Force.com Web Service API Developer's Guide

To use the API, your organization must use Enterprise Edition, Unlimited Edition, or Developer Edition. If you are an
existing Salesforce.com customer and want to upgrade to either Enterprise or Unlimited Edition, contact your account
representative.

I trial a Salesforce Professional Edition. Some apps I can integrated but others can not. If I want to develop an App that can be used in all editions of Salesforces, what api should I use and how should I with it?

Anyone let me know this.

Force.com Web Service API Developer's Guide

To use the API, your organization must use Enterprise Edition, Unlimited Edition, or Developer Edition. If you are an
existing Salesforce.com customer and want to upgrade to either Enterprise or Unlimited Edition, contact your account
representative.

I trial a Salesforce Professional Edition. Some apps I can integrated but others can not. If I want to develop an App that can be used in all editions of Salesforces, what api should I use and how should I with it?

Anyone let me know this.