The border row crop in strip intercropped maize is often exposed to heterogeneous light conditions, resulting in increased photosynthesis and yield decreased. Previous studies have focused on photosynthetic productivity, whereas carbon allocation could also be one of the major causes of decreased yield. However, carbon distribution remains unclear in partially shaded conditions. In the present study, we applied heterogeneous light conditions (T), and one side of plants was shaded (T-30%), keeping the other side fully exposed to light (T-100%), as compared to control plants that were exposed entirely to full-light (CK). Dry weight, carbon assimilation, ¹³C abundance, and transport tissue structure were analyzed to clarify the carbon distribution in partial shading of plants. T caused a marked decline in dry weight and harvest index (HI), whereas dry weight in unshaded and shaded leaves did not differ. Net photosynthesis rate (Pn), the activity of sucrose phosphate synthase enzymes (SPS), and sucrose concentration increased in unshaded leaves. Appropriately, 5.7% of the ¹³C from unshaded leaves was transferred to shaded leaves. Furthermore, plasmodesma density in the unshaded (T-100%) and shaded (T-30%) leaves in T was not significantly different but was lower than that of CK. Similarly, the vascular bundle total area of T was decreased. ¹³C transfer from unshaded leaves to ear in T was decreased by 18.0% compared with that in CK. Moreover, ¹³C and sucrose concentration of stem in T were higher than those in CK. Our results suggested that, under heterogeneous light, shaded leaves as a sink imported the carbohydrates from the unshaded leaves. Ear and shaded leaf competed for carbohydrates, and were not conducive to tissue structure of sucrose transport, resulting in a decrease in the carbon proportion in the ear, harvest index, and ear weight.

To ensure yield in strip-intercropped maize with soybeans (SM), it is crucial to plant at a density comparable to that of monoculture maize (MM). This requires reducing spacing by more than half, increasing intraspecific competition, and altering stalk lodging resistance traits compared with MM. Nitrogen fertilization can effectively mediate stalk lodging resistance. However, it is still unclear how nitrogen rates influence SM’s stalk lodging resistance under high-density conditions and how that resistance compares to MM. The experiment involved four N fertilizer treatments with two planting densities: medium density (60,000 plants/ha) and high density (75,000 plants/ha). Additionally, different planting patterns of strip-cropped (S) and monoculture (M) were implemented. The N fertilizer application rates were N0 (0 kg/ha), N225 (225 kg/ha), N300 (300 kg/ha), and N375 (375 kg/ha). The stalk lodging resistance was represented by the breaking strength of the third basal internode. The study revealed that, at the same planting density, the third basal internode of the stalk exhibited consistent results in terms of its diameter, crushing strength, total number and area of vascular bundles, and N content. Notably, all these traits exhibited a significant positive relationship with breaking strength. The highest values for these parameters and yield were observed under N225 and N300 fertilization rates for medium-density monoculture and strip-cropped maize, respectively. In contrast, the high-density monoculture and strip-cropped maize showed peak performance under N300 and N375 fertilization rates. At both medium and high planting densities, the strip-cropped maize exhibited 8.9% and 10.9% lower breaking strength than the monoculture maize under N225 treatment. However, increasing the N fertilizer application resulted in comparable lodging resistance between the strip-cropped maize and the maximum values of the monoculture maize, at N300 treatment for medium density and N375 treatment for high density. Hence, strip-cropped maize planted at high density (75,000 plants/ha) with a lower nitrogen rate had lower lodging resistance than monoculture maize, but it can be improved to match the monoculture maize by increasing the nitrogen rate.