Annals of Botany 81 : 195–201, 1998 Dry Weight and N Partitioning in Relation to Substrate N Supply, Internal N Status and Developmental Stage in Jack Pine (Pinus banksiana Lamb.) Seedlings : Implications for Modelling W E I X I N G T A N* and G A R Y D. H O G A N Natural Resources Canada, Canadian Forest Serice, Ontario Region, P.O. Box 490, 1219 Queen St. E., Sault Ste. Marie, Ontario, Canada, P6A 5M7 Received : 3 September 1996 Returned for revision : 4 February 1997 Accepted : 9 September 1997 Dry weight and nitrogen (N) partitioning of sand-cultured young jack pine (Pinus banksiana Lamb.) seedlings under controlled environments were studied 3, 6, 9, 12 and}or 15 weeks after the initiation of six dynamic N supply treatments. The supply of other nutrients was maintained at optimal levels. Total dry weight varied widely among treatments and whole plant total N concentration ranged from 10 to 32 mg g−" d. wt at most sampling intervals. Whole plant N concentration changed, with time, according to three distinct patterns : (1) stable ; (2) rapidly increasing ; or (3) gradually declining. Regardless of N treatment and sampling interval, whole plant N concentration was linearly and positively correlated with root, needle and stem N concentration. Dry weight and N weight ratios of needles declined, whereas those of roots increased linearly with decreasing whole plant N concentration (r# ¯ 0±43 to 0±76) regardless of N regime. Dry matter partitioning to stems, however, was better explained by developmental stage than by whole plant N concentration. With the decline in internal N status, N was increasingly concentrated in roots at the expense of needles and stems. These results suggest : (1) dry weight and N partioning may be largely a function of the internal N status of plant rather than root and shoot activities ; (2) both shoot and root specific activities may have a close, positive association with whole plant N concentration ; (3) N-partitioning may be an active process itself and may warrant separate consideration from dry weight ; and (4) developmental stage may be a significant determinant of partitioning, particularly to stems. # 1998 Annals of Botany Company Key words : Developmental stage, dry weight, internal N status, jack pine, modelling, nitrogen, partitioning, Pinus banksiana, substrate N supply. I N T R O D U C T I ON The nutrient status of a plant has been recognized as an important factor in the determination of dry weight partitioning (Levin, Mooney and Field, 1989 ; A/ gren and Wikstro$ m, 1993). Because of its role in regulating plant growth and partitioning, nitrogen (N) has been included in most recent attempts to model dry weight allocation (e.g. Reynolds and Thornley, 1982 ; Johnson, 1985 ; A/ gren and Ingestad, 1987 ; Levin et al., 1989 ; Hilbert, 1990 ; Burns, 1994 ; Luo, Field and Mooney, 1994). Although these models vary in design, and could be generally described as empirical, teleonomic, or mechanistic (Thornley, 1991), they contain empirical and}or simplified assumptions, or hypotheses, that relate to N status (A/ gren and Wikstro$ m, 1993). Firstly, in models using the functional balance approach, it is assumed that partitioning between roots and shoots takes place in the context of a balanced ratio of root N uptake activity, to shoot carbon (C) uptake activity (e.g. Davidson, 1969 ; Hilbert, 1990 ; Cannell and Dewar, 1994). Secondly, in opposition to the first assumption, it has been suggested that shoot}root ratio relates to internal N and C status (Johnson, 1985). Thirdly, it is assumed that the * For correspondence. Fax 705 759 5700 0305-7364}98}02019507 $25.00}0 balance between roots and shoots is maintained through changes in the partitioning of dry weight, and shoot and}or root specific activities remain unchanged (Hilbert and Reynolds, 1991). Lastly, N and}or C concentrations are often assumed to be similar throughout the whole plant (e.g. Johnson, 1985 ; Hilbert, Larigauderie and Reynolds, 1991 ; Hilbert and Reynolds, 1991). Although these assumptions appear to be biologically sound, careful testing against experimental results over a wide range of N supply}internal N conditions is warranted. By manipulating N supply regime while maintaining a common, optimal supply of other nutrients, the current study sought to achieve a wide range of internal N conditions at different ages and sizes in young jack pine (Pinus banksiana Lamb.) seedlings. The study had two objectives : (1) to examine how changes in N supply (external), plant N status (internal) and developmental stage (internal) could affect plant dry weight and N partitioning