Table_5_Maize Canopy Apparent Photosynthesis and 13C-Photosynthate Reallocation in Response to Different Density and N Rate Combinations.doc

Appropriate planting density and nitrogen (N) supply are critical factors optimizing yield in crop cultivation. To advance the knowledge of maize plants under different density and N rate combinations, responses of canopy apparent photosynthesis (CAP), and assimilate redistribution characters (by 13CO2 stable isotope tracing) were investigated. In this study, two maize varieties DH618 and DH605 were grown at various planting densities (6.75, 8.25, 9.75, and 11.25 pl m−2) and N application rates (0, 180, 270, 360, and 540 kg ha−1) during 2013–2015. Maize grain yield (GY) was maximized at a density of 9.75 pl m−2 with 180–360 kg ha−1 N during the three study years. Maize GY, biomass, CAP, leaf area index (LAI), and 13C-photosynthate reallocation all responded more intensively to density than N rate, but the N response differed between varieties. We established links among CAP, LAI and biomass, and GY and kernel number per unit area (KNA). CAP depended on high LAI and enzyme activities for photosynthesis, yet both N deficiency and N excess had inhibitory effects. Besides, relations between 13C-photosynthate reallocation and yield components were executed. High density increased the 13C-photosynthate distribution in vegetative organs but reduced the allocation in ear, while N supply moderated the response. Based on our results, maize plants with greater CAP, more 13C-photosynthate distribution to ears, and less 13C-photosynthate distribution to stems under different density and N rate combinations could improve KNA and achieve a greater GY consequently.

适宜的种植密度与氮素供应是作物栽培中优化产量的关键因素。为深化对不同密度与施氮量组合下玉米植株生理响应的认知，本研究针对冠层表观光合速率（canopy apparent photosynthesis, CAP）及同化物分配特征（通过13CO2稳定同位素示踪技术）展开了系统探究。
本试验于2013—2015年开展，选用玉米品种DH618与DH605为供试材料，设置4个种植密度梯度（6.75、8.25、9.75及11.25 株·m⁻²）与5个施氮量梯度（0、180、270、360及540 千克·公顷⁻¹）。
三年试验结果显示，当种植密度为9.75 株·m⁻²、施氮量处于180—360 千克·公顷⁻¹区间时，玉米籽粒产量（grain yield, GY）可达最大值。相较于施氮量，种植密度对玉米籽粒产量、生物量、冠层表观光合速率、叶面积指数（leaf area index, LAI）及13C-光合产物再分配的调控效应更为显著，但不同品种对施氮的响应存在差异。
本研究构建了冠层表观光合速率、叶面积指数、生物量与籽粒产量及单位面积穗粒数（kernel number per unit area, KNA）之间的关联关系。冠层表观光合速率依赖于高叶面积指数与光合相关酶活性，但氮素匮乏与氮素过量均会对其产生抑制作用。此外，本研究还解析了13C-光合产物再分配与产量构成因素间的关联。
高密度处理会提高13C-光合产物向营养器官的分配比例，降低其向果穗的分配量，而施氮可缓解这一响应趋势。综合本研究结果可知，在不同密度与施氮量组合下，若玉米植株具备更高的冠层表观光合速率、更多的13C-光合产物向果穗分配以及更少的向茎秆分配量，即可提升单位面积穗粒数，最终实现籽粒产量的提升。