Journal of Experimental Botany, Vol. 49, No. 318, pp. 59–68, January 1998
Ontogenic changes in the construction cost of leaves,
stems, fruits, and roots of tomato plants
Christian Gary1,5, Jacques Le Bot2, Jean-Sylvain Frossard3 and Jerônimo Luiz Andriolo4
1 Institut National de la Recherche Agronomique, Unité de Bioclimatologie, Domaine St-Paul, Site Agroparc,
F-84914 Avignon Cedex 9, France
2 Institut National de la Recherche Agronomique, Unité de Recherches en Ecophysiologie et Horticulture,
Domaine St-Paul, Site Agroparc, F-84914 Avignon Cedex 9, France
3 Institut National de la Recherche Agronomique, Unité Associée Bioclimatologie-PIAF, Site de Crouël,
234 avenue de Brézet, F-63039 Clermont-Ferrand Cedex 2, France
4 Universidade Federal de Santa Maria, Centro de Ciências Rurais, Departamento de Fitotecnia,
97119–900 Santa Maria, RS, Brasil
Received 31 January 1997; Accepted 3 September 1997
Abstract
The construction cost of a plant tissue, i.e. the amount
of photoassimilates used in the synthesis of a unit
weight, varies with its biochemical composition. Crop
modellers use standard values published for a few
groups of cultivated species. Yet, there are also intraspecific variations in the construction cost in relation
with the development of the plant or organ. This
research aimed at analysing the ontogenic changes in
the construction cost of leaves, stems, roots, and fruits
of tomato plants and the specific contribution of the
mineral content to these changes. For that purpose,
samples were harvested from the vegetative phase to
the beginning of fruit production. The estimation of
the construction cost was based on the contents of
carbon, nitrogen and ash. In leaves, the construction
cost decreased with the physiological age whereas, in
stem internodes, it varied with the sympod number.
These ontogenic changes could partly be explained by
different accumulations of minerals. In contrast, the
construction cost and the mineral content of fruits and
roots remained fairly stable. On a whole plant basis,
the construction cost of the bulk of each category of
organs varied much less. Most of the increase in the
mean construction cost of the whole plant during the
experiment was due to changes in the allocation ratio
between the vegetative parts and the fruits. Attention
of crop modellers is drawn to the importance of a
precise estimation of the construction cost and to the
existence of ontogenic changes at the whole plant and
organ levels.
Key words: Lycopersicon esculentum Mill., construction
cost, mineral content, ontogeny, carbon content.
Introduction
In crop models, the net production of assimilates is
converted into an increment of dry matter through a
parameter C named ‘construction cost’ and generally
expressed in grams equivalent glucose per gram dry
matter, or through a parameter Y named ‘growth yield’
g
that is the inverse of the construction cost (for an inventory of the terminology used in this field, see Gary et al.,
1995). The construction cost varies with the composition
of the synthesized plant material. For example, a unit
weight of lipids is made from about 2.5 times more
glucose than a unit weight of carbohydrates as an extra
amount of glucose has to be metabolized to produce the
higher amounts of reducing power (NADH ) and energy
(ATP) that are involved in the synthesis of the former.
Tables of values of the construction cost are available in
the literature for a limited number of species. For vegetative organs ( leaves, stems), typical values of construction
cost have been proposed by Penning de Vries et al. (1989)
for the bulk of three groups of species: leguminous crops,
rice crops and ‘non leguminous except rice’ crops. For
storage organs, as there is more variation in their bio-
5 To whom correspondence should be addressed. Fax: +33 4 90 89 98 10. E-mail: [email protected]
© Oxford University Press 1998
60
Gary et al.
chemical composition, Penning de Vries et al. (1983)
calculated median construction costs for 23 species on
the basis of proximal analyses found in the literature.