interactively, and (2) to use the results to examine various assumptions commonly applied in modelling dry weight and N allocation. It was considered that a study of this kind was needed to improve current understanding, and promote further discussion, since none of the existing allocation models could satisfy current requirements (A/ gren and Wikstro$ m, 1993). bo970539 # 1998 Annals of Botany Company 196 Tan and Hogan—Dry weight and N Partitioning in Jack Pine Jack pine (Pinus banksiana Lamb.) seedlings were grown from pre-germinated seeds in plastic tubes (SC-10 Leach tube ; 143 cm$ ; Stuewe & Sons Inc., Corvallis, OR, USA) containing sand free from organic matter, as described previously (Tan and Hogan, 1995). They were kept in a controlled environment room under the following day}night conditions : temperature, 24}18³1 °C ; relative humidity, 60}80³10 % ; photoperiod, 16}8 h ; and PPFD, 450 µmol m−# s−" at the top of plastic tubes. The seedlings were watered four times daily during the photoperiod using a semi-automatic, drip irrigation system. The amount of solution applied in each irrigation (100 cm$ for the first daily irrigation and 50 cm$ for the others each day) was more than adequate to saturate the substrate (Tan and Hogan, 1995). To achieve a precise control over nutrient supply, deionized (reverse osmosis) water containing less than 0±03 g m−$ of major nutrients was used throughout the experiment (Tan and Hogan, 1995). Seedlings were grown without the addition of nutrients (deionized water only) for 2 weeks after germination, at which point they had an average dry weight of 10±4 mg per seedling, and an N concentration of 12±9 mg g−" d. wt. N supply treatments The nutrient solutions used in this experiment ensured that N (NH NO ) concentrations varied while maintaining % $ a common, optimal availability of other macro- and micronutrients, as detailed previously (Tan and Hogan, 1995). The balance of nutrients, relative to 100 g m−$ N, followed the suggestion of Ingestad (1979) for pine species. Six different (dynamic) rates of N supply were applied as shown in Fig. 1. Solution N concentration increased gradually every 2 d in four treatments : in treatments I and II, N supply increased linearly from the initial concentrations of 46±4 and 27±8 g N m−$ during week 1 to 86±3 and 40±3 g N Solution N conc. (g m–3) 80 I II 40 20 V 15 VI III 10 5 0 IV 3 6 9 12 Weeks after N addition 15 V 2.5 I III II 2.0 IV 1.5 VI 1.0 0.5 0 3 6 9 12 15 18 15 18 35 30 I II 25 III 20 IV V 15 VI 10 5 0 3 6 9 12 Weeks after N addition F. 2. Time courses of total dry weight (g) per seedling and whole plant N concentration (mg g−" d. wt) in jack pine seedlings subjected to six N supply treatments. Vertical bars are s.e.m. (n ¯ 6) and those smaller than symbols are not shown. See text for details of treatments I to VI. m−$, respectively, during week 9 ; in treatment III, it increased linearly from a concentration of 9±2 g N m−$ during week 1 to 11±8 g N m−$ during week 12 ; and in treatment IV it increased linearly from 3±1 g N m−$ (week 1) to 3±4 g N m−$ (week 15) (Fig. 1). In contrast, the solution N concentration increased exponentially for treatments V (10 % d−") and VI (6 % d−") from the initial 0±17 and 0±10 g N m−$, respectively, to 100 g N m−$ for treatment V during week 12, and 50±2 g N m−$ for VI during week 15. The use of different initial N concentrations, and increasing rates in each treatment, was intended to achieve : (1) variation in internal N status with age (or size) ; and (2) variation in the temporal pattern of internal N concentration (i.e. increasing, declining or unchanging (stable) with time ; see Fig. 2). 100 60 3.0 0.0 Whole plant N conc. (mg g–1 DM) Plant culture Total dry weight per seedling (g) MATERIALS AND METHODS 18 F. 1. Time courses of solution N concentration (g m−$) in six different treatments initiated 2 weeks after the germination of jack pine seedlings. See text for details of treatments I to VI. Seedling harest and nutrient analysis Six seedlings from each of the six N treatments were harvested 3, 6, 9, 12 (four treatments only) or 15 (two treatments) weeks after the start of N treatments, depending 197 Tan and Hogan—Dry weight and N Partitioning in Jack Pine 35 r 2 = 0.961 30 25 Needle on the amount of N supplied and growth rate (Figs 1 and 2). Seedlings were dried for 48 h at 80 °C, separated into needles, roots and stems and weighed. Total N concentration (mg N g−" d. wt) of needle, root and stem dry samples was determined by micro-Kjeldahl digestion, and needle major macro-nutrients (for week 9 samples only) by plasma spectrometry (Tan and Hogan, 1995). Dry weight (g g−") or N weight (g N g N−") ratios of needles, roots and stems were calculated as ratios of component dry, or N, weight to total dry, or N, weight in each seedling. Nitrogen concentration of the whole plant was determined from needle, root and stem component dry weights and N concentrations. 