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An effect of water stress on ethylene production by intact cotton petioles.

The concept that ethylene is an endogenous growth regu- lator has evolved in the past few years (16). This concept has been strengthened by recent findings that internal concentra- tions of endogenous ethylene in vegetative tissues reach physio-logically active levels (3, 12, 13). These internal concentrations have been directly related to the corresponding production rates of excised tissues (13), although parallel data on produc- tion rates of intact plant tissues are not available. Using excised abscission zones from primary bean leaves, Jackson and Osborne (9) presented evidence that the timing of abscission of explant petioles can be related to the extent of ethylene pro- duction adjacent to the separation zone. They suggest that the ethylene production is coupled to a particular stage of senes- cence. Further, they proposed that in natural leaf abscission ethylene initiates the biochemical sequences leading to separa- tion, but the mechanism is not clear. Beyer and Morgan (4) have recently shown that ethylene production by and internal levels in detached cotton cotyledons increases as auxin transport declines. Amounts of exogenous ethylene necessary to in- duce abscission and inhibit auxin transport were similar. They propose that the rise in ethylene production and decline in auxin transport capacity are causally related and that reduced auxin transport is one of the ethylene mediated actions which precede induction of hydrolytic enzymes in the separation layer. Their measurement of ethylene involved whole cotyle- dons and was not restricted to the petiole. McAfee and Morgan (13) found internal ethylene levels and production rates were several times higher in petioles than leaf blades. Since auxin must be translocated through the petiole to the abscission zone, this observation