato (Lycopersicon esculentum Mill. cv. Capita) were germinated on moistened vermiculite at 21 °C. Eight days after sowing (DAS), the plants were transferred into 2·7 dm3 containers with aerated P-free nutrient solution (Steiner 1984), one plant per container. KH2PO4 was added to bring the P concentration to 55 µm. The containers were placed in a growth chamber with a photosynthetically active radiation (PAR) of 300 µmol m−2 s−1 for 16 h per day (TL-D-HF, Philips, Eindhoven, The Netherlands), followed by 30 min of incandescent light. The relative humidity was 70%, with a day/night temperature of 23/21 °C. The composition of the macronutrients and trace elements of the nutrient solution was as described by Steiner (1984), with the exception that in the P-free nutrient solution, phosphate was replaced by nitrate and sulphate without changing the ratio between these ions (Table 1). At the beginning of the treatments, the nutrient solution in the containers was replaced by the P-free solution. During the experiment, the pH was adjusted regularly to 5·5–6·0 with a 1 : 1 mixture of sulphuric and nitric acids.
Table 1. Composition of the nutrient solutions. Trace elements as described by Steiner (1984)
Concentration in nutrient solution (mm)
Free access to P
P and light treatments
At the beginning of the treatments (15 DAS), two levels of irradiance (70 and 300 µmol m−2 s−1), in combination with seven rates of P supply, were applied. P was supplied as KH2PO4 at a growth-saturating rate of 1 mm H2PO4– (free-access treatment, FA; Table 1) or daily at relative addition rates of 70, 120, 170, 220, 270 and 320 mg g−1 d−1 to a P-free nutrient solution (Table 1). The initial amount of P in the plant was estimated at 0·24 mg per plant. In order to shorten the time needed for the plants of the four lowest P-supply treatments to adjust to the lower P supply, these plants were starved of P for a short period (treatments 220, 170, 120 and 70 mg g−1 d−1 for 1, 2, 3 and 4 d, respectively). The plants were shaded by four layers of cheesecloth, with a total light transmittance of 24%, to create the low irradiance level.
Harvests and chemical analyses
Plants were harvested at the start of the experiment (15 DAS) and on every fourth day thereafter. The last harvest was at 35 DAS. RGR and NAR at a harvest date t were calculated over the (t − 4) to (t + 4) DAS interval (see below). At harvest, roots were rinsed three times in tap water and blotted dry between absorbing paper. Plants were divided into leaf blades, roots and stem (including petioles). The number of leaves (of the main shoot), leaf area (separately for main shoot and lateral shoots), fresh mass and dry mass (after at least 48 h at 70 °C) were determined. At 31 DAS, two leaflets of the third leaf (counted from bottom to top) were sampled at the end of the light period and two were sampled at the end of the dark period. Leaf area and fresh mass of these leaflets were determined. The samples were freeze-dried, weighed and stored for starch determination.
Tissue concentrations of total P were determined in each plant organ at 31 DAS. The plant organs were pooled into one sample at the other harvest dates. Total P was determined colourimetrically after digestion with a 1 : 1 (v/v) mixture of concentrated sulphuric acid and concentrated nitric acid (Humphries 1956).
Starch was determined on freeze-dried leaf material collected at 31 DAS; 25 mg dry mass was extracted four times with 80% (v/v) ethanol at 85 °C. The residue was used for starch determination. Starch was determined enzymatically using a test combination for starch (Boehringer, Mannheim, Germany), modified for use with a microtiter plate reader (Bio kinetics reader EL 340; Bio-Tek Instruments, Winooski, VE, USA).
Calculations and statistics
Growth was determined as RGR based on dry mass. RGR at 31 DAS was calculated as 1000 × [lnW(t35DAS) − lnW(t27DAS)]/[t35DAS−t27DAS]. NAR was calculated as [W(t35DAS) − W(t27DAS)]/[t35DAS−t27DAS]×[lnLA(t35DAS) − lnLA(t27DAS)]/[LA(t35DAS) − LA(t27DAS)]. W = dry mass (g); LA = leaf area (m2); LAR = leaf area/plant mass (m2 kg−1) (Hunt 1978).
