Nearly all agricultural crops respond to increases in the air's CO2 content by exhibiting increases in photosynthesis and biomass production, as well as their ability to deal with various environmental stresses. In this summary, we review some recent reports of these phenomena in rice (Oryza sativa L.).
DeCosta et al. (2003a) grew two crops of rice at the Rice Research and Development Institute of Sri Lanka from January to March (the maha season) and from May to August (the yala season) in open-top chambers in air of either ambient or ambient plus 200 pppm CO2, determining that 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. Likewise, in the study of Gesch et al. (2002), where one-month-old plants were maintained at either 350 ppm CO2 or switched to a concentration of 700 ppm for ten additional days, the plants switched to CO2-enriched air immediately displayed large increases in their photosynthetic rates that at the end of the experiment were still 31% greater than those exhibited by unswitched control plants.
With respect to the opposite of photosynthesis, Baker et al. (2000) reported that rates of carbon loss via dark respiration in rice plants decreased with increasing nocturnal CO2 concentrations. As a result, it is not surprising that in the study of Weerakoon et al. (2000), rice plants exposed to an extra 300 ppm of atmospheric CO2 exhibited a 35% increase in mean season-long radiation-use efficiency, defined as the amount of biomass produced per unit of solar radiation intercepted. In light of these several observations, therefore, one would logically expect rice plants to routinely produce more biomass at elevated levels of atmospheric CO2. And they do.
In conjunction with the study of DeCosta et al. (2003a), DeCosta et al. (2003b) found that CO2-enriched rice plants produced more leaves per hill, more tillers per hill, more total plant biomass, greater root dry weight, more panicles per plant and had harvest indices that were increased by 4% and 2%, respectively, in the maha and yala seasons, which suite of benefits led to ultimate grain yield increases of 24% and 39% in those two periods. In another study, Kim et al. (2003) grew rice crops from the seedling stage to maturity at atmospheric CO2 concentrations of ambient and ambient plus 200 ppm using FACE technology and three levels of applied nitrogen -- low (LN, 4 g N m-2), medium (MN, 8 and 9 g N m-2), and high (HN, 15 g N m-2) -- for three cropping seasons (1998-2000). They found that "the yield response to elevated CO2 in crops supplied with MN (+14.6%) or HN (+15.2%) was about twice that of crops supplied with LN (+7.4%)," confirming the importance of N availability to the response of rice to atmospheric CO2 enrichment that had previously been determined by Kim et al. (2001) and Kobaysahi et al. (2001).
Various environmental stresses, however, can significantly alter the impact of elevated CO2 on rice. In the study of Tako et al. (2001), for example, rice plants grown at twice-ambient CO2 concentrations and ambient temperatures displayed no significant increases in biomass production; but when air temperatures were raised by 2°C, the CO2-enriched plants produced 22% more biomass than the plants grown in non-CO2-enriched air. In contrast, Ziska et al. (1997) reported that CO2-enriched rice plants grown at elevated air temperatures displayed no significant increases in biomass; but when the plants were grown at ambient air temperatures, the additional 300 ppm of CO2 boosted their rate of biomass production by 40%. In light of these observations, rice growers should select cultivars that are most responsive to elevated CO2 concentrations at the air temperatures likely to prevail in their locality in order to maximize their yield production in a future high-CO2 world.
Water stress can also severely reduce rice production. As an example, Widodo et al. (2003) grew rice plants in eight outdoor, sunlit, controlled-environment chambers at daytime atmospheric CO2 concentrations of 350 and 700 ppm for an entire season. In one set of chambers, the plants were continuously flooded. In another set, drought stress was imposed during panicle initiation. In another, it was imposed during anthesis; and in the last set, drought stress was imposed at both stages. The resultant drought-induced effects, according to the scientists, "were more severe for plants grown at ambient than at elevated CO2." They report, for example, that "plants grown under elevated CO2 were able to maintain midday leaf photosynthesis, and to some extent other photosynthetic-related parameters, longer into the drought period than plants grown at ambient CO2."
