Plant, Cell and Environment (1988) 11, 1-8
OPINION
The accumulation and storage of nitrogen by herbaceous
plants
p. MILLARD Departtnent of Soil Fertility, The Macaulay Land Use Research Institute, Abetdeen, U.K.
Received 9 April 1987; accepted for publication 3 September 1987
Abstraet. Accumulation of nitrogen (N) by plants in
response to N supply outstripping demand is
contrasted with storage of N, which itnplies that N in
one tissue can be reused for the growth or
maintenance of another. Storage can, therefore,
occur in N-deficient plants; accutnulation can not.
The consequence of accumulation and storage of N
is considered, particularly in relation to the
reproductive growth of annual plants, which can
often use a great deal of stored N. Nitrate and
proteins are the fonns of N most often stored in
vegetative tissues and, quantitatively, ribulose 1,5bisphosphate carboxylase/oxygenase is often the
most important protein store. While storing nitrate
will be less costly to the plant in terms of energy,
protein stores offer several possible advantages.
These advantages are (i) maximizing the potential for
carbon assimilation, (ii) avoiding problems with the
regulation of leaf turgor and (iii) allowing the
reduction on nitrate to occur in the young, fully
illuminated leaf.
Key-words:
accumulation:
reproduction; senescence.
storage;
nitrogen;
proteins;
Introduction
The N economy of plants can be considered as a
balance between the need to produce vegetative
(capital) and reproductive growth. It has been known
for some titne that many annual plants reuse
nitrogen (N) which was first assimilated during
vegetative growth for reproduction (Williams, 1955).
Such internal-N cycling can provide most of the N
used during reproduction, and be particularly
important to the N econotrty of plants growitig with
a poor nutrient supply. Indeed, leaf senescence is
often associated with N deficiency, as a consequence
of the remobilization of N for reproductive growth
(Thomas & Stoddart, 1980).
Plants can take up N faster than is needed for
current growth, allowing N to accutnulate in their
tissues. Such luxury consumption of N is often
considered as a definition of N storage (e.g. Pate,
1983). However, such a definition is misleading, as it
Correspondence: Dr P. Millard. Department of Soil Fertility.
The Macaulay Land Use Research Institute. Craigiebuckler,
Aberdeen AB9 2QJ. U.K.
ignores the capacity of N-deficient plants to also
store N, while assutning that all the N accumulating
in response to luxury uptake is in a form that the
plant can reuse. The following discussion considers
the differences between accumulation and storage of
N. The fomis of N that can accumulate, or be
stored, in the vegetative tissues of plants are
considered in relation to plant growth and possible
advantages of using proteins as opposed to nitrate as
N stores are discussed.
The Definition of Accumulation and Storage
Accumulation of N occurs if the supply of N outstrips the demand by the plant and so can only be
considered in relation to the growth rate of the plant.
For exatnple, solution culture experiments designed
to maintain a constant internal N concentration (A^J
in the plant have shown that, under these steadystate conditions, there is a linear relationship
between A^, and the relative growth rate (Ingestad,
1982; Fig. 1). There is an optimum TVj, above which
the telative growth rate is not increased. If the supply
of N allows uptake of more N than is needed to
maintain the optimum A'j (Fig. 1), accumulation
occurs. The rate of N uptake does not necessarily
have to be rapid for accumulation to occur. If plant
growth is severely restrained by a lack of light, or
some other nutrient, N accutnulation can occur with
a low supply and rate of N uptake due to the
necessity of maintaining a lower optimum N-,
(Fig. 1). For plants gt owing in soil and not under
steady-state conditions, accutnulation can also occur
at suboptimal relative growth rates and with
changing values of A';, as long as growth is not
constrained by the rate of N uptake. While
accumulation may only occur in certain plant tissues,
it can be considered only in relation to the growth
rate of the whole plant, and may well be independent
of the gtowth rate of individual organs.
Nitrogen is stored if it can be mobilized from one
tissue and subsequently reused for the growth or
maintenance of another. Storage is, therefore,
independent of growth rate. The capacity for the
subsequent use of the N must be demonstrated, even
though it need not be expressed under all conditions.
