Set instanceName to selected label of select widget

Hi,
I have build a form for collecting species in nature conservation with odk Build and downloaded the XML File.
Now I want to change the instanceName to the name of a selected species.
this works, but only with the selected value, not with the selected label by adding the following line:

<bind nodeset="/data/meta/instanceName"  calculate="concat('XY ',/data/art)" type="string" />

Here is my select control:

<select1 appearance="autocomplete"  ref="/data/art">
  <label ref="jr:itext('/data/art:label')"/>
  <item>
    <label ref="jr:itext('/data/art:option0')"/>
    <value>6910_Acheta_domesticus</value>
  </item>
  <item>
    <label ref="jr:itext('/data/art:option1')"/>
    <value>171862_Aiolopus_thalassinus</value>
  </item>
   ...
</select1>

What is the correct syntax to get the label, not the value?
I have tested this and other variants, always without success

<bind nodeset="/data/meta/instanceName"  calculate="concat('XY ',/data/art:label)"  type="string"/>

Thanks a lot for any help!
Sven