Journal of Experimental Botany, Vol. 47, Special Issue, pp. 1245-1253, August 1996
Journal of
Experimental
Botany
Plant nutrition, dry matter gain and partitioning at the
whole-plant level
A. James S. McDonald 1 ,3 , Tom Ericsson 1 and Carl-Magnus Larsson 2 ,4
Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences Box 7072
S-750 07, Uppsala, Sweden
'
,
1
2
Department of Botany, University of Stockholm, S-106 91 Stockholm, Sweden
Received 13 August 1995; Accepted 10 February 1996
Differential flows of photoassimilate result in patterns
of dry matter distribution among plant organs. The
extent to which these patterns are dependent on the
flux of different mineral nutrients entering the root
and the extent to which the distribution of dry matter
in the whole plant is affected by differentials in nutrient (primarily nitrate) flux among parts of the root
system is considered. It is concluded that patterns of
dry matter distribution and nutritional status may
depend on how nutrient supply has been manipulated
about the root. Where the flux density of nutrient has
been decreased and has become limiting to plant
growth, two categories of response have been
observed. In the case of N, P or S, limiting flux density
results in a proportionately greater amount of plant
dry matter in roots than is found at higher flux densities. This contrasts with the case of limiting K, Mg or
Mn supply, where proportionately less plant dry matter
is found in roots at lower nutrient flux densities than
at higher flux densities. In the case of N, particular
attention is paid as to how sink strength may be related
to differences between root and leaf cells in their
capacity for loosening and synthesis processes in the
primary cell wall.
Key words: Plant nutrition, whole-plant partitioning,
dry-matter partitioning, nutrient supply, growth.
Introduction
The distribution of dry matter among plant organs is one
of the key variables which affects the survival, competitive
ability and performance of single plants. It is the net
result of carbon assimilation, partitioning of photoassimilates among organs and losses via respiration, exudation
and organ mortality. It is affected by many variables,
including the availability of mineral nutrients in the
rhizosphere and flux of nutrients into the root. The fate
of photoassimilates in the production and maintenance
of assimilatory organs is generally thought to be more
limiting to the dry matter productivity of the plant than
is the specific, carbon-assimilation rate of single leaves.
Emphasis is made in this paper that, by appropriate
nutritional control of plant growth, it is possible to assess
the contribution of process regulation at a lower level of
hierarchy to the higher level of whole-plant response. It
is further argued that, unless such nutritional control of
growth is practised, then a proper assessment of the
relevance of process regulation to plant growth can not
be made. This philosophy has been adopted in our own
research in discussing process physiology in the context
of whole-plant growth. For example, the significance of
apparent regulation in the nitrate uptake system with
respect to nitrate supply, root growth and the nitrate
requirement for plant growth has been investigated by
Larsson and his colleagues (Larsson, 1994). Here, the
merits and problems associated with different approaches
to manipulating the supply of mineral nutrients in studies
of photoassimilate partitioning and transport are discussed, and the importance of distinguishing between
shorter- and longer-term responses is emphasized. This is
important in the context of testing any apparent change
(e.g. via molecular-genetic manipulation) at the level of
pathways and mechanisms in the context of whole-plant
growth. Evidence is reviewed for two major classes of
3 Present add~ess and to whom correspondence should be sent: Department of Plant and Soil Science, University of Aberdeen, Cruickshank Building,
St Machar Dnve, Aberdeen AB24 3UU, UK. Fax: + 44 1224 272703.
4 Present address: National Chemicals Inspectorate, S-171 22 Solna, Sweden.
© Oxford University Press 1996
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Abstract
1246
McDonald et al.
dry-matter distribution, but the pattern which characterizes each class may have very different functional origins,
depending on which nutrient is most limiting to the
overall growth of the plant. Mention is made of the use
of split-root studies, where the flux of nutrients around
individual roots can be varied without necessarily
affecting the total flux of nutrient into the plant. In
particular, a source of acclimation at the cellular level is
proposed which might account for immediate differentials
in dry matter gain among organs following N deprivation.
