Beyond the Model Species: Comparative Salt‐Stress Responses in Phylogenetically Related Mosses Physcomitrium patens and P. eurystomum (Funariaceae)

Habitat salinization is an increasing global challenge that adversely affects plant growth, development, and photosynthetic performance. Although salt responses have been extensively investigated in tracheophytes, and to some extent, in the model moss Physcomitrium patens , comparable information for other mosses—even closely related species—remains scarce. This study compared the effects of increasing NaCl substrate concentrations (0, 10, 50, and 250 mM) on morphogenesis, photosynthetic pigment composition, and chlorophyll fluorescence in P. patens and P. eurystomum grown under in vitro conditions. Morphogenetic traits were monitored continuously over 4 weeks, whereas pigment content and chlorophyll fluorescence parameters were determined after 28 days of cultivation. Both species maintained 100% survival across all treatments, indicating that salinity primarily affected growth and development rather than viability. However, their morphogenetic responses differed markedly. P. patens maintained all investigated morphological traits under low and moderate NaCl stress, whereas P. eurystomum exhibited lower morphogenetic performance under the experimental conditions. At 250 mM NaCl, bud (and gametophore) formation was strongly inhibited in both species despite the persistence of viable protonemal tissue. Photosynthetic pigment and fluorescence responses also differed between species. Physcomitrium eurystomum accumulated significantly higher chlorophylls and carotenoids under 50 mM NaCl, whereas pigment concentrations remained largely unchanged in P. patens . Chlorophyll fluorescence measurements indicated that PSII function was largely preserved under low and moderate NaCl stress, whereas severe stress reduced the maximum quantum yield of PSII and elicited species‐specific changes in nonphotochemical quenching. These findings demonstrate that closely related Physcomitrium species exhibit distinct developmental and physiological acclimation response patterns under NaCl stress, emphasizing that developmental plasticity and photosynthetic regulation represent complementary components of bryophyte salinity responses.