The solute or osmotic potential is always negative but the p

The solute (or osmotic) potential is always negative, but the pressure potential can be either negative or positive. Give an example of a situation in a plant where water potential is negative and one where it is positive.

Solution

Water potential is helpful to transport the water from roots to leaves for photosynthesis. The water always flows passively due to differences of water potential. Water always move from high potential (_w) to lower potential and it always can be measured as pressure, the pressure is in the range from 0.1 Mpa (mega pascals) to 0.2Mpa.

Water potential can be broken down into 4 components

_w = _s + _p + _m + _g

_s = solute potential

_{p =} pressure potential

_m = matrix potential

_g = gravity potential

Solute potential (_s) is also called osmotic potential and the solute potential is always negative due to the solutes in water, ex: sugar solution. Thus decreases the water potential(_w) in leaf relative to the root.

Pressure potential is also called the turgor pressure or turgor potential, it may be positive or negative. As water is transpired from leaves it creates a negative pressure like a sucking of water with straw

Positive pressure however, as with guttation. When there are so many solutes in a root that water accumulates and pushes up. Ions continue to be pumped into the xylem even though transpiration has ceased at night. The increased concentration results in a lower osmotic potential in the xylem, and a gradient is developed across the root. Water moves into the xylem in response.