Cavitation: a blessing in disguise? New method to establish vulnerability curves and assess hydraulic capacitance of woody tissues.

The hydraulic performance of woody species during drought is currently of high interest in the context of climate change. It is known that woody species have the capacity to mitigate water shortage by using internally stored water. Elastic shrinkage of living cells and also water release during cavitation contribute to the so-called 'hydraulic capacitance' (C) of the plant, which adds water to the transpiration stream and buffers fluctuations in water potential. Although sap-conducting conduits may ultimately serve as a water pool, cavitation will hamper the conduction of sap. Both hydraulic conductivity and C are thus inextricably linked and the interaction between both should be studied to better understand hydraulic functioning of woody species during drought. However, measurements of C are scarce and no distinction is usually made between C from elastic storage and C supplied by cavitation. In this paper, we propose a new method to assess both the decrease in hydraulic conductivity and the change in C during bench dehydration of a whole-branch segment using continuous measurements of acoustic emissions, radial diameter shrinkage and gravimetrical water loss. With this method we could establish proper vulnerability curves for grapevine (Vitis vinifera L. 'Johanniter') and quantify C during dehydration. Our results showed that loss in hydraulic conductivity during the cavitation phase was accompanied by 22-92% gain in hydraulic capacitance; therefore, a certain degree of cavitation may be tolerated in grapevine during periods of drought stress.