Thus glycine betaine accutnulated during drought
1
p. MILLARD
0.2-
0.1 -
25
50
Internal N concentration
100
( % ot optimun )
Figure 1. The relationstiip between the internal N concentration
(A^i) and relative growth rate of plants growing under conditions
which allow the maintenance of a constant A'j. Redrawn and
modifted from Ingestad (t982).
cannot be considered as a N store. Despite
translocation to expanding leaves upon relief of
water stress in Hordeum vulgare, there is no further
metabolism of the glycine betaine which thus
behaves as an inert end product (Ladyman, Hitz &
Hanson, 1980). In contrast, proline can rapidly
disappear from leaves after the relief of drought due
to translocation and incorporation into protein and
metabolism to form other amino acids (Stewart &
Hanson, 1980). The ability of a plant to store N is
not dependent upon its N status. Although N
deficiency causes foliar senescence due to
remobilization of N, plants also use programmed
senescence in a strategic role for avoiding or
withstanding stress. For example, monocarpic
senescence and the subsequent transfer of N from the
leaves for reproductive growth is vital to the N
economy of many plants, this occurs in N-replete as
well as N-deficient plants. Thus N which was taken up
early in the plant's hfe cycle is stored in the leaves
and later used for reproductive growth. If flowers
and developing fruits and seeds are removed it is
possible to delay, or even reverse, leaf senescence in
many species (Stoddart & Thomas, 1982). This form
of N storage, using metabolically active leaf proteins,
can be particularly important for the reproductive
growth of N-deficient plants (Derman, Rupp &
Nooden, 1978; Millard & MacKerron, 1986). In Ndeficient plants, N had not been accumulated in
order for it to be stored, since plant growth had been
constrained by a low external N supply.
Nitrogen accumulation
The relative proportion of nitrate to organic N
translocated from roots in the transpiration stream
depends upon the site of nitrate reduction, and the
extent of root to shoot cycling of reduced N. If
nitrate reduction occurs predotninantly in the shoot,
tnore than 95% of the total xylem sap N can be
recoverable as nitrate (Pate, 1983). Nitrate does not
appear to be phloem mobile (Pate, 1980). In the few
cases where nitrate has been recovered from phloem
sap, it represented a small fraction of the total N
being translocated (Hayashi & Chino, 1986). In
eonsequence many tissues which have low
transpiration rates contain no appreciable quantities
of nitrate, even when plants are grown with
excessively high levels of N fertilizer (Greenwood &
Hunt, 1986). The distribution of accumulated nitrate
in the shoots of plants is variable; the examples in
Table 1 have been selected because they
demonstrated that growth was not limited by the
external availability of N, such that nitrate was
accumulated. The data for Lolium perenne (Alberda,
1965; Clement, Hopper & Jones, 1978) shows nitrate
can accumulate in roots as well as shoots.
Concentrations of accumulated nitrate tend to be
higher in stems and petioles than in leaves
(Armstrong et al., 1986; Millard & MacKerron,
1986).
Nitrate in plant tissues is believed to be divided
between a metabolic and a storage pool (Ferrari,
Yoder & Filner, 1973). Studies of nitrate reduction
suggest that most of the nitrate in leaves is contained
within the vacuole (Smirnoff & Stewart, 1985), and
isolation of leaf cell vacuoles has enabled estimates
of the size of the vacuolar nitrate pool to be tiiade.
These range frotn 58% (Granstedt & Huffaker, 1982)
to 99% (Martinoia, Heck & Wiemken, 1981) ofthe
total leaf nitrate, despite the possibility of nitrate
leakage from vacuoles during their isolation. Thus
most of the nitrate in the leaf, sequestered in the
vacuole, probably represents N that has been
accutnulated. Nitrate accumulation can have several
physiological consequences. The high concentration
of nitrate in some tissues means that nitrate will
make a significant contribution to the solute
potential. Also, assimilation of nitrate during water
stress may contribute to the avoidance of
photoinhibition in drought tolerant species (Smirnoff
& Stewart, 1985). It has also been suggested that
stitnulation of leaf growth by N may be greater in
those species that transport most of their nitrate
directly to the shoot, where accutnulation can be
used as part of the osmotic force driving cell
expansion (Sprent & Thomas, 1984). Nitrate has
been shown to stimulate leaf growth in broad-leaved
plants by affecting the rate, rather than the duration
of expansion (Artnstrong et al., 1986), possibly
due to an increase in hydraulic conductivity (Radin
& Boyer, 1982).
