Microscopic Study of the Effect of Boron Spraying on the Pollen Tube Penetration in Apple Crosses

Favorable pollen transition on the stigma and pollen tube penetration is the most important for fruit setting in the apple. Boron is the main factor for pollen tube growth. A probable responsibility for boron in pollen tube growth may include vesicle making, transportation, fusion or the successive formation of the pollen cell wall. Pollen tube cell walls are rich in polypeptides, glycoprotein, polysaccharides and arabino furanosyl which are known to form physically potent complexes with boron. Maximum cell boron content is located in the cell wall where it is associated with pectin compounds. However, under definite field conditions, foliar applications of boron have been resulted to raise fruit set by as much as 100% in many researches.

In this study effects of acid boric by 0 (as a control), 1000 and 2000 ppm concentrations was studied on the pollen tube penetration percentage to style and ovary in six crosses among four apple cultivars including "Fuji", "Gala", "Goldn delicious" and "Red delicious' by florescence microscopy method, 72 and 120 hours after pollination. All of the trees were 12 year old on EM126 rootstocks and foliar sprayed by H3BO3 as the boron source (0, 1000 and 2000 mg.L) in the October. Crosses among the cultivars were programmed as ♀Red delicious × Golden delicious ♂, ♂ ♀Gala× Fuji, ♀Red delicious × Fuji♂, ♀Red delicious × Gala ♂, ♂ ♀Golden delicious × Fuji and ♀Golden delicious × Gala ♂. For each cross four repeats in all direction of the tree were regarded and in each repeat at least two branches with 60 – 100 were labeled in winter. Selected female cultivar's flower buds at ‘D’ stage were bagged to prevent the entrance of foreign pollens on the closed pistils. Pollens collected from the male cultivar flower buds and maintained in freezer until using in the field pollination time. Pollen germination was tested in an in vitro medium before field application on the pistils. Selected female cultivar's flowers were pollinated with selected male parent pollen when the pistils were acceptable for pollens and repeated after 24 hours. Determination of pollen germination percentage and tube growth on the pistil and different parts of the styles was studied by the florescence microscopy method 72 and 120 hours after pollination healthy pistils were separated from the branches and fixed in FAA solution and prepared for fluorescence microscopy observation as indicated in Ortega and Dicenta (2006). In each pistil the number of germinated pollen grains on the stigma, the number of pollen tubes in the first, second and third section and so, in the ovary were determined by a fluorescent microscope. Because of the five partitions of the apple flower stigma and style; the mean of the five parts was evaluated for each of them. The experiment was carried out as a factorial based on completely randomized design with three factors including B in three levels (0, 1000 and 2000 mg.L), time (0, 72 and 120 hr after pollination) andsix crosses in five replications (at least 10 style per cross). Data was analyzed following SAS (26) software. Mean values were analyzed by Duncan’s multiple range test.

Results showed that all of the boron concentrations, time after pollination and crosses were significantly affected pollen germination percentage on the stigma and tube penetration to different parts of the style and ovary at P<.01 level. Maximum pollen germination percentage on the stigma (33%) was observed in ♀Goldn delicious × Gala ♂ cross which treated by 1000 ppm boron 120 hr after pollination. Also, maximum pollen tube penetration to ovary was observed in ♀Red delicious × Golden delicious ♂ cross (32.06%). However, best compatibility between them and boron effects on the pollen germination percentage and tube growth in the style. It was revealed that in all of the crosses pollen germination percentage and tube growth in the style was increased by boron treatments.

However, boron is one of the main growth elements by its role in the cytokinin biosynthesis which is the most important hormone affects pollen tube growth and penetration. In this research the foliar application of boron increased pollen germination and pollen tube growth in all of the studied four apple cultivars crosses. However, increasing the boron concentration led to increased pollen germination and pollen tube growth linearly.