I think it's the interruption of the Xylem I'm thinking of. I'm sorry to keep bringing this up, but I have to defend my assertion that the process at work is more like a siphon effect than osmosis. The water is being pulled up through channels in between the cell structure of the stem. If you get an air bubble into that passage, it will no longer pull up water or nutes. Just like breaking a siphon, and for the same reason. It's the attraction of the water molecules to each other that moves the water up the plant. If you break the chain, it stops working, and you cannot remove the air. McLoadie already mentioned the solution though. You CAN make another cut higher up, ie above the air bubble.
Denots, I hope you don't think I'm just trying to prove you wrong. We're all here to learn, and if I'm wrong, this is a great learning opportunity for me! So far though, I haven't seen any evidence to suggest I'm wrong. I understand where your confusion is coming from. When you are siphoning a liquid with a hose, yes, the output height needs to be lower than the input height. You are using gravity to do this. Gravity is acting on the water, but the water is still being pulled through the pipe the same way. In the plant, it's working against gravity, so a different force is acting on the water. As the plant respires a water molecule, that exerts a 'pull' on the long chain of water molecules leading down to the roots. Same mechanism is moving the water, it's just being driven differently.