;
Many plants can accumulate high concentrations
of amino acids and atnides in their vegetative tissues
(Pate, 1983). Atnide accumulation is comtnon if
plants experience a luxurious N supply due to the
ACCUMULATION AND STORAGE OF NITROGEN
Table 1. Accumulation of nitrate in plant tissues
Species
Tissue
Nitrate-N/
Total N
Lotium perenne
Leaves
Roots
0.21
0.19
Alberda (1965)
L. perenne
Shoots
Roots
0.36
0.29
Clement. Hopper & Jones (t978)
Phleuin pratense
Shoots
0.08
MacLeod & MacLeod (1974)
Source of data
Plantago major
Shoots
O.t9
Woldendorp (t983)
Solanuin tuherosum
Leaves
Stems
0.17
0.6t
Millard & Marshall (1986)
Brassica otercieea
Shoots
0.16
Greenwood & Hunt (1986)
Laetuca sp.
Shoots
0.15
Greenwood & Hunt (1986)
Spinaeea sp.
Shoots
O.t4
Greenwood & Hunt (1986)
imposition of a severe constraint upon their gtowth.
For example, sulphur or copper deficiency causes the
accumulation of asparagine atid glutamine in shoots
(Cheshire et aL, 1982; Millard, Sharp & Scott, 1985).
Like nitrate, free amino acids in leaves accumulate
predominantly in vacuoles (Boudet, Canut & Alibert,
1981). An exception to this is the accutnulation of
the amino acid proline and the quaternary
ammonium cotnpound glycine betaine in tissues of
some drought or salt-tolerant plants. Concentrations
of both compounds are higher in the cytoplastn than
the vacuole, consistent with their proposed function
^s compatible osniotica (Leigh, Ahtnad & Wyn
Jones, 1981).
Nitrogen can also accumulate in vegetative tissues
3s proteins. For example, nearly all the organic
N accumulating in Solanum tuberosum leaves as a
result of luxury consumption of N can be recovered
as soluble proteins (Millard & Marshall, 1986).
Many of these proteins are active catalytically.
Quantitatively, the tnost itnportant of these leaf
proteins is probably ribulose 1,5-bisphosphate
carboxylase/oxygenase (RUBISCO), constituting
between 40-80% of the total soluble leaf protein in
C3 plants (Huffaker, 1982). Providing tnote N to the
plant increases the concentration of RUBISCO and
•"esults in an increase in the ratio of RUBISCO to
total soluble leaf protein in H. vulgare (Huffaker,
1982) and Morus albus (Yamashita, 1986), but does
"ot change " this ratio in the leaves of
Trlticwn aestivum (Lawlor et aL, 1987a) and Orvza
miva (Makino, Mae & Ohira, 1983). While
'ncreasing the N supply to plants increases the
concentration of RUBISCO in the leaves, there is no
effect upon the specific carboxylation activity of the
^zyme tneasured in vitro (Makino et al., 1983;
Yamashita, 1986). Since RUBISCO activity is
subject to complex regulation in the intact leaf,
•nvolving both specific inhibitors and a soluble
protein RUBISCO-activase (Ogren, Salvucci &
Portis, 1986), caution is needed in the interpretation
of in vitro studies. However, accutnulation of
RUBISCO protein without a concomitant increase in
the rate of carbon assitnilation was found in the
leaves of T. aestivum by Lawlor et al. (1987b), in
response to an increase in the N supply. Since
photorespiration was a constant proportion of
carbon assitnilation, these workers suggested that
under conditions of high N some 50% of the
RUBISCO protein was not activated, or that only
half of the catalytic sites were functional, suggesting
accumulation of the protein (Lawlor et al. 1987b).
The C4 plants contain substantially less RUBISCO
(only 4-10% of their soluble leaf protein) than C3
plants. In consequence, C4 plants have a lower N
capital cost for growth than C3 species, due to the
CO2-concentrating mechanism of C4 photosynthesis
(Schmitt & Edwards, 1981). In a study of N
partitioning within the C4 Zea mays, Sugiyama,
Mizuno & Hayashi (1984) showed by immunochemistry that increasing the N supply to the plant
increased both the leaf protein content and the
relative amounts of phosphoenolpyruvate carboxylase and pyruvate orthophosphate dikmase
cotnpared with that of RUBISCO. However, since
the content of RUBISCO in the leaf was greater than
that of the other enzymes, increasing the N supply to
the plant resulted in a greater investment of the extra
plant N in RUBISCO than in the other enzymes.
