The Effect of Decreasing Water Potential on Net CO2 Exchange of Intact Desert Shrubs

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Abstract
This study compares the effects of seasonal (in situ) and experimentally—induced drought on three desert shrubs of different growth types. The effect of decreasing water potential on CO2 uptake was used as the response indicating differences in desert adaptation. Larrea divaricata Cav., Encelia farinosa Gray, and Chilopsis linearis Cav. were selected as representing evergreen, drought—deciduous, and winter—deciduous species respectively. Field measurements of water potential and photosynthesis were made simultaneously on plants of all three species. Data were collected in the field at monthly or bimonthly intervals for a year. Additional experiments were conducted in a transplant garden and under controlled environments in a phytotron. Larrea has a high protoplasmic tolerance to drought stress and maintains net photosynthesis throughout extended periods of low water potential. The photosynthetically active period of Encelia is prolonged, and water loss reduced, by seasonal leaf variability and the drought deciduous habit. These phenomena permit a greater photosynthetic efficiency over a wider range of water potentials than would be possible with a single leaf type. Decreasing water potentials affect Chilopsis the most, but the presence of xeromorphic leaf tissue augments the phreatophytic habit in maintaining relatively high water potentials. A transpiration—retarding mechanism in Chilopsis is also hypothesized.

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