EVIDENCE FOR 2 INDOLEACETIC ACID-INDUCED GROWTH-RESPONSES IN THE AVENA STRAIGHT-GROWTH INDOLEACTIC ACID ASSAY

Floating Avena sativa L. cv Victory coleoptile segments were used to d etermine whether the straight-growth indoleacetic acid (IAA) assay can be reconciled with the Avena curvature assay and the Cholodny-Went th eory of photo- and gravitropism. Measurements of segment length after 5 h yield sigmoid-shaped IAA dose-response curves with the growth rate leveling off at 1 mum. However, measurements made at 24 h generate be ll-shaped curves with maximal growth being induced by 10 mum IAA. The difference between short- and long-term IAA dose-response curves is no t due to IAA degradation; instead, it is the result of two growth resp onses to IAA. The initial one is rapid, responds to low concentrations of IAA, and lasts for 12 h. The second response is less sensitive to IAA than the first one. It appears after 6 h but is not obvious until the last 12 h of a 24-h incubation. The profile of short-term IAA dose -response curves reflects the initial growth response, whereas that of the 24-h curve is the sum of both growth responses. Linear-linear plo ts of 5- and 24-h dose-response curves show that coleoptile segment gr owth rate is proportional to IAA concentration up to 0.3 mum. When the efficiency of IAA action is taken into account, it is found that the most effective IAA concentration for short and long incubations is 0.4 mum. It is concluded that the Avena straight-growth IAA assay is as s ensitive as the Avena coleoptile curvature assay, and that it is consi stent with the Cholodny-Went theory.