Relationships Between Vessel Size And Vulnerability To Embolism In The Xylem Of Populus Species

Samples were collected from genus Populus and the embolism vulnerability in the xylem of stems were determined by Cochard cavitron centrifuge, and the vulnerability curves of different stems were constructed and the P50 was calculated. The hydraulic conductivity and percentage loss conductivity of twigs were measured using"Low Pressure Flow Meter". The vessel diameter was determined by means of stain method and the vessel length by silicone rubber technique. The objectives of this study were to probe into the effect of vessel size on vulnerability to embolism in the xylem, to quantify the relationship between vulnerability to cavitation and vessel diameter within a species, to construct vulnerability curves for vessel-diameter size class, to examine a computational algorithm addressing how vessel length might depend on vessel diameter, to reveal the relationship between vessel diameter and vessel length, to verify the cavitron theory and to clarify the patterns of embolism in stems spun in a Cochard cavitron. The purpose of this study was to provide scientific bases for selecting embolism-resistant woody species, and for breeding and selecting drought-resistant woody species in arid and semi-arid areas.The main results are as follows:1. Vessel diameter distributions fit well with Weibull probability density function (PDF). Different branches have different vessel diameter PDF distributions, variation in vessel diameter accounts for part of the variation in P50 between branches.2. The PDF of embolized vessels was shifted towards the large diameter size classes compared with conducting vessels, which proved that large conduits tend to cavitate before small conduits within a given stem. The stems with vessel-diameter PDFs shifted to the large size (bigger vessel diameter) will be more vulnerable to cavitation (low P50).3. Create a method to construct vulnerability curves for each diameter size-class. From these VCs it is found that as diameter decreased, P50 shifted to higher tension and the slope of the Weibull CDF decreased. Vessel-based VCs were also fitted to Weibull cumulative distribution functions (CDF), which provided best-fit values of Weibull CDF constants (c and b) and P50. We get that P50=6.166D-0.3134 (R2 = 0.995) and that b and 1/c are both linear functions of D with R2 > 0.95, these results allow us to predict the Weibull CDF constants (ci and bi) from vessel diameter, D.4. A discrete vessel-diameter model for PLC is given by This equation can be used to compute a VC within a stem and can compare with VC got by centrifuge technique.5. Vessel diameter cannot explain all the variation in VCs between stems. Two other factors of importance might be (1) pathway redundancy for water flow in stems much longer than the median vessel length and (2) the impact of pit area fraction on VCs.6. Using a centrifuge to create vulnerability curves places limits on testing the pit area hypothesis when vessels are longer than 3-4 cm, vessel diameter is a proxy for pit membrane area when using a centrifuge to measure P50 , the amount of pathway redundancy could also influence P50 within certain limits.7. The new computational algorithm can get the relationship between mean vessel length in a diameter size class ( Lc ) versus vessel diameter size class (Dc) within one species. Wide vessels tend to be long vessels. This information has some useful implications provided the pit area hypothesis is true. Our data lends additional support to the notion that there is a trade-off between vessel efficiency for water transport and vulnerability to drought-induced xylem embolism. The trade-off between efficiency and vulnerability to cavitation might drive a shift in vessel-length of wide vessels away from the optimum value for hydraulic efficiency within a species.8. The pattern of embolism we found approximating that expected from cavitron theory showed: the central segments were emboized more than the distal and basal segment. The PLC of stem segments near the axis of rotation does exceed the stem averaged PLC reported by the cavitron. PLC was very quite high near the end of stems, even though tension ought to be zero, the PLC of stem segments farthest from the axis of rotation did have non-zero PLCs in excess of the native PLCs before spinning. Large vessels cavitated before small vessels. There is a spatial asymmetry in PLC, distal stem samples had lower PLC than basal samples.9. The scaled-up (computed) value of PLC from segments was significantly different from cavitron values. Scaled up values of maximum Kh were not significantly different from cavitron values. More work needs to be done to identify the underling processes that explain why scaled-up values of PLC disagree with cavitron-measured values.