Annals Of Agric. Sc., Moshtohor, Vol 35(3): 1613-1632, (7997) TRIALS FOR IMPROVING FRUIT SET, YIELD AND FRUIT QUALITY OF WASHINGTON NAVEL ORANGE BY APPLICAION OF SOME GROWTH REGULATORS AND YEAST EXTRACT AS A NATURAL SOURCE OF PHYTOHORMONES. BY Atawia, A, A. R. * and EL-Desouky, S. A.** * Horticulture Dept. ** Agric. Bot. Dept. Faculty of Agric., Moshtohor, Zagazig Univ. ABSTRACT Foliar application to Washington Navel Orange trees at fiillbloom stage with Yeast extract (YE) at 100 & 200 ml/1., 25 & 50 ppm of Benzyl Amino Purine (BA) or Gibberellic acid (GA3) exhibited significant increase in fruit set and also significantly decreased both June drop and the biennial bearing index. At harvest, YE followed by BA and GA3 treatments; showed significant increase of the total fruit numbers and yield in Kg/tree. Besides, fruit weight, rind thickness and juice percentage were significantly increased. In addition, total soluble solids (T.S.S) and ascorbic acid contents were increased. The only exception was the reduction of T.S.S and T.S.S/acid ratio that existed with GA3 treatment. The accumulation of both Macro and Micro nutrient elements were obviously increased in leaves and fruits with the application of the different treatments. The yeast extract treatment increased cell wall thickness of fruit pedicle and retarded transverse division in the site of abscission zone. Fruit dropping was accompanied with significant increase of auxin and gibberellin inhibitors. Meanwhile, significant increase of cytokinin, auxin and gibberellin activities were correlated to fruit set and its growth as well.