The climatic effects of atmospheric carbon dioxide on plants- Wilfred Akah
This report outlines the climatic effects of atmospheric carbondioxide on plants as part of
the Group Project work in MEA 760: Biogeochemistry; at the Prairie Ridge in Raleigh.
The areas of concern of this report are the geologic perspective on plants and atmospheric
CO2, plant species response to CO2, photosynthesis, respiration, and growth. I finally
looked at the interactions of plants with some of the environmental parameters.
The 1992 annual mean value of 356 µbar (Keeling et al., 1991) represents a dynamic
CO2 compensation point for the atmosphere. Terrestrial photosynthesis and oceanic
dissolution take up an estimated 120 and 115 Gt C yr-1, but this is approximately
balanced by CO2 evolution from respiration, decomposition, fires, and oceanic release
(Post et al., 1990). Human activities of fossil-fuel burning, cement manufacture, and land
use add 5-8 Gt C yr-1 to the atmosphere (Post et al., 1990). These anthropogenic sources
comprise a small fraction of the planet’s carbon budget but are the major components
destabilizing the atmospheric CO2 compensation point.
Photosynthesis underpins the uptake side of the global carbon budget. Other influences of
CO2 on plants are becoming apparent. Some are unexpected, such as effects on
respiration and ethylene production (Amthor, 1991). Thus, CO2 influences metabolism in
ways unrelated to being a photosynthetic substrate. The effects at the cellular level ripple
out to the ecosystem, to feed back eventually on the global carbon cycle and the rise in
atmospheric CO2 itself.
Thus, CO2 influences metabolism in ways unrelated to being a photosynthetic substrate.
A GEOLOGICAL PERSPECTIVE ON PLANTS AND ATMOSPHERIC CO2
Although the prospect of a doubling in atmospheric CO2 seems extreme, from a geologic
and biologic perspective, the rapidity of change is more significant than its degree. Trees,
200 years old have encountered a 28% increase in CO2, and are facing the prospect of
over a 100% increase, during their lifespan (Post et al., 1990). Analysis of ancient air
trapped in polar ice indicate CO2 has fluctuated from 190 to 300 µbar over the past
160,000 years. After the last glacial period it stabilized at about 280 µbar CO2-that is,
until the 19th century, when it began to rise in a disturbingly logarithmic manner
(Post et al., 1990).
Views about the composition of earlier atmospheres conflict, but the last 30 million years
seem atypically low in CO2 (Budyko et al., 1987). Major atmospheric changes are not
novel, but previously they occurred over thousands to millions of years, not decades.
Thus for much of the planet’s history, terrestrial vegetation lived in an atmosphere that
saturated photosynthesis. In contrast, the present CO2/O2, regime restricts most
vegetation to only 60-70% of its photosynthetic potential, because of kinetic constraints
imposed by rubisco.
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SPECIES RESPONSES TO CO2: PHOTOSYNTHESIS, RESPIRATION, AND
GROWTH
CO2 enrichment enhances the photosynthesis and growth of C3 plants. However, there is
debate about whether enhancement can occur when factors other than CO2 limit growth,
about the degree to which enhanced photosynthesis translates into improved growth and
yield, and about whether positive responses at the plant level can be extrapolated to
communities and ecosystems. Disagreement exists over whether initially high
photosynthetic rates are sustained. There are many reports that leaf photosynthetic rates
decline with exposure to high CO2. It is not known whether this down-regulation is
species-specific, due to growth conditions, or both.
The photosynthetic mechanism of a species is the major determinant of how it will
respond to changes in CO2. Most research in CO2 enrichment has focused on terrestrial
plants. However, the environment is an important, and sometimes the primary factor.
Terrestrial Species: C3, C4, and CAM
Approximately 95 % of terrestrial plants are C3 species, while about 1% is C4, and 4%
use the CAM pathway. The present CO2:O2 ratio in the atmosphere and C3 rubisco
specificity factors (Jordan et al., 1983) translate into photorespiratory losses 25% for C3
plants (Keys, 1986.). A doubling of atmospheric CO2 in the next century should more
than halve this deleterious effect of O2 on C3 photosynthesis but have negligible effects
on C4 plants. In general, this projection is supported by growth data.
