<Ul> <Li> Pressure flow hypothesis : Sugars produced in the leaves and other green tissues are kept in the phloem system , creating a solute pressure differential versus the xylem system carrying a far lower load of solutes - water and minerals . The phloem pressure can rise to several MPa , far higher than atmospheric pressure . Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure . </Li> <Li> Transpirational pull : Similarly , the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant . This causes millions of minute menisci to form in the mesophyll cell wall . The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil . </Li> <Li> Root pressure : If the water potential of the root cells is more negative than that of the soil , usually due to high concentrations of solute , water can move by osmosis into the root from the soil . This causes a positive pressure that forces sap up the xylem towards the leaves . In some circumstances , the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation . Root pressure is highest in the morning before the stomata open and allow transpiration to begin . Different plant species can have different root pressures even in a similar environment ; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus . </Li> </Ul>
<Ul> <Li> Pressure flow hypothesis : Sugars produced in the leaves and other green tissues are kept in the phloem system , creating a solute pressure differential versus the xylem system carrying a far lower load of solutes - water and minerals . The phloem pressure can rise to several MPa , far higher than atmospheric pressure . Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure . </Li> <Li> Transpirational pull : Similarly , the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant . This causes millions of minute menisci to form in the mesophyll cell wall . The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil . </Li> <Li> Root pressure : If the water potential of the root cells is more negative than that of the soil , usually due to high concentrations of solute , water can move by osmosis into the root from the soil . This causes a positive pressure that forces sap up the xylem towards the leaves . In some circumstances , the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation . Root pressure is highest in the morning before the stomata open and allow transpiration to begin . Different plant species can have different root pressures even in a similar environment ; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus . </Li> </Ul>
water rises in the xylem tubes due to
water rises in the xylem tubes due to
Pressure flow Transpirational pull Root pressure
Pressure flow Transpirational pull Root pressure