Internal tissue aeration in Limonium narbonense and Sarcocornia fruticosa during partial and complete submergence in saline water

Limonium narbonense Mill. and Sarcocornia fruticosa (L.) A.J. Scott are two perennial saltmarsh species sharing distribution in the Mediterranean coastal region. L. narbonense has basal leaves in rosettes and a deep root system connected to a vertical rhizome. S. fruticosa is instead a semi-woody plant with modified photosynthetic stems and superficial roots. The distribution of these two species, ranging from elevated saltmarsh sites to low and daily inundated areas, arose the interest around the flooding tolerance mechanisms involved and the possible adaptation to different levels of flooding stress. Field collected plants were conditioned in a controlled tidal system in a greenhouse, imposing two treatments: (1) changing from drained to waterlogged conditions and (2) changing from waterlogged to complete submergence. Internal tissue aeration was monitored using O2 microelectrodes during waterlogging, partial and complete submergence in darkness as well as in light. Underwater photosynthesis and dark respiration were measured by measuring the O2 balance from excised green stems and leaves while porosity measurements were performed in different epigeous and hypogeal plant tissues. O2 dynamics in soil showed a clear and rapid increase of O2 during drainage condition (1) and steady anoxic conditions in the submerged treatment (2), which adversely affected L. narbonense leaf production but stimulated the development of new articles in S. fruticosa. In both species, internal O2 declined upon partial and complete submergence in darkness: L. narbonense showed values closed to zero already during partial submergence, both in petioles (9.1 μmol∙l-1 in average) and in roots (5.6 μmol∙l-1), while S. fruticosa displayed better root aeration during partial submergence (44.4 μmol∙l-1). In light, internal O2 concentrations of the articles of S. fruticosa and the petioles of L. narbonense increased, entailing also a gradual increase in O2 in roots, especially during the re-establishment of waterlogging conditions (over 50 μmol∙l-1 in both species). At air equilibrium of CO2, rates of underwater photosynthesis and dark respiration differ significantly between treatments only when based on dry mass, highlighting a morphological acclimation in leaf thickness of the plants during submergence (higher SLA). Despite the low photosynthetic rates, intact plants were able to cover the O2 requirement during inundation under light conditions (up to 179.7 and 73.0 μmol∙l-1 in S. fruticosa and L. narbonense, respectively). High tissue porosity is a common trait providing a low-resistance internal pathway for O2 diffusion to roots. S. fruticosa displayed high tissue porosity both in lignified stems (up to 25%) and in roots (up to 27%) but for L. nabonense tissues had even higher porosity (in petioles and leaves up to 63%). These findings suggest that the low internal O2 recorded during submergence of L. nabonense must be due to unidentified bottlenecks for O2 diffusion in e.g. root-rhizome junctions or rhizome-petiole junctions, restricting internal aeration. Our findings demonstrate the importance of waterlogging and submergence tolerance in these two species inhabiting saltmarshes, providing additional knowledge to flooding tolerance of halophytes.