Abstract
To enhance breeding efficiency for iron (Fe) toxicity tolerance and boost lowland rice production in sub-Saharan Africa, we have characterised the morphological, physiological and biochemical responses of contrasting rice varieties to excess iron. Here, we report the capacity of four varieties (CK801 and Suakoko8 (tolerant), Supa and IR64 (sensitive)) to oxidise iron in the rhizosphere and control iron-induced oxidative stress. The experiments were conducted in hydroponic conditions using modified Magnavaca nutrient solution and 300 ppm of ferrous iron (Fe2+) supplied in the form of FeSO4. Severe oxidative stress was observed in sensitive varieties as revealed by their high levels of lipid peroxidation. Histochemical and biochemical analyses showed that tolerant varieties exhibited a better development of the aerenchyma and greater oxygen release than the sensitive varieties in response to excess Fe. Both suberin and lignin deposits were observed in the root, stem and leaf tissues but with varying intensities depending on the variety. Under iron toxic conditions, tolerant varieties displayed increased superoxide dismutase (SOD), glutathione reductase (GR), peroxidase (POX) and ascorbate peroxidase (APX) activities in both the roots and shoots, whereas sensitive varieties showed increased APX and catalase (CAT) activities in the roots. This study had revealed also that Suakoko8 mainly uses root oxidation to exclude Fe2+ from its rhizosphere, and CK801 possesses a strong reactive oxygen species scavenging system, in addition to root oxidation ability. Key traits associated with these tolerance mechanisms such as a well-developed aerenchyma, radial oxygen loss restricted to the root cap as well as strong activation of antioxidative enzymes (SOD, GR, POX and APX) could be useful selection criteria in rice varietal improvement programs for enhanced Fe toxicity tolerance.
Highlights
Iron (Fe) toxicity is a complex nutrient disorder that greatly limits lowland rice production (Becker and Asch 2005; Sikirou et al 2015; Onaga et al 2016; Onyango et al 2018), in Africa where it was ranked as the second most important abiotic stress after drought in rice production areas
We have previously reported differential morphological, physiological and metabolic responses of two tolerant rice varieties (CK801 and Suakoko8) and two sensitive varieties (IR64 and Supa) in response to Fe toxicity (Onyango et al 2018)
Comparative assessment of root oxidation ability and oxidative stress control in four rice varieties contrasting in Fe toxicity tolerance showed that different mechanisms could contribute to tolerance of a particular genotype
Summary
Iron (Fe) toxicity is a complex nutrient disorder that greatly limits lowland rice production (Becker and Asch 2005; Sikirou et al 2015; Onaga et al 2016; Onyango et al 2018), in Africa where it was ranked as the second most important abiotic stress after drought in rice production areas (van Oort 2018). In response to the diversity of conditions under which Fe toxicity occurs (Becker and Asch 2005) and its seasonal and spatial variability (both in severity and duration), rice plants have developed a wide range of reactions to circumvent the harmful effects of Fe toxicity These reactions can be classified into three main adaptation strategies: (1) exclusion of Fe2+ at the root level; (2) inclusion of Fe2+ in the root and shoot tissues but subsequent avoidance via internal compartmentation in less photosynthetically active tissues or storage in less reactive forms; and (3) inclusion of Fe2+ and tolerance to reactive oxygen species (ROS) formed during the Fenton’s reaction induced by Fe (Becker and Asch 2005; Engel et al 2012; Onaga et al 2016). Suberin is believed to prevent the penetration of soil-derived toxins, such as reduced metal ions, into the roots by reducing cell wall permeability
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