Rapidly Induced Wound Ethylene from Excised Segments

Wound-induced ethylene synthesis by subapical stem sections of etiolated Pisum saivum L., cv. Alaska seedlngs, as described by Saltveit and Dilley (Plant Physiol 1978 61: 447450), was half-saturated at 3.6% (v/v) 02 and saturated at about 10% 02. Corresponding values for CO2 production during the same period were 1.1% and 10% 02, respectively. Anaerobiosis stopped all ethylene evolution and delayed the characteristic pattern of wound ethylene synthesis. Exposing tissue to 3.5% CO2 in air in a flowthrough system reduced wound ethylene synthesis by 30%. Enhancing gas diffusivity by reducing the total pressure to 130 mm Hg almost doubled the rate of wound ethylene synthesis and this effect was negated by exposure to 250 #1 liter-1 propylene. Applied ethylene or propylene stopped wound ethylene synthesis during the period of application, but unlike N2, no lag period was observed upon flushing with air. It is concluded that the characteristic pattern of wound-induced ethylene synthesis resulted from negative feedback control by endogenous ethylene. No wound ethylene was produced for 2 hours after excision at 10 or 38 C. Low temperatures prolonged the lag period, but did not prevent induction of the wound response, since tissue held for 2 hours at 10 C produced wound ethylene immediately when warmed to 30 C. In contrast, temperatures above 36 C prevented induction of wound ethylene synthesis, since tissue cooled to 30 C after 1 hour at 40 C required 2 hours before ethylene production returned to normal levels. The activation energy between 15 and 36 C was 12.1 mole kilocalories degree-'. A rapidly induced, transitory increase in the rate of ethylene synthesis has been observed in a variety of excised tissues (1, 24). The wound response was recently characterized in Pisum sativum L., cv. Alaska (24). In subapical stem tissue wound-induced ethylene production at 25 C increased linearly after a lag period of 26 min from 2.7 nl g-' hr-' to the first maximum of 11.3 nl g-' hr-' at 56 min. The rate of production then decreased to a minimum at 90 min, increased to a lower second maximum at 131 min, and then declined over a period of about 100 min to about 4 nl g-' hr-'. The magnitude of the response varied slightly from experiment to experiment, but the time sequence was constant for tissue excised from a given region of the pea seedling. In this study we examined the 02 and temperature dependency of woundinduced ethylene synthesis by etiolated 'Alaska' pea stem sections. Vegetative, ripening, and senescent tissue require 02 for ethylene synthesis (1, 3, 6, 13, 21, 29). Sfakiotakis and Dilley (25) found that induction of autocatalytic ethylene production by apples (Malus domestica Borkh.) occurred above 6.5% 02 and that anaerobiosis prevented induced ethylene production. Lieberman and I Michigan Agricultural Experiment Station Journal Article No. 8284. Spurr (17) showed that ethylene synthesis by broccoli (Brassica oleracea L.) was prevented by anaerobiosis and saturated at between 1 and 2.5% 02. Mature green banana fruit (Musa spp.) maintained normal, though reduced, basal metabolism at 02 tensions of 5%, but were unable to produce ethylene in atmospheres of less than 7.5% 02 (18). Burg and Thimann (10) showed that ethylene production by plugs of climacteric apples decreased at around 10% 02 and was half-maximal at around 2% 02. Yang (31) postulated that ethylene may be formed by an 02dependent reaction. He suggested that under anaerobic conditions the proposed intermediate accumulated and that a surge of ethylene production occurred upon exposure to 02. Such a surge has been reported for apples (3, 10) and pears (14). The rate of ethylene production increased in plants which had been partially frozen (13, 28, 32), cold-stressed at 5 C (11, 26, 30), or heat-stressed at 40 C (15). Plants continually exposed to 40 C produced very little ethylene, and required several hr at cooler temperatures to recover ethylene-synthesizing capacity (10, 14). Substrate availability was not limiting, since ethylene production by apple tissue fed methionine was also severely inhibited at 40 C (16). In general, ethylene production is reduced at temperatures above 35 C, and completely prevented at 40 C (1, 5). MATERIALS AND METHODS Plant Material. Seven-day-old etiolated seedlings of 'Alaska' pea were grown and prepared as previously described (24). Subapical stem sections (9-mm) excised 9 mm from the top of the apical hook were used in all kinetic studies. Some experiments employed tissue which had dissipated their initial wound response (aged sections). The procedures for identifying and quantifying ethylene, C02, and 02 were the same as previously described (24). Data points are not shown in Figures 2 through 7 because the short sampling interval of 45 sec produced data which generated a smooth curve. 02 Dependency. The 02 requirement for wound-induced ethylene synthesis was studied using 20 subapical sections enclosed in 25-ml Erlenmeyer flasks with moist filter paper and a CO2 absorber. The flasks were immediately purged with N2 and an appropriate volume of air or pure 02 was injected by syringe to give a specified O2 concentration. After 2 hr the gaseous contents of the flasks were sampled and analyzed for ethylene, 02, and CO2. Effect of Endogenous Gases. Equilibration of endogenous and ambient gas concentrations in aged subapical sections was accomplished in a flow-through system by inserting the needle of a 50ml syringe through the top serum stopper of an 0.8-ml opaque glass sample chamber. A partial vacuum was created by closing the inlet and outlet ports of the sample chamber and pulling the plunger of the syringe to the 50-ml mark. After a few sec the inlet tube was opened and ethylene-free air entered the evacuated 675 www.plantphysiol.org on June 27, 2017 Published by Downloaded from Copyright © 1978 American Society of Plant Biologists. All rights reserved.