In controlled experiments where environmental conditions are optimal for growth, plants subjected to elevated levels of atmospheric CO2 almost always exhibit greater rates of photosynthesis and biomass production than they do in ambient air. In addition, under less-than-favorable growth conditions (low soil moisture, poor soil fertility, high soil salinity, high air temperature), plants generally continue to exhibit some degree of CO2 -induced growth enhancement, which is often greater (percentage-wise) than what occurs in the absence of these stresses. However, concerns have been raised that the results of CO2 -enrichment experiments conducted in growth cabinets, greenhouses and open-top chambers may not always reflect the real-world responses of plants to atmospheric CO2 enrichment, due to perturbations in microclimate caused by the experimental enclosures. Thus, FACE technology was developed as a means of enriching the air around plants with CO2 while having minimal effect on their microclimate; and in this summary we highlight the results of a few of these FACE experiments conducted on agricultural crops.
A great deal of FACE research has been directed to studying various responses of wheat to atmospheric CO2-enrichment. In the study of Osborne et al. (1998), for example, spring wheat plants growing at 550 ppm CO2 consistently displayed higher rates of net photosynthesis than plants growing at 360 ppm CO2, regardless of leaf position in the canopy. Garcia et al. (1998) determined this CO2-induced photosynthetic enhancement to be approximately 28%. In addition, they noted that elevated CO2 increased crop water-use efficiency by about one-third, due in part to a 36% CO2-induced reduction in stomatal conductance.
When spring wheat experienced water stress, Wechsung et al. (1999) reported that plants subjected to 550 ppm CO2 produced in-row and inter-row root dry weights that were 22% and 70% larger, respectively, than those produced by ambiently-grown plants, thus illustrating the tendency for atmospheric CO2 enrichment to help wheat plants cope with water-stressed conditions by preferentially stimulating root growth to obtain more water from the soil. Elevated CO2 under water-stressed conditions also increased final grain yields in the upper and lower sections of the main stems of wheat plants by 10% and 24%, respectively (Li et al., 2000). In contrast, under well-watered conditions atmospheric CO2 enrichment increased final grain yield only in the lower sections of the main stems and by only 14%.
In a FACE study of potato, Miglietta et al. (1998) grew plants in plots receiving atmospheric CO2 concentrations of 360, 460, 560, and 660 ppm. They reported that every 100-ppm increase in the air's CO2 content generated an additional 1.5 tubers per plant, which amounted to a 40% yield increase for plants growing at 600 ppm vs. plants growing at the ambient CO2 concentration (360 ppm).
The results derived from these FACE experiments demonstrate that the ongoing rise in the air's CO2 content should positively impact photosynthesis and biomass production in the studied agricultural crops, even if they are growing under stressful conditions imposed by insufficient soil moisture. In addition, they generally corroborate data obtained from CO2 enrichment studies on agricultural species that were performed in growth chambers, greenhouses and other enclosures, thereby demonstrating the adequacy of these other experimental approaches.
References
Garcia, R.L., Long, S.P., Wall, G.W., Osborne, C.P., Kimball, B.A., Nie, G.Y., Pinter Jr., P.J., LaMorte, R.L. and Wechsung, F. 1998. Photosynthesis and conductance of spring-wheat leaves: field response to continuous free-air atmospheric CO2 enrichment. Plant, Cell and Environment 21: 659-669.
Li, A.-G., Hou, Y.-S., Wall, G.W., Trent, A., Kimball, B.A. and Pinter Jr., P.J. 2000. Free-air CO2 enrichment and drought stress effects on grain filling rate and duration in spring wheat. Crop Science 40: 1263-1270.
Miglietta, F., Magliulo, V., Bindi, M., Cerio, L., Vaccari, F.P., Loduca, V. and Peressotti, A. 1998. Free Air CO2 Enrichment of potato (Solanum tuberosum L.): development, growth and yield. Global Change Biology 4: 163-172.
Osborne, C.P., LaRoche, J., Garcia, R.L., Kimball, B.A., Wall, G.W., Pinter, P.J., Jr., LaMorte, R.L., Hendrey, G.R. and Long, S.P. 1998. Does leaf position within a canopy affect acclimation of photosynthesis to elevated CO2? Plant Physiology 117: 1037-1045.
Wechsung, G., Wechsung, F., Wall, G.W., Adamsen, F.J., Kimball, B.A., Pinter Jr., P.J., LaMorte, R.L., Garcia, R.L. and Kartschall, T. 1999. The effects of free-air CO2 enrichment and soil water availability on spatial and seasonal patterns of wheat root growth. Global Change Biology 5: 519-529.