What was done
The authors grew rice (Oryza sativa [L.] cv. IR-72) 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.

What was learned
In the elevated CO2 treatment, midday leaf photosynthetic CO2 exchange rates (CER) and chlorophyll concentrations were higher at most sampling dates.  In addition, the CO2-enriched plants exhibited enhanced midday leaf sucrose and starch accumulation during early reproductive phases.

Near the end of the imposed drought periods, water deficits caused substantial decreases in midday leaf CER and chlorophyll concentrations, along with concomitant reductions in the primary products of photosynthesis.  These drought-induced effects, according to the authors, "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," as has also been observed for a number of other plants (Rogers et al., 1984; Jones et al., 1985; Idso, 1988; Bhattacharya et al., 1990; Chaves and Pereira, 1992; Clifford et al., 1993; Baker et al., 1997; Vu et al., 1998).

Recovery from drought-induced water stress was also more rapid in the elevated CO2 treatment.  At panicle initiation, for example, the authors 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."

What it means
In the simple summary words of the authors, their results demonstrate that "rice grown under future rising atmospheric CO2 should be better able to tolerate drought situations."

References
Baker, J.T., Allen Jr., L.H., Boote, K.J. and Pickering, N.B.  1997.  Rice responses to drought under carbon dioxide enrichment. 2. Photosynthesis and evapotranspiration.  Global Change Biology 3: 129-138.

Bhattacharya, N.C., Hileman, D.R., Ghosh, P.P., Musser, R.L., Bhattacharya, S. and Biswas, P.K.  1990.  Interaction of enriched CO2 and water stress on the physiology of and biomass production in sweet potato grown in open-top chambers.  Plant, Cell and Environment 13: 933-940.

Chaves, M.M. and Pereira, J.S.  1992.  Water stress, CO2 and climate change.  Journal of Experimental Botany 43: 1131-1139.

Clifford, S.C., Stronach, I.M., Mohamed, A.D., Azam-Ali, S.N. and Crout, N.M.J.  1993.  The effects of elevated atmospheric carbon dioxide and water stress on light interception, dry matter production and yield in stands of groundnut (Arachis hypogaea L.).  Journal of Experimental Botany 44: 1763-1770.

Idso, S.B.  1988.  Three phases of plant response to atmospheric CO2 enrichment.  Plant Physiology 87: 5-7.

Jones, P. Jones, J.W. and Allen Jr., L.H.  1985.  Seasonal carbon and water balances of soybeans grown under stress treatments in sunlit chambers.  Transactions of the American Society of Agricultural Engineers 28: 2021-2028.

Rogers, H.H., Sionit, N., Cure, J.D., Smith, H.M. and Bingham, G.E.  1984.  Influence of elevated CO2 on water relations of soybeans.  Plant Physiology 74: 233-238.

Vu, J.C.V., Baker, J.T., Pennanen, A.H., Allen Jr., L.H., Bowes, G. and Boote, K.J.  1998.  Elevated CO2 and water deficit effects on photosynthesis, ribulose bisphosphate carboxylase-oxygenase, and carbohydrate metabolism in rice.  Physiologia Plantarum 103: 327-339.