Can continuous light mitigate the detrimental actions of salinity stress in facultative CAM plants?

Facultative CAM plants possess a unique ability to reversibly initiate and suppress β-carboxylation, a metabolic pathway characterised by nocturnal stomatal opening and high water-use efficiency. In the present study, we focused on determining how continuous light modifies the interaction of the Mesembryanthemum crystallinum, a model CAM facultative species, with salinity stress – an ever-growing global-scale plant diversity and productivity threat. Expression of stress-dependent PEPC form and RbcL, chlorophyll a fluorescence, pigment composition and antioxidant enzyme activity were analysed. Expression of both mentioned genes was predictably influenced by salinity stress, with stress-dependent PEPC amount increase accompanied by a concurrent RbcL decline. However, exposing plants already affected by salinity stress to continuous light resulted in a limited RbcL decline – a side-effect exclusively induced by salinity stress. Moreover, a limited expression of salinity-dependent PEPC was concomitantly observed; in M. crystallinum leaves, the content of cryptoxanthin, β-carotene, α-carotene, zeaxanthin, violaxanthin, antheraxanthin and neoxanthin has increased, especially in the case of salinity with continuous light treatments. The total SODs activity was increased in plants exposed to salinity combined with continuous light. This study unravels novel evidence that continuous light treatment may alleviate the detrimental effects of high salinity in facultative CAM plants; these conclusions, however, may potentially be extrapolated to other plant groups, including glycophytes.