Positive root pressure and xylem repair

Climate change is predicted to strongly influence rainfall patterns across the globe, with potentially disastrous consequences for crops and human food production. Recent work has greatly improved our understanding of plant function during drought, but has focused mostly on woody plants. There is therefore an urgent need to study herbaceous plants that comprise most of the world’s crops. In particular, it has been hypothesized that positive root pressure could play a crucial role in the survival of herbaceous species during drought, by removing gas bubbles (‘embolisms’) formed in the water conducting vessels due to severe drought stress, a process called ‘xylem repair’. While generation of positive root pressure has been verified in some species (including in the tomato plant, Solanum lycopersicum), its potential role xylem repair and in providing drought-tolerance is controversial in many cases and untested in herbs.

Different varieties of tomato plants will be grown under well-watered conditions. A subset of plants will be subjected to increasing levels of drought, and drought stress will be monitored daily by measuring soil water-content, leaf water-potential and gas-exchange parameters. After rewatering the droughted plants, we will monitor stem diameter variation and sap-flow, to estimate root pressure (following De Swaef et al. 2013). To visualize xylem refilling, these plants will be brought to the X ray scanning facility in Ghent University (Belgium), where they will be repeatedly scanned after rewatering to observe a direct link between positive root pressure and refilling. Vulnerability curves will assess differences in embolism resistance, by scanning plants at different levels of drought stress and measuring the area of embolized vessels relative to the area of total initially functional xylem.