Thus C4 plants appear to have the capacity to
accumulate N in RUBISCO, but to a lesser extent
than C3 plants.
Storage of nitrogen
It is well established that nitrate can be removed
ftom storage pools and used to support further
growth. For example, depletion of accumulated
nitrate from S. tuberosum leaves, once the initial rate
of N uptake by the plant had slowed, was shown by
Millard & MacKerron (1986). Reduced N can also
be stored, in a nutnber of ways. Roots receive at least
part of their N from the shoot via the phloem. In
some species a proportion of the N translocated
4
P. MILLARD
from the shoot enters the root xylem and is
retranslocated to the shoot. Such root-shoot cycling
of amino acids is a dynamic N store, the size of
which depends upon the balance between the
capacity of the plant to absorb and assimilate nitrate
and remobilize other N stores (for example during
leaf senescence). Measurements on Lupinus showed
25 to 50% of the total N absorbed in one day being
cycled in this manner (Pate, Layzell & Atkins, 1979),
and for T. aestivum seedlings 79%, representing 18%
of the N in the plant (Simpson, Lambers & Dalling,
1982). Such shoot-root cycling of N is a readilyaccessible store of labile reserves, this is important in
allowing the plant to direct N to the strongest sinks
for growth, as well as in adapting to N deficiency
(Oscarson & Larson, 1986).
The ability of a plant to store organic N in the
long-term is often more difficult to assess than for
nitrate. The N is often present as structural or
functional proteins and not in discrete storage
compartments. Values calculated for the proportion
of reduced-N redistributed from various vegetative
tissues during the reproductive growth of a range of
species are shown in Table 2. The contribution so
made to reproductive N has also been calculated.
Some of the values may be over estimates, where no
assessments of N losses during the senescence of
vegetative tissues, due for example to leaf abscission
or gaseous N losses (Westelaar & Farquhar, 1980)
were made. However, Table 2 detnonstrates the
substantial quantities of vegetative organic N that
can be stored for reproductive growth and the
itnportant contribution so tnade to the N economy
of the plant. Much of this N comes from the
degradation of soluble proteins and their
reutilization is often intimately associated with the
senescence of individual organs.
Nitrogen storage and foliar senescence
During leaf senescence RUBISCO is one of the
largest potential sources of N which is mobilized.
Evidence for this comes from several sources. Firstly,
Batt & Woolhouse (1975) demonstrated a rapid loss
of in vitro RUBISCO activity, together with several
other Calvin cycle enzymes, at the onset of
senescence of Perllla frutescens leaves. Much of this
initial decrease in activity may be attributable to
changes in the kinetic form of the enzyme or the level
of activation, rather than loss of RUBISCO protein
(Hall, Keys & Merrett, 1978). However, loss of
RUBISCO protein from senescing H. vulgare leaves
has been measured itnmunochemically as soon as
chlorophyll concentrations decrease (Peterson &
Huffaker, 1975), demonstrating that RUBISCO
proteolysis occurs at the onset of senescence.
RUBISCO protein appears to be degraded faster
than other soluble leaf proteins during senescence
(Wittenbach, 1979).
Table 2. The proportion of organic N redistributed from vegetative tissues during reproductive growth, and the contribution so
made to reproductive N in a range of species
Species
Tissue
Tritieum aestivum
Leaves
Stem
Roots
Redistribution
of nitrogen
from vegetative
growth
0.80
Contribution
to reproductive
growth
0.39
Source of data
Gregory, Crawford & McGowan (1979)
A CQ
T. aestivum
Leaves
u. /o
O.t4
0.86-0.89
0.23-0.48
Nair. Grover & Abrol (1978)
T. aestivum
Leaves
Stems
Roots
0.74
0.64
0.29
0.28
0.28
0.06
Dalling, Boland & Wilson (1976)
Helianthus annus
Leaves
Stems
Roots
0.56
0.26
0.07
0.12
Hocking & Steer (1983)
0.52
0.78
Zea mays
Leaves
Stems
Roots
0.43
0.5t
0.21
O.t 8-0.27
O.t 7-0.24
0.04-0.10
Pan et al. (1986)
Sotanum tuberosum
Leaves
Stems
Leaflets
Stems and
petioles
Pedunctes
Roots and
nodules
0.28-0.42
0-0.47
0.25-0.43
0-0.10
P. Mittard, unpublished data
0.34
0.08
Peoples, Pate & Atkins (t983)
Gtyelne max
Leaf
blades
0.90
Beta vulgaris
Shoot
Vigna unguieutata
—
0.02
0.07
O.I I
0-0.14
0.92
0-0.2t
Derman, Rupp & Nooden (1978)
Armstrong et al. (1986)
ACCUMULATION AND STORAGE OF NITROGEN
The second line of evidence comes frotn studies of
the turnover of RUBISCO, which appears to
undergo little or no degradation in C3 plants prior to
leaf senescence (Peterson, Kleinkopf & Huffaker,
1973; Huffaker, 1982; Ferreira & Davis, 1987).
