Comparative studies on the stress responses of two Bupleurum (Apiaceae) species in support of conservation programmes

Bupleurum tenuissimum is a rare species in Spain, which became extinct in recent decades in many localities, including protected areas. In the study area, the 'Albufera' Natural Park (SE Spain), the species was present in Limonietalia grasslands, a priority and endangered habitat, but has not been observed in the last 40 years. At present, it is included in conservation programmes aimed at its reintroduction in this territory. This study's primary objective was to establish the limits of tolerance of this species to salt and water stress under controlled greenhouse conditions. A congeneric species, B. fruticousm, was included as comparative material to get insights into its stress response mechanisms. Analysis of changes in growth parameters (stem length, number of leaves, root length, fresh weight and water content of roots and leaves) in response to the applied stress treatments confirmed that B. tenuissimum behaves as a facultative (moderate) halophyte susceptible to drought stress, whereas B. fruticosum is not severely affected by water stress but is sensitive to salinity, even at low NaCl concentrations. The stress-induced changes in several biochemical parameters (contents of photosynthetic pigments, ions, osmolytes, oxidative stress markers, and non-enzymatic antioxidant compounds, and specific activities of relevant antioxidant enzymes) explained the differential behaviour of the investigated Bupleurum species. In addition to the novelty of these findings in a genus that has rarely been studied from this perspective, these results will be relevant for implementing and managing B. tenuissimum conservation programmes, guiding the selection of reintroduction sites. The study also provides valuable information on the stress tolerance of B. fruticosum, with great potential for the ecological restoration of Mediterranean dry forest vegetation and riparian thickets.