Internal oxygen transport to below-ground parts: importance for emergent macrophytes

Abstract: This thesis examines factors regulating growth and distribution of emergent plants with focus on effects of oxygen supply to the roots. Greenhouse experiments revealed that P. australis acclimatizes to specific water depths with phenotypic plasticity. These phenotypic responses act to increase growth at specific water depths but may result in decreased tolerance to changed water depth. Oxygen needed for aerobic respiration and protection from phytotoxins is transported to the roots from aerial plant parts by diffusion and, in some species, by pressurised ventilation. In greenhouse experiments, growth and biomass allocation patterns have been compared between P. australis plants with, and with inhibited, pressurised ventilation. Inhibition of pressurised ventilation resulted in hampered growth in deep water, and also in shallow water when grown in an organic substrate. In shallow water and a minerogenic substrate, diffusion seemed to cover the oxygen demand. Thus, the ability to use pressurised ventilation is suggested to result in a competitive advantage in deep water and in organic substrates. In contrast to earlier assumptions, this thesis presents evidence for occurrence of pressurised ventilation in seedlings of P. australis. Thus, high performance of species with pressurised ventilation during establishment, as well as in mature clones, is suggested to result in site preemption during establishment as well as long-term competitive advantage in deep water. A field study confirms that plants with pressurised ventilation generally grow in deeper water than plants without the ability to use pressurised ventilation in situ. Pressurised ventilation can however not be regarded as a prerequisite for growth in deep water as species with low or non-detectable flow rates also were found in deep water.