Comparative Effects of Indoleacetic Acid and Gibberellic Acid on Growth of Decapitated Etiolated Epicotyls of Pisum sativum cv. Alaska

Abstract

AbstractMeasurements were made of the growthof the sub‐apical region of decapitated, etiolated epicotyls of Pisum sativum L. cv. Alaska after treatments with indoleacetic acid (IAA), gibberellic acid (GA) and triiodobenzoic acid (TIBA). Growth was measured either at the end of a 2‐day period, at short intervals during growth, or was monitored continuously for 2–3 h using a position‐sensing transducer.In experiments measuring growth after 2 days, high levels (0.1–10 μg/plnat) of IAA caused expansion, whereas similar levels of GA caused elongation. When both hormones were applied together, the effects of IAA were dominant and expansion ensued, even when GA was present at 100 times the amount of IAA. Very low amounts of IAA (0.5–5 ng/plant), however, caused elongation. The elongation elicited by high GA or low IAa was inhibited to a similar extent by TIBA and this inhibition of elongation was associated with an increased expansion at the extreme tip. When application of the hormones was delayed, GA‐induced elongation was reduced considerably, IAA‐induced elongation was lessened somewhat and IAA‐induced expansion was partially converted into elongation.In experiments measuring elongation at short intervals, high levels of IAA caused rapid elongation followed after 3 to 6 h by expnasion. Both GA and low levels of IAA extended the duration of elongation with little apparent effect on the rate of growth. In fast‐growth experiments, low, intermediate and high levels of IAA doubled the rate of elongation with a lag period of about 20 min, whereas GA had at most a very slight stimulatory effect on the growth rate.It is concluded that the main role of GA in this system is to maintain physiological levels of IAA in the growing zone and that the level of IAA present determines whether elongation or expansion will take place.