Information on the construction cost of tomato tissues is
still scarce. The values presently used in tomato crop
models (Bertin and Heuvelink, 1993) have generally been
considered as close to the mean value for ‘non-leguminous
except rice’ species for the vegetative parts (1.46, 1.51
and 1.44 g g−1 for leaves, stems and roots, respectively)
and to the estimation of Penning de Vries et al. (1983)
for fruits that is based on a reference of chemical composition for mature fruits (1.42 g g−1). Recent estimations
(Gijzen, 1994; Gary et al., 1998) showed that the construction cost of tomato tissues are actually lower than these
standard values. For tomato vegetative tissues, this is due
to a high mineral content, at least as high than those
observed in rice crops (0.15 g g−1 in Penning de Vries
et al., 1989), and in the case of tomato fruits, it is due to
an overestimation of the content in energy-rich compounds ( lignin, estimated from the fibre content).
In their estimations of the construction cost of plant
tissues, crop modellers have focused on interspecific variations although there is some evidence that intraspecific
variation is not negligible. It can be linked either to
environmental factors (Griffin, 1994; Poorter, 1994) or to
the development of the crop or organ. At the whole plant
level, Stahl and McCree (1988) clearly gave evidence of
an increase of the growth yield throughout the development cycle of a grain sorghum crop. They concluded that
this was the result of changes in the allocation ratios
between various organs: during the crop cycle, there is a
shift from the synthesis of leaves and roots to the synthesis
of stems, sheaths and grains. The first are richer in
proteins and their construction cost is higher. This hypothesis was integrated in a sorghum crop model (McCree,
1988). The use of specific construction costs for each
class of organs enabled a better simulation of experimental
data than the use of a single constant construction cost.
At the organ level, there are also ontogenic variations in
the biochemical composition (cf. for example the studies
of Merino et al., 1984, on leaves of chaparral shrubs and
Walton and de Jong, 1990, on kiwifruit berries) that
induce changes in their construction cost. Changes in
mineral content also need to be accounted for: VarletGrancher (1982) carried out a comprehensive study of
the time-course of the heat of combustion and of the ash
content of the various organs of maize plants. He observed
a simultaneous decrease in the ash-free heat of combustion
and in the ash content during the development of ears
and leaf blades and sheaths. The decrease in the dilution
of organic biomass by minerals compensated for the
decrease in the energy content of organic biomass, which
explained a fairly stable construction cost of the biomass.
The estimations of the construction cost of tomato tissues
published in Gijzen (1994) and Gary et al. (1998) were
obtained on plants in production, i.e. composed of a set
of about eight sympodial units with leaves, stem internodes and fruits of different physiological ages. Even
though organs of different age classes were present on
the plants, the question of the ontogenic changes of the
construction cost at the scale of the whole plant was not
fully addressed by these authors.
In the present study, samples of leaves, stems, fruits
and roots were analysed five times during the 2 months
from the end of the vegetative phase until the beginning
of harvest of tomato plants in order to (1) be able to
observe possible ontogenic changes in the construction
cost during the development of several sympodial units
and (2) analyse the contribution of the mineral content
in these changes. These observations were carried out
during the time when the plant experienced major changes
in the pattern of dry matter allocation. It must be stressed
that the expected information was not the construction
cost of new biomass at specific points of time that is
needed in crop models. It was the energy value (in
equivalent glucose) of the biomass accumulated since the
appearance of the analysed organs, i.e. the cost of synthesizing biomass possibly altered by subsequent changes (e.g.
storage, translocation, turn-over of some compounds).
Such estimations of the construction cost were based on
the contents in carbon, nitrogen and ash, an approach
proposed by Vertregt and Penning de Vries (1987) and
modified by Walton et al. (1990), Poorter (1994) and
Gary et al. (1995). Ontogenic changes in the construction
cost of organs could be observed, particularly in leaves
where they could be associated with the accumulation of
minerals. At the whole plant level, ontogenic changes of
the construction cost could be linked to changes in the
allocation ratio of biomass among vegetative and generative organs. Because of the sensitivity of the dry matter
production to the construction cost in crop models, the
importance of estimating its actual value and possible
sources of variation is stressed.
