Differential counteraction of ethylene effects by gibberellin a(3) and n(6)-benzyladenine in senescing citrus peel.

Treatment of mature citrus fruit (Citrus sinensis) with ethylene induced rapid chlorophyll destruction, a rise in respiration, a release of free amino acids, an accumulation of reducing sugars, and an appearance of phenylalanine ammonia lyase activity. Gibberellin A(3) (GA(3)) and N(6)-benzyladenine (BA) opposed the effects of ethylene on chlorophyll, amino acids, and to a lesser extent, reducing sugar levels. The ethylene-induced respiratory rise was only slightly modified by GA(3) and BA. Phenylalanine ammonia lyase activity was not affected by GA(3).The antagonism between ethylene and the senescence-delaying regulators GA(3) and BA seems to operate mainly within the chloroplast, but might not be confined to this compartment. The accumulation of reducing sugars exhibits the antagonism although it is not apparently related to the chloroplast.

Activity of pectin esterase and cellulase in the abscission zone of citrus leaf explants.

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 abscission and for a certain period after excision. IAA and cycloheximide delay abscission and cellulase activity, while ethylene and, to a lesser extent, GA(3) accelerate 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.

Ethylene-controlled Induction of Phenylalanine Ammonia-lyase in Citrus Fruit Peel.

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 containing 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.

Promotion of flower formation and fruit set in Citrus by antimetabolites of nucleic-acid and protein synthesis.

Shoots of the summer flush of Shamouti orange trees, which generally bear only few flowers in the following spring, were caused to produce an abundant number of flowers and eventually many mature fruits by treatments with chloramphenicol-succinate, 5-fluorodeoxyuridine and 5-bromo-3-sec-butyl-6-methyluracil (an herbicide). Increased bloom was accompanied by the shortening of internodes characteristic of flowering branchlets.

Native growth inhibitors from citrus shoots. Partition, bioassay, and characterization.

A partition method has been devised to separate auxins, gibberellins, and their respective inhibitors in plant extracts. Inhibitors counteracting gibberellin activity have been detected by a modified barley endosperm bioassay. An inhibitor found in young citrus shoots counteracts both auxin and gibberellin activities and behaves during partition and ohromatography like abscisin II.

The IAA-oxidase system of citrus roots.

The activity of indoleacetic acid oxidase in citrus plants (roots of mature trees and roots and stems of seedlings) was demonstrated. The interrelationships between 2,4-dichlorophenol, Mn(++) and the enzymatic activity were examined. The presence of Mn(++) was not found necessary, even in dialyzed enzyme reaction mixtures. However, Mn(++) promoted the enzymatic activity at the very high concentration of 10(-1)M. The inhibitory effect induced by excessive amounts of 2,4-dichlorophenol (10(-3) M) was removed in the presence of Mn(++).The enzyme showed a specificity to IAA as compared with other synthetic auxins in a wheat coleoptile bioassay. Moreover, the enzyme acted selectively on some endogenous auxins found in citrus tissues.The existence of endogenous inhibitors in stems and cofactors in roots was also demonstrated.