The dynamics of embolism formation and repair in xylem conduits: from bubble scale to loss in plant hydraulic transport capacity

Duke University, Durham NC

Investigators

Abstract

A clear understanding of water usage and resistance to mortality following drought is necessary across tree species. However, despite decades of research, describing plant water movement remains a formidable scientific challenge, and in the absence of this fundamental knowledge, sustainable management of forests will be impossible. All type of plants, including hardwoods and coniferous trees, have core bundles of vascular tissue inside their stems. These circulatory vascular bundles are composed of hollow conduits known as xylem, or wood, that are specialized to transport water efficiently while allowing maximum resistance to drought (hydraulic safety). Sufficient water transport in plants is not guaranteed when water demand increases during hot and dry days. Under those conditions, water in wood becomes highly unstable and trees can die once an air bubble enters and spreads inside those hollow cells. By integrating modelling activities with visual and physiological processes related to air entry and air bubble formation in wood, plant water transport and resistance to drought will be established. The primary impacts of the proposed project will be to advance discovery in the field of water relations and hydraulic architecture of vascular plants by predicting tree mortality following drought, while promoting training for the next generation of scientists and educators. Results from this research can be directly used in future predictions of vegetation responses to climate. The findings here will also be of interest to policy makers, plant breeders and land managers concerned with potential drought impacts on productivity and distributions of certain species. The overall objective of this project is to link the dynamics of embolism formation and removal under tension in xylem of woody plants starting from the bubble scale and integrating the resulting dynamics along individual conduits to arrive at whole-plant vulnerability to cavitation curves. The specific objectives are to 1) measure and model the timescale required for the gas phase induced by cavitation to fully embolize an entire xylem conduit; 2) investigate the conditions under which bubble formation can spread from one conduit to another; 3) investigate the possibility of bubble resorption under tension to occur; and 4) determine the relation between embolism formation and loss of hydraulic conductivity. To achieve the project objectives, a series of science questions will be addressed: Research question 1 (bubble scale): what are the mechanisms and dynamics of embolism formation and removal in a single xylem conduit? What is the time required for a gas bubble to completely fill and to drain a conduit for varying conduit structures and xylem tensions? Research question 2 (conduit connectivity): Is the time needed for the embolism to fill a conduit dependent on conduit size, and the rigidity of the end-wall pit membrane? Research question 3 (Up-scaling to whole-plant): Do the dynamics of bubble formation in a single conduit explain the whole organ vulnerability to embolism curves. The formulation of bubble formation and spread in transpiring plants will represent a major step forward as the theoretical computational methods, and experimental techniques are now ripe for making this progress. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.