Girdling a tree will cut thru the phloem and cambium, killing the tree. They actively load minerals into their root cells and then water comes across the semipermeable membrane via osmosis. The sun literally sucks water out of tree. In phloem transport, sugars are loaded into the sieve tubes. In general, a growth stimulator by increasing cell elongation.
Auxin migrates to the dark side of a stem and the stem elongates more on that side.