Plasticity versus specialization in two Amazonian Isoëtes: anatomical, physiological, and metabolic responses to seasonal water-level drawdown

Ancient aquatic lycophytes of the genus Isoëtes are increasingly threatened by seasonal hydrological variability in the Amazon. We investigated the anatomical, physiological, and metabolic responses of two threatened species from the Carajás region, the amphibious Isoëtes serracarajensis and the subaquatic specialist I. cangae , to seasonal water-level drawdown. Submerged I. serracarajensis leaves bore dense cuticular protuberances across the entire adaxial epidermis; upon emersion, total protuberance density fell 4.9-fold and the remaining protuberances were mostly restricted to anticlinal cell junctions, with a 28-fold reduction on the exposed periclinal face, while stomatal complexes were induced. In contrast, emerging I. cangae leaves showed surface relief consistent with periclinal wall thickening but failed to produce stomatal complexes. Both species exhibited three diurnal phases in efficiency of photosystem II (Φ PSII ) along the leaf blade when submerged, with I. cangae displaying greater spatial heterogeneity. Upon emersion, I. serracarajensis maintained this tripartite Φ PSII pattern, whereas I. cangae exhibited marked apical photoinhibition despite sustained high basal Φ PSII . Both species employed Crassulacean acid metabolism (CAM), with a diel malate pattern in emerged I. serracarajensis consistent with a shift toward C3-like photosynthesis. Vacuolar H⁺-ATPase activity diverged most strongly between species: under submersion it was higher in I. cangae than in I. serracarajensis and declined during the light period in both, but emersion reversed this ranking, leaving emerged I. cangae with 4.6-fold lower activity in the dark and 11-fold lower in the light. The diel response of emerged I. serracarajensis also inverted, activity being 56% higher after the light than after the dark period. Collectively, these findings reveal divergent adaptive trajectories: I. serracarajensis displayed marked physiological plasticity suited to an amphibious lifestyle, while I. cangae exhibited specialization to permanent submersion with limited emersion tolerance. This mechanistic framework is essential for predicting species responses to hydrological change and guiding conservation efforts for these endangered Amazonian endemics.