在两种水分供给(干旱胁迫和适宜水分,土壤含水量分别为田间持水量的30%~40%和70%~80%)下,研究了耐旱树种元宝枫(Acer truncatum)和中生树种女贞(Ligustrum lucidum)木质部栓塞(以导水率(Percentage loss of hydraulic conductivity, PLC)损失程度衡量)对P素添加的响应。结果发现,两个树种PLC的日变化均呈现出先上升后降低的规律,表明木质部栓塞的形成与恢复是植物体的一种平常事件;除适宜水分条件的女贞外,P素可以显著提高元宝枫和遭受干旱胁迫时女贞的PLC;两种水分条件下,干旱胁迫时元宝枫木质部栓塞明显高于适宜水分供给时。女贞的PLC在两种水分状况下无显著差异;树种间,干旱胁迫促进了元宝枫木质部的栓塞形成,明显高于同等水分条件下的女贞。该研究结果证实了“木质部限流耐旱假设”。

Aims Xylem embolism is a physiological response of tree species to adverse environmental factors, such as water deficit. However there is little research on relationships between phosphorus and xylem embolism. Our objectives were to 1) explore whether phosphorus could increase xylem embolism occurrence and 2) test the hypothesis of “drought tolerance by restricting sap flow in xylem".
Methods The tree species Acer truncatum and Ligustrum lucidum were chosen to study the response of xylem embolism (measured as percentage loss of hydraulic conductivity, PLC) to phosphorus addition under drought-stressed and well-watered treatments (soil water content of about 30%-40% and 70%-80% of field moisture capacity, respectively).
Important findings Daily PLC ranged from 60.7% to 70.7% in A. truncatum and 43.9%-72.3% in L. lucidum, indicating that occurrence and refilling of xylem embolism was a usual event in these woody plants. Phosphorus addition raised PLC significantly for A. truncatum at each water status and L. lucidum was subjected to drought. A. truncatum had more xylem embolism occurrence with drought than when well-watered, but there was no significant difference within two water status in L. lucidum. Drought could increase the embolism vulnerability of A. truncatum, causing higher percentage loss of hydraulic conductivity than in L. lucidum under the same water stress. Consequently, drought-resistant trees are not always invulnerable to water deficit. This might be a strategy for a water-limited environment. This study suggests that xylem embolism might be a “hydraulic signal" in plants. Once a plant is under water stress, its xylem conduits become cavited or embolized, resulting in decreased hydraulic conductivity and closure of stomata to maintain water balance by reducing water transportation and consumption. This study supports the hypothesis of “drought tolerance by restricting sap flow in xylem".