Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki, Finland.
Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki, Helsinki, Finland.
Plant Biol (Stuttg). 2018 Sep;20(5):834-847. doi: 10.1111/plb.12838. Epub 2018 Jun 5.
Positive sap pressures are produced in the xylem of birch trees in boreal conditions during the time between the thawing of the soil and bud break. During this period, xylem embolisms accumulated during wintertime are refilled with water. The mechanism for xylem sap pressurization and its environmental drivers are not well known. We measured xylem sap flow, xylem sap pressure, xylem sap osmotic concentration, xylem and whole stem diameter changes, and stem and root non-structural carbohydrate concentrations, along with meteorological conditions at two sites in Finland during and after the sap pressurisation period. The diurnal dynamics of xylem sap pressure and sap flow during the sap pressurisation period varied, but were more often opposite to the diurnal pattern after bud burst, i.e. sap pressure increased and sap flow rate mostly decreased when temperature increased. Net conversion of soluble sugars to starch in the stem and roots occurred during the sap pressurisation period. Xylem sap osmotic pressure was small in comparison to total sap pressure, and it did not follow changes in environmental conditions or tree water relations. Based on these findings, we suggest that xylem sap pressurisation and embolism refilling occur gradually over a few weeks through water transfer from parenchyma cells to xylem vessels during daytime, and then the parenchyma are refilled mostly during nighttime by water uptake from soil. Possible drivers for water transfer from parenchyma cells to vessels are discussed. Also the functioning of thermal dissipation probes in conditions of changing stem water content is discussed.
在北方地区，桦树的木质部在土壤解冻和芽突破之间的时间内会产生正的树液压力。在此期间，冬季积累的木质部栓塞会被水重新填充。木质部汁液增压的机制及其环境驱动因素尚不清楚。我们在芬兰的两个地点测量了木质部汁液流动、木质部汁液压力、木质部汁液渗透压、木质部和整个茎直径变化以及茎和根的非结构性碳水化合物浓度，以及气象条件，在此期间和之后的树液增压期。在树液增压期间，木质部汁液压力和汁液流量的昼夜动态变化不同，但通常与芽爆发后的昼夜模式相反，即当温度升高时，汁液压力增加，汁液流速大多降低。在树液增压期间，茎和根中的可溶性糖向淀粉的净转化发生了。与总汁液压力相比，木质部汁液渗透压较小，且不受环境条件或树木水分关系的变化影响。基于这些发现，我们建议木质部汁液增压和栓塞填充是通过白天从薄壁细胞向木质部导管转移水分逐渐发生的，持续数周，然后在夜间主要通过从土壤中吸水来填充薄壁细胞。讨论了从薄壁细胞向导管转移水分的可能驱动因素。还讨论了在改变的茎含水量条件下热耗散探头的功能。
