Kinetics of inhibition of foliar gas exchange by exogenous ethylene: an ultrasensitive response*

Abstract

summaryThe objective of this study was to examine the inhibition of foliar gas exchange as a function of ethylene (C2H4) concentration in order to evaluate the potential role of stress C2H, in mediating changes in gas exchange in response to environmental stress. Net photosynthesis (PN) and stomatal conductance to H2O vapour (gs H2o) were measured in soybean (Glycine max (L.) Merr cv. Davis) seedings after a 4 h exposure to one of a range of exogenous C2H4 concentration (0–20μII−1)in a controlled environment system. Declines in both Ps and gs H2O shifted from first‐order (liner decline) to zero‐order (saturation response) with increasing C2H4 concentration. The response of gs H2O to C2H4, was more pronounced than the response of Ps, with maximal declines for gs H2O of 32–48%, and half‐maximal response at 0.15μII−1. Corresponding parameters for PN were 18–29%, inhibition and half‐maximal response at 0–35μII−1. Both response curves were similar to those documented for C2H4, binding and for hormonally mediated C2H4 actions. Kinetic analysis of the saturation patterns by the Hill equation showed both responses to be ultrasensitive, i.e. they provide a relatively large change in gas exchange for a small change in C2H4 concentration. Based on this ultrasensitivity, we propose that endogenous stress C2H4 mediates some of the responsiveness of gas exchange to environmental stresses of natural and anthropogenic origin.