The gravitational forces generated by dropping plants and the response of Sitka spruce seedlings to dropping

Two-year-old Sitka spruce (Picea sitchensis (Bong.) Carr.) seedlings were lifted from the nursery at different times of the ear and subjected to known forces by dropping bags of plants from a height of 10, 100, or 300 cm onto a concrete floor for a varying number of times (0 to 135). The response of the seedlings to dropping was assessed by their growth and survival in field experiments, root growth potential, root electrolyte leakage, and triphenyl tetrazolium chloride reduction. The forces were measured by attaching an accelerometer linked to an oscilloscope to the root collar of trees in the centre of each bag. Typically the forces generated by a 10-cm drop were about 11 g, while those generated by a 300-cm drop were about 140 g. All seedlings lifted in mid-February and mid-March survived one growing season in a cultivated, weed-free nursery soil, but height growth was significantly reduced by 300-cm drops. The survival of seedlings lifted in August was reduced by 5 and 15 drops particularly from 300 cm. Root growth potential was decreased while electrolyte leakage and triphenyl tetrazolium chloride reduction activity of the fine roots 2 days after dropping were increased by dropping. Growth and physiological differences associated with dropping were more closely and frequently correlated with the maximum force exerted than to the mean or the total force