Short-Term Effects

Owing to the numerous biochemical processes with which aluminum can interfere, researchers have attempted to determine the primary phytotoxic event by searching for the earliest responses to aluminum. Symptoms of aluminum toxicity that occur within a few hours of aluminum exposure are inhibition of root elongation, disruption of root cap processes, callose formation, lignin deposition, and decline in cell division.

Inhibition of Root Elongation

The first, easily observable symptom of aluminum toxicity is inhibition of root elongation. Elongation of adventitious onion (Allium cepa L.) roots (54), and primary roots of soybean (55,56), corn (57,58), and wheat (59–61) were suppressed within 1 to 3 h of aluminum exposure. The shortest time of aluminum exposure required to inhibit elongation rates was observed in seminal roots of an aluminum-sensitive corn cultivar BR 201F after 30 min (62).

Application of aluminum to the terminal 0 to 3 mm of corn root must occur for inhibition of root elongation to occur; however, the presence of the root cap was not necessary for aluminum-induced growth depression (63). Using further refinement of techniques, Sivaguru and Horst (58) determined that the most aluminum-sensitive site in corn was between 1 and 2 mm from the root apex, or the distal transition zone (DTZ), where cells are switching from cell division to cell elongation.

Lateral root growth of soybean was inhibited by aluminum-containing solutions to a greater extent than that of the taproot (64,65). Interestingly, Rasmussen (49) observed greater aluminum accumulation in lateral roots that emerged from the root surface, breaking through the endodermal layer. Similarly, root hair formation was more sensitive to aluminum toxicity than root elongation in white clover (Trifolium repens L.) (66).

Disruption of Root Cap Processes

The Golgi apparatus is the site of synthesis of noncellulosic polysaccharides targeted to the cell wall (67). Activity of the Golgi apparatus in the peripheral cap cells of corn was disrupted at 18 µM Al, a concentration below that necessary to inhibit root growth (68). In wheat, mucilage from the root cap disappeared within 1 h of aluminum exposure, and dictyosome volume and presence of endoplasmic reticulum decreased within 4 h (69). Death of root border cells (a component of root mucilage) occurred within 1 h of exposure to aluminum in snapbean roots (70).

Callose Formation

Callose is a polysaccharide consisting of 1,3-β-glucan chains, which are formed naturally by cells at a specific stage of wall development or in response to wounding (67). An early symptom of aluminum toxicity is formation of callose in roots. Using fluorescence spectrometry, callose could be quantified in soybean root tips (0 to 3 cm from root apex) after 2 h of exposure to 50 µM Al (55). In root cells surrounding the meristem of Norway spruce roots, distinct callose deposits were observed after 3 h of exposure to 170 µM Al (71). Zhang et al. (72) showed that callose accumulated in roots of aluminum-sensitive wheat cultivars exposed to 75 µM Al and they proposed using callose synthesis as a rapid, sensitive marker for aluminum-induced injury. However, callose was not accumulated in two aluminum-sensitive arabidopsis (Arabidopsis thaliana Heynh.) mutants exposed to aluminum, indicating no obligatory relationship between callose deposition and aluminum-induced inhibition of root growth (73). Sivaguru et al. (74) showed that aluminum-induced callose deposition in plasmodesmata of epidermal and cortical cells of aluminum-sensitive wheat roots reduced movement of micro-injected fluorescent dyes between cells.

Lignin Deposition

Lignins are complex networks of aromatic compounds that are the distinguishing feature of secondary walls (67). Deposition of lignin in response to aluminum was found in wheat cortical cells located 1.4 to 4.5 mm from the root tip (elongating zone [EZ]) after 3 h of exposure to 50 µM Al (75). Lignin occurred in cells with damaged plasma membranes as indicated by staining with propidium iodide, and Sasaki et al. (61) proposed that aluminum-induced lignification was a marker of aluminum injury and was closely associated with inhibition of root elongation. Interestingly, Snowden and Gardner (76) showed that a cDNA induced by aluminum treatment in wheat exhibited high homology with the gene for phenylalanine ammonia-lyase, a key enzyme in the pathway for biosynthesis of lignin.

Decline in Cell Division

A decrease in abundance of mitotic figures was observed in adventitious roots of onion after 5 h of exposure to 1mM Al (54). Similarly, a decrease in the mitotic index of barley root tips was found within 1 to 4 hours of exposure to 5 to 20 µM AI (pH 4.2) (77).