空穴化过程的物理分析和栓塞修复的新机制

1、空穴化过程的物理分析 对存在于木质部树液中的微小气泡的膨胀进行力学平衡分析，得出：气泡有稳定和非稳定两种平衡态 随着水势降低，处于稳定平衡的气泡可以平稳地长大 只有水势下降到一定程度，气泡才破裂。推出破裂时的压力势阈值公式。将实际数值代入，其结果比以往的值更接近实验结果。进入管状细胞的“空气种子”处于非稳定平衡态，受到扰动，有可能马上膨胀、破裂，发出（超）声波。发生空穴化的压力势阈值与以往的结果一致。 对气泡和树液组成的系统进行自由能判据分析 由自由能极值处二阶导数的正负，判断处于平衡的气泡的稳定性，其结论与力学分析的一致。此外还得出：当木质部压力势 时，能够进入管状细胞的空气泡处于稳定平衡态，不会立即膨胀。随着水势降低，它平稳地膨胀，达到（上限值），它才破裂。该值优于以往计算的值。当时，水分还将渗出。只有（为大气压）时，“空气种子”进入管状细胞后，才有可能马上膨胀、破裂，发出（超）声波。这应该就是一般情况下，水势降低到才从实验中开始检测到空穴化事件的原因。 2、栓塞修复的新机制 完成与路易斯等人的干旱切片复水时栓塞修复的类似实验，发现新现象: ①最初管状大细胞内的长形泡伸长，而后缩短、缩小，并消失 ②木射线的横断面出现或跳出（胞外）球形泡 ③大的胞外球形泡不断长大，小的则缩小、消失。从而得出：木质切片复水实验中，气体并没有如以往所云，溶解到水中之后主要进入了大气，而是由短半径气泡移动到长半径气泡，使大半径气泡膨胀，小气泡缩小，直至消失。 用两种物理原理解释了这一现象。 推广到活体植物，空穴化事件后，空气难以从木质部移动到大气中，而是从小的管状细胞中移动到了大细胞或其他低气压区域，导致小细胞栓塞的修复，得到了栓塞修复的新机制。

1. Physical Analysis of Cavitation Processes
Mechanical equilibrium analysis was performed on the expansion of microbubbles present in xylem sap. Two equilibrium states of bubbles were derived: stable and unstable. As water potential decreases, bubbles in stable equilibrium can grow steadily. Only when the water potential drops to a certain threshold will the bubbles rupture, and a formula for the pressure potential threshold at rupture was deduced. Substituting actual numerical values, the obtained results are closer to experimental outcomes than those from previous studies. The "air seeds" that enter tubular cells are in an unstable equilibrium state; upon disturbance, they may immediately expand and rupture, emitting ultrasonic sound waves. The pressure potential threshold for cavitation occurrence is consistent with previous research findings.
A free energy criterion analysis was conducted for the system composed of bubbles and xylem sap. The stability of equilibrated bubbles was judged by the sign of the second-order derivative at the free energy extremum, and the conclusion was consistent with that from the mechanical analysis. Additionally, it was determined that when the xylem pressure potential is [omitted value], the air bubbles capable of entering tubular cells are in a stable equilibrium state and will not expand immediately. As the water potential decreases, they grow steadily until reaching the (upper limit value), at which point they rupture. This upper limit value is more accurate than those calculated in prior studies. When [another omitted condition], water will also seep out. Only when [the pressure potential equals atmospheric pressure] will the "air seeds", after entering tubular cells, immediately expand and rupture, emitting ultrasonic sound waves. This should explain why, under general circumstances, cavitation events can be detected experimentally only when the water potential drops to [omitted threshold value].
2. New Mechanism of Embolism Repair
Similar experiments on embolism repair during rehydration of drought-stained xylem sections were carried out following the protocol of Louis et al., and new phenomena were observed: ① Elongated vesicles within initial large tubular cells first elongate, then shorten, shrink and finally disappear; ② Extracellular spherical vesicles appear or emerge from the cross-sections of xylem rays; ③ Large extracellular spherical vesicles continue to grow, while small ones shrink and vanish. It was thus concluded that in xylem section rehydration experiments, rather than dissolving into water and mainly entering the atmosphere as previously proposed, the gas was transferred from short-radius bubbles to long-radius bubbles, causing the long-radius bubbles to expand and small bubbles to shrink until they disappeared.
This phenomenon was explained using two physical principles.
Extrapolating to living plants, after cavitation events, air is difficult to move from the xylem to the atmosphere, but instead migrates from small tubular cells to large cells or other low-pressure regions, leading to the repair of embolism in small cells. A new mechanism for embolism repair was thus proposed.