Effects of Zinc Foliar Spray on Pollen Tube Growth in Pistils of Some Apple Cultivars Crosses

Apple is one the most important horticultural crops in Iran. As the most apple cultivars are self-incompatible, having a good fruit set requires favorable cross-pollination. However, this phenomenon is related to a series ofcomplex processeswhich happens on thestigma. Zinc is one of the main factors that affects pollination and has an important role in increasing the percentage of pollen germination thereby increasing fruit set. It has been reported that zinc is referred to be as a main growth element because of its role in hormone production in buds specially auxin biosynthesis. Auxin is the most important hormone which affects pollen tube penetration to the style and so, fruit set in plants. Zinc deficiency in the apple orchards primarily affects the above ground portions of trees, resulting in poor leaf and shoot growth, reduced flowering and fruit set, and reduced size and coloring of fruit. Zinc is also important in the movement of calcium within the tree. Using zinc sprays is the most consistent and cost effective way to apply this element in apple tree orchards.
In this research, the effect of zinc solution in three levels (0, 3000 and 5000 mg.l) was studied on pollen penetration to the ovary in the crosses among four apple cultivars including Golden Delicious, Red Delicious, Gala, Fuji. Pollen penetration was studied 72 and 120 hours using fluorescent microscopy technique in the laboratory after field pollination. Crosses among the cultivars included 1- ♀Red delicious × Golden delicious♂, 2- ♀Galla × Fuji♂, 3- ♀Red delicious × Fuji♂, 4- ♀Golden delicious × Fuji♂ and ♀Golden delicious × Galla♂.
Results showed that the effects of zinc, crosses and their interaction were significant on pollen germination and tube penetration in the primary, middle of the stigma and top of the ovaries. The highest percentage (59.4%) of germination on the stigma was observed in the cross (♀Golden Delicious × Gala ♂), in 120 hours and the highest penetration of pollen tube in ovary (18.08%) was observed at the cross point of 5000 ppm of Zinc, in 120 hours respectively.
In this research the foliar application of Zinc (until 3000 mg.l) increased pollen germination and tube growth on the stigma in all of the studied four apple cultivars. Although, increasing zinc concentration to 5000 mg.l showed negative effects on the both characteristics which showed the tolerance range of the cultivars to the zinc concentration based on their genetic traits. However, increasing the Zinc concentration to 5000 mg.l led to increased pollen tube growth and penetration to the different parts of the styles in all of the crosses linearly.