20 15 10 Data analysis RESULTS Growth and internal nutrient status Significant differences in total dry matter accumulation found among treatments reflected the differences in N supply. From weeks 3 to 9, seedlings in treatment I (high N supply) were the heaviest, whereas those in treatments V and VI (low initial N supply) had the lowest rates of growth (Figs 1 and 2). By week 3, total dry weight per seedling in treatment I (0±06 g) was three times that of treatment VI (0±02 g) and the difference had increased to more than 16fold by week 9 (2±11 and 0±13 g, respectively ; Fig. 2). During this period the seedlings in treatments III and IV had intermediate growth rates, but the dry weight per seedling was invariably greater in treatment III than in IV. Although N supply in treatment I was double that in II (Fig. 1), dry weight per seedling did not differ between these treatments from weeks 3 to 9 (Fig. 2). From weeks 9 to 12, seedling growth in treatment V increased sharply in response to the increase in substrate N supply, such that the seedling dry weight at week 12 (2±71 g) surpassed that of treatments III (2±06 g) and IV (1±28 g). Seedlings in treatment VI remained the smallest at week 12 (0±45 g per seedling) despite the large increase in N supply from around week 9. The total dry 40 N concentration (mg g–1 DM) Root r 2 = 0.805 35 30 25 20 15 10 35 r 2 = 0.722 30 25 Stem The differences among treatments in total dry weight and whole plant N concentration were determined by analysis of variance, and the least significant difference between means by Duncan’s multiple range test. Means of whole plant N concentration in each treatment, and sampling interval, were correlated with the respective means of needle, root or stem N concentration. Means of dry weight and N weight ratios for needles, roots and stems were analysed against means of whole plant N concentration by regression. To determine whether plant developmental stage (age and size) could be related to partitioning, dry weight and N weight ratios of needles, roots and stems were also examined against sampling interval, and the reciprocal of total dry weight, by analysis of covariance using whole plant N concentration as a covariate. The relative contribution of the reciprocal of total dry weight to total variation of the ratios was calculated as a proportion of sequential sum of squares to total sum of squares (Searle, 1971 ; SAS, 1988). All the analyses were completed using general linear model and regression procedures from SAS}PC software (SAS, 1988). 20 15 10 5 5 10 15 20 25 30 Whole plant N conc. (mg g–1 DM) 35 F. 3. Correlations between whole plant N concentration and needle, root and stem N concentration (mg g−" d. wt) in jack pine seedlings subjected to six N supply treatments (+, I ; *, II ; y, III ; x, IV ; E, V ; D, VI ; see text for details) for 3 to 15 weeks. Each point represents the mean of six seedlings. The regression equations are : needles, y ¯®1±1061±063 x, P ! 0±0001 ; roots, y ¯ 2±7820±922 x, P ! 0±0001 ; and stems, y ¯®4±7301±021 x, P ! 0±0001. weight per seedling was similar in treatments IV (1±64 g) and VI (1±45 g) at week 15, but remained lower than that reached in treatments I and II in 9 weeks, and treatments III and V in 12 weeks (Fig. 2). By manipulating substrate N supply in sand-culture, this experiment generated three distinct temporal patterns of whole plant N concentration : (1) stable over time (treatments I and II) ; (2) declining (III and IV) ; and (3) increasing 198 Tan and Hogan—Dry weight and N Partitioning in Jack Pine 1.0 2 r = 0.589 0.8 0.8 0.6 0.6 Needle Needle 1.0 0.4 0.2 2 r = 0.432 0.4 0.2 0.0 0.0 r 2 = 0.755 r 2 = 0.610 0.8 Nitrogen weight ratio Root Dry weight ratio Root 0.8 0.6 0.4 0.2 0.6 0.4 0.2 0.0 0.0 r 2 = 0.158 2 r = 0.313 0.6 0.6 Stem 0.8 Stem 0.8 0.4 0.4 0.2 0.2 0.0 5 10 15 20 25 30 Whole plant N conc. (mg g–1 DM) 35 0.0 5 10 15 20 25 30 Whole plant N conc. (mg g–1 DM) 35 F. 4. Correlations between whole plant N concentration (mg g−" d. wt) and needle, root and stem dry weight ratios in jack pine seedlings subjected to six N supply treatments (+, I ; *, II ; y, III ; x, IV ; E, V ; D, VI ; see text for details) for 3 to 15 weeks. Each point represents the mean of six seedlings. The regression equations are : needles, y ¯ 0±4670±00677 x, P ! 