RGR was factorized into NAR and LAR. LAR is the product of LMR and SLA (Evans 1972). In order to determine the effect of P limitation on the role of RMR in determining growth, RGR has been factorized into RMR, the inverse of the plant P concentration and net P uptake rate per gram root mass (NPUR) (Garnier 1991). NPUR =[lnRM(t35DAS) − lnRM(t27DAS)]/(t35DAS−t27DAS)]×[Pa(t35DAS) − Pa(t27DAS)]/[RM(t31DAS) − RM(t27DAS)]. RM = root mass (g), Pa = the amount of P in the plant (mg) (Scheurwater et al. 1998).
The experiment was conducted twice, each time with three replicate plants per treatment per harvest, giving a total of six replicate plants. Within a light level, the three plants per P treatment were divided over three blocks. Data were analysed with analysis of variance (anova). Curves were fitted using regression analysis (GENSTAT 5 release 4·1, Lawes Agricultural Trust, IACR-Rothamsted, UK).
Growth response coefficients
To determine the relative importance of the growth parameters in determining RGR, we calculated the GRCs. The GRC of a growth parameter x, where x is one of the parameters in which RGR is factorized, is the relative change in x scaled to the relative change in RGR. In formula: GRCx= (dx/x)/(dRGR/RGR) (Poorter & Van der Werf 1998; Poorter & Nagel 2000). The first part (dx/x) can be rewritten as: dx/x= 1/x× dx=x=x0∫x=x1lnx+c= lnx1− lnx0. Similarly, dRGR/RGR = lnRGR1− lnRGR0. So GRCx= (dx/x)/(dRGR/RGR) = (lnx1− lnx0)/(lnRGR1− lnRGR0), which is the slope of the ln-transformed values of growth parameter x plotted against the ln-transformed RGR values (Poorter & Van der Werf 1998). The GRCs were estimated from fitting a linear relation between the ln-transformed values of the growth parameter x and the ln-transformed values of RGR: ln(x) = a + GRC × ln(RGR). GRCx will be 1 if the change in x is fully proportional to the change in RGR. GRCx will be 0 if a difference in RGR is not accompanied by a change in x. A value above 1 indicates that an increase in RGR is accompanied by a stronger increase in x. Negative values indicate that an increase in RGR is accompanied by a decrease in x (Poorter & Van der Werf 1998; Poorter & Nagel 2000). The sum of the GRCs of the parameters making up RGR, for example RGR = NAR × LAR, should be 1 (Poorter & Van der Werf 1998; Poorter & Nagel 2000).
Unless otherwise stated, the data presented are those of plants harvested at 31 DAS.
Plant P concentration
At low P supply, an increase in the P supply rate resulted in only a marginal increase in plant P concentration. At P supply rates > 120 mg g−1 d−1 (for low light) and > 220 mg g−1 d−1 (for high light), plant P concentration increased distinctly. Low-light plants reached a maximum plant P concentration of 9·3 mg g−1 at a P supply rate of 270 mg g−1 d−1. Plant P concentration continued to increase with increasing supply rate for the high-light plants (Fig. 1a). At low light and low P supply rates (70 and 120 mg g−1 d−1), plant P concentration increased by only 1% with the almost two-fold increase in P supply rate (Fig. 1a). The P concentrations of the roots, stem and leaves increased by only 2, 0·5 and 1%, respectively (Fig. 1b).
At high light and P supply rates of 70–220 mg g−1 d−1, plant P concentration increased by 8% with a three-fold increase in P supply rate (Fig. 1a), while the P concentrations of the roots, stem and leaves increased by 14, 7 and 6%, respectively (Fig. 1c). The higher increase in root P concentration (14%) did not result in an increase in plant P concentration because relatively less roots were produced and RMR decreased (Fig. 2).
