Cytokinin Activity in Avocado Seeds during Fruit Development

Abstract

The soybean callus bioassay was used to determine levels of cytokinin activity in avocado (Persea americana) seeds. In the embryo, levels are high during the early stages of development, but diminish as the fruit grows. The level of cytokinin activity in the endosperm is very high throughout the period that this tissue exists. The seed coats have very high activity levels while the fruit is young, reaching values comparable with those found in the endosperm. The activity level falls as the rate of fruit growth slows down and disappears completely by the time the seed coats shrivel at approximately the same time the fruit reaches "horticultural maturity". coats (integuments), endosperm (when found), and embryo were separated. The samples were cut up into pieces of approximately 0.2 g; then they were frozen in liquid air and freeze-dried in bottles containing 5 or 10 g of the material. When drying was complete, the bottles were sealed under vacuum and were stored. There was a time interval of approximately 2 hr from the time of picking until freezing. The dried samples were extracted with 80% methanol; after filtration, the methanol was evaporated. The aqueous residue was first shaken with petroleum ether, then acidified to pH 2.5, and shaken with ethyl acetate to remove inhibitors (2, 4). The acidic aqueous fraction was chromatographed by ascending paper chromatography by using isopropanol-ammonia-water (10 :1:1) as the solvent. The chromatogram strips were cut transversely into 10 equal sections, and the cytokinin activity of each was assayed by Miller's soybean bioassay technique (6). RESUtLTS The seeds greatly influence the development of many fruits. In some, the rate of fruit growth decreases during periods of rapid growth of the embryo (10). Fruit shape is sometimes influenced by the presence or absence of seeds, or by their number (9). In most plants, the fruit does not grow without the development of normal seeds, but a number of important species constitute exceptions to this general rule. How seeds influence fruit development is not clearly understood, but it is usually assumed that one of the main factors is via growth substances which generally occur in the seed at high levels of activity. The presence of auxins, gibberellins, and cytokinins (8) has been demonstrated in seeds of many fruit species; however, no correlation between the level of activity of these substances in seeds and the development of fruits could be established in many cases. In the present study, we have explored the relationship between cytokinin activity in avocado seeds and fruit growth. The results of our studies on other growth substances will be published separately.