活立木生理干燥过程中水分传输和散失机制探讨

Discussion on Mechanism of Water Transmission and Loss for Standing Trees during Physiological Drying

木材中的水分会严重影响木材加工和使用性能,必须通过干燥使木材含水率控制在适宜范围内。常规蒸汽干燥耗能大、干燥缺陷多,热泵除湿干燥、太阳能干燥等新型节能干燥技术工业化应用尚不理想,因此,本文从木材水分来源和树木水分生理特性出发,探讨基于蒸腾作用降低木材水分的活立木生理干燥理论和技术,并从水分与植物生理的角度阐述活立木生理干燥的理论基础。通过分析树叶水分蒸发研究进展,总结叶内水分可能的3种蒸发位点,即暴露在内部气体空间的所有叶肉细胞和表皮细胞、气孔下腔室周围大部分区域的叶肉细胞和表皮细胞以及气孔下腔室周围其他区域的叶肉细胞和表皮细胞。通过分析植物体内水分传输机制研究进展和现状,总结植物叶内水分传输的3种可能途径,即通过胞间连丝的共质体传输途径、通过水孔蛋白的跨细胞传输途径以及通过未栓化细胞壁的质外体传输途径。阐明被广泛用于解释木质部水分长距离运输的内聚力-张力学说,分析其目前存在的争论及一些新提出的学说,如补偿压学说、多驱动力学说或水门学说,并分析木质部水分运输过程中时常发生的空穴和栓塞现象及其可能的恢复机制。在此基础上,提出今后研究活立木生理干燥过程中水分传输和散失机制的几个重点和方向:一是探讨生理干燥过程中处于水分胁迫状态下树叶叶孔蒸腾和角质层蒸腾之间的关系;二是探讨生理干燥过程中处于严重水分胁迫状态下树木叶内水分传输途径和蒸发位点;三是探讨纹孔等微观构造在木质部水分长距离传输中的作用以及在空穴和栓塞产生和恢复过程中的作用;四是探讨生理干燥过程中木质部内空穴和栓塞的产生和恢复机制及其对水分长距离传输的作用和影响。

Moisture in wood gives serious effect on wood utilization and processing,and moisture content of wood should therefore be adjusted to an appropriate range through drying.Conventional drying can be improved due to great energy consumption and considerable drying defects,while the industrialization of new energy-saving drying technologies such as heat pump drying and solar drying are far from perfection.Considering the source of water in wood and the physiological properties of water in trees,the present work discusses a new drying theory and technology for living trees,namely,physiological drying based on transpiration.The basic theory of physiological drying for living trees from the viewpoint of water and plant physiology are expounded.Three possible evaporation locations summarized from the researches on water evaporation in leaves are presented as:all the mesophyll cells and epidermis exposing into the inner air spaces,the most mesophyll cells and epidermal cells around the substomatal cavity nearby the stomatal pore,and other mesophyll and epidermal cells around the substomatal cavity.By analyzing the research on water transmission theory in plant,three potential water transport ways inner leaves are presented as:the symplastic pathway via plasmodesmata,the transcellular pathway by aquaporin and the apoplastic pathway through the non-suberized cell wall.Studies on the cohesion-tension theory(CT/CTT)widely used to explain the dynamics of long-distance water transport in xylem with the current arguments and some rising theories,such as the multi-force theory and watergate theory are reported.In addition,the common phenomenon of cavitation and embolism and its recover mechanics during the process of long-distance water ascent is discussed.On this basis,4 focal points for further studies are suggested as:1)To explore the relationship between stomatal transpiration and cuticular transpiration when the trees were subjected to severe water stress in the process of physiological drying;2)To locate the water evaporation points within leaves;3)To understand the effect of the microstructure of pits on the process of long-distance water transportation in xylem and on the forming and recovering processs of the cavitation and embolism;4)To reveal the mechanism of the formation and recovery of cavitation and embolism in physiological drying,and their effects on the long-distance water transportation.