在干旱条件下维持木质部水分运输对植物至关重要，但当干旱诱导的栓塞形成并逐渐通过木质部传播时，木质部会发生衰竭。水力法被广泛认为是量化干旱诱导木质部栓塞的金标准。该方法通过将水推过树枝样品，测定脱水至特定干旱水平的树枝样品的水力传导率（Kh）。这项技术因其可靠的Kh测量而被广泛认可，但关于如何定义稳定的Kh以及它与木质部栓塞形成程度的关系，文献中存在一些不确定性。因此，该方法最常见的设置被扩展到测量四个参数：（i）入口Kh，（ii）出口Kh，（iii）从木质部到周围活组织的径向流动，以及（iv）样品的压差。从严格的理论观点来看，水力稳定状态，即流入等于流出，径向流量为零，将导致稳定的Kh。该装置在海棠上的应用表明，达到水力稳定状态需要相当长的时间（可达300分钟），达到稳定状态的时间随着木质部水势的下降而增加。在每次实验过程中，Kh和木质部水势动态增加，这得到了X射线计算机显微断层成像显示的栓塞在高压（8 kPa）和低压（2 kPa）下重新填充的支持。因此，供应加压水会导致人工重新填充容器，这使得在部分栓塞的木质部中很难获得真正稳定的Kh。

关键词：海棠；X射线计算机显微断层扫描；声发射；干旱胁迫；水力法。

The stability enigma of hydraulic vulnerability curves: addressing the link between hydraulic conductivity and drought-induced embolism

Abstract

Maintaining xylem water transport under drought is vital for plants, but xylem failure does occur when drought-induced embolisms form and progressively spread through the xylem. The hydraulic method is widely considered the gold standard to quantify drought-induced xylem embolism. The method determines hydraulic conductivity (Kh) in cut branch samples, dehydrated to specific drought levels, by pushing water through them. The technique is widely considered for its reliable Kh measurements, but there is some uncertainty in the literature over how to define stable Kh and how that relates to the degree of xylem embolism formation. Therefore, the most common setup for this method was extended to measure four parameters: (i) inlet Kh, (ii) outlet Kh, (iii) radial flow from xylem to surrounding living tissue and (iv) the pressure difference across the sample. From a strictly theoretical viewpoint, hydraulic steady state, where inflow equals outflow and radial flow is zero, will result in stable Kh. Application of the setup to Malus domestica Borkh. branches showed that achieving hydraulic steady state takes considerable time (up to 300 min) and that time to reach steady state increased with declining xylem water potentials. During each experimental run, Kh and xylem water potentials dynamically increased, which was supported by X-ray computed microtomography visualizations of embolism refilling under both high- (8 kPa) and low-pressure (2 kPa) heads. Supplying pressurized water can hence cause artificial refilling of vessels, which makes it difficult to achieve a truly stable Kh in partially embolized xylem.

Keywords: Malus domestica Borkh; X-ray computed microtomography; acoustic emissions; drought stress; hydraulic method.