Alder N N, Sperry J S, Pockman W T
Department of Biology, University of Utah, 84112, Salt Lake City, UT, USA.
Oecologia. 1996 Feb;105(3):293-301. doi: 10.1007/BF00328731.
The objective of this study was to determine how adjustment in stomatal conductance (g ) and turgor loss point (Ψ) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993-1994), the slope site had substantially lower xylem pressures (Ψ) and g than the riparian site, particularly during the drought year of 1994. The Ψ was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday Ψ and the critical pressure triggering catastrophic xylem embolism (Ψ). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from Ψ averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (g =9±2 versus 79±15 mmol m s at riparian site) and made any further osmotic adjustment of Ψ non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum Ψ for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.
本研究的目的是确定大齿槭河岸（湿润）种群与相邻斜坡（干燥）种群之间气孔导度（g）和膨压损失点（Ψ）的调整如何与根系和茎木质部对栓塞的敏感性相关。在为期两个夏天的研究（1993 - 1994年）中，斜坡位点的木质部压力（Ψ）和g显著低于河岸位点，尤其是在1994年干旱年份。斜坡处的Ψ（-2.9±0.1 MPa；所有误差为95%置信限）也低于河岸位点（-1.9±0.2 MPa）；但它在1994年干旱期间并未下降。不同位点的茎木质部在栓塞易感性方面没有差异。尽管在1994年干旱期间，斜坡位点的茎比河岸茎损失了更大比例的水力导度（46±11%对27±3%），但它们在中午Ψ与触发灾难性木质部栓塞的临界压力（Ψ）之间仍保持至少1.7 MPa的安全边际。根系木质部比茎木质部更容易受到栓塞影响，且不同位点之间存在显著差异：河岸根系在-1.75 MPa时完全空穴化，而斜坡根系为-2.75 MPa。栓塞易感性与导管间纹孔膜的孔径有关，与导管直径无简单关系。河岸和斜坡位点根系的Ψ安全边际平均均小于0.6 MPa。1994年干旱期间斜坡位点的最小安全边际可能导致气孔几乎完全关闭（g = 9±2与河岸位点的79±15 mmol m⁻² s⁻¹相比），并使Ψ的任何进一步渗透调节变得不适应。秋季降雨后，根系中的栓塞至少部分得到逆转。尽管根系中的灾难性栓塞可能会限制气体交换的最小Ψ，但部分（且可逆）的根系栓塞在土壤水分耗尽时限制水分利用方面可能是适应性的。
