Seed developmental patterns in Banksia attenuata R. Br. and B. laricina C. Gardner in relation to mechanical defence costs

summary A comparison is made of the phenologies of infructescence, follicle and seed growth development of two species of Banksia with particular reference to nutrient provision of dissimilarly sized seeds and associated mechanical defence costs. Follicles and seeds of B. attenuata developed sequentially while the smaller seeded B. laricina , exhibited an almost synchronous pattern of development. Seed nitrogen loading, assayed by temporal changes in amino pool sizes in different parts of the seed, was confined to a relatively short period (± 3 months) prior to infructescence desiccation in both species. Prolonged infructescence development in the larger seeded B. attenuata cannot therefore be interpreted simply as a mechanism for extending time available for mobilization of scarce nutrients within the plant, but may be a result of selective pressures related to the development of large woody structures around the seeds which provide mechanical defence against predators. Both Banksia spp. make relatively large investments of carbon, but not of nitrogen, in defence structures (up to 5·5 g per seed in B. attenuata ) which in plants with low photosynthetic rates could have associated penalties in terms of N required for photosynthetic enzymes. Sequential development of follicle and then seed assumes significance, since nitrogen used in photosynthesis to provide carbon to the follicle can subsequently be mobilized from the leaf and redeployed in seed protein synthesis. The often held view that larger seeds require longer maturation periods is not universal as, in the Banksia spp. examined, defence costs and photosynthetic capacity might play a more dominant role in determining seed developmental patterns.