Oxygen dynamics during submergence in the halophytic stem succulentHalosarcia pergranulata

Abstract

This study elucidated O 2 dynamics in shoots and roots of submerged Halosarcia pergranulata (Salicornioideae), a perennial halophytic stem succulent that grows on floodprone mudflats of salt lakes. Oxygen within shoots and roots was measured using microelectrodes, for plants when waterlogged or completely submerged, with shoots in light or in darkness, in a controlled environment. Net photosynthesis ( P N ) when underwater, at a range of dissolved CO 2 concentrations, was measured by monitoring O 2 production rates by excised stems. The bulky nature and apparently low volume of gas-filled spaces of the succulent stems resulted in relatively high radial resistance to gas diffusion. At ambient CO 2 , quasi-steady state rates of P N by excised succulent stems were estimated to be close to zero; nevertheless, in intact plants, underwater photosynthesis provided O 2 to tissues and led to radial O 2 loss (ROL) from the roots, at least during the first several hours (the time period measured) after submergence or when light periods followed darkness. The influence of light on tissue O 2 dynamics was confirmed in an experiment on a submerged plant in a salt lake in south-western Australia. In the late afternoon, partial pressure of O 2 (pO 2 ) in the succulent stem was 23.2 kPa (i.e. ~ 10% above that in the air), while in the roots, it was 6.2-9.8 kPa. Upon sunset, the pO 2 in the succulent stems declined within 1 h to below detection, but then showed some fluctuations with the pO 2 increasing to at most 2.5 kPa during the night. At night, pO 2 in the roots remained higher than in the succulent stems, especially for a root with the basal portion in the floodwater. At sunrise, the pO 2 increased in the succulent stems within minutes. In the roots, changes in the pO 2 lagged behind those in the succulent stems. In summary, photosynthesis in stems of submerged plants increased the pO 2 in the shoots and roots so that tissues experience diurnal changes in the pO 2 , but O 2 from the H 2 O column also entered submerged plants.

Flooding tolerance: suites of plant traits in variable environments

“…With shoots emergent above long-lasting, shallow floods, these root aeration traits would enhance root growth and functioning, as described previously for waterlogged plants. When floods are prolonged and too deep for shoots to reach the surface, existing roots would still benefit from these traits enabling some O 2 supply, depending on entry from the water column and/or underwater photosynthesis (Pedersen et al 2006). Availability of O 2 in completely submerged plants shows marked diurnal cycles, as dependent on underwater photosynthesis (Pedersen et al 2006;Colmer and Pedersen 2008a).…”

“…When floods are prolonged and too deep for shoots to reach the surface, existing roots would still benefit from these traits enabling some O 2 supply, depending on entry from the water column and/or underwater photosynthesis (Pedersen et al 2006). Availability of O 2 in completely submerged plants shows marked diurnal cycles, as dependent on underwater photosynthesis (Pedersen et al 2006;Colmer and Pedersen 2008a). Interestingly, when some species remain completely submerged, hardly any new adventitious roots emerge into the sediment (Van der Sman et al 1993).…”

Two Rumex Species from Contrasting Hydrological Niches Regulate Flooding Tolerance through Distinct Mechanisms

“…Profiles of internal O 2 partial pressure of the petiole were measured in light (175 µmol photons m 22 s 21 ) and O 2 and CO 2 of the submergence solution were kept at atmospheric equilibrium (20°C) by bubbling with air. A motorized micromanipulator (MC-232; Unisense) mounted with a microelectrode (tip diameter = 25 µm, OX25; Unisense) was used with a step size of 25 µm (Pedersen et al, 2006).…”

Biophysical probing of Spartina maritima photo-system II changes during prolonged tidal submersion periods

“…Many coastal marshes are prone to storm surges and river flooding, driven by extreme climate events, that in nowadays are becoming more frequent (Egan and Ungar, 2000;Pedersen and Colmer, 2006). Also the projections of sea level rise point out to increased inundations of coastal marshes (Reed, 2002).…”

“…At the leaf level several constrains arise. Not only the CO 2 gas slow diffusion in the aquatic environment limits its uptake by leaves (Smith and Walker, 1980), but also the decreased light availability, due to light attenuation along the water column (Pedersen and Colmer, 2006), impairs photosynthesis (Sand-Jensen, 1989). This photosynthetic impairment due to low CO 2 or low light, leads to a depletion in the O 2 production, with less O 2 to be diffused from the leaves to the roots, and thus increasing the anoxia driven stress at the sediment level (Colmer and Flowers, 2008).…”