Question

What is positive hydrostatic pressure gradient and a negative hydrostatic pressure gradient

Answer 1

Positive hydrostatic pressure gradient can be understood with the help of an example;in plants the leaves have large amount of stored foods in the form of sugar or the source has high concentration.As a result,water moves from the neighbouring cell and sink into the source cell(leaves).This moves out of the source  along with sugars.
Negative hydrostatic pressure gradient can be understood by the help of plant roots,where roots absorb water and minerals from soil.This means that soil has high solute concentration still the water enters into the root hairs along with minerals.