Materials and methods
Cultivation of plants and preparation of samples
Tomato seeds (Lycopersicon esculentum Mill. cv. Rondello) were
sown in small rockwoll cylinders on 5 February 1993. After
three leaves had appeared, the seedlings were transplanted
under a greenhouse located in southern France (INRA,
Avignon). The plants grew in a recirculating hydroponic system
made of series of 10 l pots linked with a 2 m3 reservoir and in
which the nutrient solution continuously circulated. The
composition in the major elements of the nutrient solution was
4.0 mol m−3 KNO , 0.9 mol m−3 K SO , 1.5 mol m−3 KH PO ,
3
2 4
2 4
3.75 mol m−3 Ca(NO ) , and 1.0 mol m−3 MgSO . pH was
3 2
4
checked daily and controlled to 5.5–6.0 using HNO . The
3
composition of the nutrient solution in the major cations and
anions was determined twice a week by atomic absorption
spectrophotometry and HPLC in order to identify the drifts in
the mineral balance. Temperature set-points were 20 °C during
Ontogenic changes in construction cost
the day and 16 °C at night. The development of plants was
sympodial and indeterminate. All axillary buds were removed
so that sympodial units made of three leaves and one
inflorescence (or truss) appeared successively, once every 8–10 d,
along the single stem. The pruning policy was to keep four
fruits per truss and remove all axillary buds.
Three plants were harvested every 2 weeks from flowering
until harvest of the first inflorescence (or truss), i.e. from the
end of the vegetative phase until the beginning of the production
period. At the end of the experiment, the tenth truss (on top of
the canopy) was setting fruits while fruits on the first truss (in
the bottom of the canopy) were maturing. In each sympodial
unit, all fruits were pooled in one ‘fruit’ sample, all leaf blades
in one ‘leaf ’ sample and all stems and petioles in one ‘stem’
sample. The first sympodial unit was split in two fractions: the
first truss with the surrounding leaves (two under and one
above the truss) and the corresponding internodes belonged to
sympod unit 1 and all the leaves and internodes appeared
before belonged to sympod unit 0. The physiological age of the
sympodial units was expressed in number of trusses from the
truss that was setting fruits at the moment of plant harvest. All
samples were dried in an air-forced oven at 70 °C during 72 h,
except the fruits which were dried for up to a week. They were
subsequently ground to about 60 mm.
Determination of the carbon, nitrogen and mineral content
Carbon and nitrogen contents were determined on the three
replicas wheareas mineral content was on one only. Total
carbon and nitrogen contents were measured directly on plant
samples using an automatic elemental analyser (Carlo-Erba
model 1500, Milan, Italy). The analysis was performed on 4 mg
aliquots after a flash combustion at 1030 °C in a pure oxygen
atmosphere. After reduction of all carbon and nitrogen oxides
by copper at 750 °C, carbon dioxide and free nitrogen gas were
separated by gas chromatography and analysed by thermal
conductivity. A residual moisture determination was carried
out on aliquots (24 h at 105 °C ) to enable the carbon and
nitrogen results to be converted to a 100% dry matter basis.
Nitrate concentration in the plant tissues was measured on
water-extracted samples using an automatic chemical analyser
(Quanta, Nice, France) based on the principle of reduction of
NO− into NO− by a mixture of copper/hydrazine sulphate,
3
2
and subsequent reaction with sulphanilamide and NNED to
form a coloured compound absorbing light at 520 gm.
