Annals of Botany 83 : 631–636, 1999
Article No. anbo.1999.0872, available online at http:\\www.idealibrary.com on
Relationships Between Nitrogen and Water Concentration in Shoot Tissue of
Molinia caerulea During Shoot Development
B. T H O R N T O N*†, G. L E M A I RE‡, P. M I L L A RD* and E. I. D U F F§
* Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK, ‡ Station d’Ecophysiologie,
INRA, 8660 Lusignan, France and § Biomathematics and Statistics Scotland, Macaulay Land Use Research
Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK
Received : 16 November 1998
Returned for revision : 20 January 1999
Accepted : 15 February 1999
Plants of Molinia caerulea were supplied with either a low (0n2 mol m−$) or high (10 mol m−$) supply of nitrogen over
two growing seasons. A total of 14 destructive plant harvests were made : when plants were in an over-wintering state
prior to the second season ; immediately following bud burst ; and on 12 further occasions throughout the second
season. The relationships between shoot nitrogen concentration on a dry mass basis, shoot water content and plant
developmental stage were investigated. Shoot nitrogen concentration on a dry mass basis fell as the growing season
progressed. In contrast, the concentration of nitrogen in tissue water after bud burst showed only a slight reduction.
The concentration of nitrogen both on a dry mass basis and in tissue water was greater for plants receiving the higher
supply of nitrogen. Shoot water content was highest immediately following bud burst then declined as the season
progressed, with plants receiving the low nitrogen supply having slightly greater shoot water contents. It was
concluded that the decline in shoot nitrogen concentration of M. caerulea on a dry mass basis as the mass increased
was mainly explained by changes in shoot water content. The observed increase in the rate of decline of both shoot
nitrogen concentration and water content with increased shoot mass coincided with the cessation of leaf tissue
production and was therefore due to a switch from the production of leaves to other tissues.
# 1999 Annals of Botany Company
Key words : Molinia caerulea (L.), purple moor grass, nitrogen, water content, shoot development.
INTRODUCTION
In many plant species the shoot concentration of nitrogen,
expressed on a dry mass basis, declines as shoot dry mass
increases (Caloin and Yu, 1984 ; Leigh and Johnston, 1985 ;
Greenwood et al., 1990, 1991 ; Justes et al., 1994). One
generally accepted explanation for this decline is that, as the
shoot grows and the amount of photosynthetic tissue
increases, proportionally more structural tissue is required
to support it. As structural tissue contains a lower nitrogen
concentration than photosynthetic tissue, this results in an
overall reduction in the nitrogen concentration of the shoot
(Lemaire et al., 1992 ; Lemaire and Gastal, 1997). In barley
(Hordeum Šulgare L.) the nitrogen concentration of the
shoot, expressed both on a dry mass and tissue water basis,
was found to decline as the season progressed (Leigh and
Johnston, 1985). The decline in nitrogen concentration on a
dry mass basis was greater than that in tissue water ; in fact,
the nitrogen concentration of tissue water increased after
anthesis (Leigh and Johnston, 1985). This suggests that
while some of the decline in shoot nitrogen concentration
on a dry mass basis is due to the change in nitrogen
concentration of tissue water, changes in shoot water
content may also have been partly responsible.
Different relationships between shoot nitrogen concen† For correspondence. Fax j44 (0) 1224 311556, e-mail
b.thornton!mluri.sari.ac.uk
0305-7364\99\060631j06 $30.00\0
tration on a dry mass basis and shoot mass exist for isolated
plants and those growing in dense canopies (Lemaire and
Gastal, 1997). A linear relationship, with a negative gradient,
between ln(shoot nitrogen concentration) and ln(shoot
mass) was found for isolated plants of sweet sorghum
during vegetative growth. Individual plants growing in a
dense canopy also gave the same relationship during the
early period of growth, but when the leaf area index of the
crop approached one, a more negative relationship than
that for equivalent isolated plants was observed (Lemaire
and Gastal, 1997). The difference between isolated and
individual plants in a dense canopy was thought to be due
to differing proportions of structural tissue in the plant
shoot. Lemaire and Gastal (1997) considered that, as
competition for light increased, individual plants within
dense canopies developed a greater proportion of structural
tissue, and consequently lower nitrogen concentrations,
than isolated plants of equivalent shoot mass. However,
care should be taken in comparing plants of the same mass
subjected to differing treatments as they may be at different
stages of development.
