Stomatal conductance and net photosynthesis measurements were made over a period of a week in mid-October on two stands of water hyacinths floating in sunken metal stock tanks at Phoenix, AZ. On the second day of the experiment, all free water in one of the tanks was removed. Foliage temperature measurements were subsequently used to quantify the water stress experienced by the water-robbed plants; and a plant water stress index derived from the foliage temperature and air vapor pressure deficit data was used to study the effects of developing water stress on the plant physiological parameters being measured. The results obtained were practically identical to those derived from two independent season-long studies of water stress effects in cotton: net photosynthesis decreased linearly to become negative at a plant water stress index of 0.9 (where 1.0 represents the maximum possible stress), while a parameter related to plant water use efficiency first increased with increasing stress to reach a maximum at a plant water stress index of 0.6, after which it dropped off rapidly to zero with additional stress. © 1984.
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