0±0001 ; roots, y ¯ 0±446®0±00829 x, P ! 0±0001 ; and stems, y ¯ 0±08690±00152 x, P ! 0±0542. F. 5. Correlations between whole plant N concentration (mg g−" d. wt) and needle, root and stem N weight ratios in jack pine seedlings subjected to six N supply treatments (+, I ; *, II ; y, III ; x, IV ; E, V ; D, VI ; see text for details) for 3 to 15 weeks. Each point represents the mean of six seedlings. The regression equations are : needles, y ¯ 0±4440±00817 x, P ! 0±0001 ; roots, y ¯ 0±515®0±01063 x, P ! 0±0001 ; and stems, y ¯ 0±04190±00246 x, P ! 0±0045. (V and VI) (Fig. 2). The difference in whole plant N concentration among treatments I to IV was in the order : I " II " III " IV throughout the entire experiment, reflecting the differences in substrate N supply (Figs 1 and 2). Similarly, the seedlings in treatment V maintained a higher whole plant N concentration than VI. A wide range of whole plant N concentrations was achieved at most sampling intervals. For example, an almost three-fold difference among treatments in whole plant N concentration (10–30 mg g−" d. wt) was obtained at weeks 3, 9 and 12 (Fig. 2). The concentrations of total P, K, Ca, Mg and S in needles at week 9 exceeded 2±2, 8±6, 1±1, 1±2 and 1±7 mg g−" d. wt, respectively, in all samples. Needle, root and stem N concentrations increased linearly with increasing whole plant N concentrations (Fig. 3). However, the slopes for needles and stems were slightly Tan and Hogan—Dry weight and N Partitioning in Jack Pine T 1. Effects of sampling interal or the reciprocal of seedling total dry weight (TDM −", g−") on dry weight and N weight ratios of needles, roots and stems in young jack pine seedlings using whole plant N concentration (mg g−" d. wt) as a coariate F-ratio Ratio Dry weight ratio N weight ratio Component Interval TDM−" Needle Root Stem Needle Root Stem 1±44 1±52 4±25** 4±87** 3±11* 2±98* 6±71** 0±12 19±00*** 19±12*** 17±76*** 0±83 *** P ! 0±01, ** P ! 0±05, * P ! 0±1. greater than unity (1±063 and 1±021, respectively) whereas that for roots was less than unity (0±922). Stem N concentration was consistently lower than that of the whole plant by 3 to 5 mg g−" d. wt (Fig. 3). Dry matter and N partitioning in relation to whole plant N The relationships between whole plant N concentration and dry weight ratio of needles, roots and stems were best described as linear for all samples, regardless of substrate N supply and sampling interval (Fig. 4). The variation in whole plant N concentration explained up to 59 and 76 % of the variation in needle and root dry weight ratios, respectively. However, stem dry weight ratio was only weakly correlated with whole plant N concentration (r# ¯ 0±146). A decrease in whole plant N concentration from 32 to 10 mg g−" was associated with a decline in needle dry weight ratio from 0±677 to 0±538, but a doubling of root dry weight ratio (from 0±190 to 0±358). Stem dry weight ratio varied from 0±069 to 0±171 and appeared to increase slightly with increasing whole plant N concentration (Fig. 4). Nitrogen partitioning followed a qualitatively similar, but quantitatively different, pattern from that of dry weight (Figs 4 and 5). Although needle N weight ratio decreased (r# ¯ 0±432), and root N weight ratio increased (r# ¯ 0±610) linearly with declining whole plant N concentration (Fig. 5), the slopes of the lines for N weight ratio (0±00817 for needles and ®0±01063 for roots) were steeper than those for dry weight ratio (0±00677 and ®0±00829 for needles and roots, respectively). Stem N weight ratio was related weakly to whole plant N concentration (r# ¯ 0±313) and varied from 0±054 to 0±156 (Fig. 5). Dry matter and N partitioning in relation to deelopmental stage When the effect of internal N status was taken into consideration by using whole plant N concentration as a covariate, the sampling interval (age) still had a slightly significant effect (P ! 0±1) on jack pine N weight partitioning, but the more significant effect (P ! 