Studies on the Growth of Coleoptile and First Internode Sections. A New, Sensitive, Straight-Growth Test for Auxins

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Abstract
Among the various test objects tried for quantitative auxin assay, coleoptiles and first internodes of grasses were found suitable. The hulless var. of Avena sativa, "Brighton," was used extensively. Many factors affecting response to indole-3-acetic acid (IAA) were studied, especially location of the section on seedlings, length of initial section, age, length of time in auxin soln., quality of light, pretreatment in various soln., vol. of soln., sucrose concn., pH and concn. of buffer, inorganic ions, CO2 concn., organic acids, amino acids and glutathione, vitamins, nature of the auxin, etc. Two tests are proposed: (1) a more sensitive coleoptile straight-growth test, (2) a first internode test. The main features are: a rigorous selection of age, location and initial length of sections; an accurate sectioning; a pretreatment by floating sections on a MnSO4.H2O soln. (1 mg /I.) for 3 hours (oat coleoptiles), on H2O for 3 hours (wheat coleoptiles) or on H2O for 1 hour (first internodes); use of small vols. (0.5 to 1 ml) of buffered soln. (IO-2m K-phosphate-citric acid, pH 5.0) plus 2% sucrose; rotation of tubes around an horizontal axis at 1 rpm; performance of test at 25[degree]C under dim green light (546 m[mu]). First internode sections of oat are about 10 times more sensitive than coleoptiles and can detect 10-3[mu]g of IAA. Elongation is proportional to auxin concentration from 1 to 1000 [mu]g/l. Internodes do not respond to arginine, methionine, glutamic acid and glutathione as do coleoptile sections in the absence of added auxin. They respond to IAA, its nitrile, amide and ester, to cis-cinnamic acid and gibberellin, the latter compound having also a small auxin activity on coleoptile sections.