Relationships Between Vulnerability To Xylem Embolism And Soil Moisture In Poplar Cuttings

The one-year-old cuttings of Populus euramericana were tested. Water control was addressed, the poplar can be divided into three groups, well-watered plants(75~100% of field capacity), moderately water-stressed plants(45~70% of field capacity), severely water-stressed plants(25~40% of field capacity). We used Cochard cavitron centrifuge to determine the embolism vulnerability(P50). Vessel diameter was measured by staining techniques. The hydraulic conductivity and the loss of hydraulic conductivity under natural conditions were measured by “Low Pressure Flow Meter”. Pressure chamber was used to calculate the leaf water potential, portable photosynthesis system was used to determine the diurnal variation of photosynthesis. The determinations reveal the differences among embolism vulnerabilities of three groups. With studying the changes of xylem structure characteristic, hydraulic characteristic and physiological characteristic caused by different water stress, we can illustrate the resistance and adaptability of the xylem to water stress. The scientific bases were provided for us to understand the physiological and ecological characteristics of drought-enduring plants, which were propitious to establish the index systems of drought enduring.The main research conclusions are as follows:1. The order of P50 was well-watered plants(-1.36 MPa)> moderately water-stressed plants(-1.68 MPa)>severely water-stressed plants(-1.89 MPa). The conclusion indicated severely water-stressed plants were more difficult to embolize than well-watered plants and moderately water-stressed plants. Namely, severely water-stressed plants had a higher drought resistance.2. The order of vessel diameter was well-watered plants(35.36 um)> moderately water-stressed plants(34.06 um)>severely water-stressed plants(33.47 um). Positive correlation was occurred between vessel diameter and P50(R2 = 0.6562), with the increase of vessel diameter, the embolism vulnerability also increased and embolism phenomenon was easier to happen.3. The order of hydraulic conductivity was well-watered plants(5.31E-06 g·m·MPa-1·s-1)> moderately water-stressed plants(3.54E-06 g·m·MPa-1·s-1)>severely water-stressed plants(2.62E-06 g·m·MPa-1·s-1). Negative correlation was occurred between embolism level and P50(R2 = 0.5522). Plants with a long-term water stress had a higher embolism but a smaller vulnerability. The embolism may prevent excessive transpiration and maintain water balance.4. Leaf water potentials of severely water-stressed plants declined more slowly, that is to say, severely water-stressed plants had a higher drought resistance than the well-watered plants and moderately water-stressed plants.5. With the increase of water-stress, the photosynthetic rate was significantly decreased. Positive correlation was happened between net photosynthetic rate and P50(R2 = 0.9954). Severely water-stressed plants had a less decrease in photosynthetic rate, namely, had a better adaptability to drought.