0±05) was for dry weight partitioning to stems (Table 1). The interaction 199 between sampling interval and whole plant N concentration was non significant except for the stem N weight ratio (data not shown). The reciprocal of total dry weight contributed significantly to the variation of needle and stem dry weight ratios and needle and root N weight ratios (Table 1). The relative contribution, however, was small for needle and root dry and N weight ratios, ranging from 1 to 20 %. By contrast, the explained variation in stem dry weight ratio increased from 16 %, when whole plant N was the sole variable (Fig. 4), to 61 % when the reciprocal of total dry weight per seedling was also considered. The interaction between the reciprocal of total dry weight and whole plant N concentration was non significant (data not shown). D I S C U S S I ON Jack pine seedlings allocated their dry matter and N between needles and roots largely on the basis of internal N status, as can be seen from the significant linear relationships between the dry weight and N weight ratios and whole plant N concentration (Figs 4 and 5). These relationships were maintained, regardless of N supply, plant age and size, and temporal change in whole plant N concentration (Figs 1–5) and spanned the range of critical, low and adequate foliar N concentrations in young jack pine seedlings (Swan, 1970). Similar linear relationships have been reported for a wide variety of plant species under various culture and environmental regimes (Hirose and Kitajima, 1986 ; A/ gren and Ingestad, 1987 ; Hirose, 1987 ; Hirose, Freijsen and Lambers, 1988 ; Levin et al., 1989 ; Ingestad and A/ gren, 1991 ; Boot, Schildwacht and Lambers, 1992 ; Pettersson, McDonald and Stadenberg, 1993). The linear relationship is consistent with the prediction of Hilbert (1990) that plants function optimally with respect to the response of dry matter partitioning to substrate N availability (Findenegg, 1990). Collectively, these results support the hypothesis of Johnson (1985) and Ingestad and A/ gren (1991) that dry matter partitioning is largely a function of the internal status of plants. Root and shoot activities may play an indirect role by changing plant internal status (e.g. N and C), rather than controlling partitioning directly as originally proposed by Davidson (1969). The internal N status of plants has been described using substrate (e.g. Thornley, 1972 ; Johnson, 1985 ; Thornley, 1991) or total (structuresubstrate) N concentrations (e.g. A/ gren and Ingestad, 1987 ; Hilbert, 1990). An evaluation of N status based on substrate concentration allows investigators to relate the processes mechanistically to enzyme kinetics (Cannell and Dewar, 1994) but it is conceptually elusive, extremely dynamic, and difficult to quantify. Use of total N concentration, on the other hand, brings practical advantages : it can easily be determined, is analytically accurate, and is relatively stable over the short term. Both approaches, however, describe plant partitioning reasonably well, suggesting that there is a potential functional relationship between total plant and internal substrate N status. In support of this hypothesis, an almost linear relationship has been found between total and nitrate N concentrations in tissues of potato (Solanum tuberosum L.) 200 Tan and Hogan—Dry weight and N Partitioning in Jack Pine (Biemond and Vos, 1992) and spinach (Spinacea oleracea L.) (Smolders and Merckx, 1992). The relationship between dry matter partitioning and plant total N concentration (e.g. Figs 1–4) found here provides further support, but a better understanding of the mechanisms of the interactions among external N supply, internal N reserves, substrate dynamics, and partitioning function is needed to improve the simulation of dry matter and N partitioning in plants (Cannell and Dewar, 1994). Developmental changes in partitioning, as a function of plant age and size, were evident in young, woody jack pine seedlings (Table 1). In fact, the dry matter partitioning to stems may be more a function of developmental stage than internal N status. Similar results can be derived from the work of Hirose (1986) and Hirose and Kitajima (1986) using Polygonum cuspidatum Sieb et Zucc. Recalculation of their results showed that the percentage of the explained variation in stem dry weight ratio increased from less than 1 %, when N was the sole linear variable (Hirose and Kitajima, 1986), to approximately 48 % when the reciprocal of total dry weight was also included in the regression. These results agree qualitatively with previous findings from several other plant species (e.g. Ledig, Bormann and Wenger, 1970 ; Wilson, 1988 ; Tan, Blake and Boyle, 1995) and highlight the importance of considering the effect of developmental stage on partitioning even in young plants, an aspect that has been neglected by many modellers. The developmental shift in allocation to stems in woody trees deserves more attention because of its low association with N (Figs 4 and 5, Table 1) and the relatively long life span of trees. Jack pine needle, root and stem N concentrations all increased linearly with increasing whole plant N concentration. This agrees with results for birch (Betula pendula