Effects of salinity and short-term elevated atmospheric CO 2 on the chemical equilibrium between CO 2 fixation and photosynthetic electron transport of Stevia rebaudiana Bertoni.

The effect of water salinity on plant growth and photosynthetic traits of Stevia rebaudiana was investigated to determine its level and mechanisms of salinity tolerance. It was also attempted to assess how short-term elevated CO2 concentration would influence the boundaries and mechanisms of its photosynthetic capacity. The plants were grown in gravel/hydroponic system under controlled greenhouse conditions and irrigated with four different salinity levels (0, 25, 50 and 100 mol m-3 NaCl). Low salinity did not significantly alter the plant fresh weight, which was substantially decreased by 67% at high salinity treatment. Salinity tolerance threshold was reached at 50 mol m-3  NaCl while C50 was between 50 and 100 mol m-3  NaCl, indicating that S. rebaudiana is a moderate salt tolerant species. Salt-induced growth reduction was apparently linked to a significant decline of about 47% in the photosynthetic rates (Anet ) at high salinity treatment, leading consequently to a disequilibrium between CO2 -assimilation and electron transport rates (indicated by enhanced ETRmax /Agross ratio). Elevated atmospheric CO2 enhanced CO2 assimilation rates by 65% and 80% for control and high-salt-stressed plants respectively, likely due to significant increases in intercellular CO2 concentration (indicated by enhanced Ci /Ca ). The priority for Stevia under elevated atmospheric CO2 was not to save water but to maximize photosynthesis so that the PWUE was progressively improved and the threat of oxidative stress was diminished (decline in ETRmax /Agross ). The results imply that elevated CO2 level could ameliorate some of the detrimental effects of salinity, conferring higher tolerance and survival of S. rebaudiana, a highlydesired feature with the forthcoming era of global changes.