Short-term kinetics of elongation growth of gibberellin-responsive lettuce hypocotyl sections.

The short-term kinetics of growth of the excised lettuce (Lactuca sativa L.) hypocotyl were characterized with respect to the effects of gibberellic acid (GA3), indole-3-acetic acid (IAA), KCl and pH. A Hall-device-based, miniaturized, linear displacement transducer was developed to measure the growth of 2-mm hypocotyl sections with 1-μm resolution. Following treatment with GA3, a lag time of less than 10 min was typically followed by an increase in growth rate with two acceleration phases, reaching a final elevated rate within about 1 h. The kinetics of the response to GA1, a mixture of GA4 and GA7, and GA9 were similar to the response to GA3. There was no response to IAA treatment either in the presence or absence of GA3. KCl alone had no effect on the growth rate, but caused an increase in rate when added after GA3, with a lag time of usually less than 1 h. Responses to pH changes had lag times of a few minutes in all cases. A shift from H2O to pH 6 buffer inhibited growth, while a shift from H2O to pH 4 buffer resulted in a transient increase to a rate comparable to that induced by GA3. A shift from pH 6 to pH 5 caused an increase in growth rate, followed by a gradual decline to an H2O control rate after more than an hour. The responses to GA3 at pH 4 and pH 5 were similar to that found for addition of GA3 to water controls.