Water potential is the total potential energy of water, measured in megapascals (MPa). It's the sum of osmotic potential (negative, from solutes, e.g., -0.5 MPa in a leaf) and pressure potential (positive, from cell wall pressure, e.g., +0.3 MPa). So, that leaf's water potential would be -0.2 MPa. In our research plots, I see this when drought stress makes osmotic potential more negative—but the real pressure comes from watching my mother's vegetable patch wilt, knowing her yield and our next meal depend on these invisible numbers.
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