Chironja'This seemingly spontaneous hybrid was noticed by Carlos G. Moscoso, Fruit Specialist in Horticulture, Agricultural Extension Service, of the University of Puerto Rico, when he was interviewing citrus growers in the interior, mountainous, coffee zone of that island in November 1956. He saw a tree with large, bright-yellow fruits in contrast to the normal sweet orange and grapefruit trees grown by farmers as shade for their coffee plantations. He learned that there were several other trees of the same type on other farms in the neighborhood, some of them quite a few years old and all raised from seed and showing only slight variations in form and size, and greater variation in season of fruiting.
He described the fruit as round to pear-shaped, necked, equal to grapefruit in size; peel a brilliant yellow, slightly adherent, easy to remove; the inner peel non-bitter; pulp yellow orange, with 9-13 segments having tender walls and much juice; the mild flavor reminiscent of both orange and grapefruit, hardly acid or bitter even when immature. The seed count ranges from 7 to 15, with an average of 11, and some fruits have as few as 2. The fruit is borne singly or in clusters. The tree, reaching 22 ft (6.7 m), has leaves that smell like and resemble those of the grapefruit except that they are usually deformed. Young shoots may have prominent thorns. Flowering and fruiting may occur throughout the year, though most trees flower mainly in late spring and early summer.
By 1969, horticulturists in Puerto Rico had evaluated 500 seedlings in a test planting and selected 12 clones, 3 being considered superior. It was observed that 7-year-old trees may produce 300 to 500 fruits over a period of one year, while a 7-year-old grapefruit tree in Puerto Rico may produce about 70.
In rootstock trials, grapefruit rootstock gave best results at the Adjuntas Agricultural Experiment Substation and sour orange at the Isabela Substation. On grapefruit root-stock, the 'Chironja' is larger than ordinary but not as sweet. A planting of seedlings was made at the Corozal Substation with simultaneous planting of grafted trees for comparison. So much variation was seen in the seedlings it was concluded that the 'Chironja' must be vegetatively propagated for uniform results. Ten clones selected from the Corozal planting were grafted onto sour orange and set out at the 3 Substations. The trees reached heavy production at 6 years of age. Yield was highest at Isabela Substation and 'Clone 2-4' had the best yield, the thinnest peel and the most seeds. 'Clone 2-3' had 11 seeds and 'Clone 3-6' had 14.
Storage tests revealed that fruit in polyethylene bags at 44.5º F (7º C) and relative humidity of 90%, maintained acceptable quality for 70 days. But fruits harvested 5 months after fruit-set and stored for periods of 30 to 55 days were of the best quality. Fruits harvested 7 months after fruit-set retained high quality for only 25 days.
The 'Chironja's' productivity makes it popular with Puerto Rican growers and it is in demand on Puerto Rican markets, mainly because it is more colorful than the grapefruit, sweeter, and easy to peel.
The fruit is cut in half and eaten with a spoon as a grapefruit is eaten, or is peeled and the sections eaten individually, or they are squeezed for juice. The sections can be canned in sirup with added citric acid to enhance the flavor. The rind can be candied successfully.