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18.4.2 - Through-flows of O2 along rhizomes of some wetland plants

Pressurised through-flows of gas greatly increase the rate of O2 transport along rhizomes of several emergent and floating-leaved wetland species, compared to that achieved by diffusion alone. These through-flows increase the concentration of O2 in rhizomes above those if only diffusion occurred, and the higher rhizome O2 increases diffusion into roots arising from the rhizomes. Through-flows occur when pressure gradients are established along the aerenchymatous pathway with a low-resistance exit from the plant to the atmosphere (Beckett et al. 1988). Flows can be substantial, for example in the yellow waterlily (Nuphar luteum) gas flow rates within aerenchymatous petioles were described by Dacey (1980) as ‘internal winds’. Through-flows can result in an increase of two orders of magnitude in the effective length of aeration in culms and rhizomes above that possible via diffusion (Armstrong et al. 1991), enabling some wetland plants to inhabit areas with permanent deep waters and for rhizome growth deep into waterlogged soils. It is important to note that even in species with through-flows along rhizomes, O2 movement into and along roots occurs via diffusion (the roots are a dead-end side-path so through flows cannot occur without an ‘exit’).

The importance of through-flows for growing in deep water is especially visible in lakeshore vegetation. On lakeshores, wetland plants such as Typha spp. and common reed (Phragmites australis) that have flows along rhizomes and grow more deeply than morphologically similar plants that rely solely on diffusive movement of O2 (Vretare Strand 2002). The deeper the water, the more advantageous it is for a plant to transport gases by pressurised flow rather than diffusion. Emergent plants with efficient through-flows can readily grow in water up to 3 m depth (Sorrell and Hawes 2010), and some floating-leaved plants such as sacred lotus (Nelumbo nucifera) are found in up to 5 m water depth.

Rates of through-flow are determined by the pressure gradient and the resistance to flow along the aeration system. The pressure gradient can result from: (i) pressurisation of gas in live shoots due to gradients in water vapour concentration between the interior and exterior of an enclosed space with the surface of the enclosure containing micro-pores (e.g., several wetland species, Brix et al. 1992; Armstrong et al. 1996), or (ii) venturi-induced suction caused by wind blowing over the open-ends of tall, broken culms so that gas is sucked out, and air enters via shorter culms exposed to lower wind speeds (only documented so far in common reed, Phragmites australis, Armstrong et al. 1996). The processes for pressurisation in leaves, and for venturi-induced suction, have been evaluated using physical and mathematical models.