# Insights on the Adaptation of _Foericulum vulgar_ Mill to Iron Deficiency
## 1 Introduction  
Iron (Fe) assumes a fundamental role in agriculture worldwide, since it is extremely necessary for the growth and development of plants [1, 2]. Fe acts in several regulating life-sustaining processes, such as electron or oxygen transfer, photosynthesis and respiration functions [3]. On the other hand, plants frequently face difficulties in preserving Fe homeostasis, owing to the limited fraction of soluble Fe, particularly in calcareous soils. As an example, in Tunisia, the physico-chemical analysis of soil properties in the north revealed high concentrations of bicarbonate ions (HCO\({}_{3}\)\({}^{-}\)), which lead to the precipitation of Fe and a consequent severe limitation in plant development [4]. Fe deficiency causes chlorosis, which results from the proteolytic loss of photosynthetic components, such as PSI/PSII and the cyt b6/f complex [5]. Depending on their responses to Fe deficiency, higher plants are grouped as: (I) strategy I plants, characterized by (i) stimulation of rhizosphere acidification, (ii) accumulation of organic acids and/or (iii) secretion of chelating and reducing substances, mainly phenolic compounds; (II) strategy II plants (typically gramineous species), characterized by the extrusion of phytosiderophores, responsible for the mobilization of Fe from the rhizosphere [6]. Furthermore, in strategy I plants, exudation and/or accumulation of organic compounds such as flavins, organic acids and phenolic compounds play a crucial role in the- mobilization of Fe\({}^{2+}\)[7]. In fact, these compounds are considered as effective Fe chelators in the soil, by inducing the dissolution of ferric hydroxide, which is inaccessible to plants [8]. In this regard, polyphenols are particularly relevant, since, beside chelating properties, they simultaneously exhibit important antioxidant properties [9]. As anticipated by [PERSON] and [PERSON] [10], these bioactive metabolites are synthesized in different parts of the plant, where they contribute to maintain reactive oxygen species (ROS) below toxic levels during abiotic/biotic stresses. Depending on the stress intensity and the plant efficiency to activate several mechanisms, the production of such metabolites can increase or decrease under Fe deficiency conditions [11]. Aromatic plants are claimed to have high polyphenol contents. Among them, fennel is widely cultivated in different countries of the Mediterranean (Tunisia, Italy and Spain) and especially used for food production. In fact, fennel condiment is extremely fragrant with a characteristic aniseed flavor [12] and it is used as an ingredient of foods, such as soups, salads and herbal infusions [13]. Its consumption has also been related to health benefits, a fact that is consistent with the usage of distinct parts of this plant (bulbs, young shoots, leaves, flowering stems, infloreses and seeds) in household remedies for the treatment of digestive system complications, bronchitis, chronic cough and kidney stones [14; 15]. The bioactive properties of fennel have been closely associated with polyphenols, which are particularly abundant in its shoots [14]. Possibly, the levels of such compounds can be raised, or de novo produced, when plants are subjected to different biotic and abiotic stresses, such as Fe deficiency [16]. Environmental constraints have, in general, an opposing effect on the yields of phenolic compounds, i.e., their increment is generally followed by the reduction of biomass production [17] and optimal polyphenol yields are expected to be obtained in stress-tolerant species [18; 19]. Distinct works demonstrated the occurrence of variable responses of plant to Fe deficiency, depending on genotypes, particularly in pasture species, but little interest has been devoted to species with multi-variant proprieties subjected to metal deficiency [20].