BENEFICIAL EFFECTS OF ALUMINUM IN PLANTS

GROWTH STIMULATION
Not surprisingly, aluminum addition has a growth stimulatory effect on aluminumaccumulators. In tea, addition of aluminum and phosphorus increased phosphorusabsorption and translocation as well as root and shoot growth . Similarly, the aluminum-accumulating shrub, Melastoma malabathricum L., exhibited increased growth of leaf,stem, and roots as well as increased phosphorus accumulation when aluminum wasadded to culture solutions .  Low levels of aluminum sometimes stimulate root and shoot growth ofnonaccumulators. Turnip (Brassica rapa L. subsp. campestris A.R. Clapham) rootlengths were increased by increasing aluminum levels up to 1.2 μM at pH 4.6 . Soybean(Glycine max Merr.) root elongation and 15NO3   uptake increased with increasingaluminum concentrations up to 10 μM, but were reduced when aluminum levelsincreased further to 44 μM . Shoot and root growth of Douglas fir (Pseudotsugamenziesii Franco) seedlings were stimulated by increasing aluminum levels up to 150 μM but were reduced at higher aluminum levels . Root elongation of an aluminum-toleran trace of silver birch (Betula pendula Roth) increased as solution aluminum increased upto 930 μM Al but then decreased at 1300 μM Al . Several researchers  havehypothesized that low levels of Al3 ameliorated the toxic effects of H  on cell walls,membranes, or nutrient transport, but aluminum-toxic effects predominated at higheraluminum levels.

INHIBITION OF PLANT PATHOGENS
Aluminum can be toxic to pathogenic microorganisms, thus helping plants to avoid disease. Spore germination and vegetative growth of the black root rot pathogen,Thielaviopsis basicola Ferraris, were inhibited by 350 μM Al at pH 5 . Similarly, mycelialgrowth and sporangial germination of potato late blight pathogen, Phytophthorainfestans, were inhibited by 185 μM Al, and Andrivon  speculated that amendment of soils with aluminum might be used as a means of disease control.

ALUMINUM ABSORPTION AND TRANSPORT WITHIN PLANTS

PHYTOTOXIC SPECIES
The most phytotoxic form of aluminum is Al3 (more correctly, Al(H2O)3), which predominates in solutions below pH  4.5  . Possibly, hydroxyl-aluminum  (AlOH2 and Al(OH)2 ) ions are also phytotoxic, particularly to dicotyledonous plants . However, as pointed out by many researchers , these aluminum species are interrelated along with the pH variable, so it is difficult to rank their relative toxicity.  In contrast, Al-F, Al-SO4, and Al-P species are much less toxic or even nontoxic to plants . Barley (Hordeum vulgare L.) roots were unaffected by aluminum when2.5 to 10 μM F  was added to nutrient solution containing up to 8 μM total solublealuminum . Also using nutrient solution, Kinraide and Parker  positively demonstrated thenontoxic nature of Al-SO4 complexes (AlSO4 and Al(SO4)2  ) for wheat (Triticumaestivum L.) and red clover (Trifolium pratense L.). Soybean had longer root growthwhen increasing amounts of phosphorus were added to nutrient solutions havingconstant total aluminum concentrations . 

ALUMINUM SPECIATION IN SYMPLASM
The pH of the cytoplasmic compartment generally ranges from 7.3 to 7.6 . Once aluminum enters the symplasm, the aluminate ion, Al(OH)4 or insoluble Al(OH)3  could form . Alternatively, Al3 could precipitate with phosphate as variscite, Al(OH)2H2PO4  .Based on higher stability constants, it is likely that Al3 would be complexed by organic ligands, such as adenosine triphosphate (ATP) or citrate . Martin hypothesized thatbased on their similar effective ionic radii and affinity for oxygen donor ligands, Al3 would compete with Mg2 rather than Ca2 in metabolic processes.