Since the first demonstration of plant growth stimulation by substituted phenoxyacetic acids (1), much attention has been directed toward this series of compounds in an attempt to correlate the position and type of substituent with observed growth promotion. The results of the many investigations aimed at the elucidation of the mechanism of growthregulator action have been resolved into three general theories.One of these theories (2) supposes that there is a chemical reaction between the regulator and appropriate groups, probably nucleophilic, on some plant substrate, resulting in the formation of new convalent bonds. It has been shown that the most likely point of attack on the ring is at the positions ortho to the ether oxygen. Another theory (3)Since the first demonstration of plant growth stimulation by substituted phenoxyacetic acids (1), much attention has been directed toward this series of compounds in an attempt to correlate the position and type of substituent with observed growth promotion. The results of the many investigations aimed at the elucidation of the mechanism of growthregulator action have been resolved into three general theories.One of these theories (2) supposes that there is a chemical reaction between the regulator and appropriate groups, probably nucleophilic, on some plant substrate, resulting in the formation of new convalent bonds. It has been shown that the most likely point of attack on the ring is at the positions ortho to the ether oxygen. Another theory (3)
Fig. 1. Structural formulas.gives major importance to the shape of the regulator molecule and the specificity of its "fit" onto some receptor within the plant. In this case, the benzene nucleus with its substituents acts as a whole at the locus or point of attachment, and chemical reactions at the ring are considered unlikely. The third and most recent theory, unlike the other two, is not concerned with the relations of the regulator with an "active site," but suggests that the growth-regulating activity of a compound is primarily associated with its ability to chelate metal ions, such as Mg++ (4).:'In order to make a comparative test of these theories, we have prepared the butyl ester of 5-chloro-2-pyrimidoxyacetic acid (Fig. 1, II) (5). This compound is almost identical with the very active growth regulator, butyl p-chlorophenoxyacetate ( Fig. 1, I) in its size, shape, ring planarity, molecular weight, halogen and ester reactivity, and also in those physical constants which were measured. However, the pyrimidine offers no possibility of nucleophilic reaction in the positions ortho to the ether oxygen (positions 1 and 3), and an aromatic type of substitution has never been observed to occur in positions 4 or 6 of the pyrimidine nucleus.The plant growth regulatory activity of compounds I and II ( Fig. 1) was measured by use of the oat coleoptile, wheat coleoptile, oat first-internode, and slit-pea curvature bioassays. In each case, the phenoxy ester was shown to be a highly active growth stimulator, while the pyrimidoxy est...