Vessel contents during transpiration - Embolisms and refilling
Vessel contents during transpiration - Embolisms and refilling
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DOI：
10.2307/2446046
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发表时间：
1997-09-01
影响因子：
3
通讯作者：
Canny, MJ
中科院分区：
文献类型：
--
作者：
Canny, MJ
对先前的意外观察进行了测试，即在活跃的蒸腾过程中，栓塞的血管被重新填充。向日葵植株在蒸腾过程中每隔2小时冷冻处理后，在低温扫描电子显微镜下观察叶柄中单个维管束的含量，并评估是否存在液体或气体(栓塞物)。同时测量了辐照度、叶温、蒸腾速率和叶水势(通过压力室)。高达40%的血管已经在0900(蒸腾速率类似于5µg·cm(-2)·s(-1)，水势约-300J/kg)进行了栓塞术，这一比例在1500降至最低(低至4%)。此时是蒸腾速率最高(接近25亩·cm~(-2)·s~(-1))和最负水势(-600~-700J/kg)的时期。含有混合气体和液体的容器的图像显示，水从容器壁上的凹坑中挤出，以补充它们。数据表明：(1)水柱很弱，在相当小的张力下破裂；(2)栓子是通过在短时间尺度(分钟)全天向血管注水来修复的；(3)工厂在较长时间尺度(小时)上调整充水过程的强度以适应水压力的强度；(4)压力室平衡压力(P)不测量容器内的张力；(5)P也不是水压力的量度(通过血管栓塞术测量)；(6)磷是植物对水分胁迫反应的一种量度，也就是对灌水过程活力的量度。该测试证实了之前的观察结果，否定了衔接理论的所有假设和证据。这些数据与补偿压力理论完全一致，该理论预测了本实验中所展示的关系。利用该理论的假设，很容易勾勒出一个简单的机制，其中血管的补充可以通过反渗透实现，而(3)中的调节可以通过淀粉鞘中的渗透调节来实现。
A test was made of the previous unexpected observation that embolized vessels were refilled during active transpiration. The contents of individual vessels in petioles of sunflower plants were examined, after snap-freezing at 2-h intervals during a day's transpiration, in the cryo-scanning electron microscope, and assessed for the presence of liquid or gas (embolism) contents. Concurrent measurements were made of irradiance, leaf temperature, transpiration rate, and leaf water potential (by pressure chamber). Up to 40% of the vessels were already embolized by 0900 (transpiration rate similar to 5 mu g.cm(-2).s(-1), water potential about -300 J/kg), and the proportion declined to a minimum (as low as 4%) at 1500. This was the time of highest transpiration rate (similar to 25 mu g.cm(-2).s(-1)) and most negative water potential (-600 to -700 J/kg). Images of vessels with mixed gas and liquid contents showed water being extruded through pits in the walls of the vessels to refill them. The data indicate that: (1) the water columns are weak and break under quite small tensions; (2) embolisms are repaired by refilling the vessels with water on a short time scale (minutes) throughout the day; (3) the vigor of this refilling process is adjusted by the plant on a longer time scale (hours) to the intensity of the water stress; (4) the pressure chamber balance pressure (P) does not measure tension in the vessels; (5) P is also not a measure of water stress (as measured by vessel embolization); and (6) P is a measure of the plant's response to water stress, i.e., a measure of the vigor of the refilling process. The test confirms the previous observations and negates all the assumptions and evidences of the Cohesion Theory. The data are fully consistent with the Compensating Pressure Theory, which predicted the relations demonstrated in this experiment. Using the assumptions of that theory it is easy to outline a simple mechanism by which the refilling of vessels might be achieved by reverse osmosis, and the adjustment in (3) might be achieved by osmoregulation in the starch sheath.