Physiological and photosynthetic plasticity in the amphibious, freshwater plant, Littorella uniflora, during the transition from aquatic to dry terrestrial environments

The physiological and photosynthetic responses of Littorella uniflora (L.) Ascherson, an amphibious macrophyte of isoetid life form, to rapid and prol onged emersion onto dry land, was studied at a reservoir. Water relations w ere little affected in the short term, but declining water potential and tu rgor pressure indicated water stress after flowering. High leaf lacunal CO2 concentrations suggested continued CO2 uptake from sediments, In contrast, a switch from Crassulacean acid metabolism (CAM) to C-3 photosynthesis was indicated by much lower levels of Delta H+ (down minus dusk titratable aci dity) and phosphoenolpyruvate carboxylase (PEPC) activity in new terrestria l leaves, 7-8-fold higher activity of ribulose bisphosphate carboxylase oxy genase (Rubisco), and increased chlorophyll and soluble protein contents. A ccumulated nitrate and amino acid pools were depleted, whereas storage of c arbohydrates as soluble sugars, fructan and starch increased. Plant carbon and nitrogen isotope ratios (delta(13)C and delta(15)N) declined, perhaps r eflecting changes in C fixation processes, N metabolism, and source C and N , In leaves of plants grown half-emersed for an extended period, contrastin g activities of PEPC and Rubisco were found in submersed and emersed portio ns, Overall, L, uniflora showed considerable phenotypic plasticity, yet see med to remain poised for re-submersion; these characteristics could be adap tive in the unpredictable water margin habitat.