## 1 Introduction  
Soil acidity is a major concern in worldwide agricultural systems, influencing crop production in 30-40% of agricultural lands and dominating up to 60% of potentially arable lands, which hinders the establishment of new agricultural soils in tropic and subtropic regions [1, 2]. Soil acidity promotes the release of Al, Fe, and Mn from their soil-bound forms into bioavailable Al\({}^{3+}\), Fe\({}^{2+}\), and Mn\({}^{2+}\). The intensity of this phenomenon is influenced by pH and redox potential but also by the temperature and moisture of the soil, generally leading to co-occurring ion toxicities [3, 4]. The increase in soil H\({}^{+}\) and bioavailable Al, Fe, and Mn levels leads to a decline in soil fertility by promoting deficiencies in plant essential nutrients, namely, magnesium (Mg), calcium (Ca), and potassium (K), and a decrease in the solubility of phosphorus (P) and molybdenum (Mo) [5, 6]. Furthermore, the excessive agricultural use of ammonium-based fertilizers and natural waterlogging events can decrease soil pH and increase the detrimental effects of soil acidity on plant growth [7, 8]. In plants growing on acidic soils, a clear diagnosis of metal toxicity is particularly challenging, given that different elements can show similar symptoms in above-ground plant parts, different symptoms can be observed in different families or groups of plants, and different degrees of co-occurrent toxicities and associated nutrient deficiencies can be active at any given site [3]. The simultaneous elemental analysis of soil, plant tissues, and respective subcellular compartments can provide important information for assessing not only element toxicity but also mechanisms of plant tolerance. Wheat (_Triticum aestivum_ L.) is one of the most widely cultivated cereal crops worldwide, and its demand is projected to increase as much as 60% by 2050. Wheat productivity can be heavily impaired by soil acidity, namely, by the associated effects of high levels of bioavailable Al, Fe, and Mn in the soil [9]. For Al toxicity, symptoms are mainly detected below ground through the impairment of root growth, at the meristematic regions, and a reduction in cell elongation and root hair development. Excess of Al induces cell wall modifications, interruption of signalling pathways, disruption and depolarisation of the plasma membrane, modified transport processes, and Al binding to the DNA [10]. Secondary symptoms can be detected in the aerial parts after excess Al impairs root nutrient acquisition and water uptake [11; 12]. Plants can counter Al toxicity by preventing its uptake at the root apex or by chemical chelation (with organic acids and other compounds) and detoxifying intracellular Al in excess [2; 13]. For example, in some wheat cultivars, exclusion of Al-malate complexes appears to promote tolerance to Al toxicity [14]. Iron toxicity is mainly diagnosed in crops growing in flooded soils, and thus a large amount of research has focused on rice. Toxicity in rice is characterised by a phenomenon called \"bronzing\", the spread of scattered reddish-brown spots in older leaves that progresses to cover the entire organ [15]. This process is caused by the accumulation of oxidised polyphenols and the irreversible damage of cell structural components, induced by an elevated production of radicals. Tolerance to Fe toxicity is based on either its exclusion from the root (strategy I), its subcellular compartmentalisation and exclusion from the symplast (strategy II), or through enzymatic detoxification (strategy III) [15]. In wheat, Fe toxicity is not extensively reported, and is mainly described for flooded soils, where it can co-occur with Mn toxicity, in years of heavy rainfall. The impairment of wheat growth and a yellowing of older leaves are the most commonly reported symptoms [16; 17]. Manganese and Al toxicities are the main constraints to plant productivity on acidic soils. In wheat, excessive levels of Mn are generally translocated to the aerial parts.