Collapse of Water-Stress Emboli in the Tracheids of Thuja occidentalis L.

We report the kinetics of embolus formation and collapse in the tracheids of Thuja occidentalis L. stem segments. Radial wood sections were trimmed to 4 mm long paralleling the tracheids by 1 mm wide and 0.1 mm thick. They were observed under a dissecting microscope at 128x while sections were dehydrated and rehydrated. During dehydration, cavitations resulted in the formation of emboli in tracheids, but we concluded that the cavitated tracheids did not immediately fill with air at atmospheric pressure. This conclusion was based on the time required for the emboli to collapse after the rewetting of the dehydrated segment. By hypothesis, the time for the emboli to collapse should be proportional to the amount of air in the emboli. The time for all the emboli to collapse was a linear function of the dehydration time for times up to 15 min. For dehydrations greater than 80 min, the time for collapse after rewetting was constant, and we concluded that the tracheids have saturated with air by 80 min of dehydration. The kinetics of embolus formation is discussed in terms of the air-seeding hypothesis for cavitation, and collapse is discussed in terms of the physics of gas dissolution and diffusion. Embolus formation and dissolution in intact herbaceous and woody plants should follow the same physical laws.