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Drought stress, plant water status, and floral trait expression in fireweed, Epilobium angustifolium (Onagraceae)¹

²Division of Biological Sciences, 105 Tucker Hall, University of Missouri, Columbia, Missouri 65211 USA; and ³Department of Forestry, 203 ABNR Bldg, University of Missouri, Columbia, Missouri 65211 USA

In a controlled environment, we artificially induced drought during flowering of Epilobium angustifolium, an animal-pollinated plant. Leaf water potential (_l) and floral traits were monitored over a 12-d period of soil moisture depletion. Soil moisture depletion induced drought stress over time, as revealed by significant treatment x day interactions for predawn and midday _l. Nectar volume and flower size showed significant negative responses to drought stress, but nectar sugar concentration did not vary between treatments. Floral traits were more buffered from drought than leaf water potentials. We used path analysis to examine direct and indirect effects of _l on floral traits for plants in well-watered (control) vs. drought treatments. According to the best-fit path models, midday _l has significant positive effects on flower size and nectar volume in both environments. However, for controls midday _l also had a significant negative effect on nectar sugar concentration. Results indicate that traits influencing floral attractiveness to pollinators in E. angustifolium vary with plant water status, such that pollinator-mediated selection could indirectly target physiological or biochemical controls on _l. Moreover, under mesic conditions selection for greater nectar sugar reward may be constrained by the antagonistic effects of plant water status on nectar volume and sugar concentration.

Key Words: drought stress • Epilobium angustifolium • flower size • leaf water potential • nectar production • nectar sugar concentration • Onagraceae • phenotypic plasticity • pollinator-mediated selection

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