Calorimetric Studies of the Elongation of Avena Coleoptile Segments

Abstract

Elongation rate and heat produced by Avena coleoptile segments suspended in sucrose buffer solutions were measured at pH values from 3.5 to 8.5. The caloric effciency of elongation (CEE) was defined as the ratio of the rate of elongation to the rate of heat production. Elongation and CEE were greatest at intermediate pH values, but heat production (about 1 cal/ghr) was insensitive to pH within the limits of experimental error (±20%). Qumtitative agreement was found between the results of previous respiration studies and the rate of heat production in an aerobic atmosphere, which indicates that oxidative metabolism accounts for essentialy all energy changes in the cell, so matter flow is a dgnificant component of the bioenergetics of cell function. Indole-3-acetic acid up to 1 mm, produced about a 10-fold increase in elongation rate, a 5-fold increase of the CEE, and a 25% increase in heat production. Above this concentration, sharp drops in both elongation and heat production occurred, without altering the CEE at pH 6.5, but greatly decreasing the CEE at pH 4.5. Elongation and CEE showed marked decreases after 4 hours in an anaerobic atmosphere, but heat production did not exhibit a proportional decrease. These studies indicate that rate of cell elongation in the presence and absence of anin is not directly proportional to the overall metabolism of the cell.In contrast to respirometry, calorimetric measurements reflect the total metabolic activity within the cell, not just activities which involve molecular 02 or CO2. This investigation was undertaken to determine if reproducible enthalpy changes could be measured under standard experimental conditions in an adiabatic solution calorimeter and to test the assumption usually made when using radioactive tracers to measure total cell metabolism, that oxidative metabolism is proportional to total metabolism. Indole-3-acetic acid stimulates the rate of cell enlargement in the Avena coleoptile segment system (3), and this is dependent upon pH and respiration (5).It would also be of interest to compare the rate of heat production with the rate of cell elongation under aerobic and anaerobic conditions, and at several hydrogen ion and auxin concentrations in order to determine if the heat changes parallel the changes in respiration rates previously observed. The mechanism of cell wall extension has been recently reviewed (10).As an analytical tool, one advantage of the calorimeter is that the signal to be recorded is delivered by the studied object itself. Calorimeters are designed to measure enthalpy changes accompanying various chemical processes. This is done by correlating temperature changes of the system during the course of the reaction with the heat capacity of the system. Calorimetry has had a number of biological applications (12,(15)(16)(17)(18), and many ' The experiments were performed by H. E. B. in partial fulfillment of the requirements for the Master of Science degree.of the experimental findings have been presented in book form (9). In addition, ...