Reduced Pollen Viability On Exochorda racemosa Plants From Irradiated Seeds

Exochorda racemosa (Lindl.) Rehder, common pearlbush, is a spring-flowering shrub with white flowers that grows three to five meters tall, with an upright loose form.  Pearlbush, introduced to the U.S. from China, is a heat and drought tolerant plant.  However, it has been listed as an invasive species in at least 10 states in the Eastern U.S. In an attempt to induce sterility, seeds collected from an E. racemosa plant of unknown origin were irradiated with 0, 6, 12 or 18 kilorads of gamma radiation.  Three grams of seeds were irradiated per radiation level.  Following treatment, seeds were sown and germinated in a greenhouse.  The number of seedlings that germinated per treatment was:  0 krad = 106 plants, 6 krad = 97 plants, 12 krad = 8 plants, and 18 krad = 0 plants.  Only four plants in the 12 krad treatment survived to maturity.  Plants bloomed in the spring of 2011, and pollen was stained with fluorescein diacetate and evaluated for viability based on fluorescence. Pollen was collected from three flowers of each plant on three dates.  Three control plants were evaluated and mean pollen viability ranged from 78% to 88%.  Most pollen from control plants was uniform in size and highly fluorescent. Ten plants from the 6 krad treatment were evaluated.  Pollen grains from most of the 6 krad plants were highly variable for size.  Five of the plants had less than 50% viable pollen, while three plants had pollen viability and appearance similar to the control plants.  All four of the plants from the 12 krad treatment had reduced pollen viability. Only 4% of the pollen was viable on one of the 12 krad plants, another had 20% pollen viability, and two plants had 45% viability.  Similar to the 6 krad plants, the nonstaining pollen of 12 krad plants was highly variable for size.  No obvious morphological differences were observed in leaf or flowering characteristics from any of the radiation treatments.