Responses of rice to Fe2+ in aerated and stagnant conditions: growth, root porosity and radial oxygen loss barrier

Abstract

Lowland rice (Oryza sativa L.) encounters flooded soils that are anaerobic and chemically reduced. Exposure of the roots to high soil Fe2+ concentrations can result in toxicity. Internal aeration delivering O2 to submerged roots via the aerenchyma is well understood, but the effect of Fe2+ on O2 transport in roots is less studied. We aimed to evaluate the effects of Fe2+ on growth and root aeration. O. sativa var. Amaroo was grown in aerobic and deoxygenated solutions with 0 mM, 0.18 mM, 0.36 mM, 0.54 mM or 0.72 mM Fe2+ using FeSO4.7H2O and a control with 0.05 mM Fe-EDTA. The treatments were imposed on 14-day-old plants (28–30 days old when harvested). Dry mass, shoot Fe concentration, root porosity and patterns of radial O2 loss (ROL) along roots were determined. In the aerobic solution, where Fe2+ was oxidised in the bulk medium, root dry mass increased with higher Fe2+; this was not the case in stagnant solutions, which had no significant root growth response, although Fe oxidation near the root surface was visible as a precipitate. In the highest Fe2+ treatment, shoot Fe concentrations in aerobic (667 mg kg–1) and stagnant (433 mg kg–1) solutions were below the level for toxicity (700 mg kg–1). Rice responded to high Fe2+ in aerobic conditions by increasing root porosity and inducing strong barriers to ROL. In stagnant conditions, root porosity was already high and the ROL barrier induced, so these root aeration traits were not further influenced by the Fe2+ concentrations applied.

O2 dynamics in the rhizosphere of young rice plants (Oryza sativa L.) as studied by planar optodes

“…Flooded conditions are a widespread phenomenon among rice and it is estimated that about 75 % of all rice are grown under O 2 depleted conditions in flooded and waterlogged soils, during part or all of the cropping period (Roger et al 1993 ). Rice plants growing in O 2 depleted soils are potentially exposed to high levels of harmful reduced/toxic chemical species, like H 2 S and Fe 2+ (Armstrong and Armstrong 2005 ; Mongon et al 2014 ). To overcome these challenges a range of wetland plants, including some rice genotypes, have evolved two adaptations (I) development of a barrier against solutes and gasses e.g., by increased suberization in exodermis and/or ligninfication in sclerenchyma cells (Greenway et al 2006 ; Kotula et al 2009 ) and/or (II) leakage of O 2 to establish a protective oxygenated zone around the roots, a strategy facilitated by well-developed gas conducting aerenchyma inside the roots (Armstrong 1980 ; Kirk 1993 ).…”

Dynamics and Activity of Iron-Reducing Bacterial Populations in a West African Rice Paddy Soil under Subsurface Drainage: Case Study of Kamboinse in Burkina Faso

“…The appearance of iron toxicity symptoms in rice involves an excessive uptake of Fe 2+ by the rice roots and its acropetal translocation into the leaves. A high production of toxic oxygen radicals can damage cell structural components and impair physiological processes [9] [10]. Excessive amounts of iron in the soil in reduced conditions upset the nutrient balance in plants [11].…”

Regulation of Root Traits for Internal Aeration and Tolerance to Soil Waterlogging-Flooding Stress

“…Moreover, malic acid and VLCFAs accumulate in the rice roots forming a ROL barrier, suggesting that malic acid is used for the biosynthesis of fatty acids, which, in turn, provide substrates for suberin biosynthesis (Kulichikhin et al, 2014). Interestingly, various compounds in waterlogged soils resulting from the metabolism of anaerobic microorganisms can induce a ROL barrier in roots of wetland species; these compounds include sulfide (Armstrong and Armstrong, 2005), Fe 2+ (Mongon et al, 2014), and organic acids (Armstrong and Armstrong, 2001;Kotula et al, 2014). Further research is needed to elucidate the signaling events and biochemical regulation of root ROL barrier formation.…”