Abstract
Summary The daily rhythm of water potential, osmotic potential, and transpiration of plants from different associations on a xeric limestone site was investigated. The courses of potential evapotranspiration, light intensity, relative humidity, and temperature were measured simultaneously. Water potential, as measured with a pressure bomb, was largely dependent upon transpiration. This relationship between the daily course of water potential and transpiration was especially apparent during the midday depression on clear, cloudless days. During this period the water potential rises despite the great evaporative demand. This behaviour can be explained by a reaction of stomatal closure under increasing evaporation conditions independent of the actual plant water status. Similar results have been obtained in laboratory investigations and on desert plants. During the course of a day the water potential is often considerably below the osmotic potential. The dependence of plants upon the water availability on their respective sites is also indicated by the widely differing water potential values found for the same species but growing in associations with different microclimates.
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