Study On The Seasonal Variations Of Embolism In Robinia Pseudoacacia, Diospyros Kaki, Platanus Occidentali, And Phyllostachys Propinqua

The Cohesion-Tension theory of water transportation in xylem requires a continuous column of water in the vessels. However, recent studies have shown that there are embolisms in the water column. Embolism decreases water transport efficiency, which has a great impact on other physiological processes. In this paper, four different xylem types of plants, Robinia pseudoacacia, Diospyros kaki, Platanus occidentali, and Phyllostachys propinqua, were selected as experimental material. I investigated seasonal variations in embolisms, water potential at dawn, sugar and starch contents in twigs from April 2010 to March 2011. Cavitation vulnerability and vessel morphalogy of the four species were also measured. Main results are as follows:1) R. pseudoacacia, D. kaki, P. occidentalis had a similar seasonal variation in embolism: percentage of loss conductivity (PLC, an indicator for embolism) was higher, up to 75%, in spring and winter, and relatively lower in summer and autumn (less than 20%). The PLC of P. propinqua was 80% in spring,and 30% in autumn, while no PLC was detected in summer and winter. The results suggest that embolism can be repaired in summer, but can not in winter.2) There were similar changes of xylem water potential at dawn in R. pseudoacacia, D. kaki, P. occidentalis, P. propinqua. All of them were higher than -0.2MPa in summer. In winter, R.. pseudoacacia, D.kaki, P. propinqua were below -2.6MPa and P. occidentalis was at -1.2MPa. TheΨP50 of cavitation vulnerablity in R. pseudoacacia, D. kaki, P. occidentalis, P. propinqua. were -0.56 MPa,-0.98 MPa,-1.67 MPa,-0.64 MPa, respectively. From this point of view, it was shown that when the xylem water potential was higher in summer, the embolism was able to be repaired. when the potential was lower in winter the embilism was not able to be repaired.3) Soluble sugar content was around 10% in winter and 5%-6% in summer in R. pseudoacacia, D. kaki, P. occidentalis. There were denscends of starch content occurred in spring and later autumn. There was a rapid accumulation in xylem starch content at early autumn. The starch was decomposed into sugars for bud flush and shoot growth in the spring, and for osmotic regulation and embolism repair in later autumn. Sugar content in twigs was negatively correlated with xylem water potential at dawn.4) The dimension in vessel radius and length of twigs in the four species was R. pseudoacacia > P. propinqua > D. kaki > P. occidentalis. The maximum special hydraulic conductivity was ranged as: R. pseudoacacia > P. propinqua > D. kaki > P. occidentalis. The range of cavitation vulnerability followed as: R. pseudoacacia > P. propinqua > P. occidentalis > D. kaki. It is suggested that there is trade-off between efficiency and safety in water transport. P. occidentalis had 109 vessels mm-2, much higher than that of D. kaki (35 mm-2), and its water transport efficiency as well as safety were all higher than that of, D. kaki, suggesting that vessel density, with similar diameter, could increase water transport efficiency and safety in the same time.