Roth.) (Ingestad, 1979 ; Ingestad and Lund, 1979), Holcus lanatus L. and Festuca oina L. (Kachi and Rorison, 1989), and partially with those obtained for potato (Biemond and Vos, 1992). Although a positive association between leaf (shoot) specific activity for C uptake (g C m−# d−" ; Hilbert et al., 1991) and leaf N concentration has been well established (e.g. Evans, 1989 ; Tan and Hogan, 1995), a relationship between root specific activity for N uptake [g N (g root)−" d−" ; Hilbert et al., 1991] and root N status remains to be shown (A/ gren and Wikstro$ m, 1993). From the data of Ingestad (1979) and Ingestad and Lund (1979) using birch, however, it can be shown that root specific activity was positively related to root N concentration. Similar results could be calculated from the present experiment (data not shown). Moreover, most nutrient ions do not enter the root by simple diffusion (e.g. Gleeson, 1993 ; Luo et al., 1994) : instead several important nutrient ions move into plants through the root symplasm, requiring enzyme activity, and therefore N, for uptake (e.g. Comerford, Smethurst and Escamilla, 1994). It can therefore be argued that root and shoot specific activity usually increase with increasing whole plant N status. Root specific activity may also relate to the external supply of nutrients (A/ gren and Wikstro$ m, 1993). This contradicts a very common assumption that root and}or shoot specific activities remain constant in the event of balanced activities of shoots and roots (e.g. Thornley, 1972 ; Reynolds and Thornley, 1982 ; Johnson, 1985 ; Hilbert and Reynolds, 1991 ; Luo et al., 1994). If root specific activity is interactively controlled by internal N status and external N supply, this has important implications for current model predictions. For instance, the assumption of a constant root specific activity, in an effort to achieve a balance in shoot}root activities, could lead to an over- or under-estimation of dry matter partitioning to roots. Future research should seek to clarify the functional relationship among root specific activity, internal nutrient status and external nutrient supply (A/ gren and Wikstro$ m, 1993). As internal N status declined, N was retained in roots at the expense of needles and stems. This was apparent in N concentration ratios of root : needle : stem, and in larger differences in the change in needle and stem (decrease), or root (increase), N weight ratio per unit decrease in whole plant N concentration, when compared to those of dry weight ratio (Figs 4 and 5). Similar results were found in P. cuspidatum (Hirose and Kitajima, 1986). These results suggested that : (1) the co-ordination of N distribution within plants may be an active process, independent of, and}or in association with, dry matter partitioning ; and (2) roots may take priority over shoots not only in relation to dry matter but also, increasingly, in N partitioning under Nlimited conditions. This contrasts with the commonly-held assumption that N is passively and uniformly distributed over the whole plant (e.g. Reynolds and Thornley, 1982 ; Johnson, 1985 ; Hilbert et al., 1991) and suggests that N partitioning may merit separate consideration from dry weight during modelling. Recent biochemical evidence indicates that when internal N status changes, the processes of photosynthesis receive unequal proportions of the distributed N (Evans, 1989), thereby limiting net photosynthesis to various degrees (Tan and Hogan, 1995). Gleeson (1993) has recently demonstrated a preliminary attempt to combine C and N allocation into a single model. In conclusion, these results suggest that some assumptions currently employed in modelling dry weight}matter and N partitioning processes in plants may be too simplistic, and should be reconsidered. First, dry matter and N partitioning may be largely a function of the internal N status of plants, rather than root and shoot activities, which may affect partitioning indirectly by changing internal status (N, C etc). Second, both shoot and root specific activities may have a close, positive association with whole plant N status. Third, N partitioning may be an active process itself and therefore deserve separate consideration from dry weight. These results also demonstrate a significant developmental shift in partitioning towards stems, a process that has been largely neglected in most modelling attempts. A C K N O W L E D G E M E N TS We thank J. Ramakers for his help in conducting nutrient analysis and B. Borland in building our semi-automatic nutrient delivery system. 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