Plant water relations and ion homoeostasis of Mediterranean seagrasses (Posidonia oceanica and Cymodocea nodosa) in response to hypersaline stress

Changes in plant water relations and ionic homoeostasis can determine plant osmoacclimation and tolerance to hypersaline stress. In this study, we examined the role of ion accumulation in short-term osmotic acclimation capacities of two Mediterranean seagrass species (Posidonia oceanica and Cymodocea nodosa) with differentiated ranges of salinity tolerance. These were tested by exposing plants to a wide range of hypersaline treatments (from 37 to 59.5, practical salinity scale) simulated in a mesocosm system over 7 days. P. oceanica showed a more reduced capacity to adjust its leaf water potential (I (w)) than C. nodosa, so that the osmotic gradient towards plant tissues was reduced as external salinity increased in this species. Osmotic potential, I (pi) , was also reduced in both species, reflecting the activation of osmoregulation. Some ions (as Cl-) increased in both species, but ionic uptake (relative to ion concentration in seawater) generally decreased at increasing salinities, suggesting the activation of ion-exclusion mechanisms. In P. oceanica leaves, the percentage of participation of ions in the I (pi) was reduced as salinity increased; this indicated that osmotic acclimation was highly dependent on the accumulation of organic osmolytes (i.e. soluble sugars) in this species, which involves higher metabolic costs. The participation of ions was greater in C. nodosa plants exposed to hypersaline treatments, but the contrasting responses of non-structural carbohydrates, turgor pressure, I (pi) and ions in the most severe hypersaline treatment, suggested the activation of alternative osmotic strategies. Under normal saline conditions (i.e. controls), C. nodosa plants exhibited higher values of K+/Na+ and Ca2+/Na+ and selectivity of K+ over Na+ (S (K:Na)) than P. oceanica, which are typically related to species more tolerant to salinity stress. With regard to photochemistry, F (v)/F (m) and alpha was only reduced in P. oceanica exposed to some hypersaline treatments, although reductions in both rETR and I broken vertical bar PSII were detected in plants of both species at the most severe hypersaline treatments (59.5). Only C. nodosa exhibited the activation of photoprotective mechanisms as NPQ increments.