Long-Term Effects

Although they may not be indicative of initial, primary phytotoxic events, long-term effects of aluminum are important for plants growing in aluminum-toxic soils or subsoils. Long-term exposure to aluminum over several days or weeks results in suppressed root and shoot biomass, abnormal root morphology, suppressed nutrient uptake and translocation, restricted water uptake and transport, suppressed photosynthesis, and inhibition of symbiosis with rhizobia.

Suppressed Root and Shoot Biomass

Increasing aluminum concentrations in solution, sand, or soil decreased fine root biomass of red spruce (Picea rubens Sarg.) (78). Typically, aluminum reduces root biomass to a greater degree than shoot biomass, resulting in a decreased root/shoot ratio (78-80). In contrast, in 3-year-old Scots pine (Pinus sylvestris L.), increasing solution of aluminum up to 5.6 mM produced no obvious aluminum toxicity symptoms on roots but decreased needle length and whole shoot length, resulting in increased needle density (81).

Abnormal Root Morphology

Often, one symptom of aluminum toxicity is 'coralloid' root morphology with inhibited lateral root formation and thickened primary roots (54). Cells in the elongation zone of primary wheat roots exposed to aluminum had decreased length and increased diameter, resulting in appearance of lateral swelling (61). This abnormal root morphology combined with reduced root length could result in decreased nutrient uptake and multiple deficiencies.

Suppressed Nutrient Uptake and Translocation

Increasing aluminum levels in the medium have been reported to decrease uptake and translocation of calcium, magnesium, and potassium (78,82). Forest declines in North America and Europe have been proposed to be due to aluminum-induced reductions in calcium and magnesium concentrations of tree roots and needles (3). Excess aluminum reduced magnesium concentration of Norway spruce needles to a level considered to be critical for magnesium deficiency (3). Also, aluminum toxicity reduced calcium and magnesium leaf concentrations in beech (Fagus sylvatica L.) (83). In sorghum (Sorghum bicolor Moench), magnesium deficiency was a source of acid-soil stress (84).

In the case of phosphorus, concentrations increased in roots but typically decreased in shoots. In roots of red spruce, 32P accumulation increased but 32P translocation to shoots decreased (85). Clarkson (86) proposed that there were two interactions between aluminum and phosphorus: (a) an adsorption-precipitation reaction in the apoplast; and (b) reaction with various organic phosphorus compounds within the symplasm of the cell. Aluminum and phosphorus were shown to be coprecipitated in the apoplast of corn roots, using x-ray microprobe analysis (49). Excised corn roots exposed to 20 h of 0.1 to 0.5mM Al had decreased mobile inorganic phosphate (40%), ATP (65%), and uridine diphosphate glucose (UDGP) (65%) as shown by 31P-NMR (nuclear magnetic resonance), indicating aluminum interference with phosphorus metabolism within the symplasm (87,88).

Restricted Water Uptake and Transport

Typically, aluminum toxicity decreases water uptake and movement in plants. Stomatal closure of arabidopsis occurred after 9 h of exposure to 100 µM Al at pH 4.0 (89). In wheat, transpiration decreased after 28 days of exposure to 148 µM Al (90). Treatment of 1-year-old black spruce (Picea mariana Britton) with 290 µM Al resulted in wilting and reduced water uptake within 7 days (91). Hydraulic conductivity of red oak roots was reduced after 48 to 63 days of exposure to aluminum, although no effect was observed after only 4 days (92). In contrast, transpiration in sorghum increased after 28 days of aluminum treatment (90).

Suppressed Photosynthesis

Net photosynthesis is reported to decrease with excess aluminum relative to normal rates. Exposure to 250 µM Al for 6 to 8 weeks reduced the photosynthetic rate of red spruce, and McCanny et al. (79) attributed this effect to an aluminum-induced decrease in root/shoot ratio. Similarly, exposure of beech seedlings to 0.37mM Al for 2 months significantly decreased net CO₂ assimilation rates (83).

Inhibition of Symbiosis with Rhizobia

Biological nitrogen fixation results in release of H⁺, acidification of legume pastures, and increased solubilization of aluminum (2). Excess aluminum has an inhibitory effect on rhizobial symbiosis. In an Australian pasture, the percentage of plant nitrogen derived from the atmosphere declined in subterranean clover (Trifolium subterraneum L.) as foliar concentration of aluminum increased (93). In four tropical pasture legumes, aluminum at>25 µM for 28 days delayed appearance of nodules, decreased percentage of plants that nodulated, and decreased number and dry weight of nodules (94). In phasey-bean (Macroptilium lathyroides Urb.) and centro (Centrosema pubescens Benth.), nodulation was more sensitive to aluminum toxicity than host plant growth (94).

Aluminum also inhibited the multiplication and nodulating ability of the symbiotic bacterium, Rhizobium leguminosarum bv. trifolii Frank (66). Recent research efforts have focused on identifying aluminum-tolerant rhizobial strains. For example, strains of Bradyrhizobium spp. that were isolated from acid soils were found to more tolerant of 50 µM Al at pH 4.5 than commercial strains (95).