The Role of Auxin in the correlative Inhibition of the Development of lateral Buds and Shoots

Experimental results. 1. The development of the axillary buds of seedlings of Lupinus albus decapitated above the first or second leaf-pair was inhibited by applying — via the cut surface of the stem — aqueous hetero-auxin solutions of a concentration of 5 in 106 and 1 in 105; hetero-auxin concentrations lower than 1 in 10® mostly had no inhibiting but rather a promoting effect on the development of the axillary buds, as compared with their growth when tap water was applied. 2. No increase of length of the epicotyl, neither any swelling •or thickening of the stem was observed as a consequence of this hetero-auxin supply. 3. In the series where the growth of the axillary buds of the first leaf-pair was inhibited, as well as in the series where it was promoted, the bud in the axil of the second leaf generally developed faster than that in the axil of the first leaf, though the mean distance between the insertion of both leaves is only 2 to 3 mm. 4. In single-node cuttings of Ligustrum vulgare placed in the greenhouse the development of the still dormant lateral buds was weakly promoted by an application of hetero-auxin 1 in 107 and inhibited to a slight degree by the application of hetero-auxin 1 in 10® via the apical cut surface, in comparison with their development when tap water was applied; without any supply of liquid the development of the buds was still much smaller. 5. An application of lanolin hetero-auxin paste 1 in 106 to seedlings of Lupinus albus decapitated above the first leaf-pair caused an inhibition of the growth of the axillary buds. This inhibition was as strong in an application in the immediate surroundings of the axillary buds as when applied 12 mm higher up to the stem. 6. An application of lanolin hetero-auxin 1 in 10® as near as possible to the still dormant lateral buds of single-node cuttings of Ligustrum vulgare placed in the greenhouse did not cause any inhibition of their development. 7. In seedlings of Lupinus albus, decapitated just above the first leaf-pair, with axillary shoots of different length (so called “two-shoot plants”) the growth of the shorter or of the longer lateral shoot was inhibited by applying an aqueous heteroauxin solution 1 in 105 to the cut surface of the decapitated second (resp. longer or shorter) lateral shoot, compared with the growth of these lateral shoots when applying tap water or no liquid at all. 8. The growth of cut off shoots of Lupinus albus with one expanded leaf placed in a hetero-auxin 1 in 105 solution was inhibited, as compared with the growth in tap water; a heteroauxin solution 4 in 10® and 1 in 10® caused no inhibition. In shoots of plants with two expanded leaves a hetero-auxin solution 1 in 107 already caused a very slight inhibition of growth, this inhibition being somewhat stronger with heteroauxin 1 in 10®. 9. In Pisum sativum no inhibition of the growth could be obtained by placing the shoots of plants with 4 to 5 expanded leaves (the scales included) in hetero-auxin 4 in 10", as compared with the growth in tap water. The growth of the shoots of intact plants, however, was much stronger. 10. The development of the axillary buds of decapitated shoots of Lupinus albus was not influenced by placing the shoots in hetero-auxin solutions 1 in 10“ and 1 in 105, as compared with the growth in tap water. In decapitated control plants left in the soil, however, a much stronger development of the axillary buds was observed. The auxin content at the level of the first leaf-pair of the shoots in tap water was distinctly less than that of the plants in the soil, whilst that of the shoots in the hetero-auxin solutions was still higher. At the same time a toxic effect of the hetero-auxin 1 in 10° solution was observed. 11. The auxin content of the stem of seedlings of Lupinus albus is about equal all along the length of the stem. 12. In young seedlings of Lupinus albus the auxin content of the hypocotyl increases directly after the splitting of the cotyledons and the first development of the plumule; the plumule then contains a fairly considerable amount of auxin too. 13. In far developed plants of Lupinus albus the auxin content of the leaves decreases with increasing age. 14. The application of aqueous hetero-auxin solutions to decapitated seedlings of Lupinus albus via the cut surface of the stem brings about a higher auxin content of the stem, than when tap water is applied or no liquid at all. The auxin content corresponds, within certain limits, with the concentration of the applied hetero-auxin solutions. 15. The application of tap water to decapitated seedlings of Lupinus albus increases the auxin content of the stem, as compared with plants without any supply of liquid. 