Plant dry mass increased both with increasing P supply and at higher light intensity. However, at the lowest P addition rate (70 mg g−1 d−1), plants were mainly limited by P and light did not affect plant dry mass (Fig. 3a,b).
RGR increased with increasing plant P concentration and approached a maximum at a plant P concentration of approximately 4 mg g−1. High light resulted in a higher maximum RGR than low light (Fig. 4a). At high light, the four lowest P supply rates (70–220 mg g−1 d−1) limited growth. At low light, growth was limited by the two lowest P supply rates (70 and 120 mg g−1 d−1). When the analysis was based on plants of the same size (0·4–0·8 g plant dry mass) instead of the same age (31 DAS), the shape of the relation between RGR and plant P concentration was the same. However, at low light, the decrease in RGR at low plant P concentration was not significant (Fig. 4a).
At 31 DAS, NAR increased with increasing plant P concentration; this relation was more distinct at high light than at low light (Fig. 4b). The same was found when the data were analysed for the 0·4–0·8 g dry-mass interval, with the exception that the decrease in NAR with plant P concentration was not significant (Fig. 4b). LAR increased with increasing plant P concentration at both 31 DAS and the 0·4–0·8 g dry-mass interval, and was lower at high light than at low light (Fig. 4c).
At 31 DAS, the increase in LMR with increasing plant P concentration at low light was only marginal, while at high light, LMR increased distinctly with increasing plant P concentration. Equally shaped relations were found when the data were analysed for the 0·4–0·8 g dry-mass interval (Fig. 4d). LMR (Fig. 4d) as well as RMR (Fig. 2) were lower at low light compared with high light, as a result of a higher fraction of dry mass partitioned to the stem (data not shown). At 31 DAS, SLA increased with increasing plant P concentration at both light conditions. The SLA of high-light-grown plants was lower than that of the low-light-grown plants (Fig. 4e). At both light conditions, %LDM decreased with increasing plant P concentration at 31 DAS, and was higher at high light than at low light (Fig. 4f). The same was found when SLA and %LDM were analysed for the 0·4–0·8 g dry-mass interval (Fig. 4e,f).
The starch concentration decreased with increasing plant P concentration at low light (Fig. 5) and was related linearly to %LDM (data not shown). At high light, two phases were distinguished: a first phase at P limitation (P-supply rates of 70–220 mg g−1 d−1), in which the starch concen-tration increased sharply with increasing plant P concentration (Fig. 5), and a second in which the starch concentration tended to decrease with increasing plant P concentration at P-supply rates of 220 mg g−1 d−1 and higher (Fig. 5). At both light intensities and at all plant P concentrations, the starch concentration at the end of the day was higher than that at the end of the night (Fig. 5). Correcting total leaf dry mass for starch accumulation revealed that at low light, starch accumulation explained 14% of the decrease in SLA and 29% of the increase in %LDM with decreasing plant P concentration. For high light, starch accumulation did not explain the decrease in SLA at P-supply rates of 220 mg g−1 d−1 and higher but accounted for 54% of the increase in %LDM with plant P concentration. At P-supply rates of 70–220 mg g−1 d−1, the starch concentration decreased sharply with decreasing plant P concentration. Therefore, this part of the curve variation in SLA cannot be explained by variation in starch accumulation.
At both light intensities, GRCNAR was larger than GRCLAR. GRCLMR was close to zero (Fig. 6). The effects of light were assessed by comparing the free-access treatments of the two light intensities and by comparing the two P treatments with an equal absolute reduction of RGR of approximately 100 mg g−1 d−1 (P supply rates of 70 and 170 mg g−1 d−1 at low and high light, respectively). In both cases, GRCNAR was larger than GRCLAR (Table 2).
