Influence of photosynthetic restriction due to defoliation at flowering on seed abortion in lucerne (Medicago sativa L.)

The seed yield of lucerne is poor, in contrast to its good vegetative growth. This is partly due to a lack of pollination under natural conditions, pod abortion and the production of only a few seeds per pod. In this study, lucerne plants were grown in controlled conditions and about 30% of the inflorescences were pollinated manually, There were about 10 ovules per flower, which remained constant throughout the flowering period. There were 4.5 seeds per pod, indicating over 50% seed abortion. One group of plants was defoliated and the lack of recent assimilates at flowering caused a small but significant decrease in the number of seeds per pod (4.1). The effect of defoliation varied with seed age; the youngest fertile inflorescences were the most affected and there was no effect at around 400 degrees C d(-1). This stage marked the end of the lag phase, which was determined separately in control plants. Thus, it is assumed that this date is the abortion limiting stage (ALS) for this species. The long lag phase for lucerne could be a key influence on seed yield. The longer the ALS, the greater the risk of vegetative competition, thus reducing reproductive development.