الفهرس 
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Abstract
Citrus is one of the major fruit crops of the world. The fruit is desirable by many people for its eating qua— lit ies as well as for proper development of physical health in both children and adults. Production of citrus fruit in Egypt has become the largest single factor in the fruit business of today, and it approximately equals the production of all other major fruits including grape, mango, guava and banana.
According to the 1975 statistics inventory of the Egyptian Ministry of Agriculture, the total acreage of citrus was about 285 thousand feddans from a total area occupied by fruit crops which reached about 408 thousand feddans. Trees of navel orange cultivars occupy about one half of the total acreage grown by citrus in Egypt. The navel orange fruit is particularly good for fresh fruit market as the pulp is of fine quality, seedless, and holds together better than that of most other oranges. Yet, the tree is known to be a light bearer. Hassaballa (1964) found that after petal fall abscission of young developing fruits of navel oranges occurred in two distinct waves. He defined these two periods of DROP and stated that the second wave of DROP is of major concern to the grower since excessive DROP during this period will be reflected in a
pronounced reduction in yield. Coit and Jladgson (1919), showed that fruit DROP was excessive in “Washington’1 navel orange due to an abnormal sensitivity of the abscission mechanisms of this cultivar. Young developing fruit is particularly sensitive to envirome.ntal stresses and suffer in many years excessive ttJwl&aDROP?I that normally occur during the May—June period. The DROP of fruitlets during this stage of development is conditioned by both internal and invironmental factors • Over the years, evidence has accumulated which indicated that fruit—set, growth and development are affected largely by the nutritional status of the tree and are partially controlled by chemical substances originating in meristematic cells within the fruit (Hassaballa 1964) and possibly also Sn adjacent leaves (Luckwill, 1959). Therefore, most of the efforts directed towards increasing navel tree productivity have been tried to decrease young fruitlet DROP through the use of nutrient elements and/or growth regulators sprays timed to aff e— at fruit abscission and growth. This investigation was initiated to evaluate the effect of some nutrient elements and growth regulators sprays during the early periods of fruit development on fruitset, fruit DROP, ultimate yield and fruit quality as well as vegetative growth and leaf nutrient content of bearing “Washington” navel orange trees.