Developmental and environmental factors affecting level of self-incompatibility response in Brassica rapa L.

We performed artificial self-pollination throughout the period of sexual reproduction in six inbred lines of Brassica rapa, a plant with sporophytic self-incompatibility (SI). The level of SI changed between each pollination date in all lines, suggesting the effects of both internal and external factors. To further investigate the potential factors affecting the changes in the level of SI, multiple regression was performed, with plant line, seeding date, and temperature and humidity on the day of pollination (external factors) and plant age (internal factor) as explanatory variables. The factors affecting the change in the level of SI differed between the lines, and temperature and plant age had especially large effects. The level of SI was reduced by maximum temperatures >24°C on the pollination day, but this effect was no longer significant at maximum temperatures >28°C. High temperatures before flowering also influenced the reduction of SI. The level of SI was particularly reduced at plant age 0.75 (i.e., the last quarter of the flowering period), and a significant positive interaction between average temperature and plant age was found. This is the first study to quantify the change in the level of SI in plants across the entire period of sexual reproduction and to analyze the relationship between the change in rate of seed set and various external and internal factors.