EVALUATING DROUGHT STRESS REACTIONS WITH VULNERABILITY CURVES

The focus of this study was to assess the susceptibility of asparagus hydraulics to cavitations caused by water deficit. Since hydraulic conductivity (c_h) is regarded as sensitive towards incipient drought, loss of ch and the appropriate xylem water potential (_xyl) were determined in shoot segments. The percentage loss of conductivity (PLC) was related to _xyl in order to evaluate drought resistance of ´Gijnlim`. The shape and the slope of vulnerability curves of the herbaceous monocot indicate vulnerable hydraulics towards embolism. Targeting the long-term objective of a plant-based irrigation control, the reversal point of the vulnerability curve (~PLC₅₀) is treated as critical value preventing detrimental water loss. For the purpose of prospective transfer of _xyl-thresholds to crop conditions, two methods for dehydrating shoots were tested in regard to near-natural stress conditions. In both methods, c_h was determined as described by Schaller and Paschold, 2008. The first method involved dehydrating adherent shoots by withholding water to potted plants. During the desiccation course, ψxyl and ch of single plant were measured at predawn. Different levels of soil matrix potential in pots assured gradual values of predawn _xyl (_PD). The second method utilised excised shoots of well-watered plants in pots or in open field. After excision, shoots were rehydrated until _xyl accounted for well-balanced water status. Shoots were subsequently dehydrated by air drying in greenhouse for different periods of time, leading to a wide _xyl range. The vulnerability curve, according to the “predawn method”, showed high variance in PLC at high _xyl (R²=0.26**) and is so disqualified for evaluating drought stress in asparagus. PLC obtained with the “air drying method” resulted in PLC of –0.36 MPa (R² = 0.59**) of potted plants and in PLC of –0.31 MPa (R² = 0.73***) in open field. This method seems to be valid in sensing plant stress in this monocot herbaceous species. Through this, a step forward is taken in developing thresholds-based irrigation control for asparagus.