Abstract
Stagnant flooding (SF) is a major problem in rainfed lowlands where floodwater of 25-50 cm stagnates in the field for most of the season. We aimed to establish a system for phenotyping SF tolerance and identifying tolerant germplasm through screening of landraces. A total of 626 rice accessions were evaluated over 3 years under control conditions and two levels of SF. Floodwater was raised to 20 cm at 25 or 30 days after transplanting (DAT). In one trial, the depth was increased subsequently by 5 cm a week and in another (severe stress), it was increased to 40 cm at 37 DAT and to 50 cm at 42 DAT. In both trials, water depth was maintained at 50-60 cm until maturity. In all cases, no plant was completely submerged. Plant height, elongation rate and yield were measured at maturity. Genotypes best suited to SF showed moderate elongation of 1.3-2.3 cm day(-1) under SF. In contrast, semi-dwarf and fast-elongating types performed poorly. Subsequent trials using 18 genotypes, including six pairs of near isogenic lines (NILs) with or without SUB1 showed that all SUB1 NILs were sensitive to SF. Five of the other six genotypes contained SUB1 and were SF tolerant, suggesting the possibility of combining tolerances to complete submergence (SUB1) and SF. Stem starch and soluble sugar concentrations were similar under control conditions among the 18 genotypes, but starch was depleted by 37 % under SF, with less depletion in tolerant genotypes. SUB1 NILs contained similar concentrations of starch and sugars under SF. We conclude that survival and yield under SF are dependent on moderate elongation, high tillering, lesser carbohydrate depletion and higher fertility. The tolerant genotypes identified here performed strongly in both wet and dry seasons and will be used to identify tolerance mechanisms and alleles for use in marker-assisted breeding.
Highlights
Floods caused by heavy rain or outflow of rivers result in severe crop losses worldwide
The few tolerant genotypes identified in this study showed limited elongation and in most of cases, elongation was induced by stagnant flooding (SF)
Genotypes subjected to SF in the present study showed variation in the extent of increase in culm width, with the majority of tolerant genotypes developing thicker stems in the range of 8 –9 mm, suggesting that this range is probably optimum under SF and, together with increased aerenchyma formation, could play a role in tolerance to SF
Summary
Floods caused by heavy rain or outflow of rivers result in severe crop losses worldwide. The extent of damage to rice caused by complete submergence is dependent on floodwater characteristics such as temperature, turbidity and depth These affect various plant processes such as chlorophyll retention, underwater photosynthesis, carbohydrate accumulation, elongation and survival (Das et al 2009). Rice genotypes tolerant of complete submergence retain their chlorophyll and adopt a slow-growth strategy depicted by limited elongation when submerged This enables plants to maintain sufficient carbohydrate reserves to sustain metabolism during submergence and to recover once the floodwater recedes (Setter and Laureles 1996; Xu et al 1996; Ella et al 2003; Das et al 2005; Fukao et al 2006; Sarkar et al 2006; Perata and Voesenek 2007). The physiological mechanisms by which SUB1A regulates growth have been thoroughly investigated (Jackson 1985; Jackson et al 1987; Ella et al 2003; Fukao and Bailey-Serres 2008; Bailey-Serres et al 2010; Schmitz et al 2013)
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