Studies on Extension Growth in Coleptile Sections

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Abstract
Wheat coleoptile sections were grown at various temperatures from 1–40°C. in water and solutions of the sodium salt of β-indolylacetic acid (NaIAA) ranging from low to very high concentrations (0.001 p.p.m. to 10,000 p.p.m.). Growth-time curves are sigmoid, obviously so under conditions unfavourable to growth, less clearly as conditions either of temperature or growth-substance supply are improved, the point of inflection moving nearer to zero time. The growth-rate is reduced by temperatures below the optimum, and although growth continues for a longer time, final section length is also reduced. Such reductions are also brought about by sub-optimal levels of NaIAA and by supra-optimal levels of both factors. In the test method employed, sections are grown in closed tubes of small volume, and after a time lose turgidity and become waterlogged; the onset of this condition is accelerated by high temperature or growth-substance concentration, and greatly delayed by low temperature. Under comparable conditions, but with the tubes uncorked, flaccidity is delayed even at high temperature or high concentration of NaIAA. If sections are transferred from one temperature to another the change in growth-rate is abrupt; effects of transfer on final length are complex. A contour diagram represents the idealized relationships between section growth and temperature and NaIAA supply over their whole range of effectiveness. Relations between temperature and growth in other systems are briefly discussed.

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