Paper Reviewed
Hussin, S., Geissler, N., El-Far, M.M.M. and Koyro, H.-W. 2017. Effects of salinity and short-term elevated atmospheric CO2 on the chemical equilibrium between CO2 fixation and photosynthetic electron transport of Stevia rebaudiana Bertoni. Plant Physiology and Biochemistry 118: 178-186.

Stevia (Stevia rebaudiana) is a perennial herb grown in the highlands of northeastern Paraguay. According to Hussin et al. (2017), it has recently grown in economic value "for the pharmaceutical and food industries due to its high content of steviol glycosides," which reportedly "impart 300-400 times higher sweetness than common table sugar, and are used as non-caloric sweeteners for diabetics and diet conscious people worldwide." Stevia also provides several therapeutic benefits, with its secondary metabolites offering "antihyperglycaemic, anti-hypertensive, anti-inflammatory, anti-tumour, anti-diarrhoeal, diuretic and immunomodulatory effects." Incorporation of this plant species into world agricultural systems, however, requires improved understanding of its tolerance to biotic and abiotic conditions. And in this regard, Hussin et al. note that the response of Stevia to environmental constraints such as salinity are lacking. Therefore, this team of four researchers from Germany and Egypt set out to investigate the effects of water salinity and short-term elevated CO2 concentration on the growth and photosynthetic capacity of this unique plant species.

Their experiment was conducted under controlled greenhouse conditions in a gravel/hydroponic system where the plants were subjected to irrigation under four different salinity levels (control, low, medium and high, corresponding to 0, 25, 50 and 100 mol m^-3 NaCl, respectively). Results indicated that Stevia growth was slightly stimulated at low water salinity (compared to control), but was reduced by 40 and 67 percent, respectively, at the medium and high salinity levels. Under short-term CO2 enrichment (2000 ppm), Hussin et al. report that net photosynthesis was enhanced by 66, 466, 81 and 81 percent in the control, low, medium and high salinity conditions, respectively.

Similarly, elevated CO2 improved the gross photosynthetic rate and the photosynthetic water use efficiency of Stevia. The scientists also found a significant reduction in the ratio of maximal electron transport rate and gross photosynthetic rate of salt-stressed plants under elevated CO2, which finding, they say, "is a clear indicator that these plants indeed suffered less oxidative stress than the ones grown under ambient CO2 concentration." Consequently, given these and other findings presented in their paper, Hussin et al. conclude that Stevia "will probably benefit from rising atmospheric CO2 concentrations in the future." And that is especially good news for those interested in harnessing the sweetness and pharmaceutical traits of this plant species.