竹子水分利用的生态适应性：年龄、夜间液流和根茎补水的效应

Bamboo species with shallow root system are more sensitive and vulnerable in response to environmental water stress. Water use of bamboo species is more likely to be effected by stem age because they lack of secondary growth and can’t renew the xylem structure after maturity, which lead to gradual reduction of the hydraulic conductivity of the water transport system. Therefore, bamboo faces a contradiction between the strong transpiration required by rapid growth and xylem vulnerability. Developed root rhizome system of bamboo species plays a special role in water and nutrient compensation between culms. Large numbers of parenchyma cells and the cavity structure within culm provide important guarantee for recharging water during nighttime. Bamboo species have excellent competitiveness and adaptability to heterogeneous environment, that might be due to special hydraulic character and water use efficiency. Accordingly, it is suggested that “nighttime sap flow and water compensation between culms through rhizome are distinctive hydraulic adjustment strategy for bamboo species to cope with and to adapt to the heterogeneous habitats”, which would, to a certain extent, avoid hydraulic imbalance and maintain the safety of water transmission. This project will use self-made modified thermal dissipation probe (TDP) to monitor culm transpiration of different types of bamboo species, and to analyze their physiological structure, water use mechanism and hydraulic adaptive strategies in response to changes in the environment and their age effect, in order to explore the water use characteristic of different bamboo species and their influence to the local water cycle. All of these have both theoretical and practical meanings for cultivation resources and sustainable development of bamboo species.

浅根系的竹类植物对环境水分胁迫的响应较为敏感和脆弱，由于缺乏次生生长，竹子成熟后无法更新木质组织而逐渐降低水分传输系统的传导性，水分利用更易受年龄的影响，形成了快速生长而要求旺盛的蒸腾与输水系统脆弱性之间的矛盾。竹子发达的根茎对异株的水分和养分补偿功能发挥了特殊作用，大量的薄壁细胞和空腔结构为夜间补水的储存提供重要保障。竹子对异质性环境出色的竞争力和适应力与特殊的水力特征和水分利用效率有直接的关系，据此，我们认为“夜间液流和根茎补水是竹子水分利用适应异质性生境的特殊水力调节策略”，一定程度上避免水力功能失衡和维持水分传输的安全性。本项目将采用改进的自制式热消散液流探针监测不同根茎类型竹子的水分蒸腾，分析其生理结构特征、夜间液流的补水机制、应对环境变化的水力适应和年龄效应，探索竹林的水分利用规律和对林区水文循环的影响，对竹类植物资源的培育和可持续开发具有重要的理论价值和实践指导意义。

竹类植物旺盛的蒸腾能力是其快速生长和繁殖的有力保障，也是其适应不同环境能力的表现。但与此同时，禾本科竹类植物由于其不同于其他乔木的独特的生物学结构，水分传输系统较为脆弱性。对不同类型的竹种水分利用与环境因子之间的关系，以及它们在应对水分可用性变化时的生理生态特征和适应策略仍未可知。本项目通过对比散生型毛竹和丛生型粉丹竹的液流规律，结合不同年龄段的水力特征、光合能力指标与气象因子的关系等，研究它们的蒸腾耗水规律和生态适应性。研究发现：（1）蒸腾速率、光合能力、整株水力导度和叶片叶绿素含量等生理指标都随着竹龄增加而显著下降，因此老龄竹在面对水分胁迫时（比如空气水分亏缺（VPD）较高或者土壤水分含量减少），水力失效的脆弱性更甚；（2）散生型的毛竹通过相连的根茎系统进行异株间的水分再分配。通过切根实验，我们发现幼竹有更加旺盛的蒸腾能力，更低的叶片水势使得其从其他茎秆中获取额外的水分补充，这有助于水分需求旺盛的幼竹的生长，保证了毛竹种群的繁殖能力；（3）竹类植物旺盛的夜间液流活动补偿了日间过度损失的水分，并为次日的蒸腾活动提供储存水。本项目的研究结果为探讨竹类植物在异质环境下的适应能力提供了理论基础支持。