Researches on plant growth regulators: XVII. Structure and activity. On the mechanism of the action III

Abstract

1. 1. Continuint the investigations concerning the relation structure/activity of the plant growth substances, it is shown, that replacement of the lipophilic parts in the highly active compounds (derived from benzene, naphtalene or indole) by trichloro-acetyl-, tris-chloromethylacetyl-, di- or trialkylacetyl residues respectively, results in loss of activity. Only with high concentrations of the compounds, near the toxic level, weak curvatures are produced in the pea test, as already observed earlier for certain normal fatty acids. The acids with branched chains act more rapidly in this respect than their normal isomers. Similar relations are found with some structurally quite unrelated wetting agents, causing weak curvatures in the pea test particularly if the hydrophilic part is located in the centre of the lipophilic chain. 2. 2. The importance of the spatial relation between the ring-system and the carboxyl group in the side chain, as derived formerly for the highly active compounds and reminding of the relations in the group of wetting agents and penetrants, is demonstrated once more for naphthalene-acrylic acid and derivatives. That the spatial structures ascribed to the compounds an account of their physiological activity are the correct ones, has in the meantime also been proved by means of their ultra violet absorption spectra (Havinga, Nivard, 1948). 3. 3. Comparison of the activities in series of compounds leads to the conclusion that for maximal activity, apart from the spatial relation between non-polar and polar parts of the molecule, a very definite balance between these lipophilic and hydrophilic parts (H.L. balance) is required. In this sense an upper limit is now also indicated for the requirement: “high interface activity of the non-polar part”. 4. 4. The effect of series of growth substances and related compounds on the tissue of the red beet (Beta vulgaris rubra)—by affecting the endo- and ectoplasmic membranes, the colouring matter leaves the vacuole and can be quantitatively measured—completely parallels that on the oleate coacervate (cf. preceding paper). So also with this biological object the effect, in a quantitative sense, is for the greater part the reverse of that which one would expect on account of the view that the primary growth substance action is mainly concerned with the protoplasmic membranes (influence upon the permeability). 5. 5. If the primary reaction does indeed take place in the cytoplasm—an addional argument to those given in the preceding paper being the more rapid action observed for some neutral “precursors” of highly active acids as compared to that of these acids themselves—the result of the beet test can be explained in a plausible way. Increase of the lipophilic character of a compound beyond a certain limit may namely cause too strong an interaction with (adsorption to) the membranes, preventing the attainment of the “cytoplasmic” concentration required for maximal activity (apart from a direct effect of changed membrane properties). 6. 6. It is deduced that a properly composed mixture of “underdosed” highly active growth substance and a supplementary quantity of an acid, weakly active or inactive for the reasons indicated under 5, should equal the maximal effect of the growth substance alone, even if the total number of molecules is less. Such synergistic effects of certain types of acids could indeed be established in the pea test. From the proportions found in this respect it is concluded that only a small fraction of a growth substance in a biological system is actually required for the primary reaction and that the greater part is adsorbed to the membranes (considered to perform a physiological function), has some action on the cell wall and is possibly adsorbed aspecifically at other places (“waste”). 7. 7. These findings are discussed in relation to views already put forward earlier by F.W. Went (1939, preparatory reaction—primary reaction) and to the evidence for a two-phase growth reaction, provided by the investigations of Burnstrom (1941–1942–1945). 8. 8. A short review is given of the most important biological investigations concerning growth substance action. The data suggest that a relation exists: growth substance action/a particular fraction of respiration (oxygen absorption, cf. aerobic character of growth)/water uptake/sugar transport/changes in the properties of the cell wall. As to the function of the growth substances in the cytoplasm it is highly probable that enzymatic processes are involved. 9. 9. Arguments are given which lead to rejection of the view that the auxins are part (prosthetic group) of an enzyme, in other words, in our opinion the auxins do not act catalytically as co-enzymes. The remaining possibility then would be that they act on an enzyme system, viz., that they function as regulators of enzymatic activity. Suggestions are given concerning the way in which this may occur, taking into consideration the results of the chemical investigations. This implies that in future investigations the interaction between growth substances and proteins (enzymes) will particularly have to be studied.