Cation concentration determinations ( K, Ca, Mg) in the
plant tissues were carried out on 500 mg aliquots after dry
combustion in a muffle furnace for 12 h at 420 °C, dissolution
of the ashes in 0.1 N HNO and dilution to 50 ml in volu3
metric flasks. The determination was performed on an atomic
absorption spectrophotometer (Perkin-Elmer 2100, Uberlinger,
Germany). The ash content was measured by weighing the
whitish-grey ashes remaining from this dry combustion. It gave
a biased estimation of the mineral content. In these experimental
conditions, there was certainly no more organic material after
combustion: Gurakan et al. (1990) showed that samples of
microbial biomass burnt over 450 °C contain no more carbon,
hydrogen and nitrogen. Yet organic acids can be transformed
in carbonates and some elements like sodium, chloride or
sulphur may volatilize at high temperatures (Masle et al., 1992).
To estimate the mineral content from the ash content, several
corrections have been suggested in the literature. Vertregt and
Penning de Vries (1987) proposed an approximation of the
mineral content based on the hypothesis that inorganic ions
and organic anions would have the same weight and that the
latter would be transformed into carbonates during ashing with
61
a weight reduction of 50%; consequently, the weight of minerals
would be 2/3 of the weight of ashes. Poorter and Bergkotte
(1992) subtracted to the ash content a value (in g g−1) equal
to 30 times the ash alkalinity (in meq g−1) to correct for oxides.
When using the latter method, Gijzen (1994) obtained the
mineral content by reducing the ash content by 2% (in stem
tissues) to 18% (in leaf tissues). Masle et al. (1992) compared
the ash content of plant material to the sum of the concentrations
of all mineral elements above trace level measured by X-ray
spectrometry. On wheat leaves, the ash content overestimated
the mineral content by about 20% yet large variations were
observed among genotypes. On tomato tissues, Gary et al.
(1998) compared the ash content to the complement of the
fraction of organic material (carbon+hydrogen+oxygen+
nitrogen contents). If the ash content overestimated the mineral
content by about 15% in leaves, it was similar to it in stems
and underestimated it in fruits. On the basis of the present
uncertainty about the proper correction to apply to the ash
content, it was decided to present the results on the basis of the
measured ash content and to discuss when needed the
consequences of the subsequent ovestimation of the mineral
content on these results.
Calculation of the construction cost
The construction cost C (g g−1) was calculated from the heat
of combustion DH (kJ g−1) ( Williams et al., 1987), here
c
estimated from the carbon content c∞ (mol g−1), and the
nitrogen content n∞ (mol g−1):
A
B
180.15
1
1
DH +
kn∞
(1)
c
E
15.65
24
G
E is the growth efficiency, i.e. the fraction of the energy
G
retained in the products during the biosyntheses of the various
compounds of the biomass. Its mean value was estimated to be
0.88 by McDermitt and Loomis (1981). k is the valence of
nitrogen in the substrates used for biosyntheses; it was set here
to −3 as ammonium was considered to be the only source of
nitrogen. Coefficients 15.65 and 180.15 are the heat of
combustion (kJ g−1) and the molar mass (g mol−1) of glucose,
respectively, and 24 is the number of electrons involved in the
oxidation of one mole glucose. The heat of combustion of the
organic matter can be estimated from its carbon content with
the linear relation retained for tomato tissues in Gary et al.
(1998) and recalculated from Walton et al. (1990); for dry
matter (organic matter+minerals), this relation becomes:
C=
DH =(1−A)[648.5c∞/(1−A)−7.333]
(2)
c
where A (g g−1) is the mineral content. This method of
estimating the construction cost is based on the approach
proposed by Vertregt and Penning de Vries (1987) to which a
correction for the reduction status of nitrogen was added, as
suggested by Poorter (1994) and Gary et al. (1995). For this
correction, only the organic nitrogen (total N minus nitrate N )
values presented in Andriolo (1995) were considered.
Results
Dry matter partitioning
From the flowering of the first truss to the beginning of
harvest, the growth of the tomato plants presented a
fairly classical pattern. In 8 weeks, i.e. from the first to
the fifth plant harvest, the ratio of dry matter allocation
62
Gary et al.
increased from 0 to 41% for fruits and decreased from
41% to 25% for leaves and from 40% to 25% for stems.