INTERACTIONS WITH ENVIRONMENTAL PARAMETERS
It is agreed that the rise in CO2 will alter competitive interactions, but whether it will
enhance ecosystem carbon accumulation is still an open question because other
environmental parameters may be limiting (Kramer, 1981). The nature of CO2
enrichment differs from N or P fertilization in that, besides increasing a resource supply,
enrichment reduces inefficiency, specifically in CO2 and water losses. In so doing, it
enables other resources to be used more effectively. From experiments at the plant and
canopy level, it is evident that optimal conditions are not a prerequisite to enhanced
growth. The enhancement can be maintained when other factors are co-limiting
(Drake et al., 1991).
Light and Temperature
Most measurements of the effects of elevated CO2 on photosynthesis have used relatively
high irradiance, where CO2 supply or rubisco capacity is the major limitation. In nature,
on the other hand, photosynthesis often takes place in reduced light. Several observations
indicate that CO2 enrichment enhances photosynthesis and growth even under limiting
irradiance, and sometimes the relative enhancement is greater under this condition
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(Allen, 1990). Studies have shown that the optimum temperature for C3 is lower than for
C4 photosynthesis, because higher temperatures alter the CO2/O2 specificity of rubisco in
favor of oxygenase. By counteracting this O2 effect, CO2 enrichment increases the
temperature optimum, bringing it closer to that of C4 photosynthesis (Long, 1991).
However, temperature responses for specific physiological processes do not necessarily
correlate with growth and development because the latter integrate the effect of
temperature on overall metabolism. To complicate the issue further, leaves compensate
for increased air temperatures; CO2 enrichment itself causes higher foliar temperatures;
and species differ in response to temperature (Allen, 1990).
Despite these qualifiers, the CO2 enhancement of vegetative growth is strongly
temperature dependent, within high and low limits that vary with species (Carter et al.,
1983). Even with C4 species, positive interactions have been observed (Bazzaz, 1990).
Idso et al (1987) calculated, from responses by five C3 species, which for a 3°C rise in
mean air temperature, the CO2 growth enhancement factor rose from 1.30 to 1.56. There
is also evidence that enrichment ameliorates the adverse effects of temperature extremes
(Bazzaz, 1990).
Water and Salinity
A common response to a doubling of CO2 is a 30-60% reduction in stomatal
conductance. This decrease occurs across a variety of terrestrial and emersed C3 and C4
species (Allen, 1990). Besides aperture closure, leaf stomatal density may decline with a
rise in CO2 (Allen, 1990). The reduced stomatal conductance improves leaf water
potentials, which can accelerate leaf expansion (Gifford, 1992).
Transpiration may or may not decline as a result of the drop in stomatal conductance,
because a rise in leaf temperature can offset the effects of lower stomatal conductance.
Improvements in water use efficiency have been reported for various plants, not just C3
species, partly as a result of enhanced CO2 uptake (Allen, 1990). Along with greater
water use efficiency, CO2 enrichment enables plants to withstand drought stress better,
and delays its onset (Allen, 1990).
Although growth is reduced under drought conditions, on a relative basis the
enhancement of growth by elevated CO2 is larger. An increase in CO2 also ameliorates
the adverse effects of salinity on growth (Bazzaz, 1990). This may be accomplished with
more solutes for osmo-regulation, by reducing the transpirational intake of salt (Bazzaz,
1990). It is envisaged that CO2-enriched plants of the future will better tolerate drought
and salinity.
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Nutrients
Plant-level studies that have examined the interaction between elevated CO2 and mineral
nutrition, primarily N and P, indicate almost without exception that though growth is
depressed in nutrient-poor environments it is still stimulated by CO2 enrichment, often to
the same degree as in nutrient-sufficient conditions (Allen, 1990). Nutrient uptake is
increased by CO2 enrichment, but N uptake does not respond to the same degree as C
uptake. Based on some studies, it is confirmed that root-derived nutrients and CO2 often
co-limit growth.
Conclusion
As far as direct effects are concerned, the CO2-enriched atmosphere of the 21st century
does not look to be a bleak prospect for most plants, or for agriculture. It might usher in a
greener planet, though the species mix will change. For good or ill, change always has
been an inextricable part of our world.
Websites:
1. http://www.nc-climate.ncsu.edu/climate/ncclimate.html
2. http://www.rssweather.com/climate/North%20Carolina/Raleigh/
3. http://www.annualreviews.org/aronline
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