Do seed germination patterns in cleistogamous annual grasses reduce the risk of sibling competition?

1 The prediction that germination would be lowest and slowest for seeds which produce seedlings with the greatest probability of experiencing sibling competition (i.e. in grasses, at the lowermost tiller nodes) was tested by examining germination patterns in relation to tiller node position in two cleistogamous annual species (Sporobolus vaginiflorus and Triplasis purpurea). 2 Tillers containing mature seeds within the leaf sheaths at each node were collected from a population of S. vaginiflorus in Wisconsin, USA, and from a population of T. purpurea in New York, USA. Tillers with intact seeds were separated into fragments by node and subjected to a germination trial. Seed number at each tiller node was recorded. 3 In both species, final germination percentages and germination rates were greatest for the upper tiller nodes (i.e. those with the greatest potential for dispersal); for seeds in the lower nodes, which are unlikely to be dispersed, germination rates were extremely low. 4 Both species showed a pattern of increasing seed number per node from the lowermost (basal) to the upper tiller nodes. 5 Post-germination tests of seed viability revealed that seeds in basal tiller nodes tended to have the lowest viability, but this was only significant for S. vaginiflorus. 6 These results are in agreement with theoretical predictions that intense density-dependent sibling competition can select for a germination syndrome that delays or staggers germination in time for those seeds with the greatest likelihood of producing seedlings in close spatial proximity.