6.5.1: Leaf Water Potential

Turner (1990) and Stegman (1983) discussed the use of plant water status indicators for irrigation scheduling. Leaf water potential, a measure of the energy status of the water in a plant leaf, is an indicator of the water status of a plant. Figure 6.14 shows the relationship between the various potentials in the soil plant atmosphere continuum. Instruments used to measure leaf water include pressure chambers and thermocouple psychrometers. Stegman (1983) found that threshold levels of leaf water potential ranged from –12 to –12.5 bars for corn (midafternoon readings). The thresholds were dependent on ambient temperatures much like f_dc is dependent on ET. While leaf water potential is a direct measure of plant water status, using it as a scheduling tool has some limitations. Threshold levels must be developed for the plants in question. Also, like using threshold levels of soil water potential, it lacks in predictability. Third, measuring leaf water potential is time-consuming and must be done during a narrow time window during midday. A large number of samples are necessary for an accurate estimation of the mean.

Figure 6.14. Water potentials expected at different points of the pathway for water transport through a wheat plant growing in soil at a potential of -0.1 bars and in an atmosphere with a potential of -900 bars (adapted from Turner and Burch, 1983).

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