Cochard Hervé, Froux Fabienne, Mayr Stefan, Coutand Catherine
Unité Mixte de Recherche Physiologie Intégrée de l'Arbre Fruitier et Forestier, Institut National de la Recherche Agronomique/Université Blaise Pascal, Site de Crouelle, 63039 Clermont-Ferrand, France.
Plant Physiol. 2004 Jan;134(1):401-8. doi: 10.1104/pp.103.028357. Epub 2003 Dec 4.
Wall reinforcement in xylem conduits is thought to prevent wall implosion by negative pressures, but direct observations of xylem geometry during water stress are still largely lacking. In this study, we have analyzed the changes in xylem geometry during water stress in needles of four pine species (Pinus spp.). Dehydrated needles were frozen with liquid nitrogen, and xylem cross sections were observed, still frozen, with a cryo-scanning electron microscope and an epifluorescent microscope. Decrease in xylem pressure during drought provoked a progressive collapse of tracheids below a specific threshold pressure (P(collapse)) that correlates with the onset of cavitation in the stems. P(collapse) was more negative for species with smaller tracheid diameter and thicker walls, suggesting a tradeoff between xylem efficiency, xylem vulnerability to collapse, and the cost of wall stiffening. Upon severe dehydration, tracheid walls were completely collapsed, but lumens still appeared filled with sap. When dehydration proceeded further, tracheids embolized and walls relaxed. Wall collapse in dehydrated needles was rapidly reversed upon rehydration. We discuss the implications of this novel hydraulic trait on the xylem function and on the understanding of pine water relations.
木质部导管中的细胞壁加固被认为可以防止因负压导致的细胞壁内爆，但在水分胁迫期间对木质部几何形状的直接观察仍然非常缺乏。在本研究中，我们分析了四种松树（松属）针叶在水分胁迫期间木质部几何形状的变化。将脱水的针叶用液氮冷冻，然后用低温扫描电子显微镜和落射荧光显微镜观察仍处于冷冻状态的木质部横截面。干旱期间木质部压力的降低导致管胞在低于特定阈值压力（P（塌陷））时逐渐塌陷，该阈值压力与茎干中 cavitation 的开始相关。对于管胞直径较小且细胞壁较厚的物种，P（塌陷）更负，这表明在木质部效率、木质部塌陷易感性和细胞壁加固成本之间存在权衡。在严重脱水时，管胞壁完全塌陷，但管腔仍似乎充满汁液。当脱水进一步进行时，管胞栓塞且细胞壁松弛。脱水针叶中的细胞壁塌陷在重新水化后迅速逆转。我们讨论了这种新的水力特性对木质部功能以及对松树水分关系理解的影响。
