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Alternate bloom thinners are needed for apple are needed to replace compounds which can no longer be used or have production system limitations. The effects of 24 chemicals selected as osmotic agents, organic acids, oils, essential oils, or potential metabolic agents and their properties of pH, electrical potential (EP) and water potential were tested in vitro on `Gala' apple pollen germination, tube growth and pistil damage. Solution concentrations of 0%, 0.25%, 0.5%, 1%, 2%, 5%, and 10% were prepared and solution pH, EP, and water potential measured. To test affects on germination, pollen was placed on agar germination media in petri dishes and then treated with 10: l of chemical solution. Percentage pollen germination and tube growth was calculated 4, 12, and 24 h after treatment. Excised pistils from forced flowers were placed on glass filter papers saturated with chemical solution. Pistil damage was visually, subjectively rated for damage indicated by discoloration 24 h after treatment. Effects of solution pH, EP and water potential on pollen germination, tube growth and pistil damage was significant with pH less than ∂4.0 or greater than ∂10.0, EP > 200mv, or water potential less than ∼4.0MPa inhibited pollen germination, growth, and killed pistils. Several chemical had apparent metabolic effects beyond the chemical effects mentioned above. In vitro tests were correlated to in vivo field tests in other studies indicating the use of pollen and pistil in vitro as a useful model for screening potential alternative thinning agents.
Osmotic agents used to prevent apple pollen grain germination were studied in vitro by applying 10 μL of solutions to germinating apple pollen on germinating and growth media. Seven concentrations (0%, 0.25%, 0.5%, 1%, 2%, 5% and 10%) of the solution were prepared for each chemical and the characteristics of pH, EC, and osmotic potential were measured. Apple pollen was dispersed onto the media in petri dishes. Micro drops of solution were then applied to marked areas. Dishes were then placed in germination cabinets at 25 °C. Cumulative percentage pollen germination was calculated 4, 8, 12, and 24 h after treatment by microscopic observation. Generally, the cumulative percentage pollen germination decreased asymptotically with increasing chemical concentration. The most effective chemicals for restricting pollen germination and growth were CuSO4 (0.25%), CH3 COOH (0.25%), CaCl2 (10%), K2 S2 O5 (0.25%), Methyl Jasmonate (2%). The effect of these chemicals has also been tested on pistil viability both in vitro and on excised limbs.
A strategy of chemical crop load control has been to use chemical desiccants to prevent fertilization and cause fruitlet drop. However, little is known of the solution characteristics that reduce pollen viability, inhibit pollen germination and growth, and cause pistil damage. This project was established to determine the solution characteristics effecting those results. Apple pollen was dispersed on germination media mixed with PEG (MW 10,000) to attain osmotic tensions from 0 to -5.0 MPa to evaluate effect on pollen germination and growth. Similarly, apple pollen was dispersed on germination adjusted to a range of pH from 2.3 to 12.0 with acids and NaOH. Excised apple pistils were place on filter paper supports saturated with solutions with osmotic tension adjusted by PEG in the range of 0 to -5.0 M Pa, and pH from 3.0 to 12.0. Solutions of osmotic tension in the range of 0 to -5.0 M Pa were applied by brush to intact pistils on apple flowers in a greenhouse and under field conditions. Pollen germination decreased with increasing osmotic tension and no pollen germinated at tensions greater than 4.0. Pistils, either excised or intact, had significant desiccation and death when treated with solution osmotic tensions greater than 4.0. Fruit set of individual spurs of the cvs Jonagold, Gala, and Arkansas Black were highly related to pistil survival 48 h after treatment with PEG. When solution osmotic potential exceeded 4.0, fruit set was reduced by more than 80%. Pollen germination was reduced by more than 50% at solution pH below or equal to 4.0 and greater than 10.0 and completely inhibited at solution pH below or equal to 3.0 and greater than 11.0. Similar results were observed for excised pistil and intact viability.