In situ measurement of plant hydraulic conductance

Measuring sap flow together with stem water potential and stem water storage can provide an abundance of information about the functioning and physiology of the water-conducting xylem, including its hydraulic conductance and capacitance. As xylem grows, develops, and ages, it changes its capacity to conduct water, as new vessels and tracheids are formed and older ones become dysfunctional when filled with gas, gels, or tyloses. Environmental factors, such as drought, heat waves, irrigation, or flooding can also strongly affect hydraulic conductance and water storage in stems. Some plants even can influence hydraulic conductance by changing the ionic composition of their xylem sap. Destructive measurements using cut stems are the current standard method for measuring hydraulic conductance, but this method is prone to artifacts that could introduce significant differences to actual conductance in intact plants. Continuous, non-destructive measurements of hydraulic conductance and capacitance require simultaneous recording of sap flux density, the water potential gradient driving the flow, and accounting for radial flow into and out of stem water storage. Here, we describe a simple method to measure apparent hydraulic stem conductance in situ and account for storage flows to arrive at an estimate of actual hydraulic conductance. The method is illustrated with examples from two experiments conducted in southern California, including chaparral shrubs grown in large lysimeters and mature Fuerte avocado trees. Water potential gradients in these experiments were established by measuring soil water potentials and stem water potentials via stem psychrometry or by using a basal and distal stem psychrometer. Sap flux density was measured using heat ratio sap flow meters, and stem water storage in avocado was determined using stem dendrometers. Data from these experiments are used to show responses of hydraulic conductance in response to drying and for constructing a xylem vulnerability curve (change in conductance as a function of changing stem water potential) in situ. The method provides a promising approach for measuring important parameters of water transport in intact plants under natural conditions.