Introduction
Date palm (Phoenix dactylifera L.) is an important fruit crop in arid and semi-arid regions of the world. Today, the development of tissue culture propagation methods has enabled date palm to be rapidly propagated on a large scale. The production of genetically uniform and stable in vitro date palms has a critical importance. Low levels of fruit set and supernumerary carpels were detected in many date palm trees produced by tissue culture, especially in Barhee cultivar which leads to economic losses to gardeners. One of the best tools for date palm reproductive potential studies is the direct application of nutrient elements on inflorescences and fruits. Nutrients, such as boron and potassium on dates yield and fruit quality seems to play an important role in achieving satisfactory fruit set and fruit quality. This study was conducted to investigate the effects of boron and potassium spray on improving the fruit set of tissue culture- derived date palm cv. Barhee in grove of Date Palm and Tropical Fruit Research Center Ahvaz city.
 
Materials and Methods
The experiment was conducted in a randomized complete block design with 9 treatments of bunches spraying with nutrients and 3 replications for two years (2018-2019) and each tree was considered as an experimental unit. Experimental treatments included spraying with boric acid at 0.5 and 1% , potassium sulfate at 2 and 3% as individual application or in a combination between boric acid and potassium sulfate concentrations treatments and control (distilled water). All treatments were replicated three times: 2 hours before pollination, 2 and 5 days after pollination. In 5th weeks after pollination, fruit set percentage, parthenocarpic fruits percentage and flower and fruit drop percentage were recorded.
 
Results and Discussion
Analysis of variance showed that that bunches spraying with potassium and boron significantly increased fruit set, yield and fruit physical characteristics i.e. weight, volume, length and diameter of fruit and TSS (%) compared with the control treatment. Among the different treatments, potassium sulfate at 2% was more effective in improving fruit set and reducing parthenocarpic fruits (seedless) than other treatments. Increasing fruit yield due to boron or potassium may be attributed to their effect an increasing fruit set. Increasing fruit set due to boron may be attributed to its role in maintaining high pollen viability, germination, and pollen tube elongation. Applying the potassium element has a role in controlling cell water content and carbohydrates biosynthesis and mobilization in plant tissues, consequently, carbohydrates play a serious role in fruit set and retention.
 
Conclusion
Boron and potassium nutrients play an important role in fruit set, fruit retention and development and cause efficient yield and quality improvement. Our results revealed that spraying Barhee date palm inflorescences with boric acid and potassium sulfate as individual had a positive effect on fruit set, yield and fruit quality.