Molinia caerulea (L.) Moench (purple moor grass) is a
deciduous, perennial grass (Jefferies, 1916 ; Salim et al.,
1988) which is widespread and dominant in many areas
throughout upland Britain (Rodwell, 1991). During winter
the plants have swollen basal internodes with attached buds
and an extensive root system. In spring these buds develop
into new shoot material (Jefferies, 1916). Leaf growth in
# 1999 Annals of Botany Company
632
Thornton et al.—Nitrogen and Water in DeŠeloping Molinia caerulea Shoots
spring is concomitant with mobilization of nitrogen from
basal internodes and roots, but later in the season nitrogen
is withdrawn from leaves and directed towards developing
flowers and storage in basal internodes and roots over
winter (Thornton and Millard, 1993). The relationship
between nitrogen concentration and mass, observed in crop
plants subject to selective breeding, may not be applicable
for unimproved species adapted to less fertile environments.
The relationships between nitrogen concentration (on
both a dry mass and tissue water basis), dry mass and water
content of M. caerulea shoots were examined over a
growing season with two contrasting supplies of nitrogen.
The following hypotheses were tested : (1) the decline in
nitrogen concentration of shoots of M. caerulea, on a dry
mass basis, as their mass increases is partly explained by
changes in shoot water content ; (2) the more rapid decline in
shoot nitrogen concentration previously observed with
plants in dense canopies, compared with isolated plants of
equivalent mass, is due in part to differences in the
developmental stage of the plants.
MATERIALS AND METHODS
Full details of plant collection, conditions of growth, plant
harvesting and sample analysis are given in Thornton and
Millard (1993), and only a summary of relevant details is
presented here. Tussocks of Molinia caerulea in their overwintering state were collected on 19 Apr. 1989, from a fieldsite in the Fife Forest District, Cleish Hills, Scotland, UK.
Tillers were separated and three basal internodes with
attached roots were planted in 18 cm diameter pots
containing expanded mica. The pots were placed, for two
growing seasons, within an open-sided greenhouse, which
protected the plants from rain, but exposed them to natural
variations in environmental conditions. A total of 112 pots
were arranged in a randomized block structure and, at each
subsequent harvest, four replicates of each treatment were
removed. Each pot received 66n6 cm$ of a complete nutrient
solution three times each week throughout the growing
seasons, containing either 0n1 mol m−$ or 5 mol m−$ ammonium nitrate (0n2 or 10 mol m−$ N), and deionized water
when the expanded mica appeared visibly dry. Destructive
harvests were taken when the plants were in an overwintering state between growing seasons (Julian day 31),
immediately following bud burst (Julian day 92), and on
twelve further occasions throughout the second season
(Julian days 102, 117, 129, 142, 157, 177, 184, 198, 212, 226,
240 and 255). At harvest, plants were washed free of the
expanded mica and separated into roots, the original basal
internode planted, basal internodes produced in the first
season, basal internodes produced in the second season,
laminae plus sheaths produced in the second season (live
and dead material) and flowers plus stems. All material was
weighed fresh and after drying at 70 mC, and dried samples
were milled prior to determination of nitrogen concentration using an ANA-SIRA mass spectrometer (VG Isogas,
Middlewich, UK).
