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Transpiration (Herbaceous Plants: Crops) - Summary
Most plants respond to increases in the air's CO2 content by reducing their leaf stomatal conductances, which phenomenon typically leads to reduced rates of transpirational water loss.  The resultant water savings, in turn, often lead to greater soil moisture contents in CO2-enriched ecosystems, which consequence positively impacts plant water status and growth.  In this summary, we review the results of some studies of C3 and C4 crops that treat various aspects of this CO2-induced multi-stage interaction.

Starting with C3 crops, Dong-Xiu et al. (2002) grew spring wheat in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 700 ppm and three levels of soil moisture (40, 60 and 80% of field capacity).  In addition to increasing rates of net photosynthesis by 48, 120 and 97% at low, medium and high soil water capacities, this doubling of the air's CO2 concentration reduced rates of transpiration by 56, 53 and 63%, respectively, in the three soil water treatments.

De Costa et al. (2003) grew two crops of rice in the field in Sri Lanka -- from January to March (the maha season) and from May to August (the yala season) -- in open-top chambers maintained at either ambient or ambient plus 200 pppm CO2.  As always, leaf net photosynthetic rates were significantly higher in the CO2-enriched chambers than in the ambient-air chambers: 51-75% greater in the maha season and 22-33% greater in the yala season.  In addition, leaf stomatal conductances exhibited CO2-induced reductions of 15-52% in the maha season and 13-19% in the yala season.  However, because of the significantly greater leaf area in the CO2-enriched chambers, total canopy transpiration rate per unit land area did not differ significantly between the two CO2 treatments.  Nevertheless, leaf water potentials were higher (less negative and, therefore, more beneficial) in the CO2-enriched chambers.

Vu (2005) grew peanuts from seed to maturity in greenhouses maintained at atmospheric CO2 concentrations of 360 and 720 ppm and at air temperatures that were 1.5 and 6.0C above outdoor air temperatures.  They found that although Rubisco protein content and activity were down-regulated by elevated CO2, Rubisco photosynthetic efficiency (the ratio of midday light-saturated carbon exchange rate to Rubisco initial or total activity) of the elevated-CO2 plants "was 1.3- to 1.9-fold greater than that of the ambient-CO2 plants at both growth temperatures."  He also found that "leaf soluble sugars and starch of plants grown at elevated CO2 were 1.3- and 2-fold higher, respectively, than those of plants grown at ambient CO2."  Last of all, he found that the leaf transpirational water loss of the elevated-CO2 plants compared to that of the ambient-CO2 plants was 12% less at near-ambient temperatures and 17% less in the higher temperature regime.

Malmstrom and Field (1997) grew individual oat plants for two months in pots placed within phytocells having atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, one-third of the plants were infected with the barley yellow dwarf virus that plagues more than 150 plant species, including all major cereal crops.  They found that the elevated CO2 stimulated rates of net photosynthesis in all plants, with the greatest percentage increase occurring in diseased individuals (48% vs. 34%), and that it decreased stomatal conductance by 50% in infected plants and 34% in healthy ones, thus reducing transpirational water losses.  Together, these phenomena contributed to a CO2-induced doubling of the instantaneous water-use efficiency in healthy plants and a 2.7-fold increase in diseased plants.

Turning to C4 crops, Leakey et al. (2004) grew corn in their SoyFACE facility in the heart of the US Corn Belt while exposing different parts of the field to atmospheric CO2 concentrations of 354 and 549 ppm during a year that experienced summer rainfall "very close to the 50-year average for this site, indicating that the year was not atypical or a drought year."  On five different days during the growing season (11 and 22 July, 9 and 21 August, and 5 September), they also measured diurnal patterns of photosynthesis, stomatal conductance and microclimatic conditions.

Contrary to what many people had long assumed would be the case for a C4 crop such as corn growing under the best of natural conditions, Leakey et al. found that "growth at elevated CO2 significantly increased leaf photosynthetic CO2 uptake rate by up to 41%."  The highest whole-day increase was 21% (11 July) followed by 11% (22 July), during a period of low rainfall.  Thereafter, however, during a period of greater rainfall, there were no significant differences between the photosynthetic rates of the plants in the two CO2 treatments, so that over the entire growing season, the CO2-induced increase in leaf photosynthesis averaged 10%.

