Inundation

Many saltland habitats are also subject to inundation, but the impacts of this combination of stresses on the growth and survival of crop plants are poorly understood. Within cereal crops, nearly all inundation research has focused on rice under non-saline conditions (e.g. Setter et al, 1987, 1988). Our understanding of the kinds of physiological adaptations that are important to inundation come largely from studies of rice and

Adventitious root DM Shoot DM

Longest adventitious root (mm)

400 300 200 100 -0

400 300 200 100 -0

Fig. 6.3. Relationship between the porosity of adventitious roots and: (a) ratio of adventitious root dry mass (DM) to shoot DM; and (b) length of the longest adventitious root. Data are for ten genotypes from the Triticeae grown in stagnant nutrient solutions for 70 days (McDonald et al., 2001).

Adventitious root porosity (% volume)

Fig. 6.3. Relationship between the porosity of adventitious roots and: (a) ratio of adventitious root dry mass (DM) to shoot DM; and (b) length of the longest adventitious root. Data are for ten genotypes from the Triticeae grown in stagnant nutrient solutions for 70 days (McDonald et al., 2001).

plants that occur naturally on saline and non-saline marshland (e.g. Voesenek et al., 2004; Pedersen et al., 2006; Rich et al., 2008; Colmer et al., 2009).

Inundation damages plant growth mostly because the column of inundating water limits gas (mainly O2) exchange between leaves and the atmosphere (Colmer, 2003). Oxygen concentrations in inundating water are generally highest near the water surface, decrease with depth, and fluctuate diurnally, increasing in the day and decreasing at night (Setter et al., 1987). In addition, there can be an attenuation of light by the column of water and material suspended in it. For example, in partially inundated rice fields in Thailand, only 20-52% of ambient light reached the water surface due to shading by leaves, and this was attenuated a further 15-20% by 0.2 m of water (Setter et al., 1987). Further to this, areas that are inundated are also likely to have waterlogged soils after the inundation has receded, so the mechanisms described above for waterlogged soils are also relevant to the inundated situation.

According to Voesenek et al. (2004), the key physiological traits associated with inun dation tolerance in plants centre around factors that enable plants to:

• Avoid inundation through being present as dormant seeds or quiescent perennat-ing organs.

• Ameliorate inundation through the fast elongation of leaves that can act as 'snorkels' with the atmosphere, the development of longitudinally connected gas-filled channels and barriers to ROL to facilitate inter-organ gas diffusion, and the continuation of photosynthesis under water to generate O2 and carbohydrates.

• Tolerate inundation through the generation of energy without O2 via glycolytic fermentation and through the reduction in plant metabolic rates.

It is presently difficult to choose between these options for crops as little is known about the selectable variation that exists for these traits. Rice, arguably the world's most inundation tolerant cereal, is known to employ both tolerance and amelioration strategies. During germination the seeds use the energy from fermentation (Setter and Ella, 1994) to rapidly send a coleoptile to the water surface, which acts as a 'snorkel' delivering O2 to the seed (Kordan, 1974). Later the plants develop leaves that elongate sufficiently rapidly to be partly above water (Kende et al., 1998), and the leaves develop a non-wetting surface covered with a gas film that conveys O2 to the roots and CO2 to the leaves (Pedersen et al., 2009).

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