In the meanwhile, the ratio of dry matter allocation to
roots decreased from 18% to 8% with a decrease in dry
weight during the last 2 weeks.
Mineral content
The ash content was lower in fruits (the mean value was
0.10 g g−1) than in vegetative organs (in leaves, stems
and roots, the mean values were 0.22, 0.17 and 0.17 g g−1,
respectively). It varied more in leaves than in stems, roots
and fruits. In leaves and on all sympodial units, there
was an accumulation of minerals with time, at least until
6 May as the profiles of ash content were fairly similar
on 6 and 18 May ( Fig. 1A). In contrast, the ash content
of stems varied less with time (except between 25 March
and 8 April ), but remained higher in the first sympods
than in the others ( Fig. 1B). In fruits, the ash content
did not vary either with the physiological age of the organ
or with its position on the plant (Fig. 1C ). The root
system presented a high content in minerals all along the
experiment ( Fig. 2).
Despite the ontogenic increase in ash content of leaves,
the mean ash content per plant did not vary more in
leaves than in the other organs during the experiment
( Fig. 2).
The cumulated content of K, Ca and Mg represented
about half the ash content in all the samples. Mg content
was lower than 0.01 g g−1 whereas K and Ca contents
attained 0.09 g g−1 in stems and 0.08 g g−1 in roots,
respectively. In leaves, Ca content increased with the ash
content and was higher than K content whereas in stems
and roots K content increased with the ash content and
was higher than Ca content ( Fig. 3). In fruits, K content
was much higher than Ca content.
Carbon content
The carbon content was higher in fruits (the mean value
was 0.44 g g−1) than in vegetative organs (in leaves, stems
and roots, the mean values were 0.38, 0.36 and 0.37 g g−1,
respectively). In leaves and on all sympods, the carbon
content decreased with time until 6 May and then stabilized (Fig. 4A). In stems, the carbon content was lower in
the first sympods than in the others ( Fig. 4B). In fruits
( Fig. 4C ) and roots ( Fig. 2), it remained fairly stable all
along the experiment. There was clearly a relation between
the variations in ash and carbon content. When expressed
per unit organic biomass (dry matter minus ashes), the
carbon content presented less changes either with the
physiological age or with the position of the organ: the
standard deviation lowered from 2.6 to 1.5 g g−1 in leaves,
from 2.7 to 2.1 g g−1 in stems and increased slightly in
fruits, from 1.0 to 1.4 g g−1. Similarly, when expressed
per unit organic biomass, the carbon content presented
Fig. 1. Ash content in leaves (A), stems (B) and fruits (C ) of tomato
plants. Sympods 1 to 10 include two leaves under and one leaf above
the truss and the corresponding stem internodes; sympod 0 includes all
the leaves under sympod 1. (&) 25 March; (%) 8 April; (+) 22 April;
(6) 6 May; ($) 18 May.
Ontogenic changes in construction cost
Fig. 2. Time-course of the ash (closed symbols) and carbon (open
symbols) contents of the bulk of leaves (+), stems (&), roots (2) and
fruits ($) between the end of the vegetative phase and the beginning
of harvest of tomato plants. Carbon content=mean±95% confidence
interval.
Fig. 3. Relations between the ash content and the contents in K (closed
symbols) and Ca (open symbols) in leaves (+), stems (&), roots (2)
and fruits ($).
less differences among organs (the mean values were
0.49 g g−1 in fruits and leaves and 0.44 and 0.45 g g−1 in
stems and roots, respectively). Because of the relative
stability of the carbon content in the organic biomass
and of the mean ash content at the whole plant scale, the
mean carbon content of the bulk of leaves, stems and
fruits varied by less than 0.03 g g−1 (Fig. 2).
