Growth-Modifying & Antimetabolite Effects of Amino Acids on Chrysanthemum

The question of the significance of free amino acids in soil or other cultural media has received attention in recent years in the fields of plant physiology (1, 3, 7, 13, 14, 15, 16, 17), soil microbiology (4, 5,11, 16) and soil chemistry (9, 10,12). Although it originally appeared that free amino acids were not to be found in soils, later reports (2, 10) indicated they could be extracted with aqueous and alcoholic solutions and determinations made with chromatographic methods. The quantities of amino acids found were quite variable. Amino acids added to soil were quite rapidly transformed, and usually undetectable in extracts made after 72 to 96 hours of incubation (11, 16). D-Amino acids and L-alloisoleucine appeared more resistant to change than other amino acids studied (16). Reports indicate that amino acids are absorbed by plants (3, 13, 14) and translocated with great speed in both phloem and xylem (8). Inhibitors of respiratory processes as well as low temperatures applied to fibrovascular tissue considerably retard translocation. The potential applications of research with the so-called unnatural (D-) isomers was pointed out (6, 17) in connection with the natural occurrences of Damino acids in antibiotics and parts of microorganisms. More recently, Ikawa, et al. (6) reported the finding of D-phenylalanine, D-allothreonine and D-alloisoleucine3 in peptido-lipids of bacterial origin. The demonstrated effectiveness of amino acids applied to root zones of plants in producing changes in morphology and growth rate (1, 13, 17) suggests a need for further research along these lines. Steinberg (13) found that frenching of tobacco could be simulated by furnishing leucine, isoleucine, or alloisoleucine to tobacco plants growing in sterile cultures. He reported that relatively large amounts of these amino acids were required under non-sterile conditions to produce frenching symptoms. Woltz and Jackson (15, 16, 17) found that symptoms of yellow strapleaf of chrysanthemum could be produced by small amounts of certain isomers of leucine, isoleucine, and alloisoleucine applied to the root zones of test plants growing in sterilized and nonsterilized media. Fifteen of twenty-two test plants developed syndromes similar to those of frenching and yellow strapleaf when a mixture of alloisoleucine and isoleucine isomers was applied to the root zones. Methionine applied to the root zones of chrysanthemum plants produced a physi-ological disorder given the name methionosis (17). Two of the twenty-two plants exhibited morphological changes due to methionine. A consideration of the information about the effects of externally applied amino acids on plant growth in the light of certain antimetabolite phenomena discussed by Woolley (18) led to the hypothesis that the growth-modifying effects observed might be largely antimetabolite effects. Natural (L-) amino acids could act as antimetabolites because of their structural similarity to other natural amino acids. Unnatural amino acids (D-form & others), due to their structural similarity to specific natural amino acids, could function as antimetabolites by being incorporated into peptides and blocking further synthesis due to the failure to completely fit the pattern of the natural amino acid. Experiments were planned to examine further the effects of various natural and synthetic amino acids on the growth rate and morphology of chrysanthemum plants. Chrysanthemum was selected for these studies because of its sensitivity to isomers of leucine, isoleucine, and methionine (17). The types of cultural media, amounts of amino acids applied, and timing of application relative to the development of new growth of axillary shoots were chosen to enhance growth effects so the effects might be recorded and evaluated. Interactions of related amino acids were explored to learn the degree to which the results would fit metabolite-antimetabolite relationships.