Evaluation of a Media for Artificial Pollination of Tree Fruit

There exist persistent and emerging threats to tree fruit grower abilities to consistently pollinate their crops:  weather conditions during flowering are predicted to become increasingly variable, colony collapse disorder threatens bee populations, pollinators harbor diseases that limit fruit marketability, pollenizers are sources of virus diseases, and there exists much confusion over pollinizer density and distribution. Our research team’s goal is to develop an artificial pollination system for tree fruit based on the electrostatic application of pollen in liquid suspension. The current research was designed to assess the role of potential pollen suspension constituents on pollen viability over time. Lab studies were conducted in 2014 and 2015 on previously collected apple (Malus domestica Borkh.), sweet cherry (Prunus avium L.) and pear (Pyrus communis L.) pollen. Our trials investigated pollen germination after incubation in solution, after 5-, 30-, and 60-minute intervals, with a range of concentrations of sucrose, boric acid, calcium chloride (CaCl₂), and commercially available surfactants.  Sucrose at 1% to 4% was ineffective for improving pollen germination at all sampling times.  However, sucrose at 8% and 16% improved pollen germination by 132% and 143% compared to water alone, respectively, at 30 minutes.  Similarly, at 60 minutes 8% and 16% sucrose solutions increased germination over the control by 254% and 240%, respectively.  The effects of boric acid and calcium chloride upon pollen germination in liquid delivery media are less clear. Calcium chloride at 0.075% caused 21% higher germination than control after 5 minutes.  After 5 and 60 minutes, 0.025% CaCl₂ also improved pollen germination by 11% and 10%, respectively. Boric acid had no effect on pollen germination.  When compared with control at 5 and 60 minutes, the composite suspension with calcium chloride (8% sucrose, 0.05 boric acid, and 0.025% CaCl₂) yielded 225% and 198% germination increases as a percent of control.  The composite suspension without calcium chloride was more consistent and produced significantly higher germination than control at all three loading times: 158% at 5 minutes, 161% at 30 minutes, and 316% at 60 minutes as a percentage of control.  The commercial surfactant Regulaid® and growth regulator ReTain® did not inhibit pollen germination. The results of these trials introduce a baseline for the development of pollen carrier media for application via electrostatic spray systems in tree fruit pollination.