Waterlogging

Waterlogging refers to saturation of soils by water. This leads to the displacement of air from the soil pores and an approximate 10,000-fold decrease in the rate at which O2 diffuses into the soil (Grable, 1966). As a result, soils typically become hypoxic (O2 deficient) within a few days. Although soil hypoxia has a range of moderate- to long-term impacts on the chemistry and biology of soils, the effects on plant roots are quite rapid (see reviews by Ponnamperuma, 1972; Drew and Lynch, 1980). Plants require O2 to provide the energy for root growth and function; with O2 available, plants are able to produce 24-36 moles of the energy storage compound adenosine triphosphate (ATP) per mole of glucose; without O2, plants are only able to produce 2 moles of ATP per mole of glucose through alcoholic fermentation.

Root-zone hypoxia has a variety of effects on plant roots. Decreases in the elongation of roots and the death of non-adapted apices are observable within a few hours to a day (Thomson et al., 1990), and this can lead to substantial decreases in total root relative to shoot biomass within 1 week. For example, exposure of barley seedlings to 6 days of hypoxia (O2 concentrations ~10% of saturated) decreased the root:shoot ratio from ~0.37 (well aerated) to 0.23 (hypoxic) (calculated from data of Benjamin and Greenway, 1979). Hypoxia can decrease the uptake of nutrients by crop plants (Trought and Drew,

1980; Buwalda et al., 1988) and, therefore, transient waterlogging can decrease yields if crops are not subsequently refertilized (Robertson et al., 2009).

In addition to these effects, in waterlogged saline land, root-zone hypoxia can lead to increased Na+ and/or Cl- uptake to the shoots, which decreases plant growth and survival (reviewed by Barrett-Lennard, 1986, 2003). In a wide-ranging review of the literature, hypoxia under saline conditions caused at least 30% increases in either Na+ or Cl- concentrations in the leaves or shoots of 23 of 24 species surveyed (Barrett-Lennard, 2003).

Waterlogging tolerance in crops is primarily associated with two major physiological traits that enable plants to avoid soil hypoxia. The first of these is the formation of roots with increased porosity in the cortex (aeren-chyma) that enable O2 to be conducted down the inside of the root from the root/shoot junction to the root tip. The simplest method for determining root porosity is using Archimedes' Principle: porosity can be calculated knowing the fresh weight of root segments, and the weight of these segments when suspended in water before and after the evacuation of root air spaces under vacuum (cf. Thomson et al., 1990). These kinds of assessments show that the porosity of a plant's roots is partly constitutive (i.e. relating to the habitat in which plant naturally occurs, plants from moist habitats generally having higher porosities than plants from well-drained habitats), and is partly inducible (i.e. relating to the current growth conditions, plants growing under waterlogged conditions generally having higher porosities than plants growing under drained conditions). This principle can be illustrated using data from a survey of 91 plant species conducted by Justin and Armstrong (1987) (Fig. 6.2). In this survey, plants collected from perpetually inundated landscapes (H1) had root porosities (95% confidence interval) of 16-29% when grown under drained conditions, and 25-36% when grown under waterlogged conditions, whereas plants collected from well-drained habitats (H5) had root porosities of only 1-10% when grown under drained condi tions, and of 4-15% when grown under waterlogged conditions.

The role of increased root porosity in helping to maintain root growth can be seen in a data set based on an assessment of the effects of hypoxia on ten species from the tribe Triticeae (McDonald et al., 2001). In this work, there were significant relationships between the porosity of adventitious roots under stagnant conditions and: (i) the ratio of adventitious root dry mass to shoot dry mass; and (ii) the maximum length to which these roots grew under stagnant conditions (Fig. 6.3). These results are consistent with the view that under hypoxic conditions, the growth of the root apex becomes limited by the availability of O2 supply to the root tip; therefore, plants with higher porosity develop more roots and longer roots (cf. Armstrong, 1979).

The second physiological adaptation critical for plant growth in waterlogged soils is an ability to form a barrier to radial oxygen loss (ROL) that decreases the leakage of O2 out of the root, so that more O2 can diffuse internally and reach the root tip (Armstrong, 1979; Jackson and Armstrong, 1999; Colmer, 2003). The presence of a barrier to ROL is inferred from rates of radial oxygen flux from roots, which is measured using root-sleeving cylindrical Pt electrodes that are moved up and down the root to determine rates of O2 flux at different distances from the root tip. Colmer (2003) categorizes the barrier to ROL as falling into three general classes: (i) a 'tight' barrier is indicated by very low ROL from expanded zones of the root, but high rates near the root tip; (ii) a 'partial' barrier results in similar rates of ROL along the root; and (iii) a 'weak' barrier results in the ROL being much higher in the expanded zones of the root compared with the root tip.

There are two strong indications that the barrier to ROL is of adaptive significance to plants under waterlogged conditions: (i) adapted species appear to be able to induce the barrier when exposed to waterlogged or hypoxic conditions; and (ii) tight barriers tend to occur in wetland but not dryland species. Colmer et al. (1998) examined the effects on ROL in the roots of four rice m o o CL

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Fig. 6.2. Effects of habitat of origin on the root porosity (95% confidence interval) of plants grown under: (a) drained, or (b) waterlogged conditions (water level 10-20 mm above soil surface) for ~1 month (calculated from data of Justin and Armstrong, 1987). The plants were from the following habitats: H1 -standing water above the soil surface for all or most of the year (21 species); H2 - wet soils which are saturated with water for most of the year (59 species); H3 - damp soils which may be occasionally wet (52 species); H4 - 'normal' moist soils, such as a typical field soil (38 species); and H5 - dry soils which crumble to the touch and are usually found on high ground or above very porous rock (16 species). Plant species generally occurred across more than one habitat; we have only used species where porosity data were available for both drained and flooded conditions.

H1 H2 H3 H4 H5 Habitat

Fig. 6.2. Effects of habitat of origin on the root porosity (95% confidence interval) of plants grown under: (a) drained, or (b) waterlogged conditions (water level 10-20 mm above soil surface) for ~1 month (calculated from data of Justin and Armstrong, 1987). The plants were from the following habitats: H1 -standing water above the soil surface for all or most of the year (21 species); H2 - wet soils which are saturated with water for most of the year (59 species); H3 - damp soils which may be occasionally wet (52 species); H4 - 'normal' moist soils, such as a typical field soil (38 species); and H5 - dry soils which crumble to the touch and are usually found on high ground or above very porous rock (16 species). Plant species generally occurred across more than one habitat; we have only used species where porosity data were available for both drained and flooded conditions.

varieties exposed to aerated or stagnant nutrient solutions. The ratio of ROL at 5 mm from the root tip to ROL at 35 mm from the root tip varied from ~0.3 to 1.0 when the plants were grown in well-aerated nutrient solutions, indicating that under these conditions the plants had a weak or partial barrier to ROL. However, with three of the rice varieties, this ratio increased to values greater than 20 when the plants were grown in stagnant nutrient solutions, showing that hypoxia had induced a tight barrier to ROL (Colmer et al, 1998).

McDonald et al. (2002) examined the presence of barriers to ROL in ten species from the Poaceae and two species from the Cyperaceae representing plants from a range of wetland and dryland habitats. Using the definition above, 'tight' barriers to ROL occurred in eight out of nine wetland species grown under stagnant conditions; the single exception was in the pasture species Phalaris aquatica, which only had a 'partial' barrier to ROL. In contrast, with the three dryland species, the barriers to ROL were 'partial' or 'weak' (McDonald et al., 2002).

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