Paper Reviewed
Keramat, S., Eshghizadeh, H.R., Zahedi, M. and Nematpour, A. 2020. Growth and biochemical changes of sorghum genotypes in response to carbon dioxide and salinity interactions. Cereal Research Communications 48: 325-332.

Many are the studies examining the positive impacts of elevated levels of carbon dioxide on plant growth and yield, but fewer are the number of studies examining the impacts of elevated CO2 in conjunction with abiotic stresses, such as high soil salinity. The latest study to investigate this latter topic comes from Keramat et al. (2020), who evaluated "the possible interactions of elevated atmospheric CO2 concentration and salinity on the growth of sorghum cultivars." As a C4 plant, sorghum is not expected to benefit as much from CO2 enrichment as plants that utilize the C3 photosynthetic pathway. And so there are concerns whether or not the growth enhancements from CO2 enrichment might be cancelled out by the growth-retarding influence of salinity stress.

In assessing this possibility, Keramat et al. grew four sorghum (Sorghum bicolor) genotypes (cv. SG3, SG4, SG14 and SG28) in four separate treatments under controlled conditions in a greenhouse at Isfahan University of Technology, Isfahan, Iran. The treatments included a full-factorial design of two CO2 concentrations (380 or 700 ppm) and two salinity levels (1.55 or 10.55 dS m^-1). The plants were harvested 98 days after sowing and several growth and stress-related measurements were made to evaluate the individual and combined impacts of CO2 enrichment and salinity stress.

The results of the analysis indicated that elevated CO2 enhanced sorghum growth whereas salinity stress reduced it. More specifically, in pooling data among the four cultivars the authors report elevated CO2 increased plant root volume by 16% and 9% under non-saline and saline conditions, respectively. Moreover, elevated CO2 enhanced both plant shoot and root dry weights, increasing these two parameters by 14.9% and 14.4% under non saline conditions and by 8.6% and 9.6% under saline-stressed conditions, respectively. However, as noted by Keramat et al., "the extent of positive effects of elevated CO2 on these attributes was greater under non-saline than under the saline treatment." And in this regard they say "the reduction in shoot dry matter due to salinity effect was lower under CO2-enriched treatment (36.5%) than under ambient CO2 treatment (53.6%), implying that the alleviation of the negative effects of salt stress was more at elevated CO2 concentration."

Further evidence of a CO2-induced alleviation of salt stress was evident from measurements of antioxidant enzymes and concentrations of water-soluble carbohydrates, which "were decreased in all genotypes as a result of the increased concentration of carbon dioxide." Commenting on this secondary finding of importance, the authors write that "given the roles of these compounds in [helping plants cope] with environmental stresses, the elevated carbon dioxide treatment in this study seems to have moderated the negative effects of such stresses on plants." Consequently, the researchers conclude that elevated CO2 "has important interactions with both salinity and genotype that can be exploited beneficially in sorghum cropping management." And that is good news for future producers and consumers of sorghum!