Question

The movement of sap within the xylem can be increased by

I. Movement of water into guard cell vacuoles.

II. Increase in humidity in the surrounding air.
III. Prevention of ions from entering the xylem by root endodermis.

• A. I only
• B. II only
• C. I and II
• D. II and III
• E. I, II, and III

Answer 1

The correct option is A I only

When soil moisture is high, movement of water and dissolved minerals from soil into root hair and finally to the root xylem cells build up root pressure which in turn pushes the water up the xylem vessels. This makes III incorrect as prevention of ion entry into xylem would inhibit the formation of root pressure. The increased soil moisture and the high root pressure causes increased rate of water absorption which in turn forces water droplets to come out from hydathodes and stomata. Lenticular and stomatal transpiration creates transpiration pull which in turn carry the water and minerals up in stem thereby decreasing root pressure. This makes I correct but II incorrect as increased air humidity would decrease the transpiration which in turn would hinder the transpiration pull of water up within xylem.

Therefore, the correct answer is option A.