Ammonium ions, $\text{NH}_4^+$, may reach the xylem in plant roots by two routes: along the apoplast pathway, ammonium ions pass through cell walls until the Casparian strip blocks this route, after which they enter the cytoplasm of root cells; along the symplast pathway, ammonium ions travel through the cytoplasm of root cells. A scientist investigated the uptake of ammonium ions into the xylem of Arabidopsis thaliana. The sgn3 mutant of A. thaliana lacks a Casparian strip. In the sgn3 mutant, ammonium ions are able to enter the xylem without first entering the cytoplasm of root cells. The bar charts display the scientist’s findings. Which conclusion is correct?
- AFewer ammonium ions enter the xylem when they have to move through the cytoplasm of root cells.
- BMore ammonium ions enter the xylem at low soil concentrations of ammonium ions.
- CThe Casparian strip acts as a barrier to reduce the movement of ammonium ions into the xylem.
- DThe loss of the Casparian strip has little effect on the movement of ammonium ions into the xylem.