Institute for Ecohydrology Research, 1111 Kennedy Place Ste 4, Davis, CA 95616, USA.
Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, UT 84112, USA.
New Phytol. 2012 Feb;193(3):713-720. doi: 10.1111/j.1469-8137.2011.03984.x. Epub 2011 Dec 7.
• The rare pit hypothesis predicts that the extensive inter-vessel pitting in large early-wood vessels of ring-porous trees should render many of these vessels extremely vulnerable to cavitation by air-seeding. This prediction was tested in Quercus gambelii. • Cavitation was assessed from native hydraulic conductivity at field sap tension and in dehydrated branches. Single-vessel air injections gave air-seeding pressures through vessel files; these data were used to estimate air-seeding pressures for inter-vessel walls and pits. • Extensive cavitation occurred at xylem sap tensions below 1 MPa. Refilling occurred below 0.5 MPa and was inhibited by phloem girdling. Remaining vessels cavitated over a wide range to above 4 MPa. Similarly, 40% of injected vessel files air-seeded below 1.0 MPa, whereas the remainder seeded over a wide range exceeding 5 MPa. Inter-vessel walls averaged 1.02 MPa air-seeding pressure, similar and opposite to the mean cavitation tension of 1.22 MPa. Consistent with the rare pit hypothesis, only 7% of inter-vessel pits were estimated to air-seed by 1.22 MPa. • The results confirm the rare pit prediction that a significant fraction of large vessels in Q. gambelii experience high probability of failure by air-seeding.
• 罕见坑假说预测，环孔材中早材中广泛的管间坑洼应该会使许多这些管极易受到空气播种引起的空穴化。这一预测在山毛榉属 Gambelii 中进行了测试。
• 通过现场液流张力和脱水枝条中的天然水力传导率评估空穴化。单管空气注入通过管文件产生空气播种压力；这些数据用于估计管间壁和坑洼的空气播种压力。
• 在木质部汁液张力低于 1 MPa 时就会发生广泛的空穴化。在低于 0.5 MPa 时会重新填充，韧皮部环割会抑制其填充。其余的管在较宽的范围内发生空穴化，超过 4 MPa。同样，40%的注入管文件在低于 1.0 MPa 时空气播种，而其余的在超过 5 MPa 的较宽范围内播种。管间壁的平均空气播种压力为 1.02 MPa，与平均空穴化张力 1.22 MPa 相似且相反。与罕见坑假说一致，仅估计有 7%的管间坑通过 1.22 MPa 发生空气播种。
• 结果证实了罕见坑假说，即在山毛榉属 Gambelii 中，很大一部分大管经历空气播种的高失败概率。