Additionally, on all but the first day of measurements, stomatal conductance was significantly lower (-23% on average) in the elevated CO2 treatment, which led to reduced transpiration rates in the CO2-enriched plants; and since "low soil water availability and high evaporative demand can both generate water stress and inhibit leaf net CO2 assimilation in C4 plants," in the words of Leakey et al., they felt that the lower transpiration rates of the plants growing in the CO2-enriched air "may have counteracted the development of water stress under elevated CO2 and prevented the inhibition of leaf net CO2 assimilation observed under ambient CO2."

The bottom line, in the words of the researchers, was that "contrary to expectations, this US Corn Belt summer climate appeared to cause sufficient water stress under ambient CO2 to allow the ameliorating effects of elevated CO2 to significantly enhance leaf net CO2 assimilation."  Hence, they concluded that this response of corn to elevated CO2 "indicates the potential for greater future crop biomass and harvestable yield across the US Corn Belt," due largely to the amelioration of water stress by CO2-induced decreases in transpirational water loss.

Finally, Grant et al. (2004) adjusted the crop growth and water relations model ecosys to represent the C4 crop sorghum and ran it for a period of two growing seasons (1 May 1998 to 31 Oct 1999) under both wet and dry irrigation regimes at two atmospheric CO2 concentrations (approximately 368 and 561 ppm) using hourly meteorological data measured at a field south of Phoenix, Arizona, USA, after which its simulated energy balances and water relations, verified by measurements of energy flux and water potential, were used to infer the effects of free-air atmospheric CO2 enrichment on various plant parameters and processes.

The twelve researchers report that "model results, corroborated by field measurements, showed that elevated CO2 raised canopy water potential and lowered latent heat fluxes under high irrigation [both of which responses are beneficial] and delayed water stress under low irrigation [which is also beneficial]," or as they describe it elsewhere, the elevated CO2 "reduced transpiration and hence improved water status of sorghum [and] lowered the vulnerability of sorghum CO2 fixation to soil or atmospheric water deficits, even when irrigation was high."  Also, in applying their reality-tuned model to a scenario where the air's CO2 content was 50% higher and air temperature was 3C greater, they calculated that sorghum yields would rise by about 13%, and that "current high sorghum yields could be achieved with ~120 mm or ~20% less irrigation water if these rises in temperature and CO2 were to occur."

In conclusion, these several results would appear to suggest that if the air's CO2 content continues to rise as it has over the past few decades, the future should see substantially more food produced on each hectare of land with little to no change in per-hectare water requirements.  In the case of C4 crops, in fact, it appears that higher yields may even be produced with smaller amounts of water ... even in the face of higher temperatures.

References
De Costa, W.A.J.M., Weerakoon, W.M.W., Abeywardena, R.M.I. and Herath, H.M.L.K.  2003.  Response of photosynthesis and water relations of rice (Oryza sativa) to elevated atmospheric carbon dioxide in the subhumid zone of Sri Lanka.  Journal of Agronomy and Crop Science 189: 71-82.

Dong-Xiu, W., Gen-Xuan, W., Yong-Fei, B., Jian-Xiong, L. and Hong-Xu, R.  2002.  Response of growth and water use efficiency of spring wheat to whole season CO2 enrichment and drought.  Acta Botanica Sinica 44: 1477-1483.

Grant, R.F., Kimball, B.A., Wall, G.W., Triggs, J.M., Brooks, T.J., Pinter Jr., P.J., Conley, M.M., Ottman, M.J., Lamorte, R.L., Leavitt, S.W., Thompson, T.L. and Matthias, A.D.  2004.  Modeling elevated carbon dioxide effects on water relations, water use, and growth of irrigated sorghum.  Agronomy Journal 96: 1693-1705.

Leakey, A.D.B., Bernacchi, C.J., Dohleman, F.G., Ort, D.R. and Long, S.P.  2004.  Will photosynthesis of maize (Zea mays) in the US Corn Belt increase in future [CO2] rich atmospheres?  An analysis of diurnal courses of CO2 uptake under free-air concentration enrichment (FACE).  Global Change Biology 10: 951-962.

Malmstrom, C.M. and Field, C.B.  1997.  Virus-induced differences in the response of oat plants to elevated carbon dioxide.  Plant, Cell and Environment 20: 178-188.

Vu, J.C.V.  2005.  Acclimation of peanut (Arachis hypogaea L.) leaf photosynthesis to elevated growth CO2 and temperature.  Environmental and Experimental Botany 53: 85-95.

Last updated 1 June 2005