Relationship Between Xylem Structures And Drought Tolerance In Three Tree Species

One-year-old twigs of Hippophae rhamnoides, Robinia pseudoacacia and Corylusheterophylla were used to study the relationship between xylem structure and droughttolerance of the three tree species by means of the vulnerability curves and xylem anatomicalmethod. Cochard Cavitron, microtome and upright fluorescence microscope were adopted tomeasure vulnerability to cavitation and vessel diameter of the three tree species; the inducedembolism-dye staining techniques, silicone injection, wood maceration, Low Pressure FlowMeter and SEM to measure the xylem structures of H. rhamnoides, such as the lengths,diameter and density of vessels and fibers, as well as hydraulic features including the stemhydraulic conductivity, vessel hydraulic diameter, hydraulic resistance of vessel lumen andfiber lumen. The main conclusions are as follows.1. The average P50values of the vulnerability curves of H. rhamnoides, R. pseudoacacia,and C. heterophylla fitted by single Weibull are﹣1.94±0.19MPa，﹣0.41±0.06MPa，﹣2.52±0.03MPa, respectively, and the order of vulnerability for the three species is R.pseudoacacia＞H. rhamnoides＞C. heterophylla. The order of average vessel diameter forthe three species is R. pseudoacacia（52.15±3.51m）＞C. heterophylla（21.93±1.58m）≥H. rhamnoides（20.01±1.61m）. For the three tree species, the vulnerability to cavitationis positively related with the vessel diameters.2. The vulnerability curves of C. heterophylla can get best fit by single Weibull, whichhas a typical “s” shape. The vulnerability curves of H. rhamnoides and R. pseudoacacia canbe fitted both by single Weibull and dual Weibull, however, dual Weibull fits the curves withhigher accuracy than single Weibull.3. The average P1/2values of the first and second Weibull curves deviated from dualWeibull are-0.90MPa and-3.52MPa, respectively. The first Weibull is accounted by vesselsconnected to each other directly by pit fields, while the second Weibull curve is associatedwith vessels that are connected almost exclusively by “fiber bridges”. The efficiency andsafety of the fiber bridge as well as vestured pits of H. rhamnoides might be the reason for thestronger drought tolerance. 4. The most frequent vessels are small vessels (diameter <20m), but vessels>20mare responsible for most of the water flow. The average vessel length of H. rhamnoides is15.42cm and the average length of fiber is0.413mm. The number of fibers is55times as thenumber of vessels, nevertheless, and the lumen fraction of vessels to fibers is in the ratio of1.4, therefore, fiber lumens theoretically contribute about1.4%to the stem hydraulicconductivity the fibers. The induced embolism-dye staining experiments showed that forvessel diameter≤27.5m, the regression was negatively correlated with vessel diameter; forvessel diameter≥27.5m, the regression was positively correlated.5. The maximum vessel length of R. pseudoacacia is about25cm, therefore, to obtaincorrect vulnerability curves, at least16cm long stem from the cutting end should be cut off.For R. pseudoacacia, the average P1/2values of the first and second Weibull curves ofdeviated from dual Weibull are-0.230MPa and-1.159MPa, respectively.