November 2 2010

12:00 2320 LSB

Ecolunch: Brendan Choat
Research School of Biology, The Australian National University

Embolism resistance and refilling in plants: new insights using novel imaging techniques and data synthesis


Plants are capable of rapidly transporting water to heights in excess of 100 m, and of extracting water from extremely dry or saline substrates. To achieve this, plants have evolved a transport system that relies on water sustaining a tensile force, such that the xylem sap is at negative absolute pressures. However, this transport mechanism comes with its own set of problems, most notably that water under tension is prone to cavitation, which results in the formation of a gas bubble (embolism). Embolism reduces the capacity of the xylem tissue to deliver water to sites of gas exchange and can therefore impact the ability of the plant to maintain a net positive carbon balance. In the extreme, xylem embolism can reach lethal levels causing branch die back and ultimately plant death. Resistance to embolism is now recognised as a key trait determining the extent of woody plant mortality during drought and the limits of plant distribution with regards to water availability. Hydraulic constraints on plant growth and survival also clearly play a role in declining productivity in natural and agricultural systems during prolonged and severe droughts.
Plants can avoid extensive build up of embolism in two ways. First, by refilling conduits after they have cavitated, and second, by achieving hydraulic design that reduces the risk of embolism spreading throughout the xylem as water stress increases. The basic principles of water transport in plants are regarded as well known and there is a general consensus that the cohesion-tension mechanism is the driving force of water movement through the xylem. However, discoveries in the last two decades demonstrate that there are still fundamental gaps in our knowledge of long distance water transport in plants. Here I will address two outstanding questions to how plants transport water, (a) the ability to plants to refill embolised conduits during active transpiration and, (b) the factors that control the relative resistance to drought induced embolism in plants. Recent insights into embolism formation and refilling gained by x-ray microtomography and magnetic resonance imaging will be presented in the context of structure-function relationships. In addition, the ecological importance of xylem functional traits will be examined using a recently assembled dataset that spans a range of biomes and phylogenetic affinities.















































































































































































































































































































































































































































































































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