Effect of Some Nitrogen Fertilization and Growth Regulators Treatments on Growth and Fruiting of Orange Trees

This work was carried out in the three successive seasons of 2008, 2009 and 2010 on thirty two 15-year-old Washington navel orange trees [C. sinensis (L.) Osbeck] grown in clay soil at 5 m apart in a private citrus orchard located at Minia El-Kamh district, Sharkia Governorate. The experiment included two factors: the first consisted of spraying urea at 2% three times a year (1st Dec., 1st Jan. and 1st Feb.), GA3 and NAA at 10 ppm 3 times/ year (beginning of flowering, full bloom and after fruit set), beside water spray as a control. The second factor was adding nitrogen at 1000 g N/ tree/ year (819.5 kg ammonium sulfate 20.5% /fed.) at 3 or 4 doses. The first N dose was added at 100 kg ammonium sulfate/ fed. at early Dec. in four doses treatment. The three doses either in 3 or 4 doses treatments were equally added at February, May and August.Spraying GA3 three times a year at 10 ppm, four N applications (early Dec., Feb., May and August) and their interaction gave the maximum fruit set, fruit retention percentage, fruit weight, number of fruits/ tree and yield/ tree, but reduced total fruit DROP percentage. Juice fruits of trees sprayed with water and received four N applications contained the highest TSS and the lowest total acidity percentages and consequently the highest TSS/ acid ratio as well as total, non-reducing and reducing sugars percentages compared with those sprayed with GA3 and received the same N doses.Trees received 4 n doses/ year with or without spraying urea at 2 % three times a year caused earlier flowering for about one week in comparison with other tested treatments.