Recovery from the drought-induced water stress was also more rapid in the elevated CO2 treatment. At panicle initiation, for example, Widodo et al. observed that "as water was added back following a drought induction, it took more than 24 days for the ambient CO2 [water]-stressed plants to recuperate in midday leaf CER, compared with only 6-8 days for the elevated CO2 [water]-stressed plants." Similarly, they report that "for the drought imposed during anthesis, midday leaf CER of the elevated CO2 [water]-stressed plants were fully recovered after 16 days of re-watering, whereas those of the ambient CO2 [water]-stressed plants were still 21% lagging behind their unstressed controls at that date." Hence, they logically concluded that "rice grown under future rising atmospheric CO2 should be better able to tolerate drought situations."
In a somewhat different type of study, Watling and Press (2000) found that rice plants growing in ambient air and infected with a root hemiparasitic angiosperm obtained final biomass values that were only 35% of those obtained by uninfected plants. In air of 700 ppm CO2, however, the infected plants obtained biomass values that were 73% of those obtained by uninfected plants. Thus, atmospheric CO2 enrichment significantly reduced the negative impact of this parasite on biomass production in rice.
In summary, as the CO2 concentration of the air continues to rise, rice plants will likely experience greater photosynthetic rates, produce more biomass, be less affected by root parasites, and better deal with environmental stresses, all of which effects should lead to greater grain yields. For additional results confirming this conclusion, see Rice (dry weight, photosynthesis) in the Plant Growth Data section of our website.
References
Baker, J.T., Allen, L.H., Jr., Boote, K.J. and Pickering, N.B. 2000. Direct effects of atmospheric carbon dioxide concentration on whole canopy dark respiration of rice. Global Change Biology 6: 275-286.
De Costa, W.A.J.M., Weerakoon, W.M.W., Abeywardena, R.M.I. and Herath, H.M.L.K. 2003a. 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.
De Costa, W.A.J.M., Weerakoon, W.M.W., Herath, H.M.L.K. and Abeywardena, R.M.I. 2003b. Response of growth and yield of rice (Oryza sativa) to elevated atmospheric carbon dioxide in the subhumid zone of Sri Lanka. Journal of Agronomy and Crop Science 189: 83-95.
Gesch, R.W., Vu, J.C., Boote, K.J., Allen Jr., L.H. and Bowes, G. 2002. Sucrose-phosphate synthase activity in mature rice leaves following changes in growth CO2 is unrelated to sucrose pool size. New Phytologist 154: 77-84.
Kim, H.-Y., Lieffering, M., Kobayashi, K., Okada, M., Mitchell, M.W. and Gumpertz, M. 2003. Effects of free-air CO2 enrichment and nitrogen supply on the yield of temperate paddy rice crops. Field Crops Research 83: 261-270.
Kim, H.-Y., Lieffering, M., Miura, S., Kobayashi, K. and Okada, M. 2001. Growth and nitrogen uptake of CO2-enriched rice under field conditions. New Phytologist 150: 223-229.
Kobayashi, K., Lieffering, M. and Kim, H.-Y. 2001. Growth and yield of paddy rice under free-air CO2 enrichment. In: Shiyomi, M. and Koizumi, H. (Eds.), Structure and Function in Agroecosystem Design and Management. CRC Press, Boca Raton, FL, USA, pp. 371-395.
Tako, Y., Arai, R., Otsubo, K. and Nitta, K. 2001. Application of crop gas exchange and transpiration data obtained with CEEF to global change problem. Advances in Space Research 27: 1541-1545.
Watling, J.R. and Press, M.C. 2000. Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2. Global Change Biology 6: 919-930.
Weerakoon, W.M.W., Ingram, K.T. and Moss, D.D. 2000. Atmospheric carbon dioxide and fertilizer nitrogen effects on radiation interception by rice. Plant and Soil 220: 99-106.
Widodo, W., Vu, J.C.V., Boote, K.J., Baker, J.T. and Allen Jr., L.H. 2003. Elevated growth CO2 delays drought stress and accelerates recovery of rice leaf photosynthesis. Environmental and Experimental Botany 49: 259-272.
Ziska, L.H., Namuco, O., Moya, T. and Quilang, J. 1997. Growth and yield response of field-grown tropical rice to increasing carbon dioxide and air temperature. Agronomy Journal 89: 45-53.