Bacon (1988) commented that plants allocate very little
nitrogen to cell walls. The mean nitrogen concentration of
internodes in Lolium species, a tissue in which the cell wall
was estimated to account for 60 % of the dry matter, was
found to be 0n48 % (Wilman and Altimimi, 1982). Presumably the nitrogen concentration of the isolated cell wall
would have been less than 0n48 %. Consequently, in order to
calculate the concentration of nitrogen in tissue water in the
present experiment it was assumed that all the nitrogen was
present in the aqueous phase. This concentration represents
the weighted mean of cytoplasm and vacuole.
All statistical procedures were performed using Genstat 5
Release 3.2 (Lawes Agricultural Trust, IACR-Rothamsted,
UK). The data presented in Figs 1 to 4 were subject to
analysis of variance (ANOVA) with harvest date and
nitrogen supply as treatment factors. To stabilize the
variance, some variables required transformation (shoot
and leaf dry mass, log (nj1) ; nitrogen concentration on a
"!
dry mass basis, angular arcsine). As transformation did not
affect the interpretation of results, untransformed data are
presented for clarity. The broken stick models (Figs 5 and 6)
were fitted by minimizing the residual variance on the
y-axis. Comparison of these models with simple linear
regression models was carried out to assess the improvement
gained by fitting the broken stick models. Using the
estimated slopes and standard errors from the broken stick
models, t-tests were carried out to assess differences between
gradients within models and between gradients in different
models. Similarly, the x-values at which the ‘ break points ’
occurred in different models were compared using t-tests.
RESULTS
At the start of the second growing season, leaf material was
the major component of shoot dry mass, irrespective of the
nitrogen supply (Fig. 1). Leaf growth was complete by
Julian day 177 as no further increase in leaf dry mass
occurred after this date (P 0n05). Shoot growth did
continue beyond Julian day 177 (P 0n001). The contribution of leaf material to the total shoot mass, therefore,
decreased as the season progressed (Fig. 1).
The shoot nitrogen concentration on a dry mass basis
(shoot % N) of M. caerulea fell throughout most of the
season. However towards the end of the season (Julian day
226–255), the values remained constant (P 0n001, Fig. 2).
At any given time, shoot % N of plants receiving the high
nitrogen supply was greater than that of plants receiving the
low nitrogen supply (P 0n001, Fig. 2). From before bud
burst (Julian day 31) to the last sampling date (Julian day
255), shoot % N declined by a similar proportion irrespective
of nitrogen supply (72 % for low, compared with 73 % for
the high supply of nitrogen, Fig. 2). The concentration of
nitrogen in the shoot tissue water was greatest in the overwintering buds (Fig. 3), but fell rapidly after bud burst to
values which showed only slight but significant reduction
(P 0n001) over the rest of the season. Plants receiving the
higher nitrogen supply had greater concentrations of
nitrogen in tissue water (P 0n001, Fig. 3). The shoot water
content of M. caerulea increased from that in the overwintering buds to the highest observed values immediately
after bud burst, but then declined as the season progressed
Thornton et al.—Nitrogen and Water in DeŠeloping Molinia caerulea Shoots
0·6
7
A
[N] (percent dry mass)
0·4
0·2
Dry mass (g)
Low N
High N
6
leaf
shoot
5
4
3
2
1
0
10
0
B
7·5
633
100
200
Time (Julian days)
300
F. 2. Relationships between shoot nitrogen concentration on a dry
mass basis (% dry mass) and time (Julian days) for M. caerulea plants
receiving either a low or high supply of nitrogen. The values are means
of four replicatesps.d. except for ‘ Low N ’ plants on Julian day 31
where the value represents one analysis on material pooled from four
replicates. Bud burst occurred immediately before the harvest on Julian
day 92.
leaf
shoot
5
30
2·5
Low N
High N
100
200
Time (Julian days)
300
F. 1. Relationships between dry mass of shoot and leaves and time
(Julian days), for M. caerulea plants receiving either a low (A) or a high
(B) supply of nitrogen. Values are means of four replicatesps.d. Bud
burst occurred immediately before the harvest on Julian day 92.