Table 2. Growth response coefficients for the effects of light limitation on RGR. The free access treatments at low and high light and two treatments with an equal reduction in RGR are compared (P supply treatment: 70 mg g−1 d−1 at low light and 170 mg g−1 d−1 at high light) at 31 DAS
Free access to P
ΔRGR = 100 mg g−1 d−1
RGR was also factorized into NPUR (P uptake per gram root per day), RMR and the inverse of plant P concentration (Garnier 1991). For both light intensities, GRCNPUR was greater than 1 because the rate of P uptake was stimulated more than RGR with increasing P supply. At the same time, the increase in NPUR was counterbalanced by the decrease of the inverse plant P concentration. RMR was of secondary importance (Table 3).
Table 3. Growth response coefficients, RGR = NPUR × RMR/plant P concentration at 31 DAS
Growth response coefficient
1/plant P concentration
Leaf area development
Under both light conditions, the increase in leaf area with increasing plant P concentration (Fig. 7a) resulted from an increase in the leaf area per leaf (Fig. 7b). At high light, the number of leaves of the main shoot also increased, whereas at low light this number was not affected (Fig. 7c). No branching occurred at low light, whereas branching increased with increasing P availability at high light (Fig. 7a).
Light increased the leaf area production at high plant P concentrations (Fig. 7a); this resulted from an increase in the leaf area per leaf (Fig. 7b) and the number of leaves on the main shoot (Fig. 7c). At very low P supply rates, leaf area was lower at high light than at low light. However, when compared at a certain plant P concentration, plants grown at high light always had a higher leaf area than those grown at low light (Fig. 7a). At low P supply rates, plant P concentration was too low to support leaf area production. Since no leaves were shed, senescence of leaves did not affect these data.
In exponentially growing plants, growth parameters remain constant when plants are in steady-state (Van der Werf & Nagel 1996). However, many fast-growing plants, such as tomato plants, only have a very short period of exponential growth, and parameters can change with time and plant size. The experiment described in this paper was designed such that the effects of plant size and of the applied treatments could be distinguished. Figure 4 illustrates that the trends of the relations of the different growth parameters with increasing plant P concentration were the same whether the data were analysed for plants of the same dry-mass interval (0·4–0·8 g) or for plants of the same age (31 DAS). Furthermore, GRCs calculated for plants of the same dry-mass interval were virtually the same as those calculated for plants of the same age (compare Table 4 with Fig. 6). This justifies the use of plants of one age in this study. Thus, the data discussed here concern the plants harvested at 31 DAS.
Table 4. Growth response coefficients calculated for plants in the same size interval (0·4–0·8 g DM)
Growth response coefficient
In general, the effects of P limitation on growth were more pronounced at high light intensity than at low light. The saturation curve found in this study (Fig. 4a) is common for the relationship between the concentration of a nutrient in a plant and growth (Bates 1971). The marginal increase in plant P concentration at limiting P supply rates found in this study (Fig. 1a) has not been reported before. Ericsson & Ingestad (1988) and Ryser et al. (1997) reported a consistent increase in plant P concentration with increasing P supply rate. The same has been reported for the plant N concentration of plants grown at N limitation (Agrell et al. 1997; Sims, Seemann & Luo 1998). The sharp increase in the first part of the relation between plant P concentration and RGR (Fig. 4a) shows that extra P is used mainly for extra growth and not to increase the plant P concentration. The data suggest that the plants reached a minimum plant P concentration because of the low P-supply rates used in this study. The increase in plant P concentration in the second phase of the curve, when RGR levelled off (Fig. 4a), can be attributed to the accumulation of inorganic phosphorus (Pi) in the vacuole (Bieleski 1973; Lauer, Blevins & Sierzputowska-Gracz 1988; Ratcliffe 1994) and to a higher concentration of P incorporated in organic compounds in the cell. With the same low-P treatment, RGR was lower at high light than at low light. However, when compared at a certain plant P concentration, the RGR of high-light-grown plants was always higher than that of low-light-grown plants (Fig. 4a). Plants had a higher plant P concentration at low light than at high light (data not shown).
NAR and LAR
The increase in RGR with increasing plant P concentration was associated with an increase in both NAR (Fig. 4b) and LAR (Fig. 4c), with NAR playing a more important role than LAR, especially at high light (Fig. 6).
