The problem of determining gas exchange rates from flow system data under nonsteady state conditions is analyzed. A correction factor is presenlted for obtaining constant rates under nonsteady state conditions. A general formula for obtaining any rate under nonsteady state conditions is also given. Turnover time is defined and discussed in terms of the mathematics presented. The origins of nonsteady states and steady states in flow systems are discussed, as are some of the experimental advantages of working under nonsteadv state conditions.The study of gas exchange of plants and plant parts has been largely conducted by postharvest physiologists studying fruit respiration and by biochemists, ecologists, and physiologists studying photosynthesis. There have been and are many similarities in the gasometric methods used by these various researchers. The use of flow systems is one case in point.In a flow system, tissue is enclosed in a container and air or a modified atmosphere is passed through the container at a known rate. One of the principal advantages of a flow system over the older manometric and volumetric techniques is the ability to closely control the composition of the atmosphere surrounding the tissue. Volume, pressure, flow rate, and usually temperature are constant, and changes in the composition of the gas stream with time are related to the activities of the living tissue. Summaries of the methods for monitoring such changes along with much practical advice and standard operating procedures for some types of flow systems are available (1,14).Surprisingly, the mathematics of rate calculations in flow systems has received only passing attention in the biological literature even though the problem is not difficult. The formula usually quoted (1-5, 8, 10-14, 16) for determining the rate R, given the flow rate F and the gas concentration %C, (v/v) corrected for background, is R = %C.F/100

The relationship between abscission and the evolution of ethylene and CO2 was examined in explants and explant segments of cotton seedlings (Gossypium hirsutum L. cv. Acala SJ-1) under both static and flow system conditions, and in the presence and absence of mercuric perchlorate. Explant excision was immediately followed by increased ethylene evolution (wound ethylene); senescence was also accompanied by increased ethylene evolution (senescence ethylene Cotton plants have provided the materials for extensive abscission research, and ETH2 has been implicated in the regulation of this abscission many times over (1,8,9,19,21,24,26 In a flow system specifically designed for this work, the patterns of ETH and CO2 evolution accompanying abscission in control explants were established and then compared with those patterns in explants treated with IAA and ABA. Abscission of hormone-treated and untreated explants was compared in the presence and absence of MP. Patterns of ETH and CO2 evolution were also studied in segments of cotton explants and in individual explants. MATERIALS AND METHODSGeneral Explant Techniques. All experiments were performed with excised cotyledonary nodes of 14-day-old cotton seedlings (Gossypium hirsutum L. cv. Acala SJ-1) or with further subdivisions of this explant. Subdivision of a standard explant consisted of excising the petiole stumps and dividing them each into two segments 2 to 2.5 mm in length. The proximal segments contained the abscission zones with their surrounding tissues and were called abscission zone segments. The distal segments and the axes were referred to, respectively, as petiole stump segments and axial segments. On subdivision, each standard explant yielded two abscission zone segments, two petiole stump segments, and one axial segment. The standard growth regime and cotton explant procedures were followed with minor modifications (7,12,28). Briefly, the explants were supported in an upright position by a shallow layer (5 mm) of 1.5% agar covering the base of the explant chamber, approximately 10 g of pressure was applied to the petioles in testing for abscission, and abscised petioles were not removed from the explant chambers until all gas production data had been obtained. Agar droplets were omitted on all stem and petiole stumps unless otherwise indicated.Flow System, Chromatography, and Sampling. Most experiments were conducted in a flow system. Rates of ETH and CO2 evolution were determined from chromatographic analysis of gas samples withdrawn from the explant chambers (22,23). Instantaneous rates of gas evolution were calculated using the appropriate formula (22), and total amounts produced were estimated by cutting and weighing Xerox copies of the individual rate versus time graphs.The atmosphere of the flow system was Purafil-filtered ETHfree air. The flow system was similar to that described by Claypool and Keefer (14) for respiration studies.

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