The refilling of embolized xylem in Pinus sylvestris L.

Abstract. The hypothesis that ray parenchyma cells are actively involved in the refilling of embolized xylem of Pinus sylvestris L. was tested by killing the ray parenchyma and comparing rehydration of killed stems with that of control material. Killing of ray parenchyma was achieved using hot water or sodium azide. In most experiments, the available water for refilling was at negative water potential. Experiments were done on three kinds of plant material: small branch segments, potted seedlings and small potted trees. In all experiments, there was no indication that the azide-killed xylem was slower to refill than the control material and it was concluded that the parenchyma has no role in the refilling process, which therefore must be purely physical or physicochemical. Stems treated with hot water did not refill; we suggest that this may be caused by high temperatures decreasing the water permeability of the tracheid wall. The refilling of small branch segments may be explained by surface tension forces (capillary action), which inside the tracheid lumen may lower the water potential down to -9.7kPa; this may be enough to draw in water from the available water that in experiment one was at -2kPa. In the case of seedlings or saplings, capillary action cannot explain refilling, because the xylem water potentials were always lower than those estimated from tracheid radii. Condensation of water during diurnal cycles of warming and cooling is also unlikely to contribute to refilling significantly. To account for refilling in these cases, it is supposed that the tracheid wall may be chemically active and able to lower the water potential below the value expected by capillarity.     

Vulnerability curves by centrifugation: is there an open vessel artefact,and are ‘r’ shaped curves necessarily invalid?

Vulnerability curves using the 'Cavitron' centrifuge rotor yield anomalous results when vessels extend from the end of the stem segment to the centre ('open-to-centre' vessels). Curves showing a decline in conductivity at modest xylem pressures ('r' shaped) have been attributed to this artefact. We determined whether the original centrifugal method with its different rotor is influenced by open-to-centre vessels. Increasing the proportion of open-to-centre vessels by shortening stems had no substantial effect in four species. Nor was there more embolism at the segment end versus centre as seen in the Cavitron. The dehydration method yielded an 'r' shaped curve in Quercus gambelii that was similar to centrifuged stems with 86% open-to-centre vessels. Both 'r' and 's' (sigmoidal) curves from Cercocarpus intricatus were consistent with each other, differing only in whether native embolism had been removed. An 'r' shaped centrifuge curve in Olea europaea was indistinguishable from the loss of conductivity caused by forcing air directly across vessel end-walls. We conclude that centrifuge curves on long-vesselled material are not always prone to the open vessel artefact when the original rotor design is used, and 'r' shaped curves are not necessarily artefacts. Nevertheless, confirming curves with native embolism and dehydration data is recommended.     

8种木本植物木质部栓塞变化与生理生态指标关系的研究Ⅰ.与植物木质部水势的关系

以自然状况下生长良好的耐旱树种刺槐(Robinia pseudoacacia L.)、元宝枫(Acer truncatum Bge.)、沙棘(Hippophae rhamnoides L.)、白榆(Ulmus pumila L.)、油松(Pinus tabulaeformis Carr.)、白皮松(Pinus bungeana Zucc.ex Endl.)及中生树种女贞(Ligustrum lucidum Ait.)、柳树(Salix matsudana Koidz. f. pendula Schneid.)为研究对象,用压力室法测定木质部水势,用冲洗法测定木质部栓塞程度,研究不同生长季节木质部栓塞与水势间的关系.结果表明针叶树油松、白皮松在各季节水势均较高,水势变化幅度较小,木质部不易发生栓塞,这与其木质部由管胞构成,对木质部栓塞不敏感,在干旱时采用高水势延迟脱水的耐旱策略有关.阔叶树刺槐、元宝枫、沙棘、白榆、女贞和柳树的木质部栓塞现象是其在每天正常生长过程中不可避免的 \"平常事件\",是它们适应干旱的一种方式.它们的木质部栓塞程度与水势表现出了相反的变化趋势,即同一树种在同一季节内水势值越低,木质部栓塞程度越大,但在不同树种及同一树种的不同季节不存在这种关系.由此可见,植物木质部栓塞对水势的敏感程度(即木质部栓塞脆弱性)主要由树种的木质部结构决定,同时受到树种特性、树木生长发育时期、外界环境因子的影响,木质部栓塞的脆弱性也具有季节变化特征.     

