The xylem vessels are responsible for transporting water and nutrients from the roots to the rest of the plant. A reduction in the number of xylem vessels would therefore reduce the amount of water and nutrients that can be transported, which would in turn slow down the process of photosynthesis.

Other physiological adaptations that could also decrease the process of transportation in a plant include:

* A reduction in the size of the xylem vessels.

* A decrease in the permeability of the xylem vessels.

* An increase in the viscosity of the xylem sap.

All of these adaptations would make it more difficult for water and nutrients to be transported through the xylem vessels, which would slow down the process of photosynthesis.