16. The development of the axillary buds of the first leafpair of decapitated seedlings of Lupinus albus is, within certain limits, inversely proportionate to the auxin content of the main stem. 17. In seedlings of Lupinus albus decapitated above the second leaf-pair, in all series the axillary buds of the second leaf-pair almost do not develop, whereas those of the first leaf-pair do, though in each series the auxin content at both levels of the stem is about the same. 18. “Inhibited” axillary buds of seedlings of Lupinus albus contain less auxin than buds which are not inhibited. 19. In decapitated seedlings of Lupinus albus to which tap water has been applied or no liquid at all, parallel with the development of the axillary buds the auxin content of the neighbouring stem parts increases. 20. In intact “two-shoot plants” of Lupinus albus with lateral shoots of unequal length the auxin content of the longer shoot per length unit is higher than that of the shorter shoot. The auxin content of the adjoining part of the main stem generally is of the same order of magnitude as that of the longer shoot, and always higher than that of the shorter shoot. 21. In intact “two-shoot plants” of Lupinus albus with lateral shoots of equal length, the auxin content of both shoots is about the same and that of the main stem higher than that of each shoot separately. 22. In “two-shoot plants” of Lupinus albus of which the growth of one of the shoots has been inhibited artificially by an application of hetero-auxin 1 in 105 via the decapitated other shoot, on the whole, the auxin content of the inhibited shoot is lower than that of the uninhibited shoot of two-shoot plants in which — via the other decapitated, lateral shoot — tap water has been applied or no liquid at all. Conclusions. 1. In the correlative inhibition of the development of lateral buds and shoots auxin is the correlation carrier. 2. In Lupinus albus the terminal bud and the growing leaves are production centers of auxin. 3. As the auxin content of the inhibited organs is always lower than that of the inhibiting ones, the theory of Thimann and Skoog (1934), and also that of Czaja (1935, 1935a), postulating that the inhibition of lateral buds and shoots is caused by a direct action of auxin transported from the inhibiting shoot into the inhibited organ, must be discarded. 4. As no indication has been found for the necessity of some primary growth process — or stem thickenings and swellings — in the main stem, from which, according to the “indirect” theory of Laibach (1933) and Snow (1932, 1937), a secondary influence would inhibit the growth of the lateral buds and shoots, their theory is highly improbable. 5. Besides, against the above-mentioned theories a serious objection is that one has to assume that in the inhibition of one lateral shoot by another, either auxin or an inhibiting influence or substance from either shoot has to travel upwards into the other one. It seems very improbable that two identical factors would be transported in opposite directions in one and the same organ or even in the same cells. 6. The “diversion” theory of Went (1936) does not give an adequate explanation of the correlative inhibition of lateral buds and shoots, as it fails to explain why no — or scarcely any — auxin is produced by the lateral buds in the intact plant, and why this auxin production immediately increases as soon as the terminal bud has been eliminated. 7. In a new theory on the correlative inhibition of lateral buds and shoots, the production of auxin is assumed to depend upon the supply of a precursor, or precursors, transported acropetally and chiefly attracted to those spots where auxin is most intensively produced. Consequently, those parts which received a little more of the precursor than the other parts in the beginning, by their originally higher production of auxin, will continue to receive more of the precursor. Other organs, such as young, hardly developed axillary buds, remain deprived from the precursor and therefore dormant, since they cannot produce auxin and consequently cannot grow out. In the same way the “inhibition” of lateral shoots is caused by too deficient a supply with the precursor. 8. In experiments with artificial (hetero-) auxin supply to decapitated plants via the apical cut surface it is assumed that this (hetero-) auxin supply prevents or seriously hampers already in the basal parts of the stem the upward movement of the precursor in its tracks of transport. In the same way the growth inhibitions caused by a hetero-auxin supply from a place, morphologically below the parts concerned, are explained as a blocking up of the upward transport of the precursor. 9. Went’s assumption of the existence of a new phytohormone, called caulocaline, coming from the roots and necessary for the elongation of the stem or lateral buds (Went, 1938, 193a) seems superfluous. The phenomena described by him simply — and preferably — can be explained in terms of the new precursor theory. The investigations were carried out in the Botanical Laboratory of the State University, Utrecht. My best thanks are due to Prof. Dr. V. J. Koningsberger and Dr. M. H. van Raalte for their interest in my work and their valuable criticism.