Reduced seed set under water deficit is driven mainly by reduced flower numbers and not by changes in flower visitations and pollination

Water deficit can alter floral traits with cascading effects on flower-visitor interactions and plant fitness. Water stress induction can diminish productivity, directly resulting in lower flower production and consequently seed set. Changes in floral traits, such as floral scent or reward amount, may in turn alter pollinator visitations and behaviour and consequently can reduce pollination services resulting in lower reproduction output. However, the relative contribution of this indirect in comparison to the direct effects to changes in seed set are not fully understood. We manipulated water availability using rain-out shelters in a field experiment and measured effects on floral scent bouquet, morphology, phenology, flower-visitor interactions, pollination and seed set. Plant individuals of Sinapis arvensis (Brassicaceae) were randomly assigned to one of three treatments: mean precipitation (=control), reduced mean precipitation or drought period treatment. Our results show that decreasing water availability lowers the number of flowers and seed set. This indicates a direct link between water stress and seed set, as seed mass increases with increasing flower number. The indirect link of water stress via floral traits, pollinator visits and pollination has weaker effects on seed set. However, floral traits remain relatively stable under decreased water availability, whereas plant growth and flower abundance decrease, potentially in order to allow investment in more resources in fewer flowers to maintain pollination success. Thus, plants are able to compensate for water stress and can maintain floral trait expression, such as a stable scent emission and bouquet, to retain pollinator attraction. These findings indicate that the direct link from water stress to seed set has a stronger impact on plants reproductive success than the indirect link through altered floral trait expression and pollinator visits in a generalist plant species. Read the free Plain Language Summary for this article on the Journal blog.