Document: MAT-3-32-6

Disequilibrium between plant and soil water potential at predawn: The contribution of nighttime transpiration.

LINTON, M.J.* and L.A.DONOVAN

University of Georgia, Athens, GA 30602 USA ¹

Abstract:
Measurements of plant predawn water potential are often used as an index to indicate the water-stress level of a plant and for comparing plant water-availability in different habitats. Such use of plant predawn water potential is based on the assumption that, during the night, the water potential of a plant equilibrates with that of the soil. Recent research in our laboratory has indicated that this equilibration does not occur for the two desert shrubs, Chrysothamnus nauseosus and Sarcobatus vermiculatus. This disequilibrium for plants growing in soil at field capacity ranges from 1.01.3 MPa for C. nauseosus and 1.51.8 MPa for S. vermiculatus. Our study of diurnal leaf gas exchange indicates that nighttime transpiration is partially responsible for this disequilibrium. When nighttime transpiration is artificially eliminated (via overnight bagging of plants), the predawn water potentials of C. nauseosus are 0.87 0.11 MPa (mean SD) , compared to 1.03 0.13 MPa for untreated plants. The corresponding values for S. vermiculatus are 0.87 0.21 MPa and 1.52 0.55 MPa. The amount of water loss due to nighttime transpiration constitutes a significant fraction of the total daily water loss, equaling 42% for C. nauseosus and 36% for S. vermiculatus. We continue to study the various conditions that contribute to predawn disequilibrium but, with regards to nighttime transpiration, our research suggests that it has adverse effects on plant water status and, possibly, ecological fitness.

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This abstract is being presented at: 9:45 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.