Pollen Load, Seed Position, and Agronomic Performance in Alfalfa
Competition among male gametophytes for a limited number of ovules has been related to improved performance in some sporophytic traits. However, gametophytic competition has not been widely exploited in plant breeding. In alfalfa (Medicago sativa L.), we examined two factors associated with gametophytic competition in other species: (i) amount of pollen on the stigma (pollen load), and (ii) seed position in the pod (a measure of stigma-ovule distance). Relationships were evaluated between these two factors and survival and forage yield of transplanted seedlings for 10 harvests at Tucson, AZ. Emergence and weight of seedlings were determined in a greenhouse test. Progenies derived from both one and two generations of hand pollination with heavy or light pollen loads were compared. Alcohol emasculation procedures were used to reduce pollen loads since this process reduces pollen adhesion to the stigma in alfalfa. This followed vacuum emasculation treatment to remove maternal anthers that could obstruct pollen application to the stigma. Normal hand-pollination procedures with vacuum emasculation to remove maternal anthers were used to produce heavy pollen loads. Heavy pollen loads resulted in more than 27 times more pollen tubes per ovary than light pollen loads. Analysis of seed set and the number of pollen tubes per ovary indicated that the potential for differential gametophyte competition existed in the two types of pollination. No differences were observed between progeny from heavy and light pollen loads for any of the traits evaluated. Seed position in the pod was also not related to forage yield or survival. Our data suggest that increasing pollen loads or selecting seeds from the pedicellar end of pods may have little impact in alfalfa breeding.
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