Leaves of Rumex obtusifolius L. and R. patientia L.were exposed to combinations of mild water stress and enhanced ultraviolet-B irradiation during their ontogeny. Two UV-B treatments (enhanced UV-B and control) and three water stress treatments (-0.0 MPa, -0.2 MPa and -0.4 MPa rooting medium matric potentials) were employed. The impact of the stress interaction was assessed on the basis of changes in leaf area, average adaxial epidermal cell size, and total number of adaxial epidermal cells per leaf. Although the level of UV-B irradiation applied was insufficient to significantly alter leaf growth at any given water stress, UV-B did interact with water stress to alter the pattern o= plant response to water stress. The interaction was only apparent when the water stress was greater than -0.2 MPa root matric potential. For both species UV-B irradiation exacerbated the depression of leaf growth due to -0.4 MPa water stress. For R. obtusifolius the basis of the reduction in leaf growth was likely a reduction in the rate of cell division during the early phase of leaf growth. For R. patientia the effect of the interaction on cell division was less clear. Cell expansion was not directly affected by UV-B irradiation in either species, although the reduction in cell size with increasing water stress was apparent. In terrestrial ecosystems, mild water stress is a common occurrence and with predicted anthropogenic modifications of the atmospheric ozone layer, UV-B radiation reaching the earth's surface can be expected to increase. The effect or. higher plants of the stress interaction may thus be of considerable significance under natural conditions.

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