Background
With respect to terrestrial plants in general, the authors write that complete submergence in water "impedes exchange of O2 and CO2 with shoots (Voesenek et al., 2006)," and that underwater photosynthesis "is limited by CO2 availability owing to slow diffusion in water, and stomatal closure (Mommer and Visser, 2005)," which submergence-induced phenomena, -- if long sustained -- will lead to their demise.

What was done
To learn how the wetland plant Hordeum marinum Huds. would respond when fully submerged in water, Pedersen et al. grew several 28-day-old plants consisting of three Nordic Gene Bank accessions (H21, H90 and H546) for seven additional days while exposing them to four different treatments: "aerated root zone controls with shoots in air; stagnant root zone with shoots in air; stagnant root zone with shoots also completely submerged with 18 然 CO2 (air equilibrium); stagnant root zone with shoots also completely submerged with 200 然 CO2 (simulating CO2 enrichment in many natural flood waters)," while measuring numerous plant responses.

What was learned
The three researchers report that "plants submerged for 7 days in water at air equilibrium (18 然 CO2) suffered loss of biomass, whereas those with 200 然 CO2 continued to grow," and in this regard, they say that "higher underwater net photosynthesis at 200 然 CO2 increased by 2.7- to 3.2-fold sugar concentrations in roots of submerged plants, compared with at air equilibrium CO2." And they say that this phenomenon "is likely to have contributed to the greater root growth in submerged plants with the higher CO2 supply." In addition, they note that the latter CO2-enriched plants "tillered similarly to plants with shoots in air."

What it means
Pedersen et al. note that in addition to their findings, CO2 enrichment of submerging water to ~290 然 enhanced by 2-fold the growth of two cultivars of rice, compared to plants submerged with water in equilibrium with normal ambient air (Setter et al., 1989); and they further note that such elevated CO2 concentrations "have been reported at various field sites," citing the work of Setter et al. (1987) and Ram et al. (1999). Hence, they note the significance of this phenomenon for plants experiencing submergence during floods, indicating that under water in equilibrium with air of normal CO2 concentration, such plants typically lose mass and die, while when the water is super-saturated with CO2, they can not only survive, they can actually continue to grow.

References
Mommer, L. and Visser, E.J.W. 2005. Underwater photosynthesis in flooded terrestrial plants: a matter of leaf plasticity. Annals of Botany 96: 581-589.

Ram, P.C., Singh, A.K., Singh, B.B., Singh, V.K., Singh, H.P., Setter, T.L., Singh, V.P. and Singh, R.K. 1999. Environmental characterization of floodwater in Eastern India: relevance to submergence tolerance of lowland rice. Experimental Agriculture 35: 141-152.

Setter, T.L., Kupkanchanakul, T., Kupkanchankul, K., Bhekasut, P., Wiengweera, A. and Greenway, H. 1987. Concentrations of CO2 and O2 in floodwater and in internodal lacunae of floating rice growing at 1-2 meter water depths. Plant, Cell and Environment 10: 767-776.

Setter, T.L., Waters, I., Wallace, I., Bhekasut, P. and Greenway, H. 1989. Submergence of rice. I. Growth and photosynthetic response to CO2 enrichment of floodwater. Australian Journal of Plant Physiology 16: 251-263.

Voesenek, L.A.C.J., Colmer, T.D., Pierik, R., Millenaar, F.F. and Peeters, A.J.M. 2006. How plants cope with complete submergence. New Phytologist 170: 213-226.