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Abstract. The activity of pectin esterase and cellulase in abscission of citrus explants was studied. No relation was established between pectin esterase and abscission, while cellulase activity was markedly increased before absoission and for a certain period after excision. IAA and cycloheximide delay abscission and cellulase activity, while ethylene and, to a lesser extent, GA. acoelerate them. Application of cycloheximide during the lag period and before cellulase activity can be measured, inhibits to a certain extent the formation of 'cellulase An escape from the inhibitory effect of cycloheximide is detected when inhibitor is supplied at the end of the -lag period. Osiborne (21) claimed that pectin esterase (PE) participates in the abscission process of bean leaves. In a later report Horton and Osborne (12) mention that cellulase is also active in the albscission zone of bean leaves. While studying the role of PE in the abscission zone of citrus explants we found that cellulase rather than PE is responsible for the separation process.Very recently, Abeles (5) published a parallel study, showing the role of cellulase in the abscission zone of bean, cotton, and coleus explants which were aged for 20 hr and then transferred into ethylene environment. He concludes that the mechanism of ethylene action during cell separation induces protein synthesis which is essential for cell separation by the production of essgential enzymes for the physiological processes of abscission. The present work was carried out independently although simultaneously with that of Abeles (5). The objective was to define the time relationship which exists between the ajbscission mechanism and cellulase activity in citrus leaves. Petioles of 10 explants were tied to a glass-slide and kept for the desired period in a humidified Petri dish, 9 cm in diameter, in the dark and at 250. At the end of each experiment the percent of abscission was determined by counting the distal sections which had already abscised and those which albscised due to a gentle touch administered by forceps (7).Pectin Esterase (PE) Assay. The method employed was that of Kertez (14) with the modification of Rouse and Atkins '(25). Two sections of 2 mm were cut from 20 explants, from each side of the abscission-layer over a 8 mm wide section from the proximal to the distal end (21). Twenty similar sections, representing the same portion with respect to the abscission line, were pooled, weighed, and homogenized in 20 ml of 0.2 N NaCl in a mortar. The homogenate was transferred into 50 ml of 1 % 1717 www.plantphysiol.org on May 12, 2018 -Published by Downloaded from

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Abstract. L-Phenylalanine ammonia-lyase (PAL) activity is low in the external layers (flavedo) of intact mature grapefruit peel. Flavedo discs evince upon incubation increasing PAL activity and ethylene production. Light has no effect in enhancing PAL activity in discs. Exogenous ethylene stimulates PAL activity in the flavedo of intact mature grapefruits (half maximum stimulation at 15 ppm); such activity rapidly decreases when fruit is removed from the ethylene oontaining atmosphere. Carbon dioxide inhibits both ethylene production and PAL activity of discs; exogenous ethylene only partly relieves PAL inhibition. Cycloheximide inhibits both PAL activity and ethylene production by flavedo discs. The same concentration of cycloheximide also inhibits PAL activity of discs in the presence of exogenous ethylene. Protein synthesis seems therefore to be needed at both levels of ethylene evolution and enhancement of PAL activity.

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