How can the absence of starch in the elaborated sap be explained knowing that it is produced by photosynthesis ?

Answer 1

Your question is most misleading. Starch is not produced by photosynthesis, photosynethesis produces sugars, and when those sugars are in excess, cellular enzymes convert the sugars, mainly glucose, into starch.

Elaborated sap is sugars that were in transport in the phloyem, and excess pressure caused a rupture to form sap flow.