Ethylene : Role in Fruit Abscission and Dehiscence Processes ' 2 Received for publication

Two peaks of ethylene production occur during the development of cotton fruits (Gossypium hirsutum L.). These periods precede the occurrence of young fruit shedding and mature fruit dehiscence, both of which are abscission phenomena and the latter is generally assumed to be part of the total ripening process. Detailed study of the dehiscence process revealed that ethylene production of individual, attached cotton fruits goes through a rising, cyclic pattern which reaches a maximum prior to dehiscence. With detached pecan fruits (Carya illinoensis [Wang.] K. Koch), ethylene production measured on alternate days rose above 1 microliter per kilogram fresh weight per hour before dehiscence began and reached a peak several days prior to complete dehiscence. Ethylene production by cotton and pecan fruits was measured just prior to dehiscence and then the internal concentration of the gas near the center of the fruit was determined. From these data a ratio of production rate to internal concentration was determined which allowed calculation of the approximate ethylene concentration in the intact fruit prior to dehiscence and selection of appropriate levels to apply to fruits. Ethylene at 10 microliters per liter of air appears to saturate dehiscence of cotton, pecan, and okra (Hibiscus esculentus L.) fruits and the process is completed in 3 to 4 days. In all cases some hasteuing of dehiscence was observed with as little as 0.1 microliter of exogenous ethyl. ene per liter of air. The time required for response to different levels of ethylene was determined and compared to the time course of ethylene production and dehiscence. We concluded that internal levels of ethylene rose to dehiscence-stimulating levels a sufficience time before dehiscence for the gas to have initiated the process. Since our data and calculations indicate that enough ethylene is made a sufficient time before dehiscence, to account for the process, we propose that ethylene is one of the regulators of natural fruit dehiscence, an important component of ripening in some fruits. Our data also suggest a possible involvement of ethylene in young fruit abscission. Ethylene is now implicated as a natural regulator of leaf abscission (2, 11). Young fruit abscission is a common phe1This research was supported in part by Cotton Incorporated. A contribution of the Texas Agricultural Experiment Station. 2Preliminary progress reports of this study have appeared in: 1970 Proceedings of the Beltwide Cotton Production Research Conference. p. 47; 1970 Bulletin Southeastern Biologists. 17: 52-53; 1971 Proceedings Association Southern Agricultural Workers 68: 197-198; 1971 Plant Physiol. 47: S14. 3Present address: Texas A&M University Research and Extension Center at Overton, Drawer E, Overton, Tex. 75684. nomena in many plants. Fruit dehiscence, as a separation of tissues, is an abscission-like process. Thus, we concluded that ethylene may be a natural regulator of young fruit abscission and fruit dehiscence. Horticulturists have long known that ethylene hastens fruit wall (shuck, hull, pericarp, and accessory parts) dehiscence of walnut (6, 19) and pecan (9), but until recently there has been only indirect evidence that ethylene is produced by fruits that dehisce. Lipe and Morgan (13) found that ethylene production by pecan fruits increased to a relatively high level shortly before and during fruit wall dehiscence. Similar results have been observed with detached, dehiscing cotton fruits as well (15, 16). In these studies (15, 16), a smaller peak of ethylene production was also noted near the time of maximum young fruit abscission. While fruit dehiscence is generally considered a component of the ripening process, where the regulatory role of ethylene is well established (3, 17), other considerations indicated the need for the present study. Fruit ripening has been studied most extensively in fleshy fruits which do not dehisce, and some fruits dehisce after they are dry while others open before appreciable weight loss occurs (13, 16). In some cases, for example pecans, dehiscence also involves detachment (abscission) of the seed from its vascular connections. Dehiscence as a physiological process has not received detailed attention, and yet it is a critical phase in the production of some crops where it is the dominant visible feature of the ripening process. Natural abscission of whole, ripe fruits of the species studied here is rare; seed dispersal can promptly follow dehiscence. Production of ethylene by flowers has been rather extensively studied, primarily in association with petal fading and abscission (3, 17), and there is a presumptive association of ethylene with the natural separation of fruits from plants following ripening. All of this information was useful in our approach to the specific process of young fruit abscission. In the present investigation, a detailed monitoring of the seasonal pattern of ethylene production by intact cotton fruits was conducted, ethylene production by dehiscing cotton and pecan fruits was carefully detailed, and the capacity of physiological levels of exogenous ethylene to regulate dehiscence of detached fruits was determined. MATERIALS AND METHODS Pattern of Ethylene Production by Developing Cotton Fruits. In preliminary studies (15, 16) we observed ethylene production by detached cotton (Gossypium hirsutum L., cv. SP23) fruits from anthesis until the completion of fruit dehiscence. Triplicate samples of cotton fruits from a field on the Texas A&M University Farm, tagged on the same day at anthesis. were collected weekly, and enclosed in groups of three in a 500-ml Erlenmeyer flask on water-saturated filter paper. After 3 to 4 hr, the ethylene content of an air sample from each flask was measured gas chromatographically. Ethylene was measured with a hydrogen flame ionization detector (2). Five-milliliter air samples were injected with a