The xylem transports water from roots to shoots in response to the water deficit created by transpiration—the water vapor emitted from the stomata of the needles into the air during photosynthesis. During the day, water pressure in the xylem should be low as the roots struggle to pull water from the drying soil to meet the vapor deficit created by transpiration. The xylem pressure reading at night should be higher because the stomata are shut and the taproots are still accessing the ground water, leaving the xylem saturated and not under any water stress.
