Sperry J S, Holbrook N M, Zimmermann M H, Tyree M T
Department of Botany, University of Vermont, Burlington, Vermont 05405.
Plant Physiol. 1987 Feb;83(2):414-7. doi: 10.1104/pp.83.2.414.
Xylem vessels in grapevines Vitis labrusca L. and Vitis riparia Michx. growing in New England contained air over winter and yet filled with xylem sap and recovered their maximum hydraulic conductance during the month before leaf expansion in late May. During this period root pressures between 10 and 100 kilopascals were measured. Although some air in vessels apparently dissolved in ascending xylem sap, results indicated that some is pushed out of vessels and then out of the vine. Air in the vessel network distal to advancing xylem sap was compressed at about 3 kilopascals; independent measurements indicated this was sufficient to push air across vessel ends, and from vessels to the exterior through dead vine tips, inflorescence scars, and points on the bark. Once wetted, vessel ends previously air-permeable at 3 kilopascals remained sealed against air at pressures up to 2 and 3 megapascals. Permeability at 3 kilopascals was restored by dehydrating vines below -2.4 megapascals. We suggest that the decrease in permeability with hydration is due to formation of water films across pores in intervascular pit membranes; this water seal can maintain a pressure difference of roughly 2 megapascals, and prevents cavitation by aspirated air at xylem pressures less negative than -2.4 megapascals.
生长在新英格兰地区的美洲葡萄（Vitis labrusca L.）和河岸葡萄（Vitis riparia Michx.）的木质部导管在冬季含有空气，但在五月下旬叶片展开前的一个月内充满了木质部汁液，并恢复了最大水力导度。在此期间，测得的根压在10至100千帕之间。虽然导管中的一些空气显然溶解在上升的木质部汁液中，但结果表明，一些空气被挤出导管，然后排出葡萄藤。在前进的木质部汁液远端的导管网络中的空气在约3千帕的压力下被压缩；独立测量表明，这足以将空气推过导管末端，并从导管通过枯死的葡萄藤梢、花序疤痕和树皮上的点排出到外部。一旦湿润，先前在3千帕时透气的导管末端在高达2和3兆帕的压力下仍能防止空气进入。通过将葡萄藤脱水至-2.4兆帕以下，可恢复3千帕时的渗透性。我们认为，随着水分增加渗透性降低是由于在细胞间纹孔膜的孔隙上形成了水膜；这种水封可以维持大约2兆帕的压力差，并防止在木质部压力小于-2.4兆帕时被吸入的空气导致空穴化。
