Postharvest response of avocado fruits of different maturity to delayed ethylene treatments

The effect of 10, 100, 1000, 10,000 ppm of ethylene, applied for 3, 6, 12, and 24 hours, on the ripening rate of "Hass" avocado (Persea americana Mill.) fruits at three stages of maturity was investigated. Ethylene treatments were started either immediately after picking or 2 days later. A sharp peak of ethylene production was found to precede full softening by about 2 days and the occurrence of this peak was used to determine ripening rate. Hastening of fruit ripening was much more marked following ethylene treatment which began 2 days after harvesting than immediately after. The difference in response diminished gradually as the fruit became more mature.These results can be explained by the assumption that the factors which prevent avocado ripening on the tree, continue to exert their influence for a limited period after harvest and inhibit the ripening response in the freshly picked fruit. This inhibition decreases with advancing fruit maturity.

Water-deficit Stress, Ethylene Production, and Ripening in Avocado Fruits

Differential rates of water loss were achieved in picked avocado (Persea americana Mill.) fruits, either by controlling the evaporation rate or by supplying water through the fruit stalk. A negative linear correlation was found between the daily rate of water loss from fruits and their ripening, as determined by the time from harvest to peak of ethylene production. Ripening rate was hastened by 40% in fruits which had lost water at rate of 2.9% fresh weight per day, compared with those which lost only 0.5% per day.

Cytokinin Activity in Avocado Seeds during Fruit Development

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".

Cytokinin and inhibitor activities in the avocado fruit mesocarp

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.