Light acclimation, CO2 response and long-term capacity of underwater photosynthesis in three terrestrial plant species

To characterize underwater photosynthetic performance in some terrestrial p lants, we determined (i) underwater light acclimation (ii) underwater photo synthetic response to dissolved CO2, and (iii) underwater photosynthetic ca pacity during prolonged submergence in three species that differ in submerg ence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-toler ant) and Arrhenatherum elatius (submergence-intolerant). None of the specie s had adjusted to low irradiance after 1 week of submergence. Under non-sub merged (control) conditions, only R. crispus displayed shade acclimation, S ubmergence increased the apparent quantum yield in this species, presumably because of the enhanced CO2 affinity of the elongated leaves, In control p lants of the grass species II arundinacea and A. elatius, CO2 affinities we re higher than for R, crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosy nthesis remained constant during 1 month of submergence at normal irradianc e; at low irradiance a reduction in photosynthetic capacity was observed af ter 2 weeks, although there was no tissue degeneration, In contrast, underw ater photosynthesis of the submergence-intolerant species A, elatius collap sed rapidly under both irradiances, and this was accompanied by leaf decay, To describe photosynthesis versus irradiance curves, four models were eval uated, The hyperbolic tangent produced the best goodness-of-fit, whereas th e rectangular hyperbola (Michaelis-Menten model) gave relatively poor resul ts.