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Ethylene production or content was compared to leaflet abscission in detached, compound leaves of Mèlia azédarach L. In late autumn, when abscission was progressing from basal leaves upward, the oldest leaves both produced ethylene at the highest rates and abscised their leaflets first. When C(2)H(4) levels were measured in intercellular air removed immediately after leaves were harvested, C(2)H(4) levels were also highest in basal leaves and declined progressively in more apical leaves. Levels as high as 1.8 microliters C(2)H(4) liter(-1) air were observed. Earlier in the season groups of leaves demonstrated a pattern of sequential initiation of abscission from base to apex, but the peak rates of C(2)H(4) production followed an opposite trend, being highest in the youngest leaves. Peak rates of C(2)H(4) production occurred after the initiation of leaflet abscission and presumably are related to either the auxin content or a climacteric-like, autocatalytic phase of C(2)H(4) production not directly involved in the initiation of abscission. In these experiments, the early abscission of the older leaflets reflects their greater sensitivity to C(2)H(4), presumably due to lower auxin content. C(2)H(4) production rates in all experiments, with rare exceptions, exceeded 3 microliters per kilogram fresh weight per hour at least 24 hours before leaflet abscission reached 10%. This achieving of a threshold internal C(2)H(4) level is viewed as an initiating event in leaflet abscission. Hypobaric conditions, to facilitate the escape of endogenous C(2)H(4), delayed abscission compared to controls, and termination of hypobaric exposure allowed a normal progression of abscission as well as normal C(2)H(4) synthesis rates. All of the data indicate that C(2)H(4) initiates leaflet abscission in intact but detached leaves of Mèlia azédarach L. The seasonal patterns observed suggest that C(2)H(4), in concert with those hormones which govern sensitivity to C(2)H(4), regulate autumn leaf fall in this species. |
