Physiological response of Miscanthus genotypes to salinity stress under elevated CO2

Miscanthus is a class of C-4 perennial grasses, which can be cultivated on marginal land even with high salinity. However, the future environment may be altered by elevated atmospheric CO2 concentration ([CO2]) and knowledge is limited about the interactive impacts of CO2 enrichment and salinity on this C-4 bioenergy crop. In this study, three Miscanthus genotypes (M. sacchariflorus, M. x giganteus, and M. lutarioriparius) were grown under either ambient (400 ppm) [CO2] (a[CO2]) or elevated (800 ppm) [CO2] (e[CO2]) at five salinity levels (0, 50, 100, 150, and 200 mm NaCl denoted as S0, S1, S2, S3, and S4, respectively), and the impacts of e[CO2] on plant physiological responses to salt stress were investigated. Our results suggested that e[CO2] had no obvious effect on net photosynthetic rate (A(n)), but significantly reduced the stomatal conductance (g(s)), thus improving water use efficiency regardless of salinity levels. In addition, e[CO2] could improve water potential of plants under both control and saline conditions, but the magnitude of increase was highly genotypic dependent. The maximum quantum yield of photosystem II (F-v/F-m) was not altered by e[CO2], which, however, could alleviate the negative effect of salt on F-v/F-m. Furthermore, salt stress increased the concentration of abscisic acid (ABA) in xylem sap and leaves, while the effect of e[CO2] on ABA level was closely associated with genotypes. e[CO2] reduced Na+ concentration and had positive influences on maintaining Na+/K+ ratio, thus favoring ionic homeostasis, although such effect was genotype dependent. Collectively, our data suggested that e[CO2] could partially mitigate the detrimental effects of salinity, conferring higher salt tolerance of Miscanthus.