In a meta-analysis of the literature, consisting of 75 observations, it was reported that on average, LAR is more important than NAR in explaining the effects of nutrient limitation on growth (Poorter & Nagel 2000). The same can be concluded when only the observations in which P is the limiting nutrient are considered (H. Poorter, personal communication). However, for individual cases, NAR can also be more important than LAR in explaining the effects of nutrient or P limitation on growth (Peace & Grubb 1982; Corré 1983; Poorter & Nagel 2000) or can be of equal importance (Baas, Van der Werf & Lambers 1989). In this study, when a smaller range of P limitation was considered [by skipping the lowest treatment (70 mg g−1 d−1)], NAR and LAR became equally important at both light intensities (Table 5). Skipping the two lowest P treatments (70 and 120 mg g−1 d−1) caused GRCLAR to increase at high light; GRCNAR did not change (Table 5). At low light, the two lowest P-supply treatments were the only ones causing a reduction in RGR (Fig. 4a). Without these treatments, there is no effect of P limitation on RGR. In order to address the importance of different growth parameters in determining the effect of P limitation on RGR, there has to be an effect of P limitation on RGR. For this reason, the GRC analysis is not carried out at low light with the two lowest treatments skipped. Despite the fact that the method of calculating GRCs at high light is not exact when the two lowest P-supply treatments are skipped (GRCNAR+ GRCLAR= 1·34), it shows that the relative importance of LAR increases dramatically just by decreasing the severity of P limitation. Therefore, the relative importance of LAR and NAR in determining P effects on growth changes with the severity of P limitation. Mild P limitation mainly affects LAR, while severe P limitation mainly affects NAR. This may account for the large variation in the relative importance of NAR and LAR in explaining the effects of nutrients on growth, as reported in the literature (Peace & Grubb 1982; Corré 1983; Baas et al. 1989; Biere 1996; Poorter & Nagel 2000).
Table 5. Growth response coefficients calculated after skipping no, one or the two lowest P supply treatment at 31 DAS
P supply rate
No P limitation therefore GRC could not be calculated.
When considering the effects of light intensity on growth at sufficient P supply, NAR is more important than LAR in explaining the effects of light on RGR. This agrees with earlier reports (Poorter & Nagel 2000). Under P limitation, NAR remains more important than LAR (Table 2). P limitation does not change the relative importance of NAR and LAR in explaining the effects of light on RGR.
At low light, it is important to maximize the photosynthetically active area by maximizing LAR. This can be achieved by increasing the specific leaf area and/or the dry-mass partitioning to the leaves (Björkman 1981). In this study, LMR decreased in low-light compared with high-light plants (Fig. 4d), while the dry mass partitioned to the stem increased (data not shown). The photosynthetically active area was increased at low light by an increase in SLA (Fig. 4e). Less starch was accumulated at low light than at high light (Fig. 5), which may have contributed to the higher SLA at low light.
SLA plays a more important role than LMR in explaining effects of P limitation on growth (Fig. 6). The light effect on SLA is smallest at low P and the P effect on SLA is smallest at high light (Fig. 4e). When light is not limited, plants do not have to invest in leaf area and the SLA will decrease. At low nutrient levels, plants will not invest in leaf mass, leading to a reduced SLA (Meziane & Shipley 1999).
Although LMR decreased consistently with P limitation at high light (Fig. 4d), LMR played only a minor role in explaining the effects of P limitation on growth (Fig. 6). To be able to draw a conclusion regarding dry-mass partitioning in general, the relative importance of RMR in determining the effects of P limitation on growth has to be known. As proposed by Poorter & Nagel (2000), RGR was factorized into components – P uptake per unit root dry mass, dry-mass partitioning to the roots and plant P concentration – to judge the importance of RMR. Plant P concentration was not constant as assumed (Garnier 1991), and consequently sum GRC is 0·82 at high light and 1·38 at low light. Nevertheless, the most important factor is clearly the net P uptake rate at both high and low light. RMR is of secondary importance in explaining the effects of P limitation on growth (Table 3). These data could indicate that dry-mass partitioning is not important in determining the effects of P limitation on growth.