Fig. 4. Carbon content in leaves (A), stems (B) and fruits (C ) of
tomato plants. Sympodial units 1 to 10 include two leaves under and
one leaf above the truss and the corresponding stem internodes;
sympodial unit 0 includes all the leaves under sympodial unit 1. Each
value is the mean of three replicates. (&) 25 March; (%) 8 April; (+)
22 April; (6) 6 May; ($) 18 May.
63
64
Gary et al.
Construction cost
The measurements of carbon, nitrogen and ash content
enabled the calculation of the construction cost for the
various tissues. The content in reduced nitrogen in the
organic biomass decreased with the physiological age of
the organ in leaves (from 0.06 to 0.04 g g−1) and stems
(from 0.03 to 0.02 g g−1) and remained constant in fruits
(0.03 g g−1) and roots (0.04 g g−1).
The construction cost of leaves decreased significantly
(P<0.05) with their ageing whereas it remained fairly
constant in stems (Fig. 5) and roots ( Fig. 7) and slightly
increased in fruits ( Fig. 5). These trends were linked to
the different dynamics of mineral accumulation in the
various groups of organs. More generally, the differences
in construction cost among organs, age classes and sympodial units could partly be explained by differences in
ash content ( Fig. 6). When expressed on an ash-free basis,
the construction cost of leaves increased significantly
(P<0.05) with ageing. This trend actually depended on
the method of calculating the mineral content from the
ash content. When the rule proposed by Vertregt and
Penning de Vries (1987) was adopted (mineral content=
2/3 ash content), there was no ontogenic variation in the
construction cost of the organic fraction of the leaf
biomass (Fig. 5).
At the scale of the whole plant, the ontogenic changes
in the construction cost were buffered (Fig. 7). During
the time of the experiment, the construction cost of the
bulk of leaves decreased as the number of aged leaves
increased and the construction cost of the whole stem
increased as the ash content of the last internodes
decreased. But the mean construction cost of the whole
plant increased more because of the increasing generative/
vegetative weight ratio than because of the ontogenic
changes observed in each group of organs. This is proved
by the comparison with the time-course of the construction cost of the whole plant that is calculated from
constant values (mean of the five plant harvests) of the
construction cost of each group of organs and from the
allocation ratios of biomass ( Fig. 7).
Discussion
Ontogenic changes in construction cost at the organ and
whole plant levels
This study has verified that the construction cost of the
vegetative organs of tomato plants is fairly low, compared
to other species. On plants in production (harvested at a
later development stage than in the present study), Gijzen
(1994) found mean values of the construction cost of
1.16, 1.10 and 1.57 g g−1 for leaves, stems and mature
fruits, respectively whereas Gary et al. (1997) estimated
these mean values to be 1.05, 1.02 and 1.27 g g−1 for
leaves, stems and fruits, respectively. In both publications,
the estimation of the construction cost was based on the
carbon content. Gijzen (1994) applied Vertregt and
Penning de Vries’ method (1987) designed for high carbon
content tissues instead of that of Walton et al. (1989)
better adapted to low carbon content tissues, which
certainly led to an overestimation of the construction cost
(by 0 to 7%) and the mean values reported here from his
work do not take into account the relative weight of the
various age classes of organs in the whole plant. The
construction cost of the vegetative organs of tomato
plants is in the lower 10% of the data compiled from a
large number of sources by Poorter (1994) for leaves,
stems, roots, and seeds/fruits of herbaceous and woody
species (for comparison, our data were recalculated with
NO− as the N-source, i.e. k=+5 in Equation 1) whereas
3
the construction cost of fruits is among the upper 10%.
Ontogenic changes in the construction cost appeared
clearly at the scale of the organ. In the case of tomato
leaves, the decrease of the construction cost with the
physiological age was mainly linked to the accumulation
of minerals. Such an approach of the time-course of the
energetic value of each organ class on a plant is probably
original. Merino et al. (1984) measured the biochemical
composition and estimated from it the construction cost
of five age classes of leaves on three chaparral shrubs. In
two species, they observed an ontogenic decrease of the
construction cost of leaves. They could show that, on this
plant material, such a decrease was explained both by an
accumulation of minerals and by a shift from the synthesis
of costly compounds ( lipids) to the synthesis of cheap
structural compounds (cellulose and hemicellulose).