(Fig. 4, P 0n001). Beyond Julian day 198, no further
reduction in water content of the shoot per unit dry mass
occurred (Fig. 4). Considered over all harvest dates, plants
receiving the low nitrogen supply had slightly greater shoot
water contents compared with plants receiving the high
nitrogen supply (P 0n01, Fig. 4). Because of the observed
differences in the concentration of nitrogen in tissue water
and of shoot water content between the over-wintering buds
and the shoot tissue at other times of the season (Figs 3 and
4), the following analyses were restricted to shoot growth
after bud burst.
When shoot % N was plotted against shoot dry mass
using natural logarithmic axes, two-phase dynamics for the
decline in nitrogen concentration were observed (Fig. 5).
Similar dynamics were also observed for shoot water content
(Fig. 6). The data variance accounted for by the broken
stick models was greater than that for simple linear
regression in three out of the four models ; in the remaining
model the variance was equal to that accounted for by linear
[N] (mg N g–1 H2O)
0
20
10
0
100
200
Time (Julian days)
300
F. 3. Relationships between concentration of nitrogen (mg N g−"
water) in shoot tissue and time (Julian days) for M. caerulea plants
receiving either a low or high supply of nitrogen. Values are means of
four replicatesps.d. Bad burst occurred immediately before the harvest
on Julian day 92.
regression. In Fig. 5, the variance accounted for by the
broken stick models was 89 and 96 % for plants receiving
a low and high supply of nitrogen, respectively, compared
with 87 and 93 % using linear regression. In Fig. 6, for
plants receiving a low or high nitrogen supply, the variance
634
Thornton et al.—Nitrogen and Water in DeŠeloping Molinia caerulea Shoots
2
Low N
High N
In(shoot water content)
Water content (g H2O g–1 dry mass)
6
4
2
1·5
1
0·5
Low N
High N
0
100
200
Time (Julian days)
300
F. 4. Relationships between shoot water content (g water g−" dry
mass) and time for M. caerulea plants receiving either a low or high
supply of nitrogen. Values are means of four replicatesps.d. Bud burst
occurred immediately before the harvest on Julian day 92.
2
In(shoot %N)
1
0
Low N
High N
–1
–5
–3
–1
In(shoot dry mass)
1
3
F. 5. Relationships between the natural logarithm of shoot nitrogen
concentration (% dry mass) and the natural logarithm of shoot dry
mass (g) for M. caerulea plants receiving either a low or high supply
of nitrogen. Values are for individual replicates. Lines were fitted using
broken stick models. For low N plants at x k2n9, y l 0n263k0n20
x whereas at x k2n9, y lk0n622k0n51 x. For high N plants at
x 1n1, y l 1n21k0n22 x whereas at x 1n1, y l 1n63k0n59 x.
accounted for was 85 and 86 % using the broken stick
model compared with 85 and 83 % using linear regression.
The initial gradients (at lower dry mass) of ln(shoot % N)
Šs. ln(shoot dry mass) were similar (P 0n05) irrespective
of nitrogen supply (Fig. 5). After the ‘ break point ’, the
gradients were more negative (P 0n01 low N, P 0n001
high N), but still unaffected by nitrogen supply (P 0n05 ;
0
–5
–3
–1
In(shoot dry mass)
1
3
F. 6. Relationships between the natural logarithm of shoot water
content (g water g−" dry mass) and the natural logarithm of shoot dry
mass (g) for M. caerulea plants receiving either a low or high supply of
nitrogen. Values are for individual replicates. The lines were fitted using
broken stick models. For low N plants at x k2n7, y l 0n387k0n28 x
whereas at x k2n7, y l 0n096k0n38 x. For high N plants at x 1n5,
y l 1n15k0n24 x whereas at x 1n5, y l 1n90k0n74 x.
