大豆幼苗对套作玉米遮荫环境的光合生理生态响应

以2个耐荫性不同的大豆品种为材料,田间试验设置大豆单作和玉米-大豆套作2个种植模式处理,研究不同耐荫性大豆品种的幼苗光合生理生态特性对套作玉米遮荫环境的响应。结果表明:1)玉米-大豆套作模式中,玉米遮荫显著降低大豆冠层的光合有效辐射,导致大豆幼苗光合速率、气孔导度、蒸腾速率显著下降(P0.05),分别达37.9%、54.2%和42.4%,但品种间无显著差异;而胞间二氧化碳浓度和Fv/Fm无显著变化,且光合速率的下降与气孔导度存在显著相关关系,光合速率下降主要是由气孔限制和CO2同化过程中能量不足所致;2)玉米遮荫显著降低大豆幼苗叶面积指数、叶片碳含量、叶片和根系干重及总生物量,且品种间差异显著,相关性分析显示,叶面积指数下降是导致生物量减少的主要原因;3)玉米遮荫环境中,大豆幼苗的叶片叶绿素和氮素含量提高以增强光捕获能力,但它们并不能补偿因光截获面积降低而引起的光截获量下降。

Photosynthetic responses of soybean (Glycine max) seedlings to shading caused by maize in an intercropping system

Maize-soybean relay intercropping pattern, with highly economic, ecological and environmental significance, is widely applied in southwestern China. However, growth of soybean seedlings may be depressed by maize shade in this cropping pattern. The objective of this paper was to study the photosynthetic responses of soybean seedlings to maize shading in relay intercropping system in terms of morphological plasticity and photosynthetic parameters compared with mono-cropping system. A field experiment was conducted to compare gas exchange, chlorophyll fluorescence parameters, pigments contents, leaf carbon and nitrogen contents, leaf area index and dry matter accumulation of two soybean cultivars in two cropping patterns, i.e., soybean mono-cropping and maize-soybean relay intercropping. Compared with the mono-cropped seedlings, soybean seedlings intercropped with maize had significantly lower net photosynthetic rate (P_n)(-37.9%), transpiration rate (T_r) (-42.4%) and stomatal conductance (G_s) (-54.2%) due to low available light, but there was non-significant difference in two soybean varieties. The intercellular carbon dioxide concentration (C_i) and F_v/F_m were not changed significantly, in comparison with the seedlings under mono-cropping. Additionally, reductions in photosynthetic rate induced by maize shading was correlated significantly with stomatal conductance. The maize shading-induced depressions of photosynthetic rate may be attributed to stomatal limitation and energy deficiency in CO₂ assimilation process. Leaf carbon contents, leaf weight, leaf area index, root dry weight, total biomass and root/shoot ratio was also reduced because of maize shading, and significant difference was found between the two soybean varieties, which indicated that Nangdou12 might be a better cultivar to be relay intercropped with maize in Southwest China. The correlation analysis indicated that the relative reduction in biomass was caused by the decline in leaf area index rather than P_n. Although automatic regulation of light-harvesting efficiency in terms of total nitrogen contents, total chlorophyll contents and chlorophyll b contents was significantly improved in soybean seedlings in relay intercropping condition, it could not compensate the decline of light interception caused by the reduced light interception area. Therefore, the growth parameters may be more important index for assessing the adaptation of soybean cultivars to low light condition in intercropping system than ecophysiological parameters merely at unit leaf level.