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CO2-Induced Amelioration of Environmental Stresses

The positive effect that atmospheric CO2 enrichment has on plant productivity is well-documented in the scientific literature and is discussed in our Subject Index under the heading Growth Response to CO2, which describes the increased plant growth that typically results from an increase in atmospheric CO2 concentration.  It is often stated in cursory reviews of the subject, however, that plants may not be able to reap the many benefits of the aerial fertilization effect of an increase in atmospheric CO2 if they are simultaneously experiencing less-than-optimal growing conditions brought about by environmental stresses or resource limitations.  In evaluating this possibility, Idso and Idso (1994) reviewed the scientific literature of the ten-year period 1983-1992, finding that the percentage growth enhancement resulting from atmospheric CO2 enrichment is typically greater when plants are exposed to growth-retarding stresses -- such as those imposed by low levels of sunlight, inadequate soil moisture, high soil salinity, elevated air temperatures and the presence of aerial pollutants -- than it is under ideal growth conditions.

Alaska Pea PlantsThis phenomenon is illustrated in the accompanying figure, which describes the response of Alaska pea plants to atmospheric CO2 enrichment when water and nutrients are and are not limiting to the growth of the plants (adapted from Paez et al., 1983).  Although these environmental stresses clearly have a negative impact on the plants of both CO2 treatments, the plants exposed to the higher CO2 concentration exhibit a greater percentage growth enhancement due to the extra CO2 when they are stressed, due to lack of soil water and fertility, than when these resources are present in optimal quantities.

All StressesThe second figure reduces the findings of all of the papers reviewed by Idso and Idso (1994) down to a single presentation of two relationships: (1) the percent growth enhancement due to various levels of atmospheric CO2 enrichment for plants receiving less than adequate light, water and nutrients or experiencing stresses caused by high levels of soil salinity, air pollution or temperature, and (2) the same relationship for the same plants when experiencing none of these resource limitations or environmental stresses.  These two relationships --each one the mean result of 298 separate experiments-- clearly demonstrate that plants generally experience an even greater CO2-induced percentage increase in growth when they are under stress than when they are growing under ideal conditions.

In this edition of our Biological Reviews section, we report on the findings of five new papers that continue to demonstrate this fact.  Four of them studied the effects of atmospheric CO2 enrichment on plants that were exposed to specific environmental stresses, and one addressed the more general phenomenon of oxidative stress.

Ferris et al. (1998) investigated the interactive effects of elevated CO2, water stress, and high temperature stress on the growth of soybeans.  They reported that soybeans grown in elevated CO2 exhibited a smaller percentage reduction in photosynthesis than plants grown in ambient CO2 when either of these stresses were applied.  In addition, photosynthetic recovery from water stress was rapidly achieved for plants grown in elevated CO2, while plants grown in ambient CO2 never fully recovered.  These results suggest that elevated CO2 partially ameliorates photosynthetic reductions caused by high temperature and water stress, and that it facilitates faster recovery when the water stress is removed.  In a related study, Tuba et al. (1998) observed the response of three desiccation-tolerant plants (a woody shrub, lichen and moss) to atmospheric CO2 enrichment while subjecting them to extremely severe water stress, finding that all three species responded positively to a 700 ppm increase in atmospheric CO2 concentration during an extensive desiccation treatment by prolonging the time they were able to maintain positive rates of carbon uptake via photosynthesis.

On another front, Kerstiens (1998) reviewed fifteen previously published studies that showed that plants that typically grow in the shade of larger plants generally display greater relative CO2-induced growth increases than do the plants that shade them, demonstrating that lack of sunlight does not diminish the aerial fertilization effect of atmospheric CO2 enrichment.  Likewise, in a study of C3 and C4 grasses and C3 trees, Volin et al. (1998) found that elevated CO2 reduced or eliminated the adverse effects of high ozone concentrations on photosynthesis and growth, indicating that the aerial fertilization effect of atmospheric CO2 enrichment also operates in the presence of air pollutants.

Lastly, Polle et al. (1997) grew beech seedlings for two years in open-top chambers at ambient and elevated (700 ppm) atmospheric CO2 concentrations to determine if growth in elevated CO2 reduces the occurrence of oxidative stresses.  They discovered that leaves from trees grown in elevated CO2 exhibited significant decreases in the activities of antioxidizing enzymes relative to plants grown at ambient CO2, indicating that atmospheric CO2 enrichment reduces the severity of oxidative stress and, thus, the need for enzymes that detoxify molecules that are reactive oxidizing agents.

In summation, the ongoing rise in the air's CO2 content should continue to enhance plant growth and development, particularly in the face of resource limitations and environmental stresses that tend to do just the opposite.  In a nutshell, when it's needed most, elevated CO2 helps the most.

References

Ferris, R., Wheeler, T.R., Hadley, P. and Ellis, R.H.  1998.  Recovery of photosynthesis after environmental stress in soybean grown under elevated CO2Crop Science 38: 948-955.

Idso, K.E. and Idso, S.B.  1994.  Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research.  Agricultural and Forest Meteorology 69: 153-203.

Kerstiens, G.  1998.  Shade-tolerance as a predictor of responses to elevated CO2 in trees.  Physiologia Plantarum 102: 472-480.

Paez, A., Hellmers, H. and Strain, B.R.  1983.  CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum).  Physiologia Plantarum 58: 161-165.

Polle, A., Eiblmeier, M., Sheppard, L. and Murray, M.  1997.  Responses of antioxidative enzymes to elevated CO2 in leaves of beech (Fagus sylvatica L.) seedlings grown under a range of nutrient regimes.  Plant, Cell and Environment 20: 1317-1321.

Tuba, Z., Csintalan, Z., Szente, K., Nagy, Z. and Grace, J.  1998.  Carbon gains by desiccation-tolerant plants at elevated CO2Functional Ecology 12: 39-44.

Volin, J.C., Reich, P.B. and Givnish, T.J.  1998.  Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: species respond similarly regardless of photosynthetic pathway or plant functional group.  New Phytologist 138: 315-325.