This relates to how perturbation in N supply might affect
the rate of cell expansion differentially among plant
organs and assumes that the incorporation of carbohydrate into the primary cell wall can be a major sink for
photoassimilate.
In studies of physiology pertaining to plant nutrition, it
is often desirable that any manipulation of nutrient supply
should be effective at the root surface. For this reason,
hydroponic techniques are widely used, effectively eliminating the buffering capacity of a soil volume. These
techniques can take the form of mist or spray cultures,
or aqueous solutions with aeration around the roots. In
many cases, growth responses to decreases in nutrient
supply have been studied, where total depletion of one
or more mineral nutrients has led to general information
on nutrient deficiency and its visual symptoms. This is a
valid but extreme approach which can be mechanistically
informative in the short term, but can be less meaningful
in the context of longer term response. This is because
such severe depletion such as the sudden total absence of
a mineral nutrient, may have little counterpart in most
natural plant-soil systems.
In many instances, it is of more interest only partially
to withhold one or more nutrients from solution, and
carry out studies with respect to controlled, supply limitation. However, in a great many studies where supply
limitation has been assumed, the extent to which it has
been effective seems uncertain. Two emphases in manipulating nutrient supply can be identified. In the vast
majority of cases, the concentration of nutrient in solution
about the root has been varied. However, it has been
argued that, where this is practised, interpretation of the
growth response (and its component physiology) can be
difficult (Ingestad, 1982; Ingestad and Lund, 1986;
Ingestad and Agren, 1992). This is because plant growth
can generally be maximized at very low concentrations
of nutrient in the bathing solution (Olsen, 1950).
Therefore, assuming that the external concentration is
actually maintained, no effect of concentration on plant
growth is to be expected over the wide range of nutrient
concentrations often investigated. Plates lA and B show
plants (Alnus incana Moench) which, by repeated titration,
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Methodology in plant nutrition
were grown in nutrient solution where the intended concentration of nitrogen in solution was maintained
throughout the growth period. No obvious dependence
of plant growth on external concentration of nitrogen
was found. This contrasts with reports in the literature
where apparent dependencies of growth on external concentration (the stated nutritional variable) have often
been reported. It is assumed that, where dependencies on
external concentration have been reported, the nutrient
concentration in solution may not have been sufficiently
controlled and, as the plants have become larger, nutrient
depletion and deficiency have occurred. Typically, this
will first be observed in the low-concentration treatments.
If this happens, then the plant will experience a whole
range of nutrient availability from excess consumption to
extreme deprivation as it grows larger. It then becomes
extremely difficult to relate growth response at anyone
time to a defined level of nutrient deprivation. An exceptional case where nutritional control has been documented
at extremely low external nutrient concentrations, is discussed later.
Another approach has been to vary the flux of nutrient
to the root. With this approach, the amount of nutrient
added to the external solution over a given period of time
is manipulated. It should be noted that, in order to
maintain nutritional control over plant growth, it is not
sufficient to add a constant amount of nutrient per unit
time. Either the amount added will be in excess of the
current requirement for maximum growth or it will be
insufficient for maximum growth and result in an unspecified degree of nutrient deficiency. In neither case, has
plant-growth rate been meaningfully controlled by nutrient supply. Obviously, the size of plant with respect to
the amount added, will be critical in determining the
onset of deficiency (Plate l C, D; Fig. 1). However, by
appropriate manipulation, the nutrient-flux approach can
result in precise control of plant growth and can, therefore, be suitable for studies of growth regulation with
respect to supply limitation. Many studies with this
approach, have involved the addition of variable, exponentially-increasing amounts of nutrient to plants grown
in solution (Ingestad and Lund, 1979). Other studies
have involved combinations of variable external concentration and solution flow rate (Asher and Edwards, 1983).
