Plant–water relations of intertidal and subtidal seagrasses

This work represents the first contribution to (i) examine the changes in plant‐water relations of an inter‐tidal seagrass during air exposure (Zostera noltii), and (ii) compare the water status descriptors between inter‐tidal‐ and subtidal‐adapted species (Cymodocea nodosa, Zostera marina). Two different morphotypes of Z. noltii that develop in the highest and lowest inter‐tidal levels in the Portuguese lagoon of Ria Formosa were exposed to natural emersion periods under laboratory conditions, and the evolution of leaf water relations and osmolytes (ions, proline and non‐structural carbohydrates) was measured. Both morphotypes regulated their water potential (Ψw) by reducing the osmotic potential (Ψπ) through osmolyte accumulation, but only high inter‐tidal plants were able to do this by adjusting the turgor pressure through cell wall hardening. This is a conservative mechanism for osmotic acclimation, which occurred only after long emersion periods (7 h). After a rapid increase in ion concentration under air exposure, the high inter‐tidal morphotype replaced them by more physiologically compatible solutes (proline and non‐structural carbohydrates) to maintain the osmotic adjustment. Altered ionic homeostasis was found in low inter‐tidal plants when exposed to such unnatural, long emersion periods. Osmotic unbalances were also observed during the submerged recovery phase. Descriptors of leaf pressure–volume (P–V) curves and Höfler diagrams were derived for seagrasses for the first time. They support the divergences in water relations observed between inter‐tidal and subtidal seagrasses according to their vertical distribution. More negative water and osmotic potentials and higher rigidity of cell walls (higher elastic modulus, ε) were found to be specific osmotic adaptations of seagrasses to the inter‐tidal.