Introduction
From the lowest record of 160 to 200 micromol/mol in the Last
Glacial Maximum (about 18,000 years ago) (Delmas et al., 1980;
Neftel et al., 1985; Keeling et al., 1995), atmospheric CO2 concentration has been increasing and has become
about 360 micromol/mol recently. It is predicted that the increase
will continue and the CO2 concentration will reach more than 450
micromol/mol within next century (IPCC, 1996). C3
plants are supposed to increase their biomass with this increment
of CO2 concentration, whereas C4
plants are not. Here I show experimental results indicating that
the increase of CO2 concentration from glacial
age to the future had been and will have been altering competitive
relationships between C3 and C4
plants by changing morphology and growth rates of C3
plants.
Results and Discussion
Relative growth rate (RGR), a parameter represents rate of dry
weight increment, increased with increasing CO2
concentrations both C3 and C4
plants. Compared to the RGR in the control (M), the reduction
of RGR in the C4 plants by Low CO2
treatment (L) was much lower than that in the C3
plant. The high CO2 treatment (H) did not
enhance RGR significantly in the C3 nor the
C4 plants. The growth response of the C3 plant to CO2 concentration
([CO2]) is not agree with the general belief
that increasing [CO2] should enhance growth
of C3 plants. The reason of this uncommon
result can be clarified later in this paper. C4
plants are expected to have low ability to increase their RGR
at higher [CO2], because photosynthesis
of C4 plants is saturated with low [CO2] (lower than 200 micromol/mol intercellular
concentration in leaves). The present result is consistent with
this expectation and suggests that global [CO2]
change has little effect on growth of C4
plants.
Net assimilation rate (NAR), a representative parameter of whole
plant net photosynthetic rate per unit leaf area, of the C3 plant increased significantly with increasing
[CO2], but that of the C4
plants was not affected by [CO2] . These
results also support the general expectation that enhancement
of photosynthetic activity by global [CO2]
increase is much greater in C3 plants than
C4 plants.
Thus, global [CO2] increase is expected to
increase growth and photosynthetic activity of C3
plants. Our results imply that there was an morphological change
of C3 plants with changing global [CO2], and this change should be extended towards
the future high CO2 world. Leaf area ratio
(LAR), a morphological parameter showing the leaf area per unit
plant weight, in the C3 plant decreased with
increasing [CO2], whereas that in the C4 plants showed no change. This reduction of
LAR in the C3 plant in H treatment canceled
out the increase of NAR, and thus RGR of the C3
plant was not enhanced by the H treatment. Many reports suggest
that, in C3 plants, enhancement of RGR by
high [CO2] is caused mainly by increase of
NAR.
The current results shown here support the general hypothesis
that the growth and morphology of C4 plants
are not responsive for the [CO2] from today
to the future, though the growth of the C4
plants was reduced by the L treatment, which is comparable to
[CO2] in the Last Glacial Maximum. There
is another general hypothesis that C3 plant
growth is more responsive to [CO2] than C4 plant growth. Our results suggest that this
hypothesis is not always true in the future high [CO2]
world because morphology of some C3 plants
may be altered by the high [CO2] and this
morphological change may cancel the increase of photosynthetic
activity. These findings are consistent with the previous results
of low [CO2] expeiments (Polley et al., 1993;
Dippery et al., 1995), though the number of the experiments is
quite limited yet.
It is quite likely that competitive interactions of C3
and C4 plants have been changed through geologic
time to the future (Dippery et al., 1995) . However, this change
is not so simple as being thought before. In the low [CO2] world in the last glacial age, it can be supposed
that some C3 plants had RGR comparable to
some C4 plants by oompensating reduction
of NAR with increasing amount of leaves. It can be also supposed
that, in some C4 plant, reduction of RGR
was caused by the reduction of NAR in the low [CO2].
In this case, the result of competition between C3
and C4 plants cannot be esimated srtaightly.
As shading by leaves is one of the critical factors determining
result of competition for light (Tremmel and Bazzaz, 1995), this
higher amount of leaves in the C3 plant may
be advantageous in the low [CO2] world.
In the future high [CO2] world, the competitive
interactions of C3 versus C3
and C3 versus C4 plants
may be weakened because some C3 plant reduces
their shading ability by reduction of their amount of leaves.
Thus, I have to enphasis that the alternation of competitive
interactions between C3 and C4
plants through geologic time to the future must depend on degree
of morphological change in C3 plants as well
as that of changes in photosynthetic activities in both C3 and C4 plants.