Mass Leaf Drop on Standard Photinia

I wonder if someone can assist, I currently have a Standard Photinia red robin that is currently in the growers pot waiting to be planted in the ground, during the last few days almost all of the leaves have dropped of. I have tried to keep it well watered whilst its in the pot.

Do you think I could snip out the branches and take it back down to encourage new growth, and bushiness? It is such a shame as it was a gift to me and I would hate to loose it. I am going to try and get it put in the garden ASAP as I realise it may not be happy in the pot, although I have fed it with bonemeal but wondered weather it needs a feed of something else too?

If anyone can advise I would be greatly appreciated. Thank you in advance for anyone reading and answering my query. Tracey