In managed agricultural ecosystems, the buildup of soil salinity from repeated irrigations can sometimes reduce crop yields. Similarly, in natural ecosystems where exposure to brackish or salty water is commonplace, saline soils can induce growth stress in plants not normally adapted to coping with this problem. Thus, it is important to understand how rising atmospheric CO2 concentrations may interact with soil salinity to affect plant growth.
In the study of Ball et al. (1997), it was found that two Australian mangrove species, with differing tolerance to salinity, exhibited increased rates of net photosynthesis in response to a doubling of the atmospheric CO2 concentration, but only when exposed to salinity levels that were 25%, but not 75%, of full-strength seawater. However, Mavrogianopoulos et al. (1999) reported that atmospheric CO2 concentrations of 800 and 1200 ppm stimulated photosynthesis in parnon melons by 75 and 120%, respectively, regardless of soil salinity, which ranged from 0 to 50 mM NaCl. Moreover, the authors noted that atmospheric CO2 enrichment partially alleviated the negative effects of salinity on melon yield, which increased with elevated CO2 at all salinity levels. Furthermore, Maggio et al. (2002) grew tomatos at 400 and 900 ppm in combination with varying degrees of soil salinity and noted that plants grown in elevated CO2 tolerated an average root-zone salinity threshold value that was about 60% greater than that exhibited by plants grown at 400 ppm CO2 (51 vs. 32 mmol dm-3 Cl).
In conclusion, it is clear that elevated CO2 may alleviate some of the negative impacts of high soil salinity on plant growth. In addition, as indicated in the review of Poorter and Perez-Soba (2001), there appear to be no changes in the effect of elevated CO2 on the growth responses of most plants over a wide range of soil salinities, in harmony with the earlier findings of Idso and Idso (1994). Hence, plants should respond positively to future increases in the air's CO2 content, even in areas where mild to moderate stresses may be present due to high soil salinity levels.
References
Ball, M.C., Cochrane, M.J. and Rawson, H.M. 1997. Growth and water use of the mangroves Rhizophora apiculata and R. stylosa in response to salinity and humidity under ambient and elevated concentrations of atmospheric CO2. Plant, Cell and Environment 20: 1158-1166.
Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: A review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.
Maggio, A., Dalton, F.N. and Piccinni, G. 2002. The effects of elevated carbon dioxide on static and dynamic indices for tomato salt tolerance. European Journal of Agronomy 16: 197-206.
Mavrogianopoulos, G.N., Spanakis, J. and Tsikalas, P. 1999. Effect of carbon dioxide enrichment and salinity on photosynthesis and yield in melon. Scientia Horticulturae 79: 51-63.
Poorter, H. and Perez-Soba, M. 2001. The growth response of plants to elevated CO2 under non-optimal environmental conditions. Oecologia 129: 1-20.