2017 ASHS Annual Conference

Manchurian crabapple is one of the most commonly used pollinizers in the Pacific Northwest. However, the susceptibility against Sphaeropsis pyriputrescens, Phacidiopycnis washingtonensis and other postharvest diseases led to trade issues in the past. New suitable pollinizers are needed to overcome this problem for the future. This part of the project focused on the optimization of the in vitro pollen germination procedure to evaluate new crabapple genotypes.

Pollen from five commercial apple cultivars (‘Gala’, ‘Granny Smith’, ‘Honeycrisp’, ‘Red Delicious’ and ‘Rome Beauty’) were purchased from a commercial company (Yakima, Washington, USA). Pollen was stored at - 20 °C until use. The pollen was rehydrated for 2 h at 25 °C before each experiment. 50 mg of each cultivar’s anthers were then transferred into a 2-ml tube. Afterwards, 1.5 ml with germination media containing 10 % sucrose and 0.005 % boric acid were added and the anthers were gently squeezed with a tweezer to promote the release of the pollen grains. 150 µl of the pollen suspension was transferred to an agar petri dish (60 x 15 mm). Each experiment was conducted by using four replications. At least 300 pollen grains per replication were evaluated to calculate the germination rate. During this experiment, the effect of different germination medias, incubation times, temperatures and light conditions were evaluated.

The best germination results were obtained by using 10 % sucrose, 0.005 % boric acid and 1 % agar. All genotypes reached their maximum germination rate after an incubation period of 2 hours. Germination rates ranged from 37 % (‘Granny Smith’ and ‘Rome Beauty’) to 52 % (‘Red Delicious’). Pollen tube growth increased steadily until 12 hours after incubation. The five genotypes showed the highest pollen tube growth rate 2 h after the incubation. The maximum pollen tube growth rate ranged from 154 µm/h (‘Rome Beauty’) to 238 µm/h (‘Honeycrisp’). The light conditions (light vs. dark) during the incubation (df = 1, F = 0.024, p = 0.88) showed no effect on the pollen germination rate. The incubation temperature at 15 °C lowered the germination rate for all genotypes by 9 % (df = 1, F = 64, p < 0.001).

The maximum pollen tube growth rate (measured at two hours after incubation), maximum germination rate and maximum pollen tube length (both measured 12 to 24 hours after incubation) can be used to evaluate the pollen potential of new genotypes.