Apical root zone

Aluminium toxicity is a major stress factor in many acidic soils. At soil pH levels below 5.0, intense solubilization of mononuclear A1 species strongly limits root growth by multiple cytotoxic effects mainly on root meristems (240,241). There is increasing evidence that A1 complexation with carboxylates released in apical root zones in response to elevated external Al concentration is a widespread mechanism for Al exclusion in many plant species (Fig. 10). Formation of stable Al complexes occurs with citrate, oxalate, tartarate, and—to a lesser extent— also with malate (86,242,243). The Al carboxylate complexes are less toxic than free ionic Al species (244) and are not taken up by plant roots (240). This explains the well-documented alleviatory effects on root growth in many plant species by carboxylate applications (citric, oxalic, and tartaric acids) to the culture media in presence of toxic Al concentrations (8,244,245) Citrate, malate and oxalate are the carboxylate anions reported so far to be released from Al-stressed plant roots (Fig. 10), and Al resistance of species and cultivars seems to be related to the amount of exuded carboxylates (246,247) but also to the ability to maintain the release of carboxylates over extended periods (248). In contrast to P deficiency-induced carboxylate exudation, which usually increases after several days or weeks of the stress treatment (72,113), exudation of carboxylates in response to Al toxicity is a fast reaction occurring within minutes to several hours... 

Metal complexation by carboxylates is, on the other hand, an important mechanism to exclude uptake of toxic elements, such as Al3+, in plant species and cultivars adapted to acid mineral soils. The release is restricted to the apical root zones, which are most susceptible to the toxic effects of aluminum (Ma, 2000 Brimecombe et al., 2007). 

Localized release of root exudates in apical root zones with a low density of microbial colonization and secretion peaks over a limited period of time may counteract rapid microbial degradation and thereby increase the rhizosphere concentrations of exudated compounds for mobilization of nutrients (Romheld, 1991). 

Under Fe-deficient conditions, Strategy I plant species (i.e. all plant species but grasses) show a typical enhanced release of protons (and thus rhizosphere acidification) behind the apical root zone (Marschner et al, 1982 Romheld,... 

Some suggest the concept that diversity of the hairy root tip—cell division, elongation, and maturation regions—is necessary for optimal growth rate (15). The best growth of root tips is consistent with the formation of new branches in maturation zones and enhancement of apical dominance (14). 

Developing trees contain two major types of meristems (1) terminal or apical meristems and (2) lateral meristems. Apical meristems are located at the tips of all stems and branches (both termed shoots) where they are contained within terminal buds they are also located within the tip regions of all roots. In the tip regions, the meristematic zone is usually protected by another zone of cells called the root cap. Root hairs, or microscopic roots, have no apical meristems, but these minute structures are lateral projections of roots that do have apical meristems. 

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