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

To characterize underwater photosynthetic performance in some terrestrial plants, we determined (i) underwater light acclimation (ii) underwater photosynthetic response to dissolved CO2, and (iii) underwater photosynthetic capacity during prolonged submergence in three species that differ in submergence tolerance: Phalaris arundinacea, Rumex crispus (both submergence-tolerant) and Arrhenatherum elatius (submergence-intolerant). None of the species had adjusted to low irradiance after 1 week of submergence. Under non-submerged (control) conditions, only R. crispus displayed shade acclimation, Submergence increased the apparent quantum yield in this species, presumably because of the enhanced CO2 affinity of the elongated leaves, In control plants of the grass species II arundinacea and A. elatius, CO2 affinities were higher than for R, crispus. The underwater photosynthetic capacity of R. crispus increased during 1 month of submergence. In P. arundinacea photosynthesis remained constant during 1 month of submergence at normal irradiance; at low irradiance a reduction in photosynthetic capacity was observed after 2 weeks, although there was no tissue degeneration, In contrast, underwater photosynthesis of the submergence-intolerant species A, elatius collapsed rapidly under both irradiances, and this was accompanied by leaf decay, To describe photosynthesis versus irradiance curves, four models were evaluated, The hyperbolic tangent produced the best goodness-of-fit, whereas the rectangular hyperbola (Michaelis-Menten model) gave relatively poor results.