Figure 18.8. Scanning electron micrograph of a root of Melaleuca halmaturorum after roots and the lower half of shoots had been flooded for 14 weeks. Extensive aerenchyma have formed through breakdown of cortical cell layers. Scale bar = 100 µm (Micrograph courtesy M. Denton).
Internal O2 transport from shoots to roots is essential to survival and functioning of roots in anoxic, waterlogged soils. Long-distance O2 transport through the body of plants occurs via large intercellular gas-filled spaces, termed lacunae or aerenchyma (Figure 18.8).
Movement of gases within root aerenchyma occurs via diffusion, but as will be explained below gas movements can also occur via pressure-driven mass flows in the shoots and rhizomes of some wetland species under certain conditions. When shoots are in air, atmospheric O2 enters and then diffuses into and along roots. When shoots are completely submerged, tissue O2 status will change markedly between light (i.e. O2 produced in photosynthesis during the day) and dark (i.e. night) periods. In summary, aerenchyma provides a rapid gas exchange pathway between the atmosphere and below-ground tissues, essential for survival in flooded environments.
Figure 18.9. Transverse sections of (a) adventitious root of rice showing lysigenous aerenchyma, and (b) lateral root of Rumex hydrolapathum showing schizogenous aerenchyma. (a) taken 50 mm behind the root apex, (b) taken 5 mm behind the root apex. (Micrographs courtesy of W. Armstrong).
The amount of aerenchyma within roots determines the capacity for internal O2 transport. Aerenchyma formation is constitutive in roots of many wetland species, although the amount is often further enhanced when soils are waterlogged (Justin and Armstrong, 1987). Many non-wetland species can also form root aerenchyma, but its development can take a couple of days following the onset of waterlogging. Roots of non-wetland species typically, however, form less aerenchyma than wetland species, and some dryland species cannot form aerenchyma. Nevertheless, even in relatively intolerant species such as wheat, newly-formed adventitious roots develop aerenchyma, and these new roots are important since the bulk of the seminal roots die.
Aerenchyma can form in the cortex of roots by two main, distinct, developmental processes: (i) lysigeny - the collapse of files of cortical cells to leave behind gas-filled voids (Figure 18.9a), or (ii) schizogeny - cell separation in a radial direction, so that large gas-filled channels form between cells (Figure 18.9b). Lysigenous aerenchyma results from selective programmed cell death in the cortex and this type occurs in many monocots, including important crops (e.g., barley, wheat, maize, rice). Schizogenous aerenchyma is formed by separation of cells (without death) and is found in some wetland species and particularly dicots (e.g., Rumex spp.). The ‘honeycomb-type’ schizogenous aerenchyma shown in Figure 18.9b for Rumex hydrolapathum forms due to cells being forced apart owing to oblique divisions by some of the cortical cells in radial rows.
The involvement of ethylene in formation of lysigenous aerenchyma has been studied in maize roots, and although major gaps exist in understanding of lysigenous aerenchyma, even less is known of the regulation of schizogenous aerenchyma. Evidence for the involvement of ethylene signalling in lysigenous aerenchyma formation is that inhibitors of ethylene action (e.g., silver ions) or of ethylene synthesis (e.g., aminoethoxyvinylglycine, AVG) block aerenchyma formation in hypoxic roots. Hypoxia enhances the activity of an enzyme involved in ethylene biosynthesis (1-aminocyclopropan-1-carboxylic acid (ACC synthase), ACC concentration increased in hypoxic roots, and ethylene synthesis was stimulated. The involvement of ethylene signalling in aerenchyma formation was also supported by experiments in which exogenously supplied ethylene induced aerenchyma formation in aerated roots.
Aerenchyma formation can be quantified by taking root cross-sections and measuring areas of gas-filled spaces relative to the total cross-sectional area. Many studies have quantified root porosity, the gas-filled volume per unit of root volume, and porosity includes the total gas volume in the roots (i.e. large aerenchyma channels plus the smaller intercellular spaces). Porosity in roots of plants grown in waterlogged soil varied from below 1% in some non-wetland species to as much as 53% in one wetland species, demonstrating the wide variation amongst species in their capacities for internal aeration of their roots when in waterlogged soil (Justin and Armstrong, 1987).
The importance of high porosity for root growth in waterlogged soil was demonstrated in a study of 91 species differing in aerenchyma volumes; species with roots of < 5% porosity penetrated only 30-95 mm, whereas those with > 35% porosity grew 150-345 mm into a waterlogged potting mix (Justin and Armstrong, 1987). Mathematical modelling has also highlighted the importance of root porosity for internal O2 diffusion and therefore root growth in anaerobic substrates (Armstrong, 1979). O2 supply via aerenchyma determines the respiratory activity and therefore energy status in roots, as demonstrated by measurements of adenylate energy charge (AEC) in root tips of maize seedling roots in an O2-free medium (Drew et al., 1985). For roots reliant on an internal O2 supply, the AEC was ~ 0.7 for tips of roots with aerenchyma (porosity ~13%), compared with ~ 0.4 in those without aerenchyma (porosity ~4%). Thus, O2 supplied via the aerenchyma enables respiration and the ATP produced is essential for the survival, functioning and growth of roots in waterlogged soil.
Figure 18.10. Profiles of O2 with distance behind the apex of the main axis of an intact adventitious root of Phragmites australis, when in an O2-free medium with shoots in air. O2 within the root cortex increases at positions closer towards the root-rhizome junction, consistent with higher concentrations towards the source of O2 diffusing down the root. By contrast, O2 on the exterior of the root surface is highest near the tip, and very low in the basal part of the root. A barrier to radial O2 loss largely prevents O2 movement to the rhizosphere in these basal positions (see also description in the text). Jackson and Armstrong (1999).
