A great deal of discussion about the issue of global warming has occurred as a gradual increase of atmospheric CO2 concentration has been observed. Compared to a CO2 concentration of about 280 ppm in the early 19th century, the level has increased to 369 ppm at the beginning of the 21st century. If the present rate continues, the CO2 concentration may be between 450 and 600 ppm within fifty years. Since an increase in CO2concentration is related to global warming, it is possible that increasing levels of atmospheric CO2 will create dramatic climatic changes. In a review article in the June 2002 issue of Current Opinion in Plant Biology (5(3):207-11.), F. Ian Woodward at the Department of Animal and Plant Sciences, University of Sheffield, UK discusses the effects of CO2 enrichment on plants.
The author points out that initial short-term studies indicated photosynthesis is stimulated more in C3 species compared to C4 species in response to CO2 enrichment. Later longer-term studies indicated that photosynthesis could acclimate downwards in response to elevated CO2 levels, and that photosynthetic rates may be elevated in C4 plants as well as C3 plants. One advantage of CO2 enrichment is that the extent of damage caused by photoinhibition at high light levels may be reduced in species with the C3 photosynthetic pathway.
The author describes a technique by which plants under investigation can be grown in the field in a CO2-enriched environment, while minimizing the effects of other external factors. This method is called “Free Air CO2Enrichment” (FACE). This procedure has allowed investigations on the effects of elevated CO2 on trees. Experiments were conducted to determine the net primary productivity of two populations of maturing loblolly pine grown on a soil of moderate fertility with a continuous supply of 200 ppm CO2. One study was conducted for a two-year duration, while the other parallel study was longer-term. Plants in the longer-term study showed a 34% increase in growth, compared to a 25% increase displayed by the population subjected to a two-year CO2 enrichment treatment. However, in the longer-term study, the increase in productivity and photosynthetic rates leveled off after about 3 years or so, and remained only 6% above control levels for the next four years. The author states that factors such as nitrogen availability become limiting after the initial period of increased growth, a conclusion supported by the observation that supplementation with both nitrogen fertilizer and increased CO2 caused three-fold higher growth compared to control plants. In a FACE study within a forest ecosystem, researchers also determined litter fall and nutrient flux, which increased during the early period of CO2-stimulated increased growth. Rates of litter composition, nitrogen mineralization and nitrification did not increase, however, leading to longer-term nitrogen imbalances.
CO2 concentration is a major determinant of stomatal conductance as the plant balances its need for CO2 with the need to minimize water loss. A FACE study on a grassland ecosystem indicated a reduction in stomatal conduction for 13 perennial species in response to elevated CO2. This, along with a decrease in photosynthetic rate, led to a 40% increase in instantaneous water use efficiency. However, no decrease in stomatal conductance was noted during the early phase of loblolly pine tree FACE studies. Thirteen long-term field-based studies on tree species demonstrated an overall 21% reduction in stomatal conductance. Reduced stomatal conductance was much more consistently observed in the longer-term studies than in the shorter-term studies.
Several studies have investigated the effect of CO2 enrichment on plant development and biomass partitioning. An analysis of woody plants exposed to elevated CO2 indicated no effect on biomass partitioning, and a six-year grassland study also demonstrated little change in shoot-to-root ratio in response to CO2 enrichment. In light of this, the author notes that it is particularly interesting that carbon allocation to reproduction is strongly stimulated in loblolly pine after three years of CO2 enrichment. In this study, researchers observed a three-fold increase in production of cones and seeds in trees exposed to elevated CO2 than in control trees. The author points out that a CO2-stimulated hastening of seed production onset in trees may be used to track climatic change. On the other hand, grassland species differed in flowering and seed set responses to CO2 enrichment, indicating that CO2 enrichment holds a strong potential to change the composition of plant communities. In other observations on morphological differences associated with elevated CO2, an increase in leaf thickness with a concomitant reduction in stomatal density was noted in Scots pine.
The author reiterates that long-term studies on plant responses to elevated CO2 have indicated three major types of responses: nutrient limitation of plant CO2 fluxes, responses of stomatal development and opening, and impacts on plant development. The author then discusses studies that have attempted to determine mechanisms regulating each type of response.
Plant responses to elevated CO2 may involve down-regulation of both photosynthesis and respiration. However, the situation is not straightforward, as long-term field experiments have shown that down-regulation of photosynthesis is incomplete and that leaf photosynthesis is up-regulated by a CO2 enrichment of 200 ppm. The author points out that photosynthetic stimulation varies among C3 species from 7% for legume herbs to 98% for Pinus radiata. Respiration down-regulation also varies among species. Several lines of evidence indicate that CO2 enrichment causes a shift in carbon to nitrogen balance, leading to a reduction in leaf nitrogen concentration and decline in respiration. However, CO2 enrichment also causes an increase in the average number of mitochondria per cell, possibly reflective of increased energy demand due to higher photosynthetic rates that may occur in some conditions when CO2 levels are elevated. The author notes that a number of feedback controls exist to regulate the rate of photosynthesis in response to CO2 concentration, and provides a model illustrating the influence of these mechanisms. The action of these feedback controls, which include sugar and nitrogen sensing components, may determine the actual level of photosynthetic production (which is generally lower than the potential level of production).
The author discusses studies that have focused upon the mechanism of CO2 regulation of stomatal development, highlighting the discovery of a gene designated HIC (high carbon dioxide). Disruption of this gene induces an increase in stomatal frequency in response to CO2 enrichment. The HIC gene encodes an enzyme that is involved in synthesis of long-chain fatty acids typically found in the leaf cuticle, suggesting that changes in the fatty acid profile may influence cell-to-cell signaling processes occurring during guard cell development. Longer-distance signaling in the stomatal development process may involve abscisic acid, ethylene and jasmonates.
Mechanisms for the effect of elevated CO2 levels on reproductive development in tree species are also discussed. This reproductive effect has also been observed in sour orange (Citrus aurantium), in which a CO2enrichment of 300 ppm advanced the fruiting time by about one year and enhanced total orange production. The author suggests that the enhanced sink size resulting from increased fruit production as CO2 concentration rises may allow maintenance of photosynthetic stimulation. The author also mentions that citrus trees transformed with the Arabidopsis flowering-time controlling genes LEAFY and APETALA1 flower 4-5 years earlier than untransformed controls, but flowering is still induced by environmental stimuli and drought. The effect of elevated CO2 levels on flowering in these transgenic citrus lines has not yet been determined.
The author concludes by outlining other emerging areas of research on the impact of CO2 enrichment on plants. These include study on the protective effects of CO2 enrichment against pollutants such as ozone and study on the enhanced susceptibility of forest trees to frost damage upon exposure to elevated CO2. Much work also remains to determine the mechanisms of CO2 signaling.
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