Fig. 5). The relationship of ln(shoot water content) Šs.
ln(shoot dry mass) also showed similar initial gradients
irrespective of nitrogen supply (P 0n05) at lower shoot dry
mass. After the break point, the gradient became significantly more negative only for plants receiving the high
nitrogen supply (P 0n001 ; Fig. 6).
With the low supply of nitrogen, the x-values at which the
breaks in the broken stick model occurred were similar (P
0n05) irrespective of whether ln(shoot % N) or ln(shoot
water content) was plotted against ln(shoot dry mass) (Figs
5 and 6). With the high supply of nitrogen, there was
evidence that the values of ln(shoot dry mass) at which the
breaks occurred differed (P 0n05 ; Figs 5 and 6). For the
relationship of ln(shoot % N) Šs. ln(shoot dry mass) the
break points occurred at k2n9 and 1n1 on the x-axis,
equivalent to shoot dry masses of 0n06 and 3n12 g, for plants
receiving low or high supplies of nitrogen, respectively.
From Fig. 1 the dates at which these dry masses were
achieved can be estimated : Julian day 129 for the low
nitrogen supply and between days 142 and 157 for the high
supply. For the relationship of ln(shoot water content) Šs.
ln(shoot dry mass) the breaks occurred at k2n7 and 1n5,
equivalent to shoot dry masses of 0n07 and 4n51 g. These
shoot masses were achieved between Julian days 142 and
157 for plants receiving the low nitrogen supply and
between days 157 and 177 for plants receiving the high
nitrogen supply.
The similarity, at any given nitrogen supply, of the overall
relationships between ln(shoot % N) an ln(shoot water
content) when plotted against ln(shoot dry mass) (Figs 5
and 6) is emphasized by the relationships which existed
between shoot % N and shoot water content (Fig. 7).
Thornton et al.—Nitrogen and Water in DeŠeloping Molinia caerulea Shoots
6
N concentration (% dry mass)
Low N
High N
4
2
0
2
4
Water content (g H2O g–1 dry mass)
6
F. 7. Relationships between shoot nitrogen concentration on a dry
mass basis (% dry mass) and shoot water content (g water g−" dry
mass) for M. caerulea plants receiving either a low or high supply of
nitrogen. Values are for individual replicates. Lines were obtained by
plotting back-transformations of the fitted broken stick models in Figs
5 and 6, against each other, for both the low and high supply of
nitrogen.
DISCUSSION
The observed results for M. caerulea agree with those of
Leigh and Johnston (1983, 1985) who found that, in H.
Šulgare, the seasonal decline in the shoot concentration of
both nitrogen and potassium was greater on a dry mass than
tissue water basis. In field-grown plants of M. caerulea,
Aerts and de Caluwe (1989) found that the percentage
retranslocation of nitrogen from leaves, leaf sheaths and
culms changed little with increased nutrient availability ; the
present results, showing that the proportional decline in
shoot % N was independent of nitrogen supply, support
this. Differences in the nitrogen concentration of shoots of
M. caerulea on a dry mass basis resulting from differing
supplies of nitrogen were attributable, in this investigation,
to differences in the concentration of nitrogen in the tissue
water ; such differences were not observed in H. Šulgare by
Leigh and Johnston (1985). The observed differences in the
shoot nitrogen concentration on a dry mass basis, which
occurred after bud burst, were mainly attributable to
changes in shoot water content. Lemaire and Gastal (1997)
considered that the decline in shoot nitrogen concentration,
on a dry mass basis, as shoot dry weight increased, was
caused by an increase in the proportion of structural tissue
required to support photosynthetic tissue. An accelerated
decline in shoot nitrogen concentration in plants from dense
canopies, compared with that of isolated plants, was thought
to be due to the same phenomenon (Lemaire and Gastal,
1997). The present results are consistent with the conclusions
of Lemaire and Gastal (1997), assuming that structural
tissue contains a lower water content than photosynthetic
tissue, because an increased proportion of structural tissue
in the shoot would also result in an overall reduction in
shoot water content (on a dry mass basis).
