Water - Stress - Induced Ethylene Production in Wheat ' A Fact or Artifact ? Isabelle Narayana

Effects of water stress on ethylene evolution from excised leaf segments and intact plants of wheat (Triticum aestivum L. cv Katepwa) were studied. Excised leaf segments of 8 day or 6 week old plants were dried until they lost 8% of their fresh weight (water potential about -2.3 megapascals). These and nondried control leaf segments (water potential about -1.0 megapascal) were sealed in glass tubes, and their ethylene production rates were compared by head space analysis via gas-chromatography. The dried leaves of both ages produced significantly more ethylene than the corresponding controls. However, when 6 week old intact plants were water-stressed by withholding water supply, and their ethylene production measured using a continuousflow system, no increase in ethylene was deteceted despite a drop in water potential to -2.9 megapascals over 6 days. Even the leaf segments excised from plants that had been subjected to water stress for 2, 4, or 6 days produced no more ethylene (in sealed tubes) than the leaves from well-watered plants. In fact, the ethylene production by these segments decreased with the increase in the severity of stress experienced by the plants. The results show that the commonly reported overproduction of ethylene by excised leaves subjected to rapid drying represents an artifact, which has little relevance to the water stress responses of intact wheat plants. Various types of stress have been reported to promote ethylene production from different tissues of a number of plant species (1, 17, 27). The stresses that enhance ethylene production include flooding, mechanical wounding, insect infestation, radiation, chemicals (e.g. herbicides, metals, ozone, S02, fungal exudates), chilling, freezing, high temperature, and water deficit. The diversity of these reports has led to a widespread acceptance of the occurrence of so-called stress ethylene (27). Water deficit is one ofthe more commonly reported stresses to cause an overproduction of ethylene, for example in cotton bolls and petioles (1 1, 21); broad bean leaves, stems, and roots (10, 12); orange leaves (7, 22); and wheat leaves (3, 23). In addition, acceleration of the rate of water loss from detached avocado fruits advances the peak of ethylene production associated with ripening (2). ' Supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada to H. S. S. S. L. is the recipient of a graduate fellowship from F. C. A. R. In wheat, all the reports of enhanced ethylene production consequent upon water stress are based upon experiments with either excised and partially dried leaf segments (3, 20, 23-26) or, occasionally, whole seedlings subjected to sudden water loss by lowering the osmotic potential of the root medium while blowing warm air over the shoot (24, 25). In all these cases, the leaves or the whole seedlings were sealed inside airtight containers, a condition that is certain to cause major changes in the important components (CO2, 02, C2H4, etc.) of the atmosphere around the tissue (5, 9, 16), and thus possibly affect the rates of ethylene production (6, 15, 16, 19). Using these techniques, several complex interactions among hormones and environmental conditions, and regulatory aspects of the ethylene biosynthetic pathway have been studied (13, 20, 24-26). The results of such studies have often been interpreted in terms of the regulation of drought responses of whole plants (20, 25). As a part of our ongoing research on the role of plant hormones in stress-induced reproductive disfunction in wheat, we measured ethylene production by intact waterstressed plants using a continuous-flow system that allows precise control of the gaseous environment. We found no evidence of enhanced ethylene production from stressed plants. This contradiction with what appeared to be a widely documented and accepted response to water stress (17, 27), coupled with the above mentioned technical deficiencies of the previous work, led us to the present work to reexamine the effect of water deficit on ethylene production in wheat. MATERIALS AND METHODS