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Water potential is expressed in which units, and what value is assigned to pure water?
AMegapascals, with pure water set at zero
BMegapascals, with pure water set at unity
CMillimoles, with pure water set at zero
DMillimoles, with pure water set at unity
Answer & Solution
Correct answer: A. Megapascals, with pure water set at zero
1. Water potential is denoted by the Greek letter psi and is expressed in units of pressure, because pressure is a form of energy.
2. Those units are megapascals, written MPa.
3. The potential of pure water is designated a value of zero by convenience of definition.
4. Values for root, stem or leaf water are then quoted relative to that zero, which is why they come out negative.
5. Millimoles measure amount of substance rather than pressure, so they cannot express a potential energy per unit volume.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 30 "Plant Form and Physiology", section 30.5 Transport of Water and Solutes in Plants_
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