Where nutrients are added at exponentially-increasing
amounts, it is possible to control plant-growth rate accurately such that the relative growth rate (RGR) of the
plant equals the relative rate of increase in nutrient supply
and uptake (Fig. 2). This is true irrespective of which
mineral nutrient is in most-limiting supply (Ericsson
and Ingestad, 1988; Ericsson and Kahr, 1993, 1995;
Goransson, 1993, 1994). The growth rate of a plant can,
therefore, be limited by varying the flux of any mineral
nutrient, and the significance of phenomena associated
Dry
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matter partitioning
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with, for example, partitioning and transport of photoassimilate , to plant growth at limiting supplies of anyone
nutrient, can be assessed. The maximum value of RGR
that can be maintained depends on the genotype (lngestad
and Lund, 1979; Ingestad, 1980; Jia and Ingestad , 1984;
Ingestad and Kahr, 1985) and on the growth environment (Ingestad and McDonald, 1989; Pettersson and
McDonald, 1992).
With this approach, the nutrient-flux density at the
root surface associated with a particular RGR is effectively
maintained. The approach results in a range of plant
RGR and plant nutrient concentration which is maintained throughout the exponential phase of growth . The
plants are then referred to as having steady-state nutrition.
Significantly, in the context of the present discussion, the
distribution of dry matter among vegetative organs generally assumes an equilibrium value (Plate 2), although a
dependence on ontogeny can occur (Ingestad and Lund ,
1979). Importantly, this approach allows for an unambigous assessment of partitioning and transport processes in
the context of well-defined plant growth.
As stated earlier, where external nutrient concentration
has been used, the investigated range has, for the most
part, been much too high to effect supply limitations to
growth. However, Macduff et al. (1993) ha ve compared
the external concentration and flux approaches at very
low values of external concentration. Initially, they varied
the flux of nitrate such that a range of steady-state
nutrition was achieved . It was reporte d that, for each
value of flux and its associated plant growth, there was a
unique , equilibrium value of external nitrate concentration. The crucial piece of information was that, in a
separate experiment, where values from within this range
of equilibrium nitrate concentration were chosen as the
experimental variable, plant growth and the values of all
physiological and morphological varia bles investigated
Downloaded from http://jxb.oxfordjournals.org/ at OUP site access on January 10, 2017
Plate 1. Gro wth of grey alder (ALI/liS incana Moench) in nutrient solution. (A) Plant s established in beakers where external N concentration was
maintained at either (from L-R) 10, 20,40, 80, 160 or 320 mg N 1- 1 (i .e, within the range 0.7-22.8 mM) du ring the growth period . (B) The same
plant s after 2 weeks. Note that there was no apparent dependence of growth on the external N concentra tion in the range investigated. (C) Plants
established in beakers to which an initial amount of N was added : either (from L-R) 0, 10, 20, 30, 40 or 80 mg N (i.e. within the range of
approximately 0-5 mmol N ). No further N was added during the growth period. (D) The same plants after 2 weeks. Note that the amount of N
became progressively insufficient to maintain plant growth . Eventually, even the largest initial amount of N added would have been insufficient for
maximum growth .
1248
McDonald et al.
important point is that these were extremely low nitrate
concentrations (1-10 ,uM),-much lower than have generally been used in studies where supply-limitation to
growth through the use of an external concentration
variable has often been assumed.
mg N seedling -I
20
Nutrient supply and dry matter distribution
2
1
3
4
Weeks
Fig. 1. Calculated time-course of N depletion from a nutrient solution
which was renewed at weekly intervals (constant supply, amount = 17.5
mg N available each week), compared with the exponential-increase in
N supply and uptake required for maximum growth. The data pertain
to birch plants (Betula pendula Roth) with the same initial plant size.
