Growth and Reproductive Success under Saline Conditions of Three Plantago Species with Different Levels of Stress Tolerance

Salt stress responses were studied in three Plantago species (P. crassifolia, P. coronopus and P. major), with different levels of salt stress tolerance, at growth and reproductive stages of the life cycle. Plants were treated with increasing salt concentrations (0, 100, 200 and 400 mM NaCl) under controlled conditions in the greenhouse. Fresh biomass, leaf length and reproductive parameters related to flowering were analysed. To evaluate the reproductive success of salt-stressed plants, seeds produced after two months of treatment were sampled, and the mean number of seeds per plant and mean seed weight per treatment were determined. Seeds were germinated in vitro and seed and seedling quality were assessed by determining germination percentages and rates, length of radicle, hypocotyl, cotyledon leaves, and the angle of cotyledonary leaves. The 'seedling vigour index' was also calculated. Vegetative (fresh weight and leaf length) and reproductive traits (number of inflorescences, scape and spike length, mean number of seeds per spike and mean seed weight) were less affected by salinity in the more stress tolerant species P. crassifolia and P. coronopus than in P. major. Rate and velocity of germination of seeds produced by plants grown under saline treatments were similar in all species; however seedling development was less successful in P. major, as shown by length of the radicle, hypocotyl and cotyledonary leaves. The relative sensitivity to salt of the investigated Plantago species correlated with their distribution in nature: P. major, never present in saline environments, was much more affected by salt than its halophytic counterparts, while P. crassifolia, which grows in saline habitats, exhibited the most efficient responses to salt stress, especially at the early seedling development stage.