The amount of nutrients in fruit trees and their proper nutrition is one of the most important determinants of yield. Among the micronutrients, due to the effective role of boron in germination and growth of pollen tube, it has an important role in pollination, fertilization and finally fruit set (Sharafi, 2019).Usually, the concentration of Boron (B) in plants is comparatively higher in flower, anthers, ovary and stigma as compared to vegetative parts which suggests a particular biological role of B in the reproductive phases of plants  (Agens et al., 1997; Alva et al., 2015). The B necessity for normal pollen germination and tube growth has been well known in fruit trees (Yang et al., 1999).

In this study, the effects of Boron foliar application at three concentrations of 0, 1000 and 2000 mg / l on flower buds of two cultivars of Beigi and Shahroodi apricots were investigated. After foliar application two weeks before flowering, the effect of foliar application on germination percentage, pollen tube growth, primary and final fruit set were evaluated.

The results were showed that 2000 mg / L Boron had the most effect on the rate of initial and final fruit set. The percentage of initial and final fruit set was 1000 mg / l Boron. The results also showed that the effect of Boron on germination percentage at stigma level and penetration rate of pollen tubes at the beginning of pistil, middle style and ovarian inlet was significant at 1% probability level. The germination percentage with 1000 mg / L Boron with 61.82% germination and the lowest germination with 57.91% was related to control. Also, 1000 mg / l sulfate increased the penetration of pollen tubes into the beginning and middle parts of the styles and so ovary.

 Overall foliar application of Boron increased the primary and final fruit set, and so increased germination and penetration of pollen tube to the ovary in apricot cultivars