Walton et al. (1990) carried out a similar approach on
kiwifruit berries and showed that the construction cost
decreased when organic acids accumulated then increased
during the formation of the seeds and the synthesis of
their lipid reserves without any significant contribution
of the ash content that remained low. These examples
show that the analysis of the ontogenic changes in the
construction cost of organs cannot be done without a
clear separation of the respective effects of changes in the
mineral content and of changes in the composition of the
organic fraction. The present uncertainty in the estimation
of the mineral content from the ash content does not
facilitate such an analysis.
At the scale of the whole plant, the changes in the
Fig. 5. Ontogenic changes in the construction cost of leaves (A), stems (B) and fruits (C ) of tomato plants, calculated on a dry weight basis (closed
symbols) or on an organic basis (open symbols). In the latter case, the mineral content was calculated as 2/3 of the ash content ( Vertregt and
Penning de Vries, 1987). The physiological age was calculated as the number of trusses from the truss setting fruits (which is aged 0). Mean±95%
confidence interval.
Ontogenic changes in construction cost
65
66
Gary et al.
(1982) observed on whole maize plants that variations in
the heat of combustion depended either on changes in the
allocation of biomass (when grain formation started ) or
on changes in the ash content (during the panicle differentiation). Then the patterns of ontogenic changes in the
construction cost of the whole plant and the contribution
of the mineral content vary among species.
Dilution of the organic matter by the accumulation of
minerals in greenhouse tomato crops
Fig. 6. Relation between the ash content and the construction cost of
leaves (6), stems (%), roots (1), and fruits (#). Full line: regression
for leaf samples ( y=−0.85×+1.24, R2=0.66). Dotted line: regression
for stem samples ( y=−2.05×+1.34, R2=0.72).
Fig. 7. Time-course of the mean construction cost of the bulk of leaves
(+), stems (&), roots (2) and fruits ($) and of the whole plant (bold
line) between the end of the vegetative phase and the beginning of the
production period of tomato plants. The dotted line is the construction
cost of the whole plant calculated from the mean construction cost of
each group of organs (on the five plant harvests) and from the allocation
ratios of biomass.
construction cost of tomato plants were mainly attributed
to the time-course of the allocation ratio of dry matter.
This pattern could be observed in our experiment as the
plants transited from the vegetative phase to the beginning
of fruit production. Later in the crop cycle, when the
plants are maintained in a steady-state with the development of new trusses and leaves compensated by the
harvest of mature fruits and the pruning of old leaves,
there should be no more changes in the construction cost
of the whole plant. Similarly, Stahl and McCree (1988)
attributed ontogenic variations in the construction cost
of grain sorghum plants to changes in the allocation ratio
of dry matter among organs. But Varlet-Grancher et al.
This study confirms that high amounts of minerals accumulate in tomato, and particularly in the vegetative
organs. Such a feature has been observed on other
greenhouse crops such as cucumber (Challa, 1976;
Marcelis and Baan Hofman-Eijer, 1995; Schapendonk
and Challa, 1981), sweet pepper and eggplant (Gijzen,
1994) yet it is fairly unusual among plant tissues. The
values published (Gijzen, 1994; Gary et al., 1998) or
measured in this study for tomato organs are close or
belong to the upper 10% of the compilation of data
published by Poorter (1994). The highest value measured
on shoots of 24 wild species by Poorter and Bergkotte
(1992) was 0.16 g g−1. Masle et al. (1992) measured on
leaves of field crops maximal values of 0.11 g g−1 in
sunflower, 0.09 g g−1 in wheat and 0.15 g g−1 in tobacco.
In their study, the mineral contents varied by 18–35%
among genotypes and by 10% or less under environmental
changes (vapour pressure deficit and nutrient availability).
