气生与地生根形态结构及干旱胁迫生理响应差异——以兰属附生与地生植物为例

Epiphytes are an important component of tropical forests, and are especially rich within Orchidaceae. Epiphytes are usually more sensitive to disturbance and environmental change than terrestrial plants, and water is arguably the most limiting for the growth of vascular epiphytes. However, global environmental change exert water-limited stress on this population. It is speculated that roots play important roles in adaptation of epiphytic orchids to survive in water-limited condition now and in future, as they involved in water absorption and transportation. However such knowledge is limited. Thus, the present project will mainly focus on the differences of roots in epiphytic and terrestrial Cymbidium to drought stress. The main research contents include: (1) What are the key differences in root structures and functions between epiphytic and terrestrial Cymbidium? (2) What are the differences in response to drought between epiphytic and terrestrial Cymbidium, how do the water conductance, abscisic acid and aquaporin response to drought? (3) What are the differences in plasticity to drought between epiphytic and terrestrial Cymbidium, how do the root system, root and root tip structures and physiological response to drought? By the combining of morphological, physiological and molecular data, we hope the findings will bring a better understanding of roots in epiphytic and terrestrial Cymbidium to drought, and we also hope the accomplishment of this project will shed light on how epiphytic orchids response to global change, as well as on the protection and sustainable use of Cymbidium species.

附生兰科植物是热带森林生态系统的重要组成，同时对水分限制最为敏感，但全球气候变化可能导致附生兰科植物未来经历更频繁或长期的干旱胁迫。根是参与水分获得与运输的重要器官，相较地生根，附生兰科植物的气生根缺少土壤的支持，认识其与地生根形态结构及干旱胁迫响应差异，有助于理解附生兰科植物对“当下”与“未来”附生生境的适应，申请者拟以兰属15种典型附生与9种地生植物根为研究对象，开展三方面的研究（1）气生与地生根形态解剖结构及生理功能的差异；（2）气生与地生根短期干旱响应策略差异，根导水率、水通道蛋白、脱落酸及耐旱基因在干旱胁迫过程差异变化；（3）气生与地生根长期干旱响应策略差异，根系与根尖经历干旱胁迫后生长模式及形态差异变化。以期从形态结构、短期与长期干旱生理响应策略认识气生与地生根干旱胁迫的适应差异，为附生兰科植物响应全球变化提供新的认知，同时为兰属植物资源的保护和利用提供科学依据。

附生兰科植物是热带森林生态系统的重要组成，同时对水分限制最为敏感。根是参与水分获得与运输的重要器官，相较地生根，附生兰科植物的气生根缺少土壤的支持，认识其与地生根形态结构及干旱胁迫响应差异，有助于全面理解附生兰科植物对环境的适应机制。本项目对比研究附生与地生兰科植物，通过结合多层次形态解剖观测、干旱胁迫过程生理响应监测及转录组与代谢组学深入考察生理响应的内在机制，分析附生与地生植物根形态解剖结构，及根对干旱胁迫响应差异。研究结果发现（1）位于附生兰属植物气生根最外层由数层死细胞组成的根被结构对于其胁迫适应具有重要作用，其细胞的发育及成熟过程为细胞程序化死亡过程，该过程包括细胞程序化死亡的诱导，细胞次生细胞壁的积累，及细胞死亡后的自溶清空过程。且根被细胞的成熟过程由外层细胞开始，逐渐向内层细胞推进。（2）经历短期剧烈干旱胁迫后，附生兰科植物根被表面覆盖防水层，该层物质在生物合成调控、化学组成、表面结构、减少水分流失的功能等方面均与角质层具有相似性。不同附生兰科植物气生根的类角质层的厚度存在显著差异，根被较薄的气生根的类角质层较厚。类角质层和根毛在空间上的分离发育，实现了气生根的功能分区。（3）不同兰科植物肉质化根采用不同的物理和化学防御策略，防御和储存功能协调取决于物种的年龄。肉质化根具有较强储存能力，则相应的化学防御物质较多，且具有较厚的根被和较高比例的草酸钙针晶含量。本项目揭示了根被及根被外类角质层结构对附生兰科植物干旱适应的重要作用，初级代谢产物贮藏能力与肉质化根的防御能力的协调关系。本项目丰富了对于附生兰科植物适应性的认识，为兰科植物的保护与利用提供科学依据。