The effects of elevated carbon dioxide on static and dynamic indices for tomato salt tolerance

Although there is consensus that water use efficiency increases at elevated concentrations of CO2, there are few studies on the interacting effects of elevated CO2 on plant salt tolerance. The objectives of this study were, (1) to determine the effect of ambient and twice ambient concentrations of CO2 on tomato (Lycopersicon esculentum Mill.) responses to salinity; (2) to compare the salt tolerance threshold values based on the classic root zone salinity [J. Irrig. Div. ASCE 103, (1977) 115], or ion flux to the shoot, a measure recently defined as Salinity Stress Index (SSI) by Dalton et al. [Plant Soil 192 (1997) 307; Plant Soil 219 (2000) 1; Plant Soil 229 (2001) 189]. For all salinities, the water use of the twice ambient CO2 treatment was significantly reduced. The effect of twice ambient CO2 was to increase the root zone salinity threshold value from 32 to 51 mmol dm(-3) Cl. The threshold SSI value of 1.05 mmol Cl per g shoot DW for the twice ambient CO2 treatment was almost identical to that of the ambient treatment and to those previously obtained when plant growth was modulated by root temperature (SSI = 1.19 and 1.10 at 25 and 18 degreesC, respectively [Plant Soil 192 (1997) 307]) and photosynthetic photon flux density (PPFD) (SSI = 0.97 and 1.10 at 400 and 600 mumol m(-2) s(-1) PPFD respectively [Plant Soil 229 (2001) 189]). The twice ambient CO2 treatment showed a slightly lower root/shoot ratio (0.138 +/- 0.001) than the ambient CO2 treatment (0.156 +/- 0.014). Consistent with the predictions of the SSI, leaf chloride per plant and leaf chloride concentration showed significant reduction for the twice ambient CO2 treatment which follows from the supposition that water and salt uptake are linked. Based on the SSI, it was shown that the intrinsic salt tolerance of tomato is invariant to an increase in atmospheric CO2 as has been previously shown for root temperature and solar radiation, while at the same time, the root zone salinity threshold value is dependent on environmental factors. (C) 2002 Elsevier Science B.V. All rights reserved.