The plant

Whole plant

Photosynthates acquired by leaves are used for the
production of leaves, stems, roots, and reproductive
organs. Increase in allocation to the leaf would be beneficial
for photosynthesis, but may reduce other functions
such as nutrient uptake and reproduction. We
studied changes in biomass allocation with CO2 elevation
with respect to the balance between enhanced
photosynthesis and other functions.
Maximisation of relative growth rate at elevated CO2
concentrations
Plants respond to an alteration of nitrogen availability
by changing their root/shoot (R/S) ratio. Brouwer
(1962) and Davidson (1969) proposed the ‘‘functional
equilibrium’’ hypothesis; i.e. the R/S ratio changes to
maintain the activity ratio between the shoot and root.
According to this hypothesis, any environmental changes
that increase root activity would decrease the R/S
ratio and any environmental changes that increase
shoot activity would increase the R/S ratio. As elevated
CO2 increases photosynthetic activity of the leaf, the
functional equilibrium predicts an increase in the R/S
ratio and a reduction in leaf mass ratio (LMR, the
fraction of plant mass in the leaf) in plants growing at
elevated CO2. However, LMR in actual plants does not
necessarily respond to elevated CO2 as expected (Stulen
and den Hertog 1993; Luo et al. 1999). While some
studies showed a reduction in LMR at elevated CO2 as
expected (e.g. Larigauderie et al. 1988; Wilson 1988),
others showed that LMR did not change with CO2
elevation (e.g. Pettersson et al. 1993; Curtis and Wang
1998).
Many studies have shown that leaf mass per unit
area (LMA) consistently increases under elevated CO2
(Poorter et al. 1996; Yin 2002). LMR and LMA are
important parameters to describe plant growth. Relative
growth rate (RGR, growth rate per unit plant
mass) is factorised into three components: RGR=
NAR · LMR/LMA, where NAR is net assimilation
rate (growth rate per unit leaf area). This equation
indicates that an increase in LMA reduces RGR. Increase
in LMA at elevated CO2 has been ascribed to
accumulation of non-structural carbohydrates as a result
of a source-sink imbalance (Poorter et al. 1997).
However, Luo et al. (1994) suggested a possible
advantage of increasing LMA under elevated CO2,
because it would contribute to increasing leaf nitrogen
content per unit area (Narea): Narea=Nmass · LMA,
where Nmass is leaf nitrogen concentration per unit
mass. The decrease in Nmass as a result of elevated CO2
may be compensated for by an increase in LMA to
maintain a high Narea (Luo et al. 1994; Peterson et al.
1999). However, the effect of increased LMA on wholeplant
growth has not been studied (but see Hirose
1987). Hilbert et al. (1991) studied the optimal biomass
allocation under elevated CO2, but did not consider the
effect of LMA.
To test the hypothesis that an increase in LMA at
elevated CO2 benefits plant growth by maintaining a
high Narea, we raised P. cuspidatum at ambient and elevated
CO2 concentrations with three levels of nitrogen
availability (Ishizaki et al. 2003). Elevated CO2 significantly
increased LMA but the effect on LMR was small.
The increased LMA compensated for the lowered Nmass,
leading to similar Narea between ambient and elevated
CO2 conditions. The effect of change in LMA on RGR
was investigated by means of a sensitivity analysis: LMA
Fig. 1 Photosynthetic rate versus intercellular CO2 concentration
of Polygonum cuspidatum grown either at ambient CO2 (370 lmol
mol
1, open symbols) or at elevated CO2 (700 lmol mol
1, closed
symbols) in August (a) and October (b). The model of Farquhar
et al. (1980) was fitted to the observations. Arrows indicate the
photosynthetic rate at growth CO2 concentration
245
values observed at ambient and elevated CO2 were
substituted into a steady-state growth model to calculate
RGR. In this model, NAR was assumed to be a function
of Narea. Allocation of more biomass to roots increased
Nmass via increased nitrogen uptake, but decreased leaf
mass. An increase in LMA increased Narea but decreased
leaf area. At ambient CO2, substitution of a high LMA
(observed at elevated CO2) did not increase RGR,
compared with RGR for a low LMA (observed at
ambient CO2), whereas at elevated CO2 the RGR values
calculated for the high LMA were always higher than
those calculated for the low LMA. The optimal combination
of LMR and LMA to maximise RGR was
determined for different CO2 and nitrogen availabilities
(Fig. 2). The optimal LMR was nearly constant, while
the optimal LMA increased with CO2 elevation, and
decreased at higher nitrogen availabilities. These results
suggest that the increase in LMA contributes to growth
enhancement under elevated CO2. The changes in LMR
of actual plants may be a compensation for the limited
plasticity of LMA.
Reproductive growth at elevated CO2
Although vegetative growth is enhanced by elevated
CO2, it is not always reflected by an increase in reproductive
yield (final mass of the reproductive part). From
more than 150 reports on the effect of elevated CO2 on
the reproductive yield of both crop and wild species,
Jablonski et al. (2002) found a mean relative yield
increase of 12% in fruits and 25% in seeds. These
responses were smaller than the response of total plant
mass (31%). In some cases, elevated CO2 even reduced
reproductive yield, though vegetative mass was increased
(Larigauderie et al. 1988; Fajer et al. 1991;
Farnsworth and Bazzaz 1995). Thus, the increase in
reproductive yield is not parallel to that in plant growth,
and the enhancement of vegetative growth is not a
reliable predictor of enhancement of reproductive yield
(Ackerly and Bazzaz 1995).
The difference in responses to elevated CO2 between
vegetative growth and reproductive yield should be explained
by factors involved in the process of reproductive
growth. Reproductive growth is determined not
only by biomass production but also by biomass allocation
to the reproductive part. We analysed reproductive
growth under elevated CO2 using a simple growth
model (Kinugasa et al. 2003). Reproductive mass was
expressed as the product of (1) the duration of the
reproductive period, (2) the rate of biomass acquisition
in the reproductive period, and (3) the fraction of acquired
biomass allocated to the reproductive part
(Sugiyama and Hirose 1991; Shitaka and Hirose 1998).
We raised Xanthium canadense, an annual, under
ambient and elevated CO2 concentrations with two
nitrogen availabilities. Elevated CO2 increased reproductive
yield at high nitrogen availability, but this
increase was caused by increased capsule mass without
a significant increase in seed production (Fig. 3). The
increase in total reproductive mass was due mainly to an
increase in the rate of biomass acquisition in the
reproductive period, with a delay in leaf senescence. This
positive effect was partly offset by a reduction in biomass
allocation to the reproductive part at elevated CO2. The
duration of the reproductive period was not affected by
elevated CO2.
Seed production was strongly constrained by the
availability of nitrogen for seed growth. The nitrogen

concentration in seeds was very high.