Answer
With fewer aquaporins, absorbed water from the roots cannot move quickly into the shoot. It needs a place to be stored while waiting to be transported, and this place would be the expanded mass of roots. Also, the extra water could drive extra growth of root cells, leading to the increased root mass.
Work Step by Step
Review the role of aquaporins in the movement of water as well as the role of water uptake in plant growth (Chapter 35).