砂仁光合作用的CO_2扩散限制与气孔限制分析

CO_2 DIFFUSIONAL AND STOMATAL LIMITATIONS OF PHOTOSYNTHESIS IN AMOMUM VILLOSUM

目前常用从气体交换参数计算的胞间CO2 浓度(Ci)来计算气孔限制值(Ls),但由于胁迫情况下计算的Ci偏高常导致结果不准确。该文引入扩散限制分析概念,以砂仁为例介绍了一种不需Ci 的计算扩散限制值(Ld)的新方法。同时通过叶绿素荧光参数间接估算受干旱胁迫植株的Ci(用Ci′表示)计算气孔限制值(Ls′)。采用这3种方法分析了生长在100%和40%土壤相对湿度(RSM)下的砂仁(Amomum villosum)净光合速率的限制因素。结果表明两种水分状况下砂仁午后净光合速率的限制因素不同。100%RSM下,午后砂仁Ls 没有升高,说明光合作用气孔限制并未增强;午后其Ld 升高表明光合作用的CO2 扩散限制增强,这主要是由叶肉阻力相对增大所致。40%RSM下,午后砂仁Ls′升高比Ld 升高明显,说明气孔阻力在所有扩散阻力中占主导作用,是限制净光合速率的主要原因;而其Ls 午后并未升高,暗示传统的气孔限制分析会得出非气孔限制的错误结论。Ci′低于Ci,说明干旱胁迫时传统的气体交换方法高估了Ci。上述结果都证明水分胁迫情况下传统方法不可靠,该文介绍的两种新方法比较准确可靠,同时使用两种新方法还可定性推测叶肉阻力的变化方向。

Photosynthetic rate is a function of not only the CO_2 concentration gradient between the outside and inside of the leaf, but also the CO_2 diffusional resistance. It is accepted that stomatal resistance is the greatest factor that controls CO_2 diffusional resistance and is thus a crucial factor influencing photosynthesis. Hence, adequate analysis of CO_2 diffusional resistance is necessary for understanding photosynthesis. Intercellular CO_2 concentrations (C_i) are often utilized to calculate stomatal limitation (L_s). This traditional analysis is unreliable when plants are under stress, because C_i cannot be accurately measured under such conditions. Here we introduced the concept of diffusional limitation and introduced a new method to calculate the diffusional limitation value (L_d) without using C_i. In addition, C_i, estimated indirectly through chlorophyll fluorescence parameters (C_i′), was used to calculate a new stomatal limitation value (L_s′) for plants grown under 40% relative soil moisture (RSM). We compared the L_s′, L_s and L_d for Amomum villosum grown under both 100% and 40% RSM. Photosynthetic rates (P_n) and stomatal conductance (G_s) decreased after noon in both RSM treatments, and P_n and G_s were both higher in 100% RSM than in 40% RSM. Under 100% RSM, L_s did not increase after noontime in A. villosum, indicating stomatal limitation of photosynthesis did not increase, whereas L_d increased indicating the diffusional limitation of photosynthesis increased due to the relatively high mesophyll resistance. Under 40% RSM, L_s′ increased sooner than L_d after noon, indicating stomatal resistance was the dominant factor controlling diffusional resistance. In contrast, L_s, calculated using the traditional stomatal limitation method, did not increase under 40% RSM, which might lead us to the wrong conclusion that stomatal limitation was not a factor. The estimated value, C_i′, was lower than C_i, indicating that gas exchange system was overestimated using C_i under conditions of water stress. Our results suggest that the traditional method is unreliable under soil water stress conditions, and the two new methods presented are more reliable. Furthermore, mesophyll resistance can be estimated indirectly through the joint analysis of diffusional and stomatal limitation.