Auxin Distribution in Seedlings and Its Bearing on the Problem of Bud Inhibition

Abstract

1. A method is described by which auxin was extracted from plant materials without the usual addition of acid. Acid was found to reduce the yield in some cases (corn) and in other cases (oats) to be of no benefit. The distribution of auxin concentrations in etiolated seedlings of corn, oats, and peas was determined (fig. 4). The amounts of auxin were not expressed in the usual way in arbitrary units, but in indoleacetic acid equivalents which are independent of the sensitivity of the test plant, of the test plant itself, and of the conditions under which the test is made. 2. In corn the distribution of growth was compared with the distribution of auxin (fig. 3). It was found that the region having the greatest growth rate (the upper part of the mesocotyl) has the lowest auxin concentration. This was found to be due to the great sensitivity to auxin in this region. 3. The highest auxin concentrations in the seedlings of corn and oats are found in the basal regions of the primary leaves. This may not be free moving auxin. The average auxin concentration is lowest in corn seedlings and highest in pea plants. 4. The problem of bud inhibition was analyzed in pea seedlings. The effect of decapitation on auxin concentrations in the plant was determined. It was shown that as soon as twelve hours after decapitation the auxin concentration in the lateral buds is increased (figs. 6-8). In the stem the concentration decreases after decapitation. Auxin application increases the concentration in the stem and also in the upper lateral bud. From these analyses it was concluded that a direct action of auxin on the lateral bud is excluded as a possible explanation for its dormancy. 5. From experiments with phenylbutyric acid it was concluded that high concentrations of auxin (or phenylbutyric acid) in the stem prevent the formation of auxin in the lateral bud (table 6). 6. Auxin is most effective in inhibiting lateral buds when introduced into the vascular system of the stele. 7. Auxin applied to the stump of decapitated plants some time after decapitation is less effective in bringing about bud inhibition the longer the time between decapitation and application (fig. 9). 8. The preceding evidence, in combination with many valuable data obtained from the literature, has provided material with which an attempt was made to explain the auxin controlled bud inhibition in agreement with all known data. A brief explanation as to how the mechanism of this bud inhibition might work, together with the main facts upon which it was based, is given at the end of part II.