Effects of foliar applications of nitrogen, boron and zinc on auxin contents, fruit set and fruit drop in orange (Citrus sinensis) cv. Thompson Navel

In order to investigate the effect of nitrogen, boron and zinc on auxin content and fruit set in Thompson navel orange, a factorial experiment based on complete randomized block design with 12 treatments and 3 replications was conducted. Experimental treatments included: urea at three levels (0, 3 and 6 gr/L), boric acid at two levels (0 and 0.5 gr/L) and zinc sulfate at two levels (0 and 3 gr/L). The number of flower, initial fruit set, fruit-set after the June drop and the final fruit set were measured. The length and diameter of fruit and the auxin content in the abscission zone were measured. Nutrient application had a significant effect on increasing the number of flowers, percentage of initial fruit set and yield. The lowest rate of initial and final fruit drop were obtained in combined treatments, using of urea at concentration of 6 gr/L and zinc sulfate with concentration of 3 gr/L. The highest percentage of fruit set after the June drop and amount of auxin were obtained in the combined treatments of zinc sulfate (3 gr/L) plus boric acid at 0.5 gr/L which had significant positive effect on yield increment.