what is positive hydrostatic pressure gradient and a negative hydrostatic pressure gradient
Hydrostatic pressure is the pressure exerted by liquid at rest. Hydrostatic pressure depends on height of column, density of the liquid and gravity.
Hydrostatic pressure increases with depth because of the increasing weight of fluid exerting downward force from above. This change in hydrostatic pressure with depth is known as hydrostatic pressure gradient.
The positive hydrostatic pressure gradient can be understood with the help of an example- In plants, the leaves have large amount of stored food in the form of sugar or we can say the source has high solute concentration. As a result , water moves from neighbouring cells (sink) into the source cell (leaves). This creates a pressure gradient and the water moves out of the source along with sugars thereby transporting sugars to rest of the plants.
Similarly, the negative pressure gradient can be understood with the help of an example of plant roots. As we know roots absorbs water and minerals from the soil. This means that soil has high solute concentration still the water enters into the root hairs alongwith minerals. This type of transportation is due to negative hydrostatic pressure gradient.