Water channel required to facilitate the transport of water across cell membrane. Its function is impaired by Hg(2+). May play a role in water uptake from the root surface. Active as homomers. Increased activity when heteromerization with PIP1-2. Homomers. May interact with PIP1-2 to form heteromers. Specifically expressed in roots, in the exodermis, endodermis and xylem parenchyma. Polar localization to the external periclinal side of epidermal cells in root apices. Expressed in roots with a circadian rhythm showing an increase at the end of the night period, a peak during the first part of the light period and then a decrease. Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Belongs to the MIP/aquaporin (TC 1.A.8) family. PIP (TC 1.A.8.11) subfamily.