Studies on Z. mays, however, have shown that under
conditions in which the level of RUBISCO protein
remains constant, RUBISCO is degraded at
approximately the same rate as other soluble maize
leaf proteins (Simpson, Cooke & Davies, 1981),
suggesting differences in the turnover characteristics
of RUBISCO in C3 and C4 plants. Many studies
have demonstrated a rapid turnover of RUBISCO
during leaf senescence, with only a low concomitant
incorporation of amino acids into the protein
(Huffaker, 1982). Furthermore, during senescence of
Cucumis sativis leaves. Callow (1974) tneasured
decreases in the synthesis of chloroplastic rRNA
needed for the continued production of the large
subunits of RUBISCO. Thus, both the high
concentrations of RUBISCO in leaves (resulting
particularly from a luxurious N supply) and the
turnover characteristics of the protein are suitable
attributes for a N storage compound, particularly in
C3 plants. Values for the extent of the remobilization
of RUBISCO N have been calculated for a range of
species (Table 3), where RUBISCO coticentrations
were measured by imtnunoassays. These values
demonstrate the importance that RUBISCO can
have as a N store, accounting for a large proportion
of both the soluble leaf protein lost during
senescence and much of the N exported from the
leaf.
During leaf senescence there is a rapid loss of
chlorophyll from numerous species (Sestak, 1977).
Evidence frotn light saturation curves obtained for
chloroplasts isolated from senescing Phaseolus
vulgaris leaves suggests that the average number of
chlorophyll molecules associated with each reaction
centre does not decline. In addition, each reaction
centre appears to cease functioning at the same time
t^hat their antennae chlorophyll molecules are lost
irom the thylakoid metnbrane (Woolhouse &
Jenkins, 1983). It is possible, therefore, that N frotn
the reaction centres and light-harvesting pigment
proteins, as well as that frotn RUBISCO, is
mobilized rapidly from senescing leaves.
Possible advantages of different nitrogen stores
Nitrate accumulated in leaves probably has to be
assimilated into an organic form before it can be
exported from the leaf in the phloetn (Smirnoff &
Stewart, 1985). Nitrate assitnilation occurs before
storage (in which case N is stored as atnino acids or
protein) or upon release of nitrate from a vacuolar
storage pool. The energy cost of nitrate assitnilation
will be the same regardless of the immediate origin of
the nitrate. However, storing nitrate will be less
costly than synthesizing storage proteins.
Nitrate storage involves compartmentation into a
vacuole. The energetics of nitrate transport across
the tonoplast in either direction are unknown, but
may require the mediation of ATPases. Furthertnore, retnoval of nitrate from a vacuole requires its
replacetnent by a suitable solute, if leaf turgor is to
be maintained. For exatnple, Leigh & Wyn Jones
(1986) calculated that if all the nitrate for //. vulgare
leaves was temobilized without replacement, leaf
turgor would drop by at least 0.15 MPa
(approximately 15% of leaf ostnotic potential). This
value could be twice as much if cations were removed
from the vacuole to maintain electroneutrality.
The energy costs of storing nitrogen as a protein,
such as RUBISCO, are tnuch higher than for nitrate.
Extra energy-consuming processes are involved.
