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 SerŠice, 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 harŠest 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 interŠal 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 coŠariate
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 deŠelopmental
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 oŠina 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.
L I T E R A T U R E C I T ED
AI gren GI, Ingestad T. 1987. Root : shoot ratio as a balance between
nitrogen productivity and photosynthesis. Plant, Cell and EnŠironment 10 : 579–586.
Tan and Hogan—Dry weight and N Partitioning in Jack Pine
AI gren GI, Wikstro$ m JF. 1993. Modelling carbon allocation—a review.
New Zealand Journal of Forestry Science 23 : 343–353.
Biemond H, Vos J. 1992. Effects of nitrogen on the development and
growth of the potato plant. 2. The partitioning of dry matter,
nitrogen and nitrate. Annals of Botany 70 : 37–45.
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