| The vulnerability curves of xylems(VCs)are often used to describe the correlation between xylem water potential and embolism degree of xylem,it can reveal the variation of xylem hydraulic conductivity with xylem pressure constantly increasing and quantify the sensibility of xylem to embolism "events".This will help us better understand the relationship between xylem water potential and xylem embolism,describe and compare the drought resistant ability of trees,and predict the physiological and biochemical reactions of plants to future climate change.Centrifuge method,prevalent in researches on water physiology of plant,can efficiently construct the VCs and measure the embolism degree and embolism-resistant ability of xylem.Centrifuge method is based on the relationship between tension and embolism degree of xylem under high speed rotating sample materials.Nevertheless,the particular physics-properties of flow in plants make it possible that the use of centrifuge method to build VCs is susceptible to human factors then result the VCs artifact.So,there is a lot of controversy for the accuracy of VCs which based on centrifuge method.To further investigate whether Sperry rotor can be used to accurately construct VCs of species with long-vessels and explore the possibility that xylem water extraction curves of Cochard rotor is used to constructing VCs of species with long-vessels.In this research,the species of Robina pseudoacacia L.,which with long vessels was studied,we have designed different experiment treatments to investigate the reliability and accuracy of two different centrifuge methods(Cochard rotor and Sperry rotor)for constructing the VCs of long-vessels species,and improved xylem water-extraction method was used to construct the xylem extraction curves of Robina pseudoacacia L.and compare it with natural dehydration method to verify the reliability of xylem water-extraction method.In addition,Meanwhile,the effect of dipping stem samples into ultra-pure water overnight on the measure of xylem hydraulic conductivity of current-year stems of Robina pseudoacacia L.,was also investigated which was helpful to improve the method of constructing VCs of long-vessels species.The primary results were as follows:1.When the stem samples of Robina pseudoacacia L.were continuous spun in Sperry rotor under 0.25MPa,no matter the cuvette water level at both ends of the stem is same or different,the hydraulic conductivity of stem were decrease with prolong of spin time by using both big rotor and small rotor.There was an negative linear correlation between the spun time and the hydraulic conductivity of stems xylem(R2 more than 0.99)in the same water level cuvettes experiments,and the decline degree of xylem hydraulic conductivity under different water level was greater than that of stems spun under same water level,indicating that additional embolism would be always brought into stem when Sperry rotor was used to construct VCs,no matter the cuvette water level at both ends of the stem is same or different,and different cuvette water level will result in much more additional embolism than same cuvette water level.2.The vulnerability curves of current-year stems of Robina pseudoacacia L.displayed a analogous "s" shape trend,established by one stem one tension method at same water level,the P50 value of VCs of big rotor stems was-2.333 MPa,and that of small rotor was-1.959 MPa.While,the vulnerability curves of current-year stems of Robina pseudoacacia L.displayed a typical "r" shape when stems were constantly spun under tensions at same water level and different water level in both big and small rotors,the P50 value of vulnerability curves of big rotor stems at same water level was-1.063 MPa,at different water level was-0.807 MPa,and that of small rotor stems at same water level was-1.0931 MPa,at different water level was-0.9869 MPa.The results showed whether the stems were spun constantly under different tensions had an impact on the establishment of vulnerability curves,and the length of stems had an effect on the establishment of vulnerability curves(the difference of P50 values)constructed by one stem one tensions method,however,the length of stems had little influence on the establishment of vulnerability curves when stems were constantly spun under increasing tensions.3.The vulnerability curve Robina pseudoacacia L.displayed a typical "r" shape when that was constructed by a Cochard rotor simulating Sperry rotor,i,e.on stem one tension,and the P50 value of it was-0.947 MPa,which were significant different from "s" shape vulnerability curves based on Sperry rotor.4.After flushing about 2mins under 150 kPa,the hydraulic conductivity of stems reached to Kmax when stems were not dipped into ultrapure water overnight,and the hydraulic conductivity had a decreasing tendency with the prolonging of flushing time.Similarly,the hydraulic conductivity of stems reached to Kmax when stems were dipped into ultrapure water overnight.However,the hydraulic conductivity of stems were almost stable although prolonging the flushing time,which was significantly different from stems that were not dipped into ultrapure water overnight.These results indicated that Kmax decreased with increasing flushing time could be avoided by dipping stems into ultrapure water overnight for some species.5.The rate of evaporation from water-filled cuvettes without stems increased with rotational speed and showed the evaporation rate was a linear function of RPM in Cochard rotor(R2=0.9945);Similarly,there was a linear positive correlation relationship between the evaporation rate of water in cuvette and RPM with stems spun in rotor,while the evaporation rate was visibly less than that without stems,indicating that it was necessary to correct the water evaporation in cuvettes to construct water extraction curves.6.The xylem extraction-water curves were typical 's'shape constructed by improved xylem water extraction method with Cochard rotor,the mean P50 value was-4.24 MPa(n=6),then the theoretical PLC value calculated based on the PLV and relevant equation was-3.59 MPa that similar to the P50 value of VCs by bench top dehydration,which indicated that the VCs were valid by xylem water-extraction method. |
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