What is the maximum potential sucrose concentration of plant sap? What keeps plants below this potential?
As the concentration of sugars in the sap increases, so does viscosity. At high viscosity, the sap will not flow at a sufficient rate to provide nourishment to the different plant tissues.

In addition, there are limitations to concentrating the solutes in the sap. As the sap becomes more concentrated, the plant would be working against a osmotic gradient to further increase that concentration. That energy cost may be too great.