Opens are only possible with HTML email, it's not possible to record opens for text-only emails. A recipient who clicked a link in a text-only email message will be tracked as a clicker however they will not be tracked as an opener.
Open are tracked by a small, transparent image placed in an HTML email. Once the image is rendered an open is recorded. As tracking opens relies on rendering of this image it's not possible to track opens for text-only or text versions of an email message. Additionally image blocking in the email client will prevent an open from being recorded.