GROWTH, AUXIN, AND TROPISMS IN DECAPITATED AVENA COLEOPTILES

Abstract

It was known that in the Avena coleoptile growth and tropisms decrease after decapitation and show a sudden increase 2-3 hrs. later, which is paralleled by a regeneration of growth hormone at the cut surface 2-3 hrs. after removal of the coleoptile tip. This relation was studied quantitatively by measuring the amts. of diffusible and extractable auxin, growth, and geotropism at different times after decapitation of comparable Avena coleoptiles. The growth rate and the extractable auxin both gradually decrease after decapitation, and increase again 2 hrs. later. The auxin production, and geotropism both fall off abruptly after decapitation, to rise again after 2 hrs. upon regeneration of the physiological tip. From these facts, supported by many experiments, the conclusion is drawn that the growth rate depends on the extractable or bound auxin in the cells, whereas tropisms are due to the effect of gravity or light on the diffusible or free-moving auxin in the organ. Thus the diffusion and the extraction methods of measuring auxin are not interchangeable since they determine different physiological entities.