Seed Mass Variation in the Shrub Banksia spinulosa (Proteaceae): Resource Constraints and Pollen Source Effects

Theory predicts that plants should produce seeds of a single optimal size. Contrary to theory, seed mass varied fivefold in Banksia spinulosa from two populations over a 2-yr period. Population and year did not contribute to the variation in seed mass. Most of the variation was explained by differences occurring among plants (32.3%), among infructescences within plants (10.1%), and within infructescences (45.6%). Among plants, seed mass was unaffected by plant size and seed number per plant. Mean seed mass of plants, however, was positively correlated among years, which indicates that among-plant variation resulted in part from genetic and/or maternal effects. Among infructescences within plants, seed mass was unaffected by flowering time, inflorescence size, stem diameter, and seed number. Defoliation did not influence seed number but significantly reduced seed mass by 16% compared with undefoliated inflorescences. This indicates that plants have a limited capacity to maintain seed size when resource levels are reduced. Within infructescences, seed number and mass were influenced by the position of the seeds. Fewer and smaller seeds were produced in the apical third of infructescences compared with the mid- and basal thirds following cross- and open pollination. Seed number and mass within infructescences were also affected by pollen source. Fewer seeds were produced following self-pollination compared with cross-pollination. Mass of selfed seeds was less when infructescences had both selfed and crossed seeds compared with infructescences with selfed seeds only, but crossed seed mass did not differ from infructescences with crossed seeds only. The reduction in selfed seed mass probably resulted from inbreeding depression exacerbated by stress related to competition for maternal resources. Overall, resource limitation, developmental constraints, and inbreeding depression limit the ability of maternal plants to produce seeds of a uniform size in B. spinulosa.