A Critical Study of the Auxin Theory of Growth Regulation in the Mesocotyl of Avena sativa

Abstract

A method of growing Avena seedlings is described, which allows them to be handled individually in darkness. Mesocotyls of seedlinge from which the tip of the coleoptile is repeatedly removed are as long as those of control plants not so decapitated. Mesocotyls of seedlings which are deseeded on the 3rd day of growth, followed by decapitation of the cleoptile tip on the 4th day, are, at 7 days old, as long as those of controls not so decapitated. When deseeded plants are decapitated, regeneration of auxin production occurs at the tip of the coleoptile stump. Where a reduction in the length of the mesocotyl results from decapitation, a wound reaction is probably concerned in addition to any auxin changes. Removal of the coleoptilar node causes a sharp decrease in the final length of the mesocotyl. Heating intact seedlings at 40° C. for 3 hours causes a reduction in the length of the mesocotyl but not of the coleoptile. The effect of heating is not reversed by subsequent treatment at low temperature, which instead appears to augment these effects. When seedlings are exposed to the action of KCN, iodoacetate, or anaerobic conditions, and illuminated while so exposed, perception of light takes place, resulting in a reduction in the length of the mesocotyl. Perception of light takes place in seedlings germinated at normal temperatures, but maintained at low temprature during illumination and also in seedlings grown for 6 weeks at 2° C. without any previous growth at normal temperature. Light perception takes place in embryos excised from dry grain and grown on a culture medium. No difference in free amino-acid content is apparent between dark grown and illuminated seedlings. The effects of illumination survive a period of drying down and become apparent upon subsequent germination of the grain in darkness. The drying process itself causes an additional reduction in mesocotyl length. It is concluded that auxin itself is not the primary reactant in the perception process, and that the growth of the mesocotyl is probably controlled by the coleoptilar node and plumular growing point, rather than by auxin diffusing downward from the tip of the coleoptile.