温带主要树种叶肉导度对水分胁迫和氮添加的响应及其对森林光合作用模型的改进

Photosynthesis is the basis of the forest carbon sink. Traditional photosynthetic models only include the leaf surface conductance and the stomatal conductance, while ignore the mesophyll conductance to CO2, the conductance between the sub-stomatal cavity and the chloroplast. So these models were with shortcoming in mechanism and impacted the precision of the estimation and prediction of forest carbon exchange. The fluctuation in rainfall and increase in nitrogen deposition, which are the main aspects of global change, also increase the uncertainty of conventional models of estimating forest photosynthesis. This proposal will aim at the five main species in temperate forest in northeastern China, probing the influences of water stress and nitrogen addition on mesophyll conductance of these species under the artificial modification conditions using the curve-fitting method and the combined chrorophyll fluorescence and gas exchange method. And the anatomical and physiological reasons will be studied. The traditional photosynthetic models will be revised by introducing the mesophyll conductance and the relation with CO2 concentration in intercellular and inside the chloroplasts. And the revised model will be validated by the experimental data of sapling crown gas exchange and the CO2 flux measurements over the canopy and soil respiration in broad-leaved Korean pine mixed forest in Changbai Mountain. This study is expected to provide more elaborate scientific basis for carbon exchange mechanism of the forest.

森林的光合作用是森林碳汇形成的基础，传统的光合作用模型只考虑了CO2从大气环境到叶表面以及从叶表面进入气孔这两段路径的导度，而忽略了从气孔下腔到叶绿体基质的叶肉导度，导致了模型的机理性缺陷，影响了森林碳循环估算和预测的准确性，随着全球变化影响下降水波动和氮沉降增加，加剧了森林光合作用传统计算方法的不确定性。针对上述热点科学问题，本项目拟以我国东北温带5个主要树种为研究对象，在水分胁迫和氮添加处理的基础上，采用国际通用的气体交换以及气体交换-叶绿素荧光相结合方法观测叶肉导度，探索水分胁迫和氮添加处理对叶肉导度的影响规律，分析叶肉导度变化的解剖学和生理学原因。将叶肉导度及其与胞间和叶肉CO2浓度的关系引入传统光合作用方程，对温带混交林碳交换模型进行改进，并利用幼树树冠的CO2交换观测结果、长白山阔叶红松林生态系统CO2通量和土壤呼吸观测结果进行检验，为准确理解森林碳循环机制提供科学基础。

叶肉导度gm即CO2在叶片内部的扩散能力，是表达植物光合作用机理的重要参数，但我国温带树种gm对水分和氮沉降的响应特征还不清楚，严重影响着人们对森林碳交换机制的科学理解。本研究选取长白山阔叶红松林两大优势树种—水曲柳和蒙古栎作为研究对象，探究其gm对土壤长期干旱(从20天到75天)、短期干旱(20天)+旱后复水及土壤氮添加的响应，并对其机理进行叶片解剖学和生理生化学分析。主要研究结果如下：(1)土壤长期和短期干旱期间，两树种gm均减小，且减小幅度随水分胁迫加重而增大。(2) 蒙古栎gm略高于水曲柳gm，但其随水分胁迫的降低程度略小于水曲柳gm，说明水分胁迫对水曲柳gm的影响更大。(3)两树种gm在土壤复水后均得到一定恢复，恢复程度因初始水分胁迫的强度而异, 初始水分胁迫越强，恢复能力越弱；蒙古栎gm的旱后恢复能力略高于水曲柳。(4)氮添加后两树种gm均有增加，gm增量随土壤氮添加水平升高先增大后减小，中度氮添加(46 kgNha-1a-1)下gm增量最大。（5)gm对干旱、复水和氮添加的响应可由解剖学和生物化学原因解释，在解剖学方面，干旱影响下叶肉细胞脱水萎缩黏连成团簇状，叶肉细胞面向细胞间隙的面积Smes有减小的趋势；复水后该黏连解除，细胞体积增大，Smes有增大趋势，可促进gm的恢复；在生物化学方面，叶片水通道蛋白AQP和碳酸酐酶CA活性随水分胁迫减小，复水后增大，这显著影响了CO2的跨膜运输及其向-HCO3的转化，从而促进gm的旱后恢复。氮添加同样具有增加Smes、增强AQP和CA活性、增加叶氮含量的效果，显著促进了gm的增大。(6)将gm的多因子连乘公式代入混交林冠层碳交换模型，并用长白山阔叶红松林涡动通量和土壤呼吸观测数据进行检验，发现模型模拟效果略有改善。本研究结果对于揭示全球变化背景下森林生态系统碳循环规律、准确评估和预测碳收支具有重要意义。