Cytokinin and Inhibitor Activities in the Avocado Fruit Mesocarp

Abstract

No cytokinin activity was found in methanolic mesocarp extracts after purification with petroleum ether and ethyl acetate. Cytokinin activity did appear, however, in aqueous fraction after acid hydrolysis or passage through Dowex 50 (H+) ion exchange columns. The level of this activity, the fruit growth rate, and the cell division rate were found to be positively correlated (with each other).An inhibitor to cytokinin activity in soybean callus bioassay was found in the petroleum ether fraction; this inhibitor is not abscisic acid. The activity of the inhibitor could be considerably reduced by raising the level of kinetin in the nutrient medium. The intensity of inhibition was found to be positively correlated with reduction in the rate of mesocarp growth.The reduction in fruit growth rate may be considered to be due to the diminution of bound cytokinin activity and (or) increased activity of an endogenous inhibitor. Fruit growth occurs both by cell division and by cell expansion. In most fruits, cells divide for only a short period after fruit set (10), the fruit growth after this period being due to cell expansion. The avocado is exceptional in that cell division continues as long as the fruit is attached to the tree, and even after the fruit has reached horticultural maturity (11). Since cytokinins are known to be regulators of cell division (6), it was of interest to test for their activity in avocado fruits. MATERIALS AND METHODSAvocado fruits of the Fuerte variety were picked once a month, starting 2 months after fruit set (June to November), from productive trees growing under good conditions. After weighing the fruit, the mesocarp was cut into 5-mm cubes which were immediately frozen in liquid air. Ten-gram (fresh weight) portions of the frozen material were freeze-dried, sealed under vacuum in small glass containers, and were stored for 1 to 6 months before use. The freeze-dried material was extracted by maceration with 80% methanol. The homogenate was placed in a refrigerator for 1 hr and then was filtered through a Buchner funnel. The filtrate was treated as follows. (Scheme 1). Partition Fractionation. The filtrate was concentrated under low pressure at 50 C until all of the methanol had evaporated. The aqueous residue was shaken four times with equal quantities of petroleum ether (bp 60-80 C). Immediately thereafter it was acidified to pH 2.5 with 1 N HC1, and extracted four times with equal volumes of ethyl acetate. The purified aqueous residue was tested before and after passage through an ion exchange column in the acid form, or by acid hydrolysis; all fractions were bioassayed after paper chromatography.Column Fractionation. The column used was a Dowex 50 (H+) 200-to 400-mesh ion exchange column, 150 mm long and 20 mm wide. The aqueous concentrate loaded on the column was equivalent to 5 or 10 g fresh weight of the mesocarp material. The column was eluted with 120 ml of water, 120 ml of 70% methanol, 120 ml of water, 120 ml of 1.5 N ammonia, and 120 ml of water, in that order.Hydrolysis. ...

Abscisic Acid and Cytokinin Contents of Leaves in Relation to Salinity and Relative Humidity

“…4) (3). They observed that while no cytokinin activity was found in the methanolic extract of avocado mesocarp, this activity, as assayed by soybeancotyledon callus, did appear in the aqueous fraction after either acid hydrolysis or passage through Dowex 50 (H+) ion exchange columns.…”

“…To estimate cytokinin activity, the water phase which was separated from the ethyl acetate fraction containing the ABA was passed through a Dowex column as described by Gazit and Blumenfeld (3). The appropriate eluate was dried and redissolved in methanol (70% v/v).…”

Hormonal Activity in Detached Lettuce Leaves as Affected by Leaf Water Content

“…Cytokinin activity was determined by the soybean callus bioassay (26), according to Gazit and Blumenfeld (11).…”

The Effect of Indole-3-acetic Acid and Other Growth Regulators on the Ripening of Avocado Fruits

“…In several cases, only pigment changes of chlorophyll breakdown, or lycopene synthesis have been affected by these hormones (1,11,22). Blumenfeld and Gazit (6) and Gazit and Blumenfeld (17) have demonstrated the existence of gibberellins and cytokinins, respectively, in avocado fruits. They have also pointed to the possible synthesis of these hormones in the fruit, particularly in the seed coat.…”