應(yīng)用：

在植物根系吸收水分速度不能滿足植物蒸騰需水速度的時候（如土壤水分嚴重缺乏、蒸騰強度過大、冷凍脅迫或者土壤水勢過低），植物導(dǎo)管內(nèi)水柱中溶解的空氣逸出，水分子間的內(nèi)聚力失效，水柱發(fā)生斷裂或抽空，在導(dǎo)管內(nèi)形成氣泡或氣柱，這些氣泡或氣柱被稱為氣穴。該儀器可利用高壓將空氣或氮氣通過植物枝條表面的皮孔和射線組織滲入到導(dǎo)管中，使得植物枝條的導(dǎo)管內(nèi)產(chǎn)生氣泡或氣柱，模擬脅迫產(chǎn)生氣穴。不同的壓力和壓力持續(xù)時間可以在枝條內(nèi)部產(chǎn)生不用數(shù)量的氣穴。通過其他儀器（如植物導(dǎo)水率高壓測試儀）測定不同壓力和壓力持續(xù)時間下枝條的導(dǎo)水率，建立植物導(dǎo)水率——氣壓（氣壓與產(chǎn)生的氣穴量正相關(guān)）曲線，可以獲知氣穴對該植物率的影響，從而評估該樣品導(dǎo)水能力的抗脅迫性。

該儀器裝置適用于同種植物（木本）不同條件處理（如不同生境）或者不同生理狀態(tài)下的抗脅迫能力研究，獲得植物導(dǎo)水率的脆弱曲線。

儀器描述：

1.中間的氣室大約長3.20英寸。

2.橡膠墊圈套在樣品上，并一起深入氣室。

3.金屬塞擋住墊圈，可以增加密封性。

4.螺絲帽可以增強密封并固定樣品。

5.樣品必須要達到4.25英寸，保證可以*通過氣室。

儀器包括：

1. 可以控制到100bar的壓力表

2. 3 英尺連接氣穴的管子

3. 氣穴氣室

4. 2個金屬堵頭尺寸為(5/8", 3/8", 3/16")

5. 每個金屬堵頭帶5個橡膠墊圈尺寸為(自己打口, 1/16", 1/8", 3/16")

6. 6英尺連接儀器到氣罐的管子

7. 便攜式氣瓶（可選）

在導(dǎo)水率測定方面，*使用植物導(dǎo)水率高壓測試儀。

產(chǎn)地：

美國

下面是兩篇關(guān)于氣穴與導(dǎo)水率關(guān)系的文章，用戶可以參考并試驗自己的樣品

參考1

Relationships between Hydraulic Conductance, Xylem Cavitation, and Stomatal Regulation of Transpiration Sperry, John

There is increasing evidence that avoidance of critical levels of xylem cavitation caused by dynamic (transpiration-induced) water stress is one major adaptive advantage of stomatal closure. The minimum negative xylem pressure recorded for 37 species strongly correlated with the negative pressure inducing * loss of hydraulic conductance in stems from xylem cavitation. The minimum safety margin from complete cavitation ranged from a few tenths of a megapascal in drought-susceptable plants to several megapascals in more drought-tolerant plants. Safety margins were even smaller for root xylem. Small safety margins from cavitation could not be explained by a trade-off between cavitation vulnerability and conducting efficiency.

Although avoidance of complete cavitation is achieved by stomatal closure, partial cavitation and loss of hydraulic conductance appears to be promoted, particularly in the more vulnerable root system. Field studies showed significant cavitation in root xylem of woody plants during drought, and reversal following rain. Stem xylem showed much less dramatic response to drought. Loss of hydraulic conductance in the root system may be the primary hydraulic limitation for gas exchange.

Partial cavitation during drought may be advantageous; particularly in roots where it is most readily replaced after the drought either by refilling embolized conduits, or growth of new roots. Studies with Betula occidentalis have shown that stomata close in response to a loss of hydraulic conductance because they sense the reduction in leaf water status. Thus, loss of hydraulic conductance by cavitation leads to reduced transpiration for the same drop in xylem pressure. As a drought progresses, cavitation superimposed on stomatal regulation moderates the rate of soil water extraction more than stomatal regulation alone. Reduced water use during drought prolongs its availability in soil and may allow more to be extracted by minimizing the drop in soil-to-root hydraulic conductance.

Key words: xylem cavitation, hydraulic conductance, stomatal regulation, water stress, drought tolerance.

Correspondence: John Sperry, Department of Biology, University of Utah, Salt Lake City, UT 84112, USA

參考2

Susceptibility to Xylem Embolism as an Index of Drought Tolerance in Chaparral Shrubs of California (USA) Davis, Stephen

There are two genera of chaparral shrubs in California (Arctostaphylos and Ceanothus) that contain species with very different life history characteristics ?sprouters and non-sprouters after wildfire. Non-sprouters were hypothesized to be more tolerant of the first summer drought after wildfire and thus more resistant to water stress-induced embolism of their xylem tissue. This hypothesis was tested by comparing three pairs of co-occurring sprouters and non-sprouters of Ceanothus. In all cases, non-sprouters had significantly greater resistance to water stress-induced embolism than sprouters (differences ranged between 2.5 and 3.7 MPa in water potential were 50% embolism occurred). When vulnerability to xylem embolism was compared among four dominant species of a mixed chaparral stand, susceptibility to water stress-induced embolism varied between ?1 MPa for Ceanothus megacarpus to ?.9 MPa for Malsoma laurina. Adenostoma fasciculatum (?.3 MPa) and A. sparsifolium (?.9 MPa) were intermediate. The increasing order of susceptibility to embolism (C. megacarpus < A. fasciculatum < A. sparsifolium < M. laurina) corresponded to the order in which post-fire seedlings were susceptible to summer drought: C. megacarpus (63% survival), A. fasciculatum (21% survival), A. sparsifolium (8% survival), and M. laurina (1.1% survival). A comparison of the vulnerability of xylem to embolism among 22 species of chaparral shrubs indicated that susceptibility corresponds to minimum seasonal water potentials but is weakly correlated with vessel diameter or area specific conductivity. This is consistent with the hypothesis that susceptibility to water stress-induced embolism is a function of pore size in pit membranes of vessel and tracheid cell walls and not vessel size.

Key words: xylem embolism, drought tolerance, chaparral.

Correspondence: Stephen D. Davis, Natural Science Division, Pepperdine University, Malibu, California 90263 , USA