Salinity and drought suppress C4 and induce CAM metabolism in two Portulaca species

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation that improves plant survival under salinity and drought stress. Portulaca is one of the few genera capable of switching between C4 and CAM photosynthesis, especially in response to drought. However, the physiological and root morphological responses during the C4-CAM transitions under salinity remain poorly understood. Here, we investigated CAM induction in the leaves of P. grandiflora and P. molokiniensis under salinity and drought conditions and evaluated their recovery after stress. In P. grandiflora, CAM photosynthesis was induced within one week under high salinity and severe drought and after one week under moderate salinity and drought, as indicated by nocturnal increases in CO2 uptake, titratable acidity, PEPC activity and malate content. In P. molokiniensis, CAM induction occurred after one week under high salinity and severe drought, but was delayed two weeks under moderate salinity and drought conditions. Both species exhibited reduced CO2 uptake, chlorophyll fluorescence and protein content under stress. P. grandiflora also exhibited pronounced day/night fluctuations in proline and malondialdehyde content, indicating greater oxidative stress, while P. molokiniensis was less affected. Root morphological changes in P. grandiflora were limited to severe drought, whereas P. molokiniensis responded to both high salinity and severe drought. After recovery, P. grandiflora reverted to C4 photosynthesis within one week under drought and two weeks under salinity. P. molokiniensis also returned to C4 metabolism within one week of drought recovery and two weeks of salinity recovery. These results demonstrate the timing, reversibility and species-specific plasticity of C4-CAM transitions in Portulaca and provide insights into their adaptive strategies under abiotic stress and the potential resilience of C4 plants under future climate scenarios.