There are several biochemical lines of evidence to indicate that the conversion of xanthoxin to ABA occurs very rapidly and is also independent of stress conditions. If xanthoxin or AB-aldehyde accumulated in plants the observed 18O labeling of the carboxyl group would not occur, due to exchange of the 18O-labeled carbonyl with water. In addition, cell-free extracts prepared from stressed and non-stressed tissue displayed no difference in their ability to convert xanthoxin to ABA