Plant roots generally have water potential values of ca. Plant leaves often have midday water potential values of ca. The lower atmosphere on a summer day often has a midday water potential value of ca. In the example above, there is a gradation of water potential values, with the least negative in the soil, and the most negative in the atmosphere. It is this difference in water potential values that provides the drawing force for moving water from the soil, into plants, and out of leaves into the atmosphere.
Some desert plants can tolerate much more negative leaf water potential values.
For example, creosote bush , a common desert shrub in the southwest U.S., has fully turgid leaves and detectable rates of photosynthesis at leaf water potential values of -100 bars. For some plants that are very intolerant of water deficits, a leaf water potential of ca. Typically, a growing plant evaporates ca. 600gm of water for each gram of biomass produced. This ratio is also very important in determining the very strong geographic relationship between precipitation and plant production.