The reasons why tomato plants, and other greenhouse
crops, accumulate relatively larger quantities of minerals
in their tissues than plants grown in the soil, still remain
unclear. The mineral content in the plant results from the
balance between the demand of the growing organs and
the ability of the root system to absorb nutrients. In the
present study, plants were grown in a hydroponic system
on a nutrient solution with a composition close to those
encountered in commercial greenhouses, i.e. with concentrations in elements generally higher than the standard
Hoagland solution. There is a relation between the availability of nutrients in the root medium and their concentration in plant tissues (shoots and roots of various species
in Pitman, 1988; tomato leaves in Seresinhe and Oertli,
1989). The range of concentrations of K and Ca we
observed fits with the values reported by Pitman (1988)
for the highest concentrations of these elements in the
nutrient solution. The accumulation of minerals in ageing
leaves indicates that the balance between import from the
xylem, growth and export to the phloem changed with
ontogenesis. But, at present, no rationale can be proposed
to explain and, further, model the dynamics of accumulation of minerals in the various organs of tomato plants.
Such high mineral contents dilute the organic matter
and lead to the observed low construction costs. There is
indeed a negative correlation between the ash content
Ontogenic changes in construction cost
and the energy content of biomass; the ash content is
even used as a predictor of the heat of combustion of
biomass in fuel engineering (Jenkins, 1989).
Which coefficients of construction cost should be used in
tomato crop models?
The discrepancy between the few estimations of the
construction cost of tomato organs (this paper, Gijzen,
1994; Gary et al., 1998) and the standard values that
have been used until now by tomato crop modellers
(Bertin and Heuvelink, 1993) questions the use of standard values when starting on a new species. Furthermore,
interspecific variations can be significant, because of
changes either in the biochemical composition and/or in
the mineral content of organs. Various methods are
available for the estimation of the construction cost; on
the same samples, they produce values that may differ by
up to 10% ( Walton et al., 1990). Any error in the
estimation of the construction cost will have direct consequences in crop models on the calculation of the dry
matter production from the net assimilation rate of CO .
2
The process of conversion of photoassimilates into biomass is quite fundamental and deserves at least as much
attention and experimental effort as the other processes
(such as photosynthesis and dry matter partitioning)
during the calibration of a crop model.
In this study, the analysis of the ontogenic changes in
the construction cost differed at the organ and whole
plant levels. If a tomato crop model aims at simulating
the growth of the bulk of a category of organs (vegetative
and generative organs or leaves, stems, roots and fruits),
using a mean construction cost for each category is
possible. It provided in the present study a satisfactory
estimation of the construction cost of the whole plant
( Fig. 7). At the scale of the organ, ontogenic changes in
the construction cost of leaves were observed. If a tomato
crop model is designed to simulate the growth of individual organs or sympodial units, then it should take into
account these ontogenic changes. In that case, two difficulties have to be overcome. First, the estimations of the
construction cost are based on the analysis of tissues that
are the result of the integration of growth during a period
of time, whereas the crop model needs at each time step
a conversion factor for the increment of biomass. Such a
conversion factor should integrate possible changes in the
composition of the organic fraction of the new biomass
(it has been shown that they are fairly limited, Fig. 5)
and in the accumulation of minerals (information on its
dynamics is still scarce and descriptive). Second, changes
in the composition of an organ may be due to the
remobilization of some compounds from other organs
and their synthesis should not be accounted for twice
(Stahl and McCree, 1988). The present status of our
knowledge in this field limits the ability of crop models
to simulate small units within the plant.
67
Acknowledgements
We are grateful to Claude Sarrouy for building the experimental
set-up, to Béatrice Brunel for growing plants and preparing
samples and to Monique Bonafous, Magali Augé and Laurent
Gomez for analysing them carefully. During this research,
Jerônimo Andriolo received a Brazilian CAPES fellowship.
Nadia Bertin is acknowledged for her critical reading of the
manuscript.
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