Effects of Amino Acids on Bean Leaf Abscission

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Abstract
The effects of various methyl donors, amino acids, and leaf extracts on leaf abscission of Phaseolus vulgaris L. var. Red Kidney were determined by an abscission test with petiole explants. The methyl donors methionine, sodium formate, formaldehyde, and glycine were tested and only formaldehyde gave a marked promotion. The smaller stimulations produced by the other methyl donors were not proportional to their reported methyl donating abilities, making it seem unlikely that methyl donors play a limiting role in abscission of bean leaves under the conditions of the bean abscission test. Many naturally occurring amino acids, especially alanine and glutamic acid, were found to produce marked promotions of abscission. Extracts from leaves approaching abscission were very effective in accelerating abscission of the bean explants, while similar extracts from non-abscissing leaves showed little promotion. The promotive activity of amino acid fractions of the leaf extracts were found to correlate in each case with the natural tendency for abscission. Auxin treatments (naphthaleneacetic acid) which inhibit abscission could erase the stimulatory effect of alanine. It was suggested that as the leaf senesces and the auxin content declines, the accumulation of amino acids acts to promote abscission of the bean leaf.