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Endogenous polyamine (PA) content in the seagrass Cymodocea nodosa was moni- tored at different embryo and seedling developmental stages. Total PA concentration, including putrescine (Put), spermidine (Spd) and spermine (Spm), and the Put (Spm + Spd) 1 ratio decreased during embryo development, from the immature fruit to the mature seed stage (with a sharp decline in the free PA fraction), and subsequently increased during germination and seedling development from the cotyledon to seedling stage, indicating an important role during zygotic embryogenesis and seed germination. In particular, total levels of Put and Spm significantly decreased from the fruit to the mature seed, to be restored after germination. Spd levels remained constant during all the developmental stages studied. When seedlings were subjected to hypo- osmotic stress (i.e. salinity decrease from 36 to 5 psu), fluorescence levels decreased, reaching a minimum photochemical efficiency (Fv/Fm) at 5 psu (0.582 ± 0.025, mean ± SE), indicating that seedlings were under high stress. Contrary to expectations, this was followed by a slight decrease in total PA content (showing a significance difference between 5 psu and higher salinities in bound-insoluble PA) and the accumulation of the PA cadaverine (Cad) at 5 psu (indicating cell death), which revealed the inability of this species to face the abiotic stress promoted by long-term salinity decrease. These 2 physiological indicators may suggest that C. nodosa is an obligate halo- phyte because it requires an environment with relatively high and stable salinity for its survival. Moreover, exogenous application of Spd significantly improved Fv/Fm values at the lowest salinity treatment (5 psu: 0.670 ± 0.012), which suggests a protective role of Spd in maintaining the photo- synthetic apparatus of C. nodosa seedlings under long-term hypo-osmotic stress.

Endogenous polyamine content and photosynthetic performance under hypo-osmotic conditions reveal Cymodocea nodosa as an obligate halophyte