In pressure flow theory, when water moves out of sieve tube cells by osmosis, it lowers the _.

Explanation

In the pressure flow theory of phloem transport, the movement of sugars from source (e.g. leaves) to sink (e.g. roots) requires a pressure gradient. This gradient is established when sugars are actively transported into the sieve tube cells at the source, causing an increase in hydrostatic pressure. Water then moves into the sieve tube cells by osmosis, increasing hydrostatic pressure. This high pressure at the source end of the phloem pushes the sugars and water solution towards the sink. At the sink, sugars are removed from the phloem, causing hydrostatic pressure to decrease, allowing water to move out of the sieve tube cells and back into the xylem.