The Anatomical Characteristics of Roots and Plant Response to Soil Flooding

Abstract

SUMMARYNinety-one plant species from wetland (WL), intermediate (INT) and non-wetland (NWL) habitats were grown in flooded and drained soils and responses to flooding were assessed in relation to root anatomy and fractional root porosity (FRP).Flooding response and tolerance were related to FRP. Rooting depth increased with FRP in accordance with diffusion model predictions and emphasized the ventilating efliciency of aerenchyma. Major determinants of FRP were cortical cell conflgurations, closeness of cell packing, the relative proportions of conflguration types, porous: non-porous tissue ratios, aerenchyma development and the type and degree of secondary growth. A classification of cortical types based on cell and aerenchyma patterns is presented.Aerenchyma, both schizogenous and lysigenous, developed preferentially where preaerenchymatous cortical cell configurations (in TS) were radial and particularly, cubic and radial typical of WL and INT species. Aerenchyma rarely formed from hexagonal non-radial arrays which occur chiefly in NWL and in the outer cortical zones of WL and INT plants. The ventilating potential in non-aerenchymatous tissue was shown to be greater for cubic (fractional porosity, FP, max. 0-2146) than for hexagonal arrays (FP max. 0-0931); closer packing greatly accentuated the differences. It is suggested that cubic: hexagonal zonal ratios in roots may reflect a balance between respiratory and mechanical needs.In a majority of WL and INT species, shoot weights were unaffected by or increased with flooding and maximum rooting depths usually much exceeded 50 mm. Cubic packing raised the FRPs, as did aerenchyma which was often much greater under flooding. In the dicotyledonous species, a suppression of secondary growth in some, and a highly porous phelloderm in others, helped maintain high FRP. A minority of species were anatomically and responsively similar to NWL plants; survival under flooding was attributed to shallow rooting.Under flooding, the FRP of almost all NWL species was < 0-055 due to hexagonal packing, a lack of aerenchyma and, in dicotyledonous plants, secondary growth with scanty phelloderm. Shoot weights were reduced in 50 % of cases, rooting depths were < 50 mm, and some species died. Some species were exceptional in having cubic and radial packings; a lack of aerenchyma was associated with continuing meristematic activity in the primary cortex.

Acclimation to soil flooding — sensing and signal-transduction

“…Schizogenous aerenchyma may also be found in roots, but more often in the stems and petioles of wetland plants. In both aerenchyma types, there is a large variation in the exact configuration of the cells and cell remnants, as shown by Justin and Armstrong (1987) in their extensive screening of root anatomy of wetland, intermediate and non-wetland species.…”

“…It is interesting that quite often radial files of cells remain unaffected by the lytical process (Justin and Armstrong, 1987), and also the presence of a developing lateral root may inhibit cell break-down. Apart from this, programmed cell death usually does not affect the inner-and outermost parenchyma layers of the cortex.…”

“…Aerenchyma is the specialised tissue in petioles, stems and roots consisting of longitudinal gas-filled channels, which may result from various anatomical patterns of cellular configuration (Smirnoff and Crawford, 1983;Justin and Armstrong, 1987;Visser et al, 2000). A common feature of aerenchyma is that it forms a network of gas-filled spaces between the cells that interconnects most parts of the (partly) submerged plant with the atmosphere, and thereby is able to deliver oxygen almost throughout the plant (Armstrong, 1979).…”

“…In the last decades considerable progress has been made in our understanding of the mechanisms that enable certain plant species and cultivars to withstand periods with excess soil water, or even complete submergence. Much of the research has been carried out with crop plant species, such as rice (Oryza sativa), maize (Zea mays) and sunflower (Helianthus annuus), but also wild species originating from wetland habitats have been used, mostly for comparative studies (e.g., Justin and Armstrong, 1987;Laan et al, 1989;Smirnoff and Crawford, 1983;Visser et al, 1996a).…”

Salt marsh ecohydrological zonation due to heterogeneous vegetation–groundwater–surface water interactions

“…Many studies have assumed rooting depths of 20 or 30 cm on the basis of data from Spartina alterniflora marshes [Howes et al, 1981;Ellison et al, 1986;Steinke et al, 1996;Marani et al, 2006;Darby and Turner, 2008]. We used speciesspecific rooting depth estimates from the literature and distributed the simulated ET demand linearly over these depths in our models: 10 cm for Salicornia virginica [Chapman, 1938a;Mahall and Park, 1976a;Justin and Armstrong, 1987;Seliskar, 1983], 20 cm for Spartina foliosa [Mahall and Park, 1976b;Boyer et al, 2000], and 30 cm for Distichlis spicata [Seliskar, 1983]. The linear weighting approximated the decline in root biomass with depth observed in salt marshes, to an extinction depth [Chapman, 1938a;Howes et al, 1981;Ellison et al, 1986;Steinke et al, 1996;Boyer et al, 2000], and the greater effect of soil evaporation at the surface.…”