分享到: 新浪微博 点击次数:784 点击收藏

导管空穴化和注水这种循环造成导管的 “疲劳” 的普遍性
气穴产生装置配置中可选水势测量单元,测量范围0-40 bar, 0-70bar或 0-100bar
气穴产生装置样品室常规小样品室,或者大样品室(内径2.5 或4.0 inches, 长12,18 或24 inch)
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 100% 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
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

  • 文献下载
  • 应用案例
  • 新闻中心