Increasing planting density is a key agronomic strategy to enhance maize yield; however, excessive density may result in an imbalanced population structure, reduced utilization efficiency of limited resources (e.g., light), and suppressed yield potential. Gene editing can optimize canopy architecture through targeted improvement of maize plant type, thereby enhancing adaptability to high-density planting and boosting yield. Elucidating the effects of plant type improvement on root-shoot characteristics, grain yield, and density response in spring maize, as well as the underlying mechanisms, will provide theoretical and technical foundations for optimizing plant type and achieving high-yield dense planting in spring maize.

The field experiment was conducted at Gongzhuling farm in Jilin, China. In this study, two maize hybrids, includding Jingke 968 and the improved plant types Jingke Y968, were grown with 60 000 plants/hm² (D1), 75 000 plants/hm² (D2) and 90 000 plants/hm² (D3) in 2019 and 2020, respectively. The effects of two plant types of spring maize of the same genetic background on the root-canopy characteristics and yield of spring maize were studied.

Under normal density conditions (D1), there were no significant differences in leaf area index (LAI), net photosynthetic rate (Pn), PAR utilization (PUE), dry matter accumulation and grain yield between the two different plant types spring maize cultivars. However, compared with Jingke 968, under D3 conditions, the improved plant type Jingke Y968 had a relatively high number of main roots (7.2%) and a relatively large weight of root dry matter (6.0%), which promoted the absorption of nutrients; furthermore, under D2 and D3 conditions, Jingke Y968 significantly improved the canopy structure of maize, so that the upper, middle and lower parts had relatively low leaf angles, higher leaf orientation and LAI, and the excellent canopy structure increased the Pn of mid-to-late ear leaves of (7.5% (D2) and 7.7% (D3)) and PUE (4.3% (D2) and 10.8% (D3)). The structural equation results showed that higher leaf direction values and LAI could positively and directly increase the accumulation of dry matter in the aboveground, thereby increasing grain yield (8.7% (D2) and 11.2% (D3)).

In summary, the improvement of plant type enabled Jingke Y968 to have higher main root number and larger root dry matter weight under high-density conditions, which was conducive to nutrient absorption in the underground part. Meanwhile, its leaves were more compact, Pn was significantly increased, PUE was effectively improved, and root-canopy characteristics were more reasonable, which promoted dry matter accumulation in the above-ground part. Thus, the relatively high grain yield could be obtained.

Dense planting is one of the main measures to improve the high yield cultivation of maize, and density has a significant effect on the formation of maize starch. Therefore, this study analyzed the granule size distribution and viscosity parameters of different types of spring maize kernels under different increasing density conditions for improving the quality of maize starch.

The field experiments were conducted at the Gongzhuling experimental base in Jilin province in 2019 and 2020. In the present experiments, eight main maize varieties in Northeast China, such as Xianyu 335, Zhengdan 958 and Nonghua 101, etc, were selected as experimental materials, and two planting densities of 67 500 and 97 500 plants/hm² were set. The granule size distribution and viscosity parameters of starch in different treatments were measured by diffraction particle size analyzer and viscosity analyzer, and the relative quality of maize was measured by near-infrared analyzer, and the correlation analysis was conducted to clarify the effects of increasing density on the granule size distribution and viscosity parameters of endosperm starch in spring maize.

The results showed that with the increase of planting density, maize grain yield and starch content increased significantly, and the increase of density significantly increased the volume, surface area and number percentage of large (>17 μm) starch granules, while the opposite trend was observed in the volume, surface area and number percentage of small (<3 μm) starch granules. It could be seen that with the increase of planting density, the volume and number percentage of small starch granules in maize kernels decreased significantly, and the volume and number percentage of large starch granules increased, indicating that increasing density was beneficial to the increase of the volume ratio of large starch granules, that is, increasing density promoted the accumulation of starch and increased the number of large starch granules and the formation of individual volume. At the same time, it was found that the peak viscosity, trough viscosity, breakdown and final viscosity of maize starch were significantly increased after increasing density. The results of correlation analysis showed that the starch content, yield and viscosity parameters of maize grain were negatively correlated with the volume percentage of small starch granules, significantly or extremely significantly negatively correlated with the volume percentage of medium (3-17 μm) starch granules, and significantly or extremely significantly positively correlated with large (>17 μm) starch granules.

Increasing planting density could increase maize grain yield, starch content and its viscosity parameters by mainly affecting the granule size distribution of endosperm starch, namely increasing the proportion of large starch granules and reducing the proportion of small and medium-sized ones.