Key biological factors related to outcrossing-productivity of cytoplasmic-nuclear male-sterile lines in soybean [ Glycine max (L.) Merr.]

Soybean scientists in China have tried to utilize heterosis for yield enhancement. A number of cytoplasmic-nuclear male-sterile (CMS) lines, their maintainers and restorers were developed, but the commercialized hybrid seed production is still a bottleneck. Five soybean CMS lines with their maintainers and restorers were studied for the sufficiency of viable and effective pollens from maintainers/restorers, the natural insect pollinators in fields and outcrossing capacity of CMS lines. The results showed that the maintainers/restorers may offer viable pollens during the flowering day time in Nanjing and Taiyuan (germination rate 77.8–86.5%). But the morning dew causes pollens losing effectiveness in Nanjing, while normal in Taiyuan, indicating the location and planting season without morning dew is a key for CMS/hybrid seed production. In CMS fields in Taiyuan, the major insect pollinators composed of China bee (54.3%), Mellifera bee (11.3%), Northern-cutting bee (6.0%), Corn belt hair bee (1.5%), etc. A parallel trend between insect number and flower nectar amount in a day with peak time at 12:00–13:00 implied the soybean nectar attracting the insects. The outcrossing pod-set rate of CMS lines in net-room with artificial-raised bees, net-room without bees and open-field with enhanced natural insects was 57.8, 2.1 and 47.3%, respectively, indicating that insect pollination is necessary. The realized outcrossing pod-set rate in seed production field achieved 88.9% (1460.4 kg/ha) for SXJLCMS1A × SXJLCMS1B and 145.2% (1829.5 kg/ha) for SXCMS1A × JY-31, different outcrossing capacity existed among CMS lines. Thus the CMS/hybrid seed production can be improved through optimizing the pollen effectiveness, pollinator population and talented CMS line.