Can starch accumulation explain the increase in %LDM and the decrease in SLA with increasing P limitation and increasing light? At mild P limitation (plant P concentration > 3 mg g−1), more starch accumulated at high light than at low light and at both light intensities, the starch accumulation decreased with increasing plant P concentration (P < 0·05, Fig. 5). This is consistent with earlier reports (Gent 1986; Fredeen et al. 1989; Rao & Terry 1989; Sims et al. 1998). A proposed mechanism for starch accumulation at mild P limitation is that cytosolic Pi is exchanged against triose P in the chloroplast, and hence is needed to allow export of triose P out of the chloroplast into the cytosol (Plaut, Mayoral & Reinhold 1987; Stitt 1991). At P limitation, the Pi concentration drops, the triose P exchange rate declines and the triose P concentration in the chloroplast will increase. This triose P is then used for starch synthesis in the chloroplast (Plaut et al. 1987; Stitt 1991). However, plants with antisense repression of the triose P translocator could counteract the decrease in triose P transport with a higher turnover rate of starch and so prevent (tobacco plants, Haüsler et al. 1998) or reduce (potato plants, Heineke et al. 1994) starch accumulation.
At low light, starch accumulation explains 29% of the increase in %LDM and 14% of the decrease in SLA with decreasing plant P concentration. At high light and mild P limitation, starch accumulation explains 54% of the increase in %LDM, but does not explain the decrease in SLA at mild P limitation. At high light and severe P limitation (plant P concentrations < 3 mg g−1), the starch concentration decreased sharply at high light (P < 0·05, Fig. 5) and cannot explain the decrease in SLA and the increase in %LDM. A possible explanation for the decrease in starch accumulation at severe P limitation is the reduced rate of photosynthesis (Plaut et al. 1987; Stitt 1991). As a consequence of this decreased supply of photoassimilates, the starch concentration will decrease. The decrease in starch concentration at high light and at plant P concentrations below 3 mg g−1 (Fig. 5) suggests that at this severe P limitation, growth is limited by a reduction in net leaf photosynthesis. Net leaf photosynthesis decreases with decreasing plant P concentration (De Groot et al. 2001). The sharp decrease in starch concentration at severe P limitation coincided with an increase in %LDM (Fig. 4f) and a decrease in SLA (Fig. 4e). Consequently, at severe P limitation, the increase in %LDM and the decrease in SLA are not due to starch accumulation. Nutrient limitation can decrease leaf cell size. The relatively greater amount of cell wall polymers of those smaller cells can decrease SLA (Van Arendonk et al. 1997). Another explanation for the decrease in SLA might be the negative effects of nutrient limitation (in this case P) on the hydraulic conductance of the roots (Radin & Eidenbock 1984; Carvajal, Cooke & Clarkson 1996). The decreased root hydraulic conductance might increase water deficit (Radin & Boyer 1982) and, consequently, decrease leaf expansion (Radin & Boyer 1982; Radin & Eidenbock 1984). Therefore, the reduction of SLA at low P might be the result of P effects on water relations.
Under P limitation, extra P is used for more growth, rather than to increase the plant P concentration.
Dry-mass partitioning to the leaves does not seem to play an important role in explaining the effects of P limitation on growth.
The relative importance of the morphological component (LAR) and the physiological component (NAR) changes with the severity of P limitation: mild P limitation mainly affects LAR, while severe P limitation mainly affects NAR.
At mild P limitation, transport and/or utilization of assimilates in growth, not the production of assimilates, results in an increase in starch accumulation. At severe P limitation, the production of assimilates is limited.
We are grateful to J.A. Dieleman, L.H.W. van der Plas, H. Poorter and B.W. Veen for reading the manuscript critically, and to E. Cabanes for her help with the experiments.