The impact of seed head age and orientation on seed release thresholds

1. It has been established that the timing of abscission has a major impact on the dispersal distances achieved by individual seeds, and yet there is little information available on seed release thresholds. 2. The current research used Conyza bonariensis to examine seed release thresholds of seed heads at varying ages and at varying orientations (in relation to the direction of the air flow). 3. Wind speeds of 070ms-1 were used to remove the seeds from seed heads harvested directly prior to exposure to wind. Seed head age at the time of harvest from the plant (010days old, where day 0 is the day on which the seed head opens) and seed head orientation (in relation to the direction of air) were altered. 4. While seed loss increased with increasing wind speed, average loss from seed heads increased from 49.4% on the day they opened to 92.8% from seed heads open for 10days. Further, there was an average seed loss of 76.4%, 72.5%, 73.9%, 65.3% and 58.8% for wind orientations of 0, 45, 90, 135 and 180 degrees, where 0 degrees indicates wind directed up towards the base of the hemispherical seed head (simulating an updraft), 90 degrees indicate wind directed towards the side of the seed head (horizontal wind) and 180 degrees indicate wind directed down towards the top of the seed head (simulating a downdraft). 5. Seeds were released in the strongest wind events and were more likely to be released in updrafts or horizontal winds rather than downdrafts. Both of these factors interact with seed aging in a way that will increase the potential dispersal distance. Given that minor changes to abscission can have major impacts on maximum dispersal distance; these factors need to be taken into account when formulating models on dispersal. 6. This research supports the theory that where greater dispersal confers an evolutionary advantage, wind-dispersed plants would be likely to evolve strategies to ensure maximum dispersal capability.