Root exudation composition changed by root zone but not by root types
Root exudation can change significantly along the root axis and different root types (Abenavoli et al. 2004; Tiziani et al. 2021). However, only the targeted analyses revealed significantly different metabolites in relation to the root zone. Aconitic acid, total phenols, total flavonoids, as well as glycine, alanine and threonine exudation has been significantly enhanced in maize apical root regions, compared to subapical ones (Figure 2). Generally, the root tip is considered to be very sensible to environmental stimuli (Baluška & Mancuso 2013). Some studies suggested that the root tip functions as control center for sensing external nutrient concentrations, inducing alterations of root morphology and physiology (Baluška & Mancuso 2013; Canarini, Kaiser, Merchant, Richter & Wanek 2019; Tiziani et al.2020a). Moreover, the elongation zone, which in our study is located in the apical root zone, is known to be one of the physiologically most active root zones being a sort of command center for root responses to the environment (Baluska, Mancuso, Volkmann & Barlow 2010; Baluška & Mancuso 2013; Tiziani et al. 2020a). The lack of root type effects on the release of the exudates could be due to the restricted soil volume of the potting system, which may have limited the soil exploration of single root axes.