8种木本植物木质部栓塞变化与生理生态指标关系的研究 I.与植物木质部水势的关系

以自然状况下生长良好的耐旱树种刺槐（Robinia pseudoacacia L.）、元宝枫（Acer truncatum Bge）、沙棘（Hippophae rhamnoides L.）、白榆（Ulmus pumila L.）、油松（Pinus tabulaeformis Carr.）、白皮松（Pinus bungeana Zucc.ex Endl.）及中生树种女贞（Ligustrum lucidum Ait.）、柳树（Salix matsudana Koidz.f.pendula Schneid.）为研究对象,用压力室法测定木质部水势,用冲洗法测定木质部栓塞程度,研究不同生长季节木质部栓塞与水势间的火系。结果表明：针叶树油松、白皮松在各个季节水势均较高,水势变化幅度较小,木质部不易发生栓塞,这与其木质部由管胞构成,对木质部栓塞不敏感,在干旱时采用高水势延迟脱水的耐旱策略有关。阔叶树刺槐、元宝枫、沙棘、白榆、女贞和柳树的木质部栓塞现象是其在每天正常生长过程中不可避免的“平常事件”,是它们适应干旱的一种方式。它们的木质部栓塞程度与水势表现出了相反的变化趋势,即同一树种在同一季节内水势值越低,木质部栓塞程度越大,但在不同树种及同一树种的不同季节不存在这种关系。由此可见,植物木质部栓塞对水势的敏感程度（即木质部栓塞脆弱性）主要由树种的木质部结构决定,同时受到树种特性、树木生长发育时期、外界环境因子的影响,木质部栓塞的脆弱性也具有季节变化特征。     

Dynamic changes in petiole specific conductivity in red maple (Acer rubrum L.), tulip tree (Liriodendron tulipifera L.) and northern fox grape (Vitis labrusca L.)

Diurnal variation in petiole specific hydraulic conductivity and simultaneous measurements of leaf water potential were recorded in red maple, tulip tree and fox grape. Petiole specific conductivity was determined from in situ measurements of water flow into the distal (leaf‐bearing) end of an attached petiole as a function of applied hydrostatic pressure and petiole dimensions. The hydraulic properties of the petiole dominated the measurements, indicating that this technique can be used for rapid estimates of petiole hydraulic conductivity. There was a significant decrease in petiole specific conductivity associated with increasingly more negative leaf water potentials in maple and tulip tree, but not in grape. Petiole specific conductivity increased during the afternoon while the plant was actively transpiring and the xylem sap was under tension. The recovery of petiole conductivity during the afternoon suggests that hydraulic conductivity reflects a dynamic balance between a loss of hydraulic conductivity with increasing water stress, and its restoration as tension within the xylem decreases. Three experimental manipulations were applied to red maple and tulip tree to examine the sensitivity of diurnal changes in petiole conductivity to various physiological perturbations. Both phloem girdling and application of HgCl₂ to the transpiration stream resulted in a marked decrease in the degree to which petiole specific conductivity recovered as xylem tension relaxed during the afternoon. Delivery of a surfactant to the xylem, however, did not significantly alter the relation between leaf water potential and petiole hydraulic conductivity.     

Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function

Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water-transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for > 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade-offs between efficiency and safety of water transport. However, trade-offs in hydraulic function are influenced both by pit-level differences in structure (e.g. average porosity of pit membranes) and by tissue-level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of individual pits to the whole plant.     

植物木质部水力学研究进展

植物为适应陆地环境进化出木质部维管系统,通过水力学机制高效安全的向光合器官长距离运输水分,木质部水分运输对蒸腾、气孔运动、光合碳同化等生理过程有调控和协调作用,被称为植物生理学的支柱.植物水力学作为木质部水分运输的研究内容和手段,已成为整合植物与生态系统功能的中心枢纽.该文首先概述了植物水分运输的水力学机制、运输系统的...     

三个耐旱树种木质部栓塞化的脆弱性及其恢复能力

植物在长期适应赖以生存的自然环境中 ,形成了一套最适宜自身生长发育的生理生态行为 ,采取各种方式来抵御或忍耐水分胁迫的影响。如通过具有深广而茂密的根系格局来保持水分吸收 ,通过气孔调节、角质层障碍作用和小的叶蒸发表面来减少水分散失 ,通过渗透调节和增加组织弹性来保持膨压 ,通过增强原生质耐脱水能力来免受伤害或少受伤害等等。植物遭受干旱危害时 ,首先出现表型反应的多是植物的叶片 ,因此 ,研究植物的耐旱机理多从叶入手 ,对根系类型、分布及根茎比在植物耐旱性方面也有不少报道[1,2 ],而对木质部在干旱适应性反应方面的研究…     

植物体内水分长距离运输的生理生态学机制

植物体内长距离水分运输是植物生理生态学研究中的一个重要问题,长期为植物生理学家和生理生态学家所关注。木质部探针技术的问世,掀起了近年来植物生理学界最为激烈的一场争论。提出了已经有100多年,风行40年的内聚力-张力(Cohesion-Tension, C-T)学说受到质疑。随后维护派和质疑派围绕木质部探针技术、压力室技术(C-T理论的主要支撑实验技术)的可靠性展开辩论。进一步从物理学原理和各种实验上就C-T理论的3个支柱(木质部导管或管胞中巨大的张力、沿树高的压力梯度、连续水柱)进行争论。这场争论似暂告一段落,C-T理论没有被推翻,但仍留有问题期待以后的研究。     

Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica

The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.     

New evidence for large negative xylem pressures and their measurement by the pressure chamber method

Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.