Accurately measuring cavitation resistance to understand how plants cope with drought

Guest post by Danielle Marias, Oregon State University.

Plant water transport systems from roots to stems to leaves are under negative pressure due to tension on the water column. This is caused by water loss through stomata – small pores on leaves – and is driven by how dry the atmosphere is, as described by the cohesion-tension theory. This tension or negative pressure puts plants at risk for cavitation. Cavitation is the conversion of water from liquid to vapor and can result in a gas-filled (embolized) vessel or tracheid that no longer transports water. Therefore, cavitation resistance is crucial to coping with and surviving drought.

Methods to assess cavitation resistance have been highly debated. It has been suggested that the standard centrifuge method, the most common and efficient method for measuring cavitation resistance, may have methodological artifacts and is not appropriate for roots. To investigate this, Pratt et al. (2015) compared the standard centrifuge method to two other independent types of measurements of cavitation in roots. This compelling study suggested that the standard centrifuge method accurately measures cavitation resistance and is appropriate for measuring cavitation resistance in roots. Because roots are generally more vulnerable to cavitation and embolism than stems and leaves, studies accurately measuring root cavitation are vital to understanding plant responses to drought as the severity and frequency of drought may increase with changing climate. Drought resistance and related topics in tree hydraulic functioning will also be covered in the forthcoming Special Issue in Tree Physiology.

Pratt, R.B., MacKinnon, E.D., Venturas, M.D., Crous, C.J., & Jacobsen, A.L. (2015) Root resistance to cavitation is accurately measured using a centrifuge technique. Tree Physiology, 24 February 2015 doi: 10.1093/treephys/tpv003
Abstract
Plants transport water under negative pressure and this makes their xylem vulnerable to cavitation. Among plant organs, root xylem is often highly vulnerable to cavitation due to water stress. The use of centrifuge methods to study organs, such as roots, that have long vessels are hypothesized to produce erroneous estimates of cavitation resistance due to the presence of open vessels through measured samples. The assumption that roots have long vessels may be premature since data for root vessel length are sparse; moreover, recent studies have not supported the existence of a long-vessel artifact for stems when a standard centrifuge technique was used. We examined resistance to cavitation estimated using a standard centrifuge technique and compared these values with native embolism measurements for roots of seven woody species grown in a common garden. For one species we also measured vulnerability using single-vessel air injection. We found excellent agreement between root native embolism and the levels of embolism measured using a centrifuge technique, and with air-seeding estimates from single-vessel injection. Estimates of cavitation resistance measured from centrifuge curves were biologically meaningful and were correlated with field minimum water potentials, vessel diameter (VD), maximum xylem-specific conductivity (Ksmax) and vessel length. Roots did not have unusually long vessels compared with stems; moreover, root vessel length was not correlated to VD or to the vessel length of stems. These results suggest that root cavitation resistance can be accurately and efficiently measured using a standard centrifuge method and that roots are highly vulnerable to cavitation. The role of root cavitation resistance in determining drought tolerance of woody species deserves further study, particularly in the context of climate change