Question

A) Why do most ions enter the root by active absorption?

B) How do plants regulate the type and quantity of solutes that enter the xylem?

Answer 1

a) The concentration of mineral ions in the plant sap inside the plant is much higher than that in the soil. Hence these ions cannot move in the plant by simple diffusion. They have to taken up by the plant against the concentration gradient and hence have to go via active transport.
b) The type and quantity of solutes is regulated by the pressure that is generated in its cells owing to water and ions lost in the process of transpiration.