In the case of RUBISCO these are; polypeptide
synthesis, transport of the cytoplasmically-synthesized small subunits into the chloroplast (Ellis,
1983), their processing by proteases (Robinson &
Ellis, 1984) and the assembly of hexadecameric
RUBISCO (Bloom, Milos & Ross, 1983). Associated
with this are also extra energy costs for RNA and
ribosome turnover. Analysis of phloem exudates
from the petioles of senescing leaves has shown that
the major forms of organic N exported from leaves
Table 3. Retnobiiization of N from RUBISCO during leaf senescence. A range of values are given when the
N supply to the plant* or ditlerent leaf positionsf within a canopy were considered
Remobilization of RUBISCO as
Species
Proportion of
soluble leaf
protein lost
Proportion of
nitrogen exported
from leaf
Oryza sativa
0. sativa
Tritieum aestivum
0.55-0.57*
T. aestivum
Hordeum vutgare
0.44-0.55t
0.85
—
Vigna unguieutata
Solanum tuberosum
0.45-0.77t
0.32-0.63t
0.30-0.95*
0.31-0.37*
0.67
—
—
Source of data
Makino, Mae & Ohira (1983)
Makino. Mae & Ohira (1984)
Peoples et al. (t980)
Wittenbach (1979)
Friedrieh & Huffaker (t980)
Peoples, Pate & Atkins (1983)
Mitlard & Catt (1987)
6
P. MILLARD
are amides (Stoddart & Thomas, 1982). Since this
does not reflect the amino acid composition of
RUBISCO, it is hkely that in addition to the energy
costs of RUBISCO proteolysis, utihzation of the
stored N would also involve energy expenditure on
the metabolic ititerconversion of amino acids to
amides, prior to their export from the leaf (Thomas
& Stoddart, 1982).
Why then do plants accutnulate and store N in
proteins such as RUBISCO? There are several
possible advantages. Firstly, by storing N in a
catalytically active protein the plant could be
recovering, effectively, the energy costs incurred by
peptide synthesis and RNA and ribosome turnover,
unless there were suboptimal concentrations of other
Calvin cycle enzymes. In addition, while there would
be an energy cost for protein turnover if N were
stored in many catalytically active proteins, there is
httle turnover of RUBISCO in C3 plants before leaf
senescence (Ferreira & Davis, 1987). Furthermore, if
the growth of the plant were limited by N, using
Calvin cycle enzymes as N stores would maximize the
potential for carbon assimilation. A second
advantage is that storage of N as a protein avoids
the potential osmotic embarrassment of nitrate
reutilization. A third consideration may be the time
during the life cycle of the plant that nitrate
reduction occurs. If a plant was storing N in
proteins, nitrate reduction and protein synthesis
would occur predominantly in young leaves. In
contrast, N mobilization occurs predominantly from
older, senescing leaves, which are often shaded at the
base of a canopy. Light is needed for the release of
nitrate from storage pools (Aslam, Oaks & Huffaker,
1976); nitrate reductase activity in leaves is also
stimulated by light (Smirnoff & Stewart, 1985). The
older, shaded leaves at the base of a canopy would,
therefore, be less able to export N stored as nitrate
than higher leaves receiving more light. No such
constraint upon mobilization would exist if the plant
stored N in proteins, since nitrate reduetion would
have oecurred in the young, fully-illuminated leaf.
Conclusions
Accumulation of N occurs if the supply exceeds the
demand, whereas N is stored by a plant if it can be
reutilized from one pool, for growth or maintenance
of another pool. Therefore, accumulation only
occurs if growth is not limited by the N supply,
whereas storage can happen even in severely Ndeficient plants. N can be accumulated and stored as
nitrate or in a reduced form. Proteins are often used
as a N store, of which RUBISCO is often the most
important quantitatively. Possible advantages to the
plant in storing N in proteins, rather than as nitrate,
are the potential maximization of carbon
assimilation, while avoiding problems with
regulation of leaf turgor. Furthermore, protein
storage allows for nitrate reduction to occur in the
young, fully illuminated leaf.
In many N-replete plants, nitrate accumulates
mainly in the lowermost leaves (e.g. Millard & i
MacKerron, 1986), which have the lowest potential
for nitrate mobilization. This paradox is probably a
consequence of the changing availability of nitrate \
frotn the soil during plant growth. In many
agricultural soils there is a rapid depletion of plantavailable N during the first few weeks of plant
growth (e.g. Artnstrong et aL, 1986). The high
concentrations of nitrate available initially to the j
plant means that the young plant is tnore carbonlimited than N-limited. This results in nitrate being
sequestered in vacuoles and used to help drive leaf
expansion. A few weeks later the plant has a larger
canopy and so a much greater potential for carbon
assimilation, while the supply of nitrate has declined.
This results in the use of organic cotnpounds for the
tnaintenance of leaf turgor, with N storage in |
proteins.
Acknowlegments
The author thanks Professor J. A. Raven and Dr D.
Robinson for their interest and helpful suggestions.
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