Abstract
Iron (Fe) is a fundamental element involved in various plant metabolic processes. However, when Fe uptake is excessive, it becomes toxic to the plant and disrupts cellular homeostasis. The aim of this study was to determine the physiological and biochemical mechanisms underlying tolerance to Fe toxicity in contrasting rice varieties adapted to African environments. Four varieties (CK801 and Suakoko 8 (tolerant), Supa and IR64 (sensitive)) selected from our previous work were analysed in more detail, and the first part of this study reports morphological, physiological and biochemical responses induced by Fe toxicity in these four varieties. Morphological (shoot length, root length, number of lateral roots), physiological (photosynthesis rate, stomatal conductance, transpiration rate, fluorescence, relative water content and cell membrane stability) and biochemical (tissue Fe, chlorophyll pigments, soluble sugars, protein and starch) traits were measured, as appropriate, on both shoot and root tissues and at different time points during the stress period. Fe toxicity significantly (P ≤ 0.05) reduced growth and metabolism of all the four varieties. Tolerant varieties showed more lateral roots than the sensitive ones, under Fe toxic conditions as well as higher photosynthesis rate, chlorophyll content and cell membrane stability. Strong dilution of Fe concentration in cells was identified, as one of the additional tolerance mechanisms used by CK801, whereas Suakoko 8 mainly used strong mobilisation of carbohydrates at the early stage of the stress period to anticipate metabolite shortage. Traits associated with Fe toxicity tolerance in this study could be specifically targeted in trait-based breeding programs of superior lowland rice varieties tolerant of Fe toxicity.
Highlights
Iron plays pivotal functions in plant respiration, cell division, synthesis of chlorophyll, and electron transport chain of photosynthesis (Müller et al 2015)
The tolerant varieties used in this study, CK801 and Suakoko 8, were both selected at known iron concentration in upper leaves (Fe) toxicity hotspot sites, respectively at Kilissi (Guinea) and Suakoko (Liberia) and Suakoko 8 has been extensively used as a tolerant check in different studies (Dramé et al 2011; Sikirou et al 2015, 2016)
This work shows the responses of four lowland rice varieties to Fe toxicity and their ability and strategies to adjust plant growth and metabolism to overcome the stress
Summary
Iron plays pivotal functions in plant respiration, cell division, synthesis of chlorophyll, and electron transport chain of photosynthesis (Müller et al 2015). Fe2+ acts as a catalyst in Fenton reactions, producing reactive oxygen species (ROS) that are potentially harmful to the cell and that induce oxidative damage in plants (Kao et al 2001; Onaga et al 2016). To maintain balanced levels of Fe that enable optimal plant cell functions while avoiding the harmful effects of Fe-induced oxidative damages, plants must establish tight control mechanisms of cellular Fe levels. When rice plants experience Fe toxicity, they usually develop orangebrown spots on the leaves. This symptom is commonly called leaf bronzing and it is thought to occur as a result of oxidative stress (Apel and Hirt 2004). Strong growth depression and damaged root systems can be observed on rice plants grown under excess Fe conditions
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