In addition to large volumes of aerenchyma (i.e. high porosity), roots of many wetland plants also possess a barrier to radial O2 loss (ROL) in the root exterior (Armstrong, 1979; Colmer, 2003). The ROL barrier diminishes losses of O2 to the rhizosphere and thus enhances longitudinal diffusion towards the root apex. Loss of O2 from roots to waterlogged soil can be substantial, owing to the steep concentration gradient from root-to-soil. Many, but not all, wetland species can restrict O2 losses from the basal parts of their roots. An example of a functional barrier to ROL is shown in Figure 18.10 for a root of common reed (Phragmites australis). When in a low O2 medium, the concentration of O2 at the root surface was relatively high near the tip, but it was extremely low at 30 mm and further behind the apex. This pattern of low surface O2 in basal regions, despite the internal O2 being higher towards the root base (closer to the O2 source), indicates a high resistance to ROL across the outer cell layers in sub-apical positions. This resistance to radial O2 diffusion results from suberin depositions in the hypodermis/exodermis. A ‘tight’ barrier to ROL develops constitutively in the basal zones of adventitious roots of many wetland species, but was induced by growth in stagnant deoxygenated medium in several others, including rice (Colmer, 2003).
Even for roots with a ‘tight’ barrier to ROL, losses of O2 are substantial near the root tip, and also from laterals. ROL around the root tip protects this sensitive growing point from reduced toxins, as these would be re-oxidised as the tip advances into the otherwise anaerobic soil (e.g. Fe2+ oxidised to insoluble Fe2O3). Thus, ROL may ‘protect’ the apex against reduced toxins, with the barrier to ROL loss in mature zones not only restricting exit of O2 but also excluding reduced phytotoxins in the soil.
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.
Figure 18.11. Mangroves are trees or large shrubs which grow within the intertidal zone. Mangroves have specialised structures for root aeration, such as (A) pneumatophores (vertical ‘air roots’) or (B) ‘knee roots’ (also called ‘prop roots’). The surfaces of these roots have lenticels (C; surface of knee roots) which are pores that allow gas exchange between the atmosphere and the internal tissues. At high tide these root structures are submerged whereas at low tide these parts of the roots have direct contact with air and oxygen can then enter via the lenticels. Photographs taken in north-western Australia by Ole Pedersen.
Waterlogging tolerant species tend to develop larger adventitious root systems than intolerant species, and these roots contain aerenchyma. The initiation and outgrowth of adventitious roots has been studied in wetland species such as rice and Rumex palustris; accumulation of ethylene appears to be the primary signal for this response, and auxin and H2O2 are also involved in the downstream signalling cascade (Visser et al. 1996; Steffens and Sauter 2009).
Newly-formed adventitious roots with aerenchyma can grow into anoxic waterlogged soil, but in many cases adventitious roots also grow close to the soil surface, or when floods submerge a significant portion of the stem they emerge into the water column. Surface roots are common for species with low amounts of aerenchyma – the superficial root system is therefore restricted to the surface oxidised layer of the soil. Such plants often develop a ‘sprawling’ growth form in shallow water with many adventitious roots in the water column, barely entering the soil surface, and taking up nutrients from the surface water. These plants are most common in eutrophic, relatively still water, as they rely on high nutrient concentrations in water given that they cannot exploit soil nutrients. The shallow root system means such plants are vulnerable to uprooting. Aquatic roots that grow into the floodwater are exposed to light and can form chloroplasts, with photosynthesis resulting in high endogenous O2 levels during the daytime (e.g. Rich et al. 2012).
Pneumatophores and ‘knee roots’ are specific features of mangroves (Figure 18.11), and are the point of entry for the atmospheric O2 that is transported to roots. The surfaces contain lenticels, pores which allow gas exchange between the atmosphere and these woody organs. Lenticels are also common on stems and the number and size increase in response to flooding on the trunks of flood-tolerant trees. Their openings become ‘hypertrophied’, i.e. impregnated with hydrophobic compounds, to prevent water infiltrating the aerenchyma when submerged by high tides or rising floodwaters.
Grey mangroves (Avicennia spp.) have long, horizontal roots (‘cable roots’) close to the soil surface, from which arise hundreds of ca. 1 cm thick, 30 cm long vertical aerial roots termed pneumatophores. O2 enters the pneumatophores and diffuses via aerenchyma to the underground roots. Even the pneumatophores, however, become temporarily submerged and thus cut off from the atmosphere at high tide. Early researchers suggested that aeration via the pneumatophores might involve pressurised gas flows, but this effect is negligible, and most O2 transport in mangroves is by diffusion (Beckett et al. 1988). The red mangrove (Rhizophora spp.) lacks pneumatophores, instead having aerial ‘knee’ or ‘prop’ roots that elevate the trunk above the water surface, and with hypertrophied lenticels serving as entry ports for O2 which then diffuses to underground roots.
Even with O2 transport in aerenchyma, trees in wetlands are unable to grow roots as deeply into soil as terrestrial trees, often leaving them vulnerable to toppling. Many flood-tolerant trees in freshwater swamps therefore feature extensions to their lower trunks (buttresses and knees) that provide mechanical stabilisation, and with lenticels present these enable entry of O2 into the root aerenchyma.