getOrganizer() get ServiceObjectPropertyException:You must load or assign this property before you can read its value

EWS JAVA API 1.1.5   On  Exchange server 2007

I have some java code to query  calendar events on exchange server.

it works perfectly until I try to getBody() and getOrganizer(). I searched other discuss and find for body I need this kind of code:

appt.load(new PropertySet(ItemSchema.Body));

This works for body.

But for getOrganizer(), I don't know which PropertySet I need.

 I tried those :

appt.load(new PropertySet(ItemSchema.AllowedResponseActions));

appt.load(new PropertySet(ItemSchema.Attachments));
appt.load(new PropertySet(ItemSchema.ConversationId));

appt.load(new PropertySet(ItemSchema.MimeContent));

none of them works. I really can't find which one is related to getOrganizer().

I wish I can get your help.