植物叶肉导度的测定及计算方法综述

Measuring and calculating methods of plant mesophyll conductance: A review

叶肉导度(g_m)指叶肉细胞内部的CO_2扩散能力,它是叶肉细胞阻力的倒数.光合作用研究的早期,研究者们多将叶肉细胞对CO_2的扩散阻力视为零,即假定g_m无穷大,而忽略了其对光合作用的限制.但近来的研究表明,g_m是有限的,并随着环境条件的改变而发生变化,此外,g_m的大小直接决定了CO_2在叶片内的扩散量,进而影响到植物光合效率的高低.因此,g_m的估算对于植物光合能力的评估意义重大.目前,气体交换与叶绿素荧光相结合法、曲线拟合法以及瞬时碳同位素(^(13)CO_2)辨别法已经成为植物g_m估测的3种常用方法,但国内针对这3种方法原理及其优缺点介绍的文献极少,阐述这3种方法的原理、过程并比较分析其优缺点就显得尤为必要.本文综合了相关文献,从原理、推导过程及优缺点3方面对上述3种g_m估测方法进行了详细介绍.结果表明:曲线拟合法虽然易于理解,便于操作,但其拟合模型因光合作用的发生状态不同而不同,需要研究者对光合作用不同状态进行严格划分,不利于广泛推广应用;瞬时碳同位素(^(13)CO_2)辨别法虽然提高了结果的准确性,但其测定过程比较复杂,对试验操作的要求比较严格,同时该方法对试验误差的敏感性较差,可靠性不高.相较上述两种方法,气体交换与叶绿素荧光相结合法的可操作性更强,可靠性更高,更利于多处理多重复的大样本的观测分析,叶绿素荧光技术的使用,既简化了试验步骤,又降低了试验过程的偶然误差,增加了观测结果的科学性;此外,叶绿素荧光技术还能为叶片提供饱和脉冲活化能,从而最大限度地激发叶片的光合潜能,但该方法也存在很多问题,比如,为了提高叶片叶绿素荧光参数的准确程度,试验中需要使用较低的气体流速,而流速的降低又会增大气体扩散泄漏的风险,所以该方法对选择合理气体流速的要求很高.综合来看,气体交换与叶绿素荧光相结合法在植物g_m的实际测定中的认可度最高,使用最广泛.

Mesophyll conductance(gm)refers to the diffusion capacity of CO2 inside mesophyll cells,which is the reciprocal of resistance of mesophyll cells.In the early stage of photosynthesis research,mesophyll diffusion resistance to CO2 was usually assumed to be zero,namely the gm was infinite.In recent studies,however,the gm was found to be limited and changed with external environments.As gm directly determines CO2 diffusion and affects leaf photosynthetic efficiency,it is of great significance to mechanestic research of photosynthesis.Presently,simultaneous chlorophyll fluorescence and gas exchange,the curve-fitting and instantaneous carbon isotope(13CO2)discrimination are commonly used to estimate gm,but few literature have been introduced on those methods in China.Therefore,it is particularly necessary to elaborate the principles and processes of these methods and to compare their advantages and disadvantages.We synthesized the relevant literature,and introduced the three methods in detail from the aspects of principle,derivation process and advantages and disadvantages,aiming to provide a methodological basis to promote the research on gm in China.The curve-fitting method was easy to understand and operate.Its fitting model varied with the status of photosynthesis,which is needed to be divided strictly by researchers.Consequently,it was not conducive to be widely used.Although the instantaneous carbon isotope(13CO2)discrimination method improved the accuracy of results,it was complex in measurement and strict in operation.Furthermore,it was less sensitive to test errors with low reliability.Compared with the above two methods,simultaneous chlorophyll fluorescence and gas exchange was more operable and reliable,and was more conducive to the observation and analysis for large samples with multi-processing and multi-repetition.In addition,the use of chlorophyll fluorescence technology not only simplified the test procedures,but also reduced the accidental errors,making the results more scientific.Chlorophyll fluorescence technology also provided saturated pulse activation energy to maximize leaf photosynthetic potential.But this method also had many problems,for instance,to improve the accuracy of chlorophyll fluorescence parameters,a lower gas flow rate was needed,which would increase the risk of gas diffusion and leakage.Thus,this method had a high requirement for a reasonable gas flow rate.In general,simultaneous chlorophyll fluorescence and gas exchange method was most widely used in the actual determination of plant gm.