635
Acceleration in the rate of decline of shoot nitrogen
concentration on a dry mass basis has been observed to
occur after flowering in sorghum (Greenwood et al., 1990 ;
Ple! net and Cruz, 1997). The causes of the accelerated
decline of both shoot % N and water content of M. caerulea
with increased shoot mass may be twofold : (1) an effect of
competition for light on plants as shoot mass increases, as
shown by Lemaire and Gastal (1997) for plants in dense
canopies ; or (2) a developmental change leading to
production of a greater proportion of structural tissue of
reduced nitrogen and water concentrations. The fact that
the break points in ln-ln plots of Figs 5 and 6 occurred at
very different shoot dry masses for the two nitrogen
treatments, but that these different masses were all achieved
around the time of cessation of leaf tissue production,
indicates that only the second hypothesis needs to be
considered.
Shoot growth after the break point consisted of increases
in flower stems, flowers and new basal internodes for
storage over the following winter (Thornton and Millard,
1993). In Lolium perenne and L. multiflorum, flower stems
were found to have lower nitrogen concentrations than leaf
blades (Wilman and Altimimi, 1982) ; an increase in the
proportion of flower stems would, therefore, result in a
greater reduction in shoot nitrogen concentration than an
equivalent increase in leaf material. Concomitant with
flower stem production, leaf material may senesce, with the
onset of nitrogen mobilization both from leaves to other
tissues (Thornton and Millard, 1993) and\or within leaves
(Hirose, Werger and van Rheenen, 1989). The relationship
between % N and water content of the shoot remains
remarkably unaffected by the developmental changes occurring at the time of cessation of leaf growth. The
concentration of nitrogen in tissue water, therefore, appears
to be relatively constant, irrespective of the development of
the plant, and reflects the nitrogen status of the plant
independent from its development. In contrast to Leigh and
Johnston (1985), an increase in the nitrogen concentration
of tissue water after anthesis was not observed in the present
results. These authors considered that the increase they
observed was due to development of ears containing high
concentrations of nitrogen in the tissue water (Leigh and
Johnston, 1985). The differences between these observations
could be explained by the fact that M. caerulea flowers in
the present study had not reached the same maturity as the
corresponding H. Šulgare flowers at the end of the respective
experiments.
The concept of the use of the nitrogen concentration in
tissue water as an indicator of plant nitrogen status has to
be restricted to growth associated with ‘ volume ’ expansion.
It cannot be applied during growth of storage organs such
as grain or tubers when dry mass increase is not necessarily
associated with water accumulation. It will also be of
limited use during water stress, as the variation in the
nitrogen concentration in tissue water may not reflect
nitrogen nutrition but mainly fluctuation in water content.
The concentration of nitrogen in tissue water in M.
caerulea after bud burst was similar to the preanthesis
values of between 5–10 mg N g−" tissue water observed in H.
Šulgare (Leigh and Johnston, 1985), supporting the sugges-
636
Thornton et al.—Nitrogen and Water in DeŠeloping Molinia caerulea Shoots
tion of Greenwood et al. (1991) that the nitrogen concentration of either the metabolic or structural components
of plants varied little between species of the same metabolic
group (C or C ).
$
%
In conclusion, the decline in the nitrogen concentration of
the shoots of M. caerulea on a dry mass basis, as mass
increased after bud burst, is mainly explained by changes
in shoot water content. The concentration of nitrogen in
the tissue water of shoots showed only a small decrease
beyond bud burst. A more rapid decline in shoot nitrogen
concentration was due, not to an increased proportion of
structural tissue required to support leaves competing for
light, but to a switch in the type of tissue produced from
leaves to flower stems, flowers and new basal internodes.
A C K N O W L E D G E M E N TS
This work was funded by the Scottish Office Agriculture,
Environment and Fisheries Department. We thank MR
Tyler for skilled technical assistance.
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