Root excretions in relation to the rhizosphere effect

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The exudation of amino acids by the roots of tomato, subterranean clover, and phalaris growing in nutrient solution under different environmental conditions has been studied. The amounts and balance of amino acids exuded by the different plants differed — phalaris and tomato exceeding clover. Exudation was greater from the plants during the first two weeks of growth than in the second two-week period. Light intensity had a marked effect on the exudation of amino acids by clover, the amounts of serine, glutamic acid and α-alanine in the exudate were less at the lower light intensities. In tomato root exudate aspartic acid, glutamic acid, phenylalanine and leucine decreased with decreasing light but serine and asparagine increased. Higher temperatures increased the amounts of amino acids exuded by tomato and clover roots, the latter released large amounts of asparagine at the higher temperatures. The amounts of glutamic acid from both plants increased considerably with temperature. Calcium level and aeration had little effect on exudation but the presence of micro-organisms influenced the concentration of amino acids in solution surrounding tomato roots.