Answer
At dawn, root pressure causes the water droplet to form since root pressure drives water movement from root to shoot without cohesion-tension having to operate. This does not happen at noon since, by then, the root pressure has been eliminated by transpiration.
Work Step by Step
Review the material on both root pressure and cohesion tension. The first example in the question occurs when guttation would happen on a leaf and so is analogous with that process.
