Paper Reviewed
Song, Y. and Huang, B. 2014. Differential effectiveness of doubling ambient atmospheric CO2 concentration mitigating adverse effects of drought, heat, and combined stress in Kentucky Bluegrass. Journal of the American Society of Horticultural Science 139: 364-373.

In prefacing their work, Song and Huang (2014) write that "drought and heat stress are two major environmental constraints limiting the growth of cool-season plant species," while noting that "simultaneous drought and heat can be more detrimental than either stress alone," citing the findings of Albert et al. (2011), Jiang and Huang (2001a), and Mittler (2006), specifically mentioning, in this regard, the problems of stomatal closure and declines in leaf photochemical efficiency, net photosynthetic rate and leaf relative water content, together with increases in electrolyte leakage and the induction of oxidative damages, as described in the papers of Ebdon and Kopp (2004), Jiang and Huang (2001b), Mittler (2006), Wang and Huang (2004), Yu et al. (2012) and Zhang et al. (2003). So what did they do besides bemoan these sad facts?

The two researchers describe how they studied Kentucky Bluegrass plants obtained from field plots in New Brunswick, New Jersey (USA) in controlled environment chambers maintained at ambient and double-ambient atmospheric CO2 concentrations (400 and 800 ppm, respectively), while they divided them into sub-treatments of optimum temperature and water availability, as well as drought-stressed (D) and heat-stressed (H) conditions, along with a combined D and H environment. And what did they thereby learn?

Song and Huang report that (1) "the ratio of root to shoot biomass increased by 65% to 115% under doubling ambient CO2 across all treatments with the greatest increase under D" (see figure below), while noting that (2) "high CO2 may enhance the capacity of water uptake by the root system, supplying water to maintain leaf hydration," that (3) "the positive carbon gain under doubling ambient CO2 was the result of both increases in net photosynthesis rate and suppression of respiration rate," that (4) "leaf net photosynthesis increased by 32% to 440% with doubling ambient CO2," that (5) there was a significant decline (by 18% to 37%) in leaf respiration rate under the different treatments "with the greatest suppression under D + H," all of which findings led the two scientists to conclude "the increase in carbon assimilation and the decline in respiration carbon loss could contribute to improved growth under elevated CO2 conditions," as they also note has been found to be the case with several other plants," citing the studies of Ainsworth et al. (2002), Drake et al. (1997), Long et al. (2004) and Reddy et al. (2010).

Shoot dry weight (A), root dry weight (B), and root/shoot dry weight ratio (C) of Kentucky Bluegrass grown under drought stress, heat stress, and drought and heat stress, under ambient and elevated CO2 concentrations for 28 days.

References
Ainsworth, E.A., Davey, P.A., Bernacchi, C.J., Dermody, O.C., Heaton, E.A., Moore, D.J., Morgan, P.B., Naaidu, S.L., Ra, H.-S.Y., Zhu, X.G., Curtis, P. and Long, S.P. 2002. A meta-analysis of elevated CO2 effects on soybean (Glycine max) physiology, growth and yield. Global Change Biology 8: 695-709.

Albert, K.R., Ro-Poulsen, H., Mikkelsen, T.N., Michelsen, A., van der Linden, L. and Beier, C. 2011. Interactive effects of elevated CO2, warming, and drought on photosynthesis of Deschampsia flexuosa in a temperate heath ecosystem. Journal of Experimental Botany 62: 4253-4266.

Drake, B.G., Gonzalez-Meler, M.A. and Long, S.P. 1997. More efficient plants: A consequence of rising atmospheric CO2? Annual Review of Plant Physiology and Plant Molecular Biology 48: 609-639.

Ebdon, J.S. and K.L. Kopp. 2004. Relationships between water use efficiency, carbon isotope discrimination, and turf performance in genotypes of Kentucky bluegrass during drought. Crop Science 44: 1754-1762.

Jiang, Y. and Huang, B. 2001a. Physiological responses to heat stress alone or in combination with drought: A comparison between tall fescue and perennial ryegrass. HortScience 36: 682-686.

Jiang, Y. and Huang, B. 2001b. Osmotic adjustment and root growth associated with drought preconditioning-enhanced heat tolerance in Kentucky bluegrass. Crop Science 41: 1168-1173.

Long, S.P., Ainsworth, E.A., Rogers, A. and Ort, D.R. 2004. Rising atmospheric carbon dioxide: Plants FACE the future. Annual Review of Plant Biology 55: 591-628.

Mittler, R. 2006. Abiotic stress, the field environment and stress combination. Trends in Plant Science 11: 15-19.

Reddy, A.R., Rasineni, G.K. and Raghavendra, A.S. 2010. The impact of global elevated CO2 concentration on photosynthesis and plant productivity. Current Science 99: 1305-1319.

Wang, Z.L. and Huang, B.R. 2004. Physiological recovery of Kentucky bluegrass from simultaneous drought and heat stress. Crop Science 44: 1729-1736.

Yu, J., Chen, L., Xu, M. and Huang, B. 2012. Effects of elevated CO2 on physiological responses of tall fescue to elevated temperature, drought stress, and the combined stresses. Crop Science 52: 1848-1858.

Zhang, X.Z., Ervin, E.H. and Schmidt, R.E. 2003. Plant growth regulators can enhance the recovery of Kentucky bluegrass sod from heat injury. Crop Science 43: 952-956.