Carbon Dioxide Effects on Auxin Responses of Coleoptile Sections

In the wheat cylinder bioassay technique as previously usedhere 5 sections have been enclosed in a 2 x 38 in, assay tubetogether with 0.5 ml. of the test solution. A method developedfor estimating the amount of carbon dioxide which accumulatesin these tubes through the respiration of the enclosed sectionshas shown that the level can rise to 20 per cent. after 24 hrs.at 25°C. In the presence of a 100 p.p.m. IAA(6x10^-4M.) testsolution, growth of 5 enclosed sections is depressed from 8hrs. onwardas and they eventually shrink, releasing their accumulatedIAA back into the solution. The growth of sections under various gas mixtures of carbondioxide in air has also been followed and these experimentsshow that section length is reduced approximately lineraly withrespect to increasing carbon dioxide concentration up to 20per cent. in air, both in the presence and absence of a 100p.p.m. IAA solution. The slope of the fitted regression line,however, is much steeper when the test solution contains IAA—i.e.there is a large interaction. In the presence of IAA, growth-time data show that a reductionin the growth rate, as compared with that in normal air, canbe detected after only 4 hrs, at the highest carbon dioxideconcentration. In the absence of IAA, high concentrations ofcarbon dioxide accelerate growth during the first 8 hrs. ofthe assay but depress it later. The mechanism of action of this interaction is unknown but itis not shown at very high concentrations of IAA, e.g. 1,000p.p.m. (6x10^-3M.).