Willow species (genus: Salix) with contrasting habitat affinities differ in their photoprotective responses to water stress

Although many Mediterranean and xeric plant species enhance their xanthophyll-mediated thermal dissipation under drought conditions, there has been limited research on photoprotective mechanism in droughted plants from other habitats. To investigate whether wetland plants utilise this mechanism under drought conditions, and whether species differ in their responses depending on their habitat affinities, we investigated the response of six willow (Salix) species to a short-term drought. In a greenhouse, 40 individuals per species were dried down over 4 weeks. Periodically during the drought, predawn and midday chlorophyll fluorescence measurements were taken and leaf discs were collected for pigment analysis with HPLC. Predawn water potential was also monitored throughout the experiment. All six species increased xanthophyll cycle activity and their capacity to dissipate excess energy during the drought by increasing their total de-epoxidised xanthophyll concentration and the concentration of zeaxanthin in proportion to chlorophyll. In general, habitat generalists had greater photoprotective responses than wetland specialists, while the wetland specialists had higher pre-drought nonphotochemical quenching. These differences are consistent with their contrasting photosynthetic rates. The observed variation in species drought responses suggests that their photoprotective strategies vary with habitat affinity.