Note that plants from both treatments would have sufficient N for
maximum growth during the first 3 weeks, but that plants grown with
a constant supply rate would become increasingly N-deficient during
week 4 (from Linder and Ingestad, 1977).
Betula
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Helianthus
Salix
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Fig. 2. The relationship of plant relative growth rate (d -1) to the
relative addition rate of N (d -1). The data are from different
experiments at steady-state nutrition. Each symbol pertains to one
experiment (from McDonald and Stadenberg, 1993).
were identical to those where nitrate flux had been
manipulated. In fact, in maintaining different external
concentrations over the range investigated, Macduff et al.
( 1993) would effectively have added nitrate at exponentially-increasing amounts in the growth-limiting range. The
Downloaded from http://jxb.oxfordjournals.org/ at OUP site access on January 10, 2017
o
It has often been assumed that the net result of growth
responses to decreased nutrient availability is not only a
decrease in plant growth rate, but a shift in the distribution of dry matter in favour of root biomass. This may
be true of responses to many nutrient limitations (e.g.
with respect to N supply, Plate 2) but there are many
exceptions. For example, Cakmak et al. (1994) reported
smaller fractions of plant dry matter in roots of bean
plants (Phaseolus vulgaris L.) at K and Mg deficiency
than at more optimal supplies of these nutrients. This
contrasted with responses to P deficiency where a larger
fraction of plant dry matter was found in roots than at
more optimal supply of P. In a series of experiments with
birch seedlings, Ericsson and his colleagues (Ericsson,
1995) have built upon the earlier work of Ingestad
(Ingestad and Lund, 1979) and have varied the flux of
different limiting nutrients, controlling RGR and achieving steady-state values of dry-matter distribution between
shoots and roots. Two main categories of response have
been observed (Fig. 3). In the case ofN, P and S, a larger
fraction of plant dry matter was found in roots at limiting
nutrient supplies than at more optimal supply. Responses
to limitation in Fe supply were similar, although less
marked at extreme limitation, compared with other nutrients in this category. This contrasted with responses to
limitations in Mg and Mn supply, where smaller fractions
of plant dry matter were found in roots than at more
optimal supply. The response to K limitation was similar
to that for Mg and Mn, but less pronounced at extreme
limitation. All of these responses are qualitatively consistent with those of Cakmak et al. (1994) .
In the case of nitrogen, total dry-matter increment of
the plant is a function of the total, net nitrogen uptake
by the roots. It is apparently unimportant that nitrogen
deprivation occurs about one part of the root if the
supply is increased by the same amount around another
part. In split-root studies, Samuelson et al. (1992) showed
that the growth rate of the plant (Hordeum vulgare) was
proportional to the total amount ofN absorbed, irrespective of how it was supplied to two parts of the root. The
two parts of the root system, however, grew at quite
different rates, in proportion to the amount of nitrogen
which they were absorbing. With time, this resulted in a
stable root-to-shoot ratio at the whole plant level (Fig. 4).
It was also noted that the frequency of lateral roots and
their ultimate length was unaffected by reduced supply of
nitrogen. However, laterals took much longer to develop
Dry matter partitioning
1249
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30
20
60
100
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Fig. 3. Root weight ratio (RWR) in Betula pendula seedlings as a
percentage of total plant weight at stresses of different nutrients. The
stresses were from 20% of optimum relative addition rate (R) of the
stated nutrient to no stress at optimum (= 100%) (from Ericsson, 1995).
0
5
10
15
20
Days of N addition
Fig. 4. Time-series of weight proportions of subroots in split-root plants
growing at a relative rate of increase in nitrate supply ( = 0.09 d - 1).
The nitrate addition was divided at a ratio of 20: 80 between the two
subroots. Also shown are data from an experiment with perturbed
nitrate additions (from Samuelson et al., 1992).
Downloaded from http://jxb.oxfordjournals.org/ at OUP site access on January 10, 2017
Plate 2. Time-series of plant size at two different , exponentially-increasing availab ilities of N : lower-availability, R A = 10% d - 1; higher-availability,
R A=R u=26% d - 1 . To obtain this picture series, evenly-matched plants were grown with different start dates at the same growth environment.
Note that root size was a larger fraction of plant size at lower-N availability and that the ratio of root-to-plant size was similar on all occasions
within an N treatment (from Linder, 1990).
1250
McDonald et al.
Integration of responses to nutrient deprivation:
feedback loops at the whole-plant level
The longer term acclimation of plant growth to perturbations in nutrient supply can involve a large number of
process interactions which can be described as non-linear
systems with feedback loops. Therefore, there are inherent
difficulties in the quantitative prediction of the integrated
response. In the case of shifts in dry-matter distribution
between roots and shoots, the processes which can result
in differentials in carbon partitioning and transport into
the cells of these organs are particularly interesting. Shifts
in partitioning will be associated with changes in structure
and storage in mature and expanding cells and with
differentials in the initiation of primordia and activity of
meristems in shoots and roots. Thus, dramatic and different shifts in, for example, the branching pattern (apical
dominance) of shoots and roots may be associated with
significant shifts in the partitioning of photoassimilate.
However, another important association is likely to be
that of differentials in the partitioning of photoassimilate
and its incorporation in the walls of expanding cells in
the growing tissues of shoots and roots. This is considered
in more detail with respect to N supply in the next section.
This highlights an area where current advances in the
appreciation of growth activity in the cell wall might
meaningfully be coupled to transport phenomena in the
phloem.
Differentials in cell expansion and transport of
photoassimilates: possible short-term responses
to N deprivation?
When plants experience a degree of N deprivation, the
extent to which a shift in dry matter distribution between
shoots and roots occurs can depend upon the type of
plant (Aerts, 1994). The possibility that differences in
response among plant types might relate to inherent
differences in modes of phloem loading has been discussed
by van Bel and Visser (1994).
Following N deprivation, an inhibition of expansion
growth can proceed more rapidly than the shift in dry
matter distribution (McDonald et al., 1986). Because it
was concurrent with accumulation of starch in leaves,
this rapid inhibition of shoot expansion in Betula pendula
was assumed not to be caused by a carbohydrate limitation to expansive growth. However, it has yet to be
definitively demonstrated that, following N deprivation,
carbohydrate supply is in fact non-limiting to the growth
of cells in individual, expanding leaves. Presumably, carbohydrate metabolism and transport phenomena associated with, for example, the availability of xyloglucan in
the growing-cell wall could be crucial (Fry et al., 1992).
It has been observed that N deprivation leads to rapid
reduction in the expansion of single leaves of Helianthus
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and attain final length at lower nitrogen supply than at
higher supply.
Nutrient proportions can also be important in affecting
the distribution of dry matter amongst plant organs. In
studies where the growth of birch seedlings has been
manipulated by varying the flux of Mg, the allocation of
dry matter between roots and shoots has been affected
by the amount of N absorbed (Ericsson, 1995). Where
excess uptake ofN was restricted (but where plant growth
rate was still being limited by Mg supply), the fraction
of plant dry matter in the root was greater than in plants
with larger amounts of excess N. This may indicate a
central role for nitrogen in modifying the distribution of
dry matter amongst plant organs, even where other
nutrients (at least Mg) are limiting the growth rate of the
plant and determining the general category of dry matter
distribution.
Although the data on dry matter distribution, in
response to different nutrient limitations, provide convincing evidence that the net result of acclimation to limiting
nutrient supply falls into one of two main categories of
dry matter distribution between shoots and roots, they
imply nothing about the mechanistic similarities, or otherwise, of limitations to partitioning and transport of photoassimilates within each category. It will be particularly
interesting to see the outcome of more mechanistic studies
on partitioning of assimilate, such as those described by
Cakmak et al. (1994), but where plants are grown at
controlled nutrient fluxes. The advantage of the approach
will be that the significance of any responses in the physiology of partitioning and transport to a perturbation in
nutrient supply can be assessed in a growth context where
the longer term acclimation, accounting for possible
changes in the initiation and activity of meristems, cell
division and expansion, can be considered alongside
responses in partitioning and transport of photoassimilates. This would appear to be a crucial type of information for plant breeders and agronomists, who may
understandably become increasingly interested in mutants
where expression of genes encoding proteins and enzymes
with a regulatory function in the partitioning and transport of photoassimilates has been manipulated. Obviously,
molecular approaches are providing an extremely useful
tool in furthering our understanding of partitioning and
transport mechanisms (Frommer and Sonnewald, 1995).
However, the importance of manipulated shifts in gene
expression to the overall, longer term phenotype generally
has still to be assessed. It is quite likely that feedback
mechanisms at the whole-plant level, for example, those
which result in differences in the relative amounts of
functional tissues, may effectively oppose apparent gains
at the biochemical level. This is largely uncharted territory
but, as the appreciation of a need to study the longer term
acclimation increases, the nutrient-flux approach is particularly suited to its investigation.
Dry matter partitioning
wall at the time of N decrease. Hormonal balance (ABA
and cytokinins) in the plant can be affected by perturbation in N supply (Clarkson and Touraine, 1994) and
increased ABA concentrations in plants with decreased
availability of N or all nutrients have been reported
(McDonald and Davies, 1996). The causal significance,
however, of ABA involvement in regulating growth processes in the cell wall following N deprivation, has yet to
be established. An interesting possibility is that N deficiency may accelerate the process of cross-linking wall
components, thus making the wall less yielding.
MacAdam et al. (1992) have provided evidence of a link
between growth deceleration and a dramatic increase in
the activity of peroxidase enzymes in the meristems of
tall fescue leaves. Chaloupkova and Smart (1994) have
recently reported the ABA-stimulated induction of a
novel peroxidase in Spirodela. Apparently, induction is
antagonized by high cytokinin concentrations.
Because the initial events in cell expansion are associated with loosening in the cell wall and because there is
no apparent basis for assuming a carbohydrate limitation
to the activities of wall-loosening enzymes and proteins
discussed above, it may be more logical to think of shifts
in carbohydrate partitioning between shoot and root
growth as a consequence of differential, wall-loosening
activities rather than a cause of such (Fig. 5). Following
a decrease in nitrate supply, the proportional decrease in
average cell length was found to be less in the root cortex
than in epidermal cells of the leaf in sunflower
(McDonald, unpublished results). This was associated
with a decrease in plant growth rate and an increasing
fraction of plant dry matter in the root. If the wallloosening events constitute important components of
sink strength by, for example, creating sites for the subsequent incorporation of xyloglucan, then differentials in
loosening could create differentials in sink strength, ultimately affecting the directional fluxes of sucrose. With
time, other components of sink strength such as secondary-wall thickening and the differential activity of root
and shoot meristems will presumably contribute to the
fate of sucrose in the phloem. However, part of the shift
in dry-matter distribution with respect to perturbation in
N supply may be a direct result of differences in sink
strength induced by differences in the extent to which the
expansion of root and leaf cells is inhibited by N deprivation (McDonald and Davies, 1996). This view sees shortterm shifts in the transport of photoassimilate amongst
organs as being the result of N-induced differentials in
component cell expansion.
Conclusions: testing the phenotype
The importance of testing the significance of any apparent
shifts in the mechanistic pathways pertaining to partitioning and transport of photoassimilates at the level of
Downloaded from http://jxb.oxfordjournals.org/ at OUP site access on January 10, 2017
annuus (Palmer et al., 1996). Here, reduction in leaf
expansion was attributed to reduced cell growth in older
expanding leaves, but may also have partly been attributable to a reduced rate of cell production in younger
leaves. Reduction in final leaf size was accounted for by
a decrease in the size of epidermal cells in Salix viminalis
(McDonald, 1989). However, reduced cell numbers were
more important in accounting for decreased blade extension in N-limited Festuca sp. (MacAdam et al., 1992). It
is concluded that cell division and cell expansion can
both be reduced following N deprivation and that reduced
numbers and activity of meristems may assume increasing
importance with time in inhibiting the development of
shoot area (Dale and Milthorpe, 1983). Here, consideration is given to a possible relationship between the regulation of photoasimilate transport and cell expansion
following N deprivation, which might contribute to
observed shifts in dry-matter partitioning among plant
organs.
Radin and Boyer (1982) addressed the question of
possible mechanisms of growth reduction in leaves of
Helianthus annuus when plants were grown in solutions
of lower N concentration compared with leaves on plants
at higher N concentration. (The problems with this
approach in studying the effects of N deprivation were
discussed above.) They found that the hydraulic conductivity of the root was decreased at lower N concentration
and that, during periods of high transpiration, the water
potential of growing leaves was reduced on plants at
lower-N compared to leaves of plants at higher N. They
concluded that the reduction in calculated turgor may
have accounted for the observed reduction in leaf expansion at lower N.
Presumably, this may have been attributable to either
a reduced turgor-driven rate of plastic deformation or to
expansion ceasing in a number of cells where P - Y = < 0
(where P is cell-turgor pressure and Y is the yield turgor
requirement for irreversible wall movement; McDonald
and Davies, 1996). More recently, Palmer et al. (1996)
have measured turgor with a pressure probe in epidermal
cells of growing leaves of Helianthus annuus following a
rapid depletion of N about the roots. These authors did
not find a significant difference in turgor between cells
measured either before or after N deprivation or between
cells in similar leaves of control and N-deprived plants.
Here, it would seem likely that the rapid reduction in leaf
expansion was attributable to changes in cell-wall
properties.
It is possible that effects of N supply on cell-wall
growth relate directly to N-substrate limitations to the
synthesis of wall enzymes such as XET (Fry et al., 1992)
and other wall proteins in the expansin class (McQueenMason et al., 1993; McQueen-Mason, 1995). It is also
possible that N deprivation may be more indirect,
affecting the activity of XET and expansins in the cell
1251
1252
McDonald et al.
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Fig. 5. Causal-loop diagram showing a possible cell-growth induced
flow of photoassimilate. Differences among cell-expansion rates will
cause differentials in the flow of photoassimilate. The ± symbols
indicate the assumed effect (enhancement/reduction) of increasing the
value of one variable on the value of another variable.
whole-plant performance has been emphasized here. This
is a particularly relevant consideration given the enormous number of possibilities associated with moleculargenetic manipulation. Whilst acknowledging the exciting
advances that have been made in the partitioning of
photoassimilates with molecular approaches, it is because
of the apparently enormous possibilities of feedback
interaction within the whole plant and, because of the
hierarchical nature of plant structure and function, that
there should be serious reservations about the predictive
ability in moving from the level of gene expression to the
phenotype. This may appear to be a somewhat trivial
point but it is considered to lack emphasis in the current debate.
The importance of assessing the phenotype under conditions of controlled nutrition has been emphasized,
aspects of mineral nutrition have been discussed and the
use of techniques pertaining to steady-state nutrition have
been advocated. This is believed to be particularly important in physiological studies pertaining to partitioning and
transport of photoassimilates because these phenomena
are particularly sensitive to perturbations in nutrient
supply.
Aerts R. 1994. The effect of nitrogen supply on the partitioning
of biomass and nitrogen in plant species from heathlands
and fens: alternatives in plant functioning and scientific
approach. In: Roy J, Garnier E, eds. A whole plant perspective
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\
growth promoterJ+
enzymes, proteins
+
~
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+
A closer functional emphasis in coupling the regulation
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