ANALYSIS OF ACOUSTIC SIGNALS FROM DROUGHT STRESSED PLANTS

Especially in the Northern hemisphere, global change is expected to increase the frequency of heat waves and drought periods. As a consequence, reduced water availability will affect plant productivity and growth in many areas. Knowledge about the hydraulic performance of long-living plants (woody species) under drought stress is therefore of utmost importance. This knowledge can be applied to prognosticate the survival prospects of woody species or to screen for less drought sensitive clones, cultivars or provenances. Water is transported from the roots to the crown in a metastable state (i.e. under tension) caused by transpiration from the leaves. Due to this metastable state the water column inside the tracheids is likely to break when the tension forces increase above a certain level. Cavitation, the breakage of the water column, induces a reduction in the hydraulic conductivity of the plant which impairs water supply of the transpiring leaves. Cavitation would result in a total loss of hydraulic conductance in the absence of mechanisms regulating transpiration. Plants therefore respond by closing their stomata, which should reduce transpiration and prevent cavitation. The behavior of plants under drought stress can be simulated by vulnerability curves (VC), where the percent loss in hydraulic conductivity (PLC) of the xylem is related to the pressure potential in the solution inside the conducting elements. The hydraulic method to assess PLC is however rather labor intensive. Cavitation can be also assessed directly by acoustic emission (AE) testing. Acoustic emission in the high frequency range (> 15 kHz) is induced by a sudden tension release when liquid water is replaced by water vapor. AE counting per se is a useful method to detect cavitation but gives rather poor information on the PLC, because AE also results from cavitation in non-conducting wood elements and bark. Recently, we developed a method for standard-size Norway spruce wood specimens to quantify the PLC by analysis of AE energy, a waveform feature derived from amplitude and duration. We see a high potential in this method for constructing VCs in other woody species and for drought stress monitoring of whole plants in the field. The proposed project will focus on the reliability of AE waveform feature extraction for constructing VCs and on online monitoring of drought stress in different woody species. Analysis of AE waveform features, such as the peak amplitude and the AE energy, should allow to discriminate between AE produced by cavitation events of conducting xylem elements and those from non-conducting xylem elements and bark. If so, analysis methods could be developed which permit the pinpointing of periods of severe drought stress online. We further expect a high information potential of AE waveform features in hydraulic signaling processes for stomatal closure. The research will be done on a selected set of economically important central European conifer, diffuse-porous and ring-porous woody species, such as Norway spruce, Scots pine, poplar species, beech, ash and grapevine. The testing method developed will help to select forest